ci(coderabbit): make Release-As releases explicit in the release check

The release check claimed a Release-As footer could override the bump rule, but CodeRabbit's pre-merge checker cannot see commit footers, so a forced-version release would fail with no stated way through. It now names both versions and says a maintainer must ignore the check for such a release. Co-Authored-By: Claude Opus 5.5 <[email protected]>
ci(coderabbit): decide the release check from the title and diff only
2026-11-08 01:09:46 -10:00 · 2026-11-04 10:43:23 +08:00 · 2026-11-04 10:32:44 +08:00 · 2026-11-04 10:21:17 +08:00 · 2026-11-04 10:09:35 +08:00 · 2026-11-04 09:59:39 +08:00
163 changed files with 2207 additions and 14129 deletions
@@ -5,8 +5,13 @@ tone_instructions: "Be direct and prioritize actionable correctness, safety, CLI

 reviews:
  profile: "chill"
-  # Allow explicit @coderabbitai approve / resolve commands to submit review verdicts.
-  request_changes_workflow: true
+  # CodeRabbit requests changes for blocking findings and approves the PR once
+  # they are resolved, the latest commit is reviewed, and every pre-merge check
+  # below passes. This is the current key for the deprecated
+  # request_changes_workflow; setting both keys turns approvals off. Explicit
+  # @coderabbitai approve / resolve commands keep working.
+  auto_approvals:
+    enabled: true
  high_level_summary: true
  poem: false
  # Avoid noisy "review skipped" status comments when CodeRabbit decides not to run.
@@ -32,6 +37,26 @@ reviews:
    - path: "**/*.go"
      instructions: |
        Do not request Plan 9 or other unsupported GOOS/GOARCH shims. The five native prebuilt release targets are linux-x64, linux-arm64, macos-arm64, macos-x64, and windows-x64. Android/Termux is also a supported runtime through the documented android-arm64 source build and Linux artifact fallback. Other GOOS/GOARCH combinations are not a review criterion.
+  pre_merge_checks:
+    custom_checks:
+      - name: "npm release PR validated"
+        mode: "error"
+        instructions: |
+          Merging a release PR publishes the package to npm, so CodeRabbit approves one only after this check passes.
+
+          Decide every item from the PR title and the diff alone. This check does not receive the head branch name, the PR body, or GitHub Check results, so no item depends on them. If an item still cannot be decided from the title and the diff, fail it and say what was missing; never report this check as inconclusive.
+
+          Scope: a PR is a release PR when its title starts with `chore(main): release`, or when its diff changes `version` in `package.json` or any value in `.release-please-manifest.json`. Either signal is enough, so editing the title does not take a release PR out of scope. Every other PR passes this check; say "Not a release PR."
+
+          For a release PR, NEW is `version` in `package.json` after the change, and PREVIOUS is the `"."` value of `.release-please-manifest.json` before the change. The check passes only when all of the following hold. When it fails, list each failed item with the file and the value found.
+
+          1. Title: the title is exactly `chore(main): release NEW`. A malformed or edited title fails this item; quote it.
+          2. Changed files: the diff touches only `package.json`, `package-lock.json`, `.release-please-manifest.json`, and `CHANGELOG.md`. Any other path fails, whether source, tests, workflows, scripts, dependency manifests, or lockfiles.
+          3. One version everywhere: NEW equals the top-level `version` and `packages[""].version` in `package-lock.json`, and the `"."` value in `.release-please-manifest.json`.
+          4. Version-only edits: in `package.json` only the `version` line changes, in `package-lock.json` only the two root version fields change, and in `.release-please-manifest.json` only the `"."` value changes. No dependency, `engines`, `files`, `bin`, `exports`, or `scripts` change is allowed.
+          5. Valid increment: NEW is a plain MAJOR.MINOR.PATCH with no prerelease or build suffix, and it is greater than PREVIOUS.
+          6. Bump level: this repo uses release-please defaults, without `bump-minor-pre-major`. A breaking change bumps MAJOR, even from 0.x to 1.0.0. A feat bumps MINOR and resets PATCH, and a fix bumps PATCH. Read the change types from the new CHANGELOG section: a "BREAKING CHANGES" heading means a breaking change, "Features" means a feat, and every other section, such as "Bug Fixes", "Performance Improvements", "Reverts", or "Dependencies", counts as a fix. NEW must be exactly the next version for the highest change type present. This check cannot see commit footers, so it cannot confirm a `Release-As:` override. When NEW differs from the version these rules give, fail this item and name both versions; a release forced with `Release-As:` passes only when a maintainer ignores this check.
+          7. Changelog: `CHANGELOG.md` gains exactly one new section, at the top, headed `## [NEW](https://github.com/Twigpine/zero/compare/vPREVIOUS...vNEW) (YYYY-MM-DD)`, with at least one entry. Everything below it is unchanged, so no earlier section is edited or removed.
  tools:
    github-checks:
      timeout_ms: 300000
@@ -46,10 +71,11 @@ reviews:
      - "[skip review]"
    labels:
      - "!do-not-review"
+    # github-actions[bot] is not ignored on purpose: it opens the release-please
+    # PRs, and the "npm release PR validated" check has to run on them.
    ignore_usernames:
      - "dependabot[bot]"
      - "renovate[bot]"
-      - "github-actions[bot]"
    # CodeRabbit reviews default-branch PRs by default. This extends automatic
    # reviews to PRs targeting any branch, including release and stacked branches.
    base_branches:
@@ -1 +1 @@
-{".":"0.11.0"}
+{".":"0.12.0"}
@@ -6,6 +6,30 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
 aims to follow [Semantic Versioning](https://semver.org/spec/v2.0.0.html) once the first release is
 tagged. Until then, source builds report the version `dev`.

+## [0.12.0](https://github.com/Twigpine/zero/compare/v0.11.0...v0.12.0) (2026-11-02)
+
+
+### Features
+
+* **providers:** add MiniMax-M3.1-Flash to catalog ([#1180](https://github.com/Twigpine/zero/issues/1180)) ([d99f92b](https://github.com/Twigpine/zero/commit/d99f92b81a671f186cede81614c89b8e8a7f9451))
+* **sandbox:** support self-exec pattern to eliminate separate helper binary ([#946](https://github.com/Twigpine/zero/issues/946)) ([8c4e759](https://github.com/Twigpine/zero/commit/8c4e7597989ef9b3ca67b7703dfd15d92372aed2))
+* **skills:** install a skill's bundled files and point the skill tool at them ([#1167](https://github.com/Twigpine/zero/issues/1167)) ([1641506](https://github.com/Twigpine/zero/commit/164150633a8b2e6ae864ae02fd7105b201eeb68e))
+
+
+### Bug Fixes
+
+* **agent:** mark headless guardrail stops incomplete ([#1182](https://github.com/Twigpine/zero/issues/1182)) ([5aa54fe](https://github.com/Twigpine/zero/commit/5aa54fe1d2b419c9bbef73605a77919c9007cf7b))
+* bump Go to 1.26.10 for GO-2026-6809 ([#1168](https://github.com/Twigpine/zero/issues/1168)) ([6393a90](https://github.com/Twigpine/zero/commit/6393a90ed618a3536e09c02852f4ec829b97545b))
+* **chatgpt:** refresh subscription defaults and compatibility version ([#1179](https://github.com/Twigpine/zero/issues/1179)) ([f912f80](https://github.com/Twigpine/zero/commit/f912f80d002602e11ba0ef8548ecee58442874aa))
+* **deps:** refresh the lockfile to hono 4.13.13 ([#1171](https://github.com/Twigpine/zero/issues/1171)) ([de1443b](https://github.com/Twigpine/zero/commit/de1443b2edb44fd3747cc77200d54dc8cffcd0a7))
+* **oauth:** reuse the crash-safe secret store and sync encrypted publications ([#1165](https://github.com/Twigpine/zero/issues/1165)) ([84c1b15](https://github.com/Twigpine/zero/commit/84c1b1554076679a342e9cac61f8f8cdbaf3a429))
+* **redaction:** redact credentials whose trigger a greedy span swallowed ([#1158](https://github.com/Twigpine/zero/issues/1158)) ([a97db88](https://github.com/Twigpine/zero/commit/a97db8832fdba077f2d31bdf03ecef8d7c774fe1))
+* **sessions:** capture freeform apply_patch checkpoints ([#1183](https://github.com/Twigpine/zero/issues/1183)) ([6396208](https://github.com/Twigpine/zero/commit/63962081cf130c2171407936ce3beb7021d238b8))
+* **sessions:** preserve image attachment summaries on resume ([#1152](https://github.com/Twigpine/zero/issues/1152)) ([1e84849](https://github.com/Twigpine/zero/commit/1e848492e5c1919d187fb5e1eb5e7ef9bc6140dc)), closes [#1135](https://github.com/Twigpine/zero/issues/1135)
+* **tools:** grep stops where a file's line breaks stop ([#1160](https://github.com/Twigpine/zero/issues/1160)) ([aa9ba84](https://github.com/Twigpine/zero/commit/aa9ba8412eb8917daa2ca5cb3fdbd0a39276fa88))
+* **workspaceindex:** import gitsafe under the Twigpine module path ([#1153](https://github.com/Twigpine/zero/issues/1153)) ([ef4da13](https://github.com/Twigpine/zero/commit/ef4da1323f1097863ae278224b5f304c2fff909a))
+* **workspaceindex:** run Scan's git calls through gitsafe ([#1149](https://github.com/Twigpine/zero/issues/1149)) ([2ffe183](https://github.com/Twigpine/zero/commit/2ffe1831b0950b1463ec46d487d03a5fcb679cc6))
+
 ## [0.11.0](https://github.com/Twigpine/zero/compare/v0.10.0...v0.11.0) (2026-10-15)


@@ -15,6 +15,7 @@
  <img alt="Go 1.26.10+" src="https://img.shields.io/badge/Go-1.26.10+-00ADD8?logo=go&logoColor=white">
  <img alt="25+ providers" src="https://img.shields.io/badge/providers-25+-34E2EA">
  <a href="https://discord.gg/CaQDS6wdFn"><img alt="Discord" src="https://img.shields.io/badge/Discord-join-5865F2?logo=discord&logoColor=white"></a>
+  <a href="https://vercel.com/open-source-program"><img alt="Vercel OSS Program" src="https://vercel.com/oss/program-badge-2026.svg"></a>
  <br>
  <strong>English</strong> | <a href="README_ZH.md">中文</a>
 </p>
@@ -87,9 +88,18 @@ zero exec --output-format stream-json < turns.jsonl
        </picture>
      </a>
    </td>
+    <td align="center" width="150" height="80">
+      <a href="https://vercel.com/">
+        <picture>
+          <source media="(prefers-color-scheme: dark)" srcset="docs/assets/vercel-logo-dark.svg">
+          <img src="docs/assets/vercel-logo.svg" alt="Vercel logo" width="120">
+        </picture>
+      </a>
+    </td>
  </tr>
  <tr>
    <td align="center"><a href="https://www.gmicloud.ai/"><strong>GMI Cloud</strong></a></td>
+    <td align="center"><a href="https://vercel.com/"><strong>Vercel</strong></a></td>
  </tr>
 </table>

@@ -15,6 +15,7 @@
  <img alt="Go 1.26.10+" src="https://img.shields.io/badge/Go-1.26.10+-00ADD8?logo=go&logoColor=white">
  <img alt="25+ providers" src="https://img.shields.io/badge/providers-25+-34E2EA">
  <a href="https://discord.gg/CaQDS6wdFn"><img alt="Discord" src="https://img.shields.io/badge/Discord-join-5865F2?logo=discord&logoColor=white"></a>
+  <a href="https://vercel.com/open-source-program"><img alt="Vercel OSS Program" src="https://vercel.com/oss/program-badge-2026.svg"></a>
  <br>
  <a href="README.md">English</a> | <strong>中文</strong>
 </p>
@@ -85,9 +86,18 @@ zero exec --output-format stream-json < turns.jsonl
        </picture>
      </a>
    </td>
+    <td align="center" width="150" height="80">
+      <a href="https://vercel.com/">
+        <picture>
+          <source media="(prefers-color-scheme: dark)" srcset="docs/assets/vercel-logo-dark.svg">
+          <img src="docs/assets/vercel-logo.svg" alt="Vercel logo" width="120">
+        </picture>
+      </a>
+    </td>
  </tr>
  <tr>
    <td align="center"><a href="https://www.gmicloud.ai/"><strong>GMI Cloud</strong></a></td>
+    <td align="center"><a href="https://vercel.com/"><strong>Vercel</strong></a></td>
  </tr>
 </table>

@@ -110,6 +110,8 @@ Discovery roots (earlier wins on name collisions):

 A missing directory is fine — Zero just omits it. Management commands (`zero skill add` / `zero skill remove`) always write to the primary Zero dir only; `list` / `info` search primary + `~/.agents/skills`. `skill info` reports lock source/hash when present and whether the on-disk `SKILL.md` has drifted from the recorded hash.

+For a skill installed since Zero started installing bundled files, `skill info` also reports drift in any file installed with it.
+
 ```text
 ~/.local/share/zero/skills/          # primary (install/remove target)
  run-benchmarks/
@@ -140,6 +142,15 @@ Only `name` and `description` are recognized in the frontmatter today. The `name

 The `skill` tool lets the agent load any discovered skill by name (primary, agents, or plugin).

+A skill can ship files beside its `SKILL.md`, in the [agentskills.io](https://agentskills.io/specification) layout: `scripts/`, `references/`, `assets/` or anything else the instructions refer to. `zero skill add` installs the whole skill directory, leaving out hidden entries such as `.git` and not following or copying symlinks (it lists any it skipped). A skill larger than 1,000 files or 32 MiB is refused. When the `skill` tool loads a skill that has such files, its output ends with the skill's directory and a list of the files, so instructions like "run `scripts/fill.py`" can be followed. Running a bundled script goes through the normal tool permissions; installing or loading a skill never runs anything.
+
+```text
+pdf/
+  SKILL.md
+  scripts/fill.py
+  references/form-fields.md
+```
+
 ## 4. Hooks

 Hooks fire shell commands on lifecycle events. Configure them in JSON:
@@ -0,0 +1 @@
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@@ -0,0 +1 @@
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@@ -0,0 +1,34 @@
+package agent
+
+import "github.com/Twigpine/zero/internal/tools"
+
+// CheckpointTargets returns the workspace-relative paths a tool call will mutate,
+// resolving its arguments exactly as executeToolCall does. Freeform apply_patch
+// input is normalized through the registered built-in tool, so absolute headers
+// are rewritten relative to the selected write root and cwd is set: checkpoint
+// capture and execution therefore agree on the files at stake.
+//
+// Best-effort: nil when the call has no capture targets or its arguments cannot
+// be resolved. Checkpoint capture never fails a run.
+func CheckpointTargets(workspaceRoot string, registry *tools.Registry, call ToolCall) []string {
+	if registry == nil {
+		return nil
+	}
+	args := map[string]any{}
+	if call.Freeform {
+		tool, ok := registry.Get(call.Name)
+		if !ok || !tools.IsBuiltInApplyPatch(tool) {
+			return nil
+		}
+		prepared, err := tools.PrepareFreeformApplyPatchArguments(tool, call.Arguments)
+		if err != nil {
+			return nil
+		}
+		args = prepared
+	} else if call.Arguments != "" {
+		if err := decodeToolArguments(call.Arguments, &args); err != nil {
+			return nil
+		}
+	}
+	return tools.MutationTargets(workspaceRoot, call.Name, args)
+}
@@ -0,0 +1,99 @@
+package agent
+
+import (
+	"encoding/json"
+	"os"
+	"path/filepath"
+	"slices"
+	"testing"
+
+	"github.com/Twigpine/zero/internal/tools"
+)
+
+func TestCheckpointTargets(t *testing.T) {
+	root := t.TempDir()
+	// Absolute headers must be compared against the physical root: t.TempDir
+	// differs from it on macOS (/var -> /private/var) and via Windows short names.
+	if resolved, err := filepath.EvalSymlinks(root); err == nil {
+		root = resolved
+	}
+	if err := os.WriteFile(filepath.Join(root, "main.txt"), []byte("before\n"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	relativePatch := "*** Begin Patch\n*** Update File: main.txt\n@@\n-before\n+after\n*** End Patch\n"
+	absolutePatch := "*** Begin Patch\n*** Update File: " + filepath.Join(root, "main.txt") + "\n@@\n-before\n+after\n*** End Patch\n"
+	multiFilePatch := "*** Begin Patch\n*** Update File: main.txt\n@@\n-before\n+after\n*** Add File: notes.txt\n+new\n*** End Patch\n"
+	jsonArgs, err := json.Marshal(map[string]any{"patch": relativePatch})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	registry := tools.NewRegistry()
+	registry.Register(tools.NewScopedApplyPatchTool(root, nil))
+	unbuiltinRegistry := tools.NewRegistry()
+	unbuiltinRegistry.Register(&customApplyPatchTool{})
+
+	for _, tc := range []struct {
+		name     string
+		registry *tools.Registry
+		call     ToolCall
+		want     []string
+	}{
+		{
+			name: "freeform relative header",
+			call: ToolCall{Name: "apply_patch", Arguments: relativePatch, Freeform: true},
+			want: []string{"main.txt"},
+		},
+		{
+			name: "freeform absolute header stays workspace-relative",
+			call: ToolCall{Name: "apply_patch", Arguments: absolutePatch, Freeform: true},
+			want: []string{"main.txt"},
+		},
+		{
+			name: "freeform multi-file patch",
+			call: ToolCall{Name: "apply_patch", Arguments: multiFilePatch, Freeform: true},
+			want: []string{"main.txt", "notes.txt"},
+		},
+		{
+			name: "json arguments",
+			call: ToolCall{Name: "apply_patch", Arguments: string(jsonArgs)},
+			want: []string{"main.txt"},
+		},
+		{
+			name: "freeform traversal is rejected",
+			call: ToolCall{Name: "apply_patch", Arguments: "*** Begin Patch\n*** Update File: ../escape.txt\n@@\n-x\n+y\n*** End Patch\n", Freeform: true},
+			want: nil,
+		},
+		{
+			name:     "freeform custom apply_patch is not prepared",
+			registry: unbuiltinRegistry,
+			call:     ToolCall{Name: "apply_patch", Arguments: relativePatch, Freeform: true},
+			want:     nil,
+		},
+		{
+			name: "unknown freeform tool",
+			call: ToolCall{Name: "read_file", Arguments: "raw", Freeform: true},
+			want: nil,
+		},
+		{
+			name: "malformed json arguments",
+			call: ToolCall{Name: "apply_patch", Arguments: "{"},
+			want: nil,
+		},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			reg := registry
+			if tc.registry != nil {
+				reg = tc.registry
+			}
+			got := CheckpointTargets(root, reg, tc.call)
+			if !slices.Equal(got, tc.want) {
+				t.Fatalf("CheckpointTargets() = %#v, want %#v", got, tc.want)
+			}
+		})
+	}
+
+	if got := CheckpointTargets(root, nil, ToolCall{Name: "apply_patch", Arguments: relativePatch}); got != nil {
+		t.Fatalf("CheckpointTargets(nil registry) = %#v, want nil", got)
+	}
+}
@@ -80,6 +80,9 @@ func TestRunStopsAfterConsecutiveEmptyTurns(t *testing.T) {
 	if result.FinalAnswer == maxTurnsAnswer {
 		t.Fatalf("no-output guard must stop before reaching maxTurns, got max-turns answer")
 	}
+	if result.Incomplete {
+		t.Fatalf("interactive run must not be marked Incomplete; reason=%q", result.IncompleteReason)
+	}
 }

 func TestRunResetsEmptyTurnCounterOnVisibleOutput(t *testing.T) {
@@ -513,6 +516,22 @@ func repeatedFlakyTurns(n int) [][]zeroruntime.StreamEvent {
 	return turns
 }

+// alwaysSucceedingTool is the successful counterpart to alwaysFailingTool, used
+// to put a productive tool turn before an empty-turn streak.
+type alwaysSucceedingTool struct{}
+
+func (alwaysSucceedingTool) Name() string        { return "ok" }
+func (alwaysSucceedingTool) Description() string { return "always succeeds for testing" }
+func (alwaysSucceedingTool) Parameters() tools.Schema {
+	return tools.Schema{Type: "object", AdditionalProperties: false}
+}
+func (alwaysSucceedingTool) Safety() tools.Safety {
+	return tools.Safety{SideEffect: tools.SideEffectRead, Permission: tools.PermissionAllow}
+}
+func (alwaysSucceedingTool) Run(context.Context, map[string]any) tools.Result {
+	return tools.Result{Status: tools.StatusOK, Output: "ok"}
+}
+
 func TestRunStopsAfterRepeatedToolFailures(t *testing.T) {
 	registry := tools.NewRegistry()
 	registry.Register(alwaysFailingTool{})
@@ -529,6 +548,51 @@ func TestRunStopsAfterRepeatedToolFailures(t *testing.T) {
 	if len(provider.requests) != toolFailureStopAt {
 		t.Fatalf("expected stop at %d failures, made %d requests", toolFailureStopAt, len(provider.requests))
 	}
+	if result.Incomplete {
+		t.Fatalf("interactive run must not be marked Incomplete; reason=%q", result.IncompleteReason)
+	}
+}
+
+// Under the headless completion gate both guardrail stops are unfinished runs,
+// so they must set Result.Incomplete and a reason; exec maps that to exit 4.
+// Thresholds and stop answers stay pinned by the two tests above.
+func TestHeadlessGuardStopsAreIncomplete(t *testing.T) {
+	for _, tc := range []struct {
+		name       string
+		turns      [][]zeroruntime.StreamEvent
+		wantTurns  int
+		wantReason string
+	}{
+		{"empty_turns", [][]zeroruntime.StreamEvent{emptyTurn(), emptyTurn(), emptyTurn()}, maxEmptyTurns, "3 consecutive turns with no output"},
+		{"empty_turns_after_progress", [][]zeroruntime.StreamEvent{
+			toolTurn("ok-1", "ok", "{}"),
+			emptyTurn(), emptyTurn(), emptyTurn(),
+		}, maxEmptyTurns + 1, "3 consecutive turns with no output"},
+		{"repeated_tool_failures", repeatedFlakyTurns(10), toolFailureStopAt, "flaky"},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			registry := tools.NewRegistry()
+			registry.Register(alwaysFailingTool{})
+			registry.Register(alwaysSucceedingTool{})
+			result, err := Run(context.Background(), "go", &mockProvider{turns: tc.turns}, Options{
+				Registry:                registry,
+				MaxTurns:                12,
+				RequireCompletionSignal: true,
+			})
+			if err != nil {
+				t.Fatal(err)
+			}
+			if result.Turns != tc.wantTurns {
+				t.Fatalf("guard did not trigger: turns=%d want=%d", result.Turns, tc.wantTurns)
+			}
+			if !result.Incomplete {
+				t.Fatalf("guardrail stop under the headless gate must be Incomplete; final=%q", result.FinalAnswer)
+			}
+			if !strings.Contains(result.IncompleteReason, tc.wantReason) {
+				t.Fatalf("IncompleteReason = %q, want it to mention %q", result.IncompleteReason, tc.wantReason)
+			}
+		})
+	}
 }

 func TestRunInjectsToolFailureHintWithSchema(t *testing.T) {
@@ -587,6 +587,14 @@ func Run(ctx context.Context, prompt string, provider Provider, options Options)
 			// counted toward the runaway cap so we stop before burning maxTurns.
 			if guards.observeTurn(collected) {
 				result.FinalAnswer = noOutputStopAnswer(result.Turns)
+				// Under the headless completion gate this stop is unfinished work,
+				// not a successful final answer: the guard exists precisely because
+				// the model made no progress. Interactive runs keep the gate off, so
+				// their behavior is unchanged.
+				if options.RequireCompletionSignal {
+					result.Incomplete = true
+					result.IncompleteReason = "stopped after " + strconv.Itoa(guards.emptyTurns) + " consecutive turns with no output"
+				}
 				result.Messages = copyMessages(messages)
 				return result, nil
 			}
@@ -808,6 +816,13 @@ func Run(ctx context.Context, prompt string, provider Provider, options Options)
 				messages = appendAbortedToolResults(messages, collected.ToolCalls[index+1:])
 				messages = append(messages, buildToolImageMessages()...)
 				result.FinalAnswer = toolFailureStopAnswer(call.Name, outcome.Count)
+				// Same headless contract as the empty-turn guard: halting on a
+				// repeated failure is an unfinished run, not success. Gate-off
+				// (interactive / --no-completion-gate) keeps the old behavior.
+				if options.RequireCompletionSignal {
+					result.Incomplete = true
+					result.IncompleteReason = "the " + call.Name + " tool failed " + strconv.Itoa(outcome.Count) + " times in a row with the same error"
+				}
 				result.Messages = copyMessages(messages)
 				return result, nil
 			}
@@ -92,13 +92,25 @@ func runSkillAdd(args []string, dir string, stdout io.Writer, stderr io.Writer)
 	lines := []string{}
 	if result.Updated {
 		lines = append(lines, fmt.Sprintf("Updated skill %q.", result.Name))
-		lines = append(lines, fmt.Sprintf("  hash: %s -> %s", shortHash(result.PreviousHash), shortHash(result.Hash)))
+		switch {
+		case result.PreviousHash != result.Hash:
+			lines = append(lines, fmt.Sprintf("  hash: %s -> %s", shortHash(result.PreviousHash), shortHash(result.Hash)))
+		case result.PreviousTreeHash == "":
+			// The install this replaced predates tree hashes and copied SKILL.md
+			// alone, so the bundled files are new rather than changed.
+			lines = append(lines, "  SKILL.md is unchanged; the files bundled with it are now installed too")
+		default:
+			lines = append(lines, "  SKILL.md is unchanged; the files bundled with it changed")
+		}
 	} else {
 		lines = append(lines, fmt.Sprintf("Installed skill %q.", result.Name))
 		lines = append(lines, "  hash: "+result.Hash)
 	}
 	lines = append(lines, "  source: "+result.Source)
 	lines = append(lines, "  path: "+result.Path)
+	if len(result.SkippedLinks) > 0 {
+		lines = append(lines, "  not copied (symlinks): "+strings.Join(result.SkippedLinks, ", "))
+	}
 	if _, err := fmt.Fprintln(stdout, redaction.RedactString(strings.Join(lines, "\n"), redaction.Options{})); err != nil {
 		return exitCrash
 	}
@@ -7,6 +7,8 @@ import (
 	"path/filepath"
 	"strings"
 	"testing"
+
+	"github.com/Twigpine/zero/internal/skills"
 )

 func writeSourceSkillDir(t *testing.T, dir string, content string) string {
@@ -391,3 +393,133 @@ func TestRunDistributionCommandsDoNotLaunchTUI(t *testing.T) {
 		}
 	}
 }
+
+// A reinstall that only changed a bundled script must still read as an update
+// (#584): the SKILL.md hash is the same, so "hash: X -> X" would say nothing.
+func TestRunSkillAddReportsAnUpdateToBundledFilesOnly(t *testing.T) {
+	skillsDir := t.TempDir()
+	src := writeSourceSkillDir(t, filepath.Join(t.TempDir(), "src"),
+		"---\nname: pdf\ndescription: Fill forms.\n---\nRun scripts/fill.py.\n")
+	script := filepath.Join(src, "scripts", "fill.py")
+	if err := os.MkdirAll(filepath.Dir(script), 0o755); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(script, []byte("print('v1')\n"), 0o755); err != nil {
+		t.Fatal(err)
+	}
+	deps := appDeps{skillsDir: func() string { return skillsDir }}
+
+	var stdout, stderr bytes.Buffer
+	if exit := runWithDeps([]string{"skill", "add", src}, &stdout, &stderr, deps); exit != 0 {
+		t.Fatalf("first skill add exit = %d, stderr = %s", exit, stderr.String())
+	}
+	if err := os.WriteFile(script, []byte("print('v2')\n"), 0o755); err != nil {
+		t.Fatal(err)
+	}
+	stdout.Reset()
+	stderr.Reset()
+	if exit := runWithDeps([]string{"skill", "add", src}, &stdout, &stderr, deps); exit != 0 {
+		t.Fatalf("second skill add exit = %d, stderr = %s", exit, stderr.String())
+	}
+	for _, want := range []string{`Updated skill "pdf".`, "SKILL.md is unchanged; the files bundled with it changed"} {
+		if !strings.Contains(stdout.String(), want) {
+			t.Fatalf("second add output does not say %q:\n%s", want, stdout.String())
+		}
+	}
+}
+
+// An install from before tree hashes copied SKILL.md alone and recorded no tree
+// hash. Adding the skill again over that entry must not claim bundled files
+// changed: there are none, or they are installed for the first time.
+func TestRunSkillAddOverAnEntryWithoutATreeHash(t *testing.T) {
+	for _, tc := range []struct {
+		name    string
+		script  bool
+		want    []string
+		notWant []string
+	}{
+		{
+			name:    "SKILL.md only",
+			want:    []string{`Installed skill "pdf".`},
+			notWant: []string{"Updated skill", "files bundled with it"},
+		},
+		{
+			name:    "with a bundled script",
+			script:  true,
+			want:    []string{`Updated skill "pdf".`, "SKILL.md is unchanged; the files bundled with it are now installed too"},
+			notWant: []string{"files bundled with it changed"},
+		},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			skillsDir := t.TempDir()
+			src := writeSourceSkillDir(t, filepath.Join(t.TempDir(), "src"),
+				"---\nname: pdf\ndescription: Fill forms.\n---\nRun scripts/fill.py.\n")
+			if tc.script {
+				script := filepath.Join(src, "scripts", "fill.py")
+				if err := os.MkdirAll(filepath.Dir(script), 0o755); err != nil {
+					t.Fatal(err)
+				}
+				if err := os.WriteFile(script, []byte("print('fill')\n"), 0o755); err != nil {
+					t.Fatal(err)
+				}
+			}
+			deps := appDeps{skillsDir: func() string { return skillsDir }}
+
+			var stdout, stderr bytes.Buffer
+			if exit := runWithDeps([]string{"skill", "add", src}, &stdout, &stderr, deps); exit != 0 {
+				t.Fatalf("first skill add exit = %d, stderr = %s", exit, stderr.String())
+			}
+			lock, err := skills.ReadLock(skillsDir)
+			if err != nil {
+				t.Fatal(err)
+			}
+			entry := lock["pdf"]
+			entry.TreeHash = ""
+			lock["pdf"] = entry
+			data, err := json.Marshal(lock)
+			if err != nil {
+				t.Fatal(err)
+			}
+			if err := os.WriteFile(filepath.Join(skillsDir, skills.LockFileName), data, 0o644); err != nil {
+				t.Fatal(err)
+			}
+
+			stdout.Reset()
+			stderr.Reset()
+			if exit := runWithDeps([]string{"skill", "add", src}, &stdout, &stderr, deps); exit != 0 {
+				t.Fatalf("second skill add exit = %d, stderr = %s", exit, stderr.String())
+			}
+			for _, want := range tc.want {
+				if !strings.Contains(stdout.String(), want) {
+					t.Fatalf("second add output does not say %q:\n%s", want, stdout.String())
+				}
+			}
+			for _, notWant := range tc.notWant {
+				if strings.Contains(stdout.String(), notWant) {
+					t.Fatalf("second add output says %q:\n%s", notWant, stdout.String())
+				}
+			}
+		})
+	}
+}
+
+func TestRunSkillAddListsTheSymlinksItDidNotCopy(t *testing.T) {
+	skillsDir := t.TempDir()
+	src := writeSourceSkillDir(t, filepath.Join(t.TempDir(), "src"),
+		"---\nname: pdf\ndescription: Fill forms.\n---\nbody\n")
+	outside := filepath.Join(t.TempDir(), "notes.md")
+	if err := os.WriteFile(outside, []byte("outside\n"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.Symlink(outside, filepath.Join(src, "notes.md")); err != nil {
+		t.Skipf("symlinks unavailable: %v", err)
+	}
+	var stdout, stderr bytes.Buffer
+	deps := appDeps{skillsDir: func() string { return skillsDir }}
+	if exit := runWithDeps([]string{"skill", "add", src}, &stdout, &stderr, deps); exit != 0 {
+		t.Fatalf("skill add exit = %d, stderr = %s", exit, stderr.String())
+	}
+	if !strings.Contains(stdout.String(), "not copied (symlinks): notes.md") {
+		t.Fatalf("add output does not list the skipped symlink:\n%s", stdout.String())
+	}
+}
@@ -712,7 +712,7 @@ func runExec(args []string, stdout io.Writer, stderr io.Writer, deps appDeps) in
 				"arguments": call.Arguments,
 			})
 			// Snapshot before-state of files this call will mutate (safe rewind).
-			if checkpoint, ok := sessionRecorder.captureCheckpoint(workspaceRoot, call); ok {
+			if checkpoint, ok := sessionRecorder.captureCheckpoint(workspaceRoot, registry, call); ok {
 				writer.checkpoint(checkpoint)
 			}
 		},
@@ -0,0 +1,206 @@
+package cli
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/Twigpine/zero/internal/agent"
+	"github.com/Twigpine/zero/internal/config"
+	"github.com/Twigpine/zero/internal/sessions"
+	"github.com/Twigpine/zero/internal/tools"
+	"github.com/Twigpine/zero/internal/zeroruntime"
+)
+
+func freeformApplyPatchArguments(root string, absolute bool) string {
+	target := "main.txt"
+	if absolute {
+		target = filepath.Join(root, "main.txt")
+	}
+	return "*** Begin Patch\n*** Update File: " + target + "\n@@\n-before\n+after\n*** End Patch\n"
+}
+
+// captureCheckpoint must resolve a freeform apply_patch call through the same
+// preparation execution uses, so the before-state is captured and rewind can
+// restore it. The JSON control pins the unchanged path.
+func TestCaptureCheckpointFreeformApplyPatch(t *testing.T) {
+	for _, tc := range []struct {
+		name     string
+		freeform bool
+		absolute bool
+		jsonArgs bool
+	}{
+		{name: "freeform", freeform: true},
+		{name: "freeform absolute header", freeform: true, absolute: true},
+		{name: "json arguments", jsonArgs: true},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Setenv("XDG_DATA_HOME", t.TempDir())
+			t.Setenv("ZERO_CHECKPOINTS", "on")
+			root := t.TempDir()
+			// The absolute-header case needs the physical root spelling the tool
+			// resolves (macOS /var vs /private/var, Windows short names).
+			if resolved, err := filepath.EvalSymlinks(root); err == nil {
+				root = resolved
+			}
+			if err := os.WriteFile(filepath.Join(root, "main.txt"), []byte("before\n"), 0o644); err != nil {
+				t.Fatal(err)
+			}
+			store := sessions.NewStore(sessions.StoreOptions{})
+			session, err := store.Create(sessions.CreateInput{Title: "freeform checkpoint"})
+			if err != nil {
+				t.Fatal(err)
+			}
+			if _, err := store.AppendEvent(session.SessionID, sessions.AppendEventInput{
+				Type:    sessions.EventMessage,
+				Payload: map[string]string{"role": "user", "content": "apply the patch"},
+			}); err != nil {
+				t.Fatal(err)
+			}
+			recorder := &execSessionRecorder{prepared: sessions.PreparedExec{Store: store, Session: session}}
+			registry := tools.NewRegistry()
+			registry.Register(tools.NewScopedApplyPatchTool(root, nil))
+
+			call := agent.ToolCall{ID: "call", Name: "apply_patch", Freeform: tc.freeform}
+			if tc.jsonArgs {
+				encoded, err := json.Marshal(map[string]string{"patch": freeformApplyPatchArguments(root, false)})
+				if err != nil {
+					t.Fatal(err)
+				}
+				call.Arguments = string(encoded)
+			} else {
+				call.Arguments = freeformApplyPatchArguments(root, tc.absolute)
+			}
+
+			event, ok := recorder.captureCheckpoint(root, registry, call)
+			if !ok || event.Type != sessions.EventSessionCheckpoint {
+				t.Fatalf("captureCheckpoint() = %#v, %t; want a checkpoint event", event, ok)
+			}
+			var payload sessions.CheckpointPayload
+			if err := json.Unmarshal(event.Payload, &payload); err != nil {
+				t.Fatalf("decode checkpoint payload: %v", err)
+			}
+			if len(payload.Files) != 1 || payload.Files[0].Path != "main.txt" || payload.Files[0].Blob == "" {
+				t.Fatalf("checkpoint files = %#v, want main.txt with a blob", payload.Files)
+			}
+
+			// Simulate the tool's mutation, then rewind past it.
+			if err := os.WriteFile(filepath.Join(root, "main.txt"), []byte("after\n"), 0o644); err != nil {
+				t.Fatal(err)
+			}
+			report, err := store.ApplyRewind(session.SessionID, root, 1)
+			if err != nil {
+				t.Fatalf("ApplyRewind: %v", err)
+			}
+			if report.FilesRestored != 1 {
+				t.Fatalf("restore report = %#v, want one restored file", report)
+			}
+			content, err := os.ReadFile(filepath.Join(root, "main.txt"))
+			if err != nil {
+				t.Fatal(err)
+			}
+			if string(content) != "before\n" {
+				t.Fatalf("restored content = %q, want before newline", content)
+			}
+		})
+	}
+}
+
+// freeformPatchExecProvider emits one native freeform apply_patch call, then a
+// final text turn.
+type freeformPatchExecProvider struct {
+	patch string
+	calls int
+}
+
+func (provider *freeformPatchExecProvider) StreamCompletion(ctx context.Context, request zeroruntime.CompletionRequest) (<-chan zeroruntime.StreamEvent, error) {
+	ch := make(chan zeroruntime.StreamEvent, 5)
+	calls := provider.calls
+	provider.calls++
+	if calls == 0 {
+		ch <- zeroruntime.StreamEvent{Type: zeroruntime.StreamEventToolCallStart, ToolCallID: "call_patch", ToolName: "apply_patch", ToolCallFreeform: true}
+		ch <- zeroruntime.StreamEvent{Type: zeroruntime.StreamEventToolCallDelta, ToolCallID: "call_patch", ArgumentsFragment: provider.patch}
+		ch <- zeroruntime.StreamEvent{Type: zeroruntime.StreamEventToolCallEnd, ToolCallID: "call_patch"}
+	} else {
+		ch <- zeroruntime.StreamEvent{Type: zeroruntime.StreamEventText, Content: "patched"}
+	}
+	ch <- zeroruntime.StreamEvent{Type: zeroruntime.StreamEventDone}
+	close(ch)
+	return ch, nil
+}
+
+// End-to-end: exec captures the freeform checkpoint and `sessions rewind`
+// restores the pre-patch content.
+func TestRunExecFreeformApplyPatchCheckpointsForRewind(t *testing.T) {
+	isolateCLIUserState(t)
+	t.Setenv("ZERO_CHECKPOINTS", "on")
+	root := t.TempDir()
+	if err := os.WriteFile(filepath.Join(root, "main.txt"), []byte("before\n"), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	provider := &freeformPatchExecProvider{patch: freeformApplyPatchArguments(root, false)}
+	var stdout, stderr bytes.Buffer
+	exitCode := runWithDeps([]string{
+		"exec", "--init-session-id", "freeform_rewind", "--skip-permissions-unsafe",
+		"--output-format", "stream-json", "apply the patch",
+	}, &stdout, &stderr, appDeps{
+		getwd: func() (string, error) { return root, nil },
+		resolveConfig: func(string, config.Overrides) (config.ResolvedConfig, error) {
+			return execResolvedConfig(), nil
+		},
+		newProvider: func(config.ProviderProfile) (zeroruntime.Provider, error) {
+			return provider, nil
+		},
+	})
+	if exitCode != exitSuccess {
+		t.Fatalf("exit = %d, want %d; stderr=%q stdout=%q", exitCode, exitSuccess, stderr.String(), stdout.String())
+	}
+	content, err := os.ReadFile(filepath.Join(root, "main.txt"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if string(content) != "after\n" {
+		t.Fatalf("patched content = %q, want after newline", content)
+	}
+
+	store := sessions.NewStore(sessions.StoreOptions{})
+	events, err := store.ReadEvents("freeform_rewind")
+	if err != nil {
+		t.Fatalf("ReadEvents: %v", err)
+	}
+	checkpoints := 0
+	for _, event := range events {
+		if event.Type == sessions.EventSessionCheckpoint {
+			checkpoints++
+		}
+	}
+	if checkpoints != 1 {
+		t.Fatalf("checkpoint events = %d, want 1; events=%v", checkpoints, sessionEventTypes(events))
+	}
+
+	report, err := store.ApplyRewind("freeform_rewind", root, 1)
+	if err != nil {
+		t.Fatalf("ApplyRewind: %v", err)
+	}
+	if report.FilesRestored != 1 {
+		t.Fatalf("restore report = %#v, want one restored file", report)
+	}
+	content, err = os.ReadFile(filepath.Join(root, "main.txt"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	if string(content) != "before\n" {
+		t.Fatalf("rewound content = %q, want before newline", content)
+	}
+}
+
+func sessionEventTypes(events []sessions.Event) []string {
+	types := make([]string, 0, len(events))
+	for _, event := range events {
+		types = append(types, string(event.Type))
+	}
+	return types
+}
@@ -0,0 +1,90 @@
+package cli
+
+import (
+	"bytes"
+	"context"
+	"slices"
+	"strings"
+	"testing"
+
+	"github.com/Twigpine/zero/internal/config"
+	"github.com/Twigpine/zero/internal/zeroruntime"
+)
+
+// guardStopExecProvider emits turns with no visible text and no tool calls, so
+// the empty-turn guardrail stops the run after its consecutive-empty-turn cap.
+type guardStopExecProvider struct{}
+
+func (guardStopExecProvider) StreamCompletion(ctx context.Context, request zeroruntime.CompletionRequest) (<-chan zeroruntime.StreamEvent, error) {
+	ch := make(chan zeroruntime.StreamEvent, 1)
+	select {
+	case <-ctx.Done():
+		close(ch)
+		return ch, ctx.Err()
+	case ch <- zeroruntime.StreamEvent{Type: zeroruntime.StreamEventDone}:
+	}
+	close(ch)
+	return ch, nil
+}
+
+// A guardrail stop in headless exec must use the existing INCOMPLETE contract
+// (exit 4, error event, run_end status), and --no-completion-gate must keep
+// reporting success. isolateCLIUserState also turns off the models.dev refresh
+// that runExec starts in the background (see the #1152/#1161 flake).
+func TestRunExecGuardrailStopIncompleteContract(t *testing.T) {
+	for _, tc := range []struct {
+		name       string
+		args       []string
+		wantExit   int
+		wantStatus string
+	}{
+		{"gate on", []string{"exec", "--output-format", "stream-json", "complete the task"}, exitIncomplete, "incomplete"},
+		{"gate off", []string{"exec", "--no-completion-gate", "--output-format", "stream-json", "complete the task"}, exitSuccess, "success"},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			isolateCLIUserState(t)
+			cwd := t.TempDir()
+			var stdout, stderr bytes.Buffer
+			exitCode := runWithDeps(tc.args, &stdout, &stderr, appDeps{
+				getwd: func() (string, error) { return cwd, nil },
+				resolveConfig: func(string, config.Overrides) (config.ResolvedConfig, error) {
+					return execResolvedConfig(), nil
+				},
+				newProvider: func(config.ProviderProfile) (zeroruntime.Provider, error) {
+					return guardStopExecProvider{}, nil
+				},
+			})
+			if exitCode != tc.wantExit {
+				t.Fatalf("exit code = %d, want %d; stderr=%q", exitCode, tc.wantExit, stderr.String())
+			}
+
+			events := decodeJSONLines(t, stdout.String())
+			if len(events) == 0 || events[len(events)-1]["type"] != "run_end" {
+				t.Fatalf("expected run_end as the last event, got %#v", events)
+			}
+			runEnd := events[len(events)-1]
+			if runEnd["status"] != tc.wantStatus {
+				t.Fatalf("run_end status = %v, want %q", runEnd["status"], tc.wantStatus)
+			}
+			if runEnd["exitCode"] != float64(tc.wantExit) {
+				t.Fatalf("run_end exitCode = %v, want %d", runEnd["exitCode"], tc.wantExit)
+			}
+			finalEvent := findJSONEvent(t, events, "final")
+			if text, _ := finalEvent["text"].(string); !strings.Contains(text, "no output") {
+				t.Fatalf("final text = %q, want the guard stop answer", text)
+			}
+
+			if tc.wantStatus == "incomplete" {
+				errorEvent := findJSONEvent(t, events, "error")
+				if errorEvent["code"] != "incomplete" {
+					t.Fatalf("error code = %v, want incomplete", errorEvent["code"])
+				}
+				if message, _ := errorEvent["message"].(string); !strings.Contains(message, "no output") {
+					t.Fatalf("error message = %q, want the guard reason", message)
+				}
+			} else if slices.Contains(jsonEventTypes(events), "error") {
+				t.Fatalf("gate-off run must not emit an error event: %#v", events)
+			}
+		})
+	}
+}
@@ -1,7 +1,6 @@
 package cli

 import (
-	"encoding/json"
 	"fmt"
 	"io"
 	"strings"
@@ -19,15 +18,11 @@ type execSessionRecorder struct {
 // captureCheckpoint snapshots the before-state of files a tool call will mutate.
 // Best-effort: failures never affect the run. Returns the checkpoint event and
 // true when one was recorded.
-func (recorder *execSessionRecorder) captureCheckpoint(workspaceRoot string, call agent.ToolCall) (sessions.Event, bool) {
+func (recorder *execSessionRecorder) captureCheckpoint(workspaceRoot string, registry *tools.Registry, call agent.ToolCall) (sessions.Event, bool) {
 	if recorder.prepared.Store == nil || recorder.prepared.Session.SessionID == "" {
 		return sessions.Event{}, false
 	}
-	var args map[string]any
-	if call.Arguments != "" {
-		_ = json.Unmarshal([]byte(call.Arguments), &args)
-	}
-	targets := tools.MutationTargets(workspaceRoot, call.Name, args)
+	targets := agent.CheckpointTargets(workspaceRoot, registry, call)
 	if len(targets) == 0 {
 		return sessions.Event{}, false
 	}
@@ -148,7 +148,7 @@ func runExecSpecDraft(run execSpecDraftRun) int {
 				"name":      call.Name,
 				"arguments": call.Arguments,
 			})
-			if checkpoint, ok := sessionRecorder.captureCheckpoint(run.workspaceRoot, call); ok {
+			if checkpoint, ok := sessionRecorder.captureCheckpoint(run.workspaceRoot, run.registry, call); ok {
 				writer.checkpoint(checkpoint)
 			}
 		},
@@ -174,7 +174,7 @@ func runSandboxSetup(args []string, stdout io.Writer, stderr io.Writer, deps app
 	if !setupHelper.Available() {
 		return writeAppError(stderr, "Windows sandbox setup helper is not available", exitProvider)
 	}
-	setupPlan, err := zeroSandbox.BuildWindowsSandboxSetupArgs(zeroSandbox.WindowsSandboxSetupArgsOptions{
+	setupArgs, err := zeroSandbox.BuildWindowsSandboxSetupArgs(zeroSandbox.WindowsSandboxSetupArgsOptions{
 		CommandCWD:        workspaceRoot,
 		WorkspaceRoots:    []string{workspaceRoot},
 		PermissionProfile: profile,
@@ -182,18 +182,9 @@ func runSandboxSetup(args []string, stdout io.Writer, stderr io.Writer, deps app
 	if err != nil {
 		return writeAppError(stderr, err.Error(), exitCrash)
 	}
-	setupArgs := append(append([]string{}, setupHelper.ArgsPrefix...), setupPlan.Args...)
-	// BUILDING THE ARGS ALREADY WROTE TO DISK. Selecting the runtime root takes a
-	// lease, and taking a lease creates the runtime tree and the lease file when
-	// they are not there. A helper that never publishes a marker leaves that tree
-	// behind, attested by nothing, so the undo belongs on this side of the process
-	// boundary where the record of it lives.
+	setupArgs = append(append([]string{}, setupHelper.ArgsPrefix...), setupArgs...)
 	if err := deps.runSandboxSetupHelper(setupHelper.Name, setupArgs, stdout, stderr); err != nil {
-		message := "Windows sandbox setup failed: " + err.Error()
-		if undo := setupPlan.Rollback(); undo != nil {
-			message += "\nleftover sandbox runtime state could not be removed: " + undo.Error()
-		}
-		return writeAppError(stderr, message, exitProvider)
+		return writeAppError(stderr, "Windows sandbox setup failed: "+err.Error(), exitProvider)
 	}
 	return writeSandboxSetupResult(stdout, options.json, sandboxSetupResult{
 		Platform: "windows",
@@ -0,0 +1,51 @@
+package config
+
+import (
+	"encoding/json"
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestEnsureChatGPTDefaultPreservesSavedSelection(t *testing.T) {
+	for _, saved := range []string{"", "gpt-explicit-saved"} {
+		name := "new profile"
+		if saved != "" {
+			name = "saved selection"
+		}
+		t.Run(name, func(t *testing.T) {
+			path := filepath.Join(t.TempDir(), "config.json")
+			if saved != "" {
+				body, err := json.Marshal(FileConfig{ActiveProvider: "codex", Providers: []ProviderProfile{{Name: "codex", CatalogID: "chatgpt", Model: saved}}})
+				if err != nil {
+					t.Fatal(err)
+				}
+				if err := os.WriteFile(path, body, 0600); err != nil {
+					t.Fatal(err)
+				}
+			}
+			ensured, err := EnsureCatalogProvider(path, "chatgpt")
+			if err != nil {
+				t.Fatal(err)
+			}
+			body, err := os.ReadFile(path)
+			if err != nil {
+				t.Fatal(err)
+			}
+			var cfg FileConfig
+			if err := json.Unmarshal(body, &cfg); err != nil {
+				t.Fatal(err)
+			}
+			want := "gpt-6.1-sol"
+			if saved != "" {
+				want = saved
+			}
+			if len(cfg.Providers) != 1 || cfg.Providers[0].Model != want {
+				t.Fatalf("ChatGPT default/selection = %#v, want model %q", cfg.Providers, want)
+			}
+			if saved != "" && (ensured.Name != "codex" || ensured.Created) {
+				t.Fatalf("saved subscription profile was replaced: %#v", ensured)
+			}
+		})
+	}
+}
@@ -99,77 +99,11 @@ func windowsSandboxSetupCheck(goos string, backend sandbox.Backend, workspaceRoo
 		return &result
 	}
 	profile := sandbox.PermissionProfileFromPolicy(workspaceRoot, doctorSandboxPolicy(sandboxConfig), scope)
-	// A RECORDED ROOT IS HISTORY, NOT PROOF. Doctor pins the marker's runtime
-	// root so the stamp check can run without taking a lease, but a command does
-	// not pin blindly: it derives the current cache and fallback candidates and
-	// honours the marker only when its root is one of them. Run setup with the
-	// cache at A, relocate the cache so commands derive B, and the stamped A tree
-	// remains: pinning A here reported a healthy sandbox immediately before every
-	// real command rejected A and failed on the out-of-date marker.
-	//
-	// So ask the command's own question first, through the command's own
-	// function. Only a root a command would still select is used to validate
-	// the marker; anything else is reported as out of date, with the remedy.
-	//
-	// THREE STATES, NOT TWO. The helper answers current, stale, or "I could not
-	// tell", and the last one used to fall through to the pinned branch: a valid
-	// old marker then made doctor report healthy while selectSandboxRuntimeRoot,
-	// reached by BuildCommandPlan, propagated the same resolution error and stopped
-	// every command from launching. Historical state is not a substitute for a
-	// failed current selection, so an error is surfaced as its own warning with the
-	// cause, and the pinned validation profile is built only once currentness has
-	// actually been established.
-	recorded, current, rootErr := sandbox.WindowsSandboxRecordedRuntimeRootIsCurrent(sandboxHome, workspaceRoot)
-	if rootErr != nil {
-		result := check("sandbox.backend", "Sandbox backend", StatusWarn, fmt.Sprintf("Native sandbox backend %s is installed, but the runtime root a command would select cannot be resolved (%s), so the recorded setup cannot be trusted and commands are likely to fail to launch.", backend.Name, rootErr.Error()), map[string]any{
-			"backend":      string(backend.Name),
-			"platform":     goos,
-			"supportLevel": string(backend.SupportLevel()),
-			"setupStatus":  "runtime-root-unresolved",
-			"error":        rootErr.Error(),
-			"remedy":       "run `zero sandbox setup` to prepare the Windows native sandbox for the current cache location",
-		})
-		return &result
-	}
-	if recorded != "" && !current {
-		result := check("sandbox.backend", "Sandbox backend", StatusWarn, fmt.Sprintf("Native sandbox backend %s is installed, but the runtime root setup recorded (%s) is not one a command would select now, so the setup is out of date.", backend.Name, recorded), map[string]any{
-			"backend":      string(backend.Name),
-			"platform":     goos,
-			"supportLevel": string(backend.SupportLevel()),
-			"setupStatus":  "runtime-root-stale",
-			"runtimeRoot":  recorded,
-			"remedy":       "run `zero sandbox setup` to prepare the Windows native sandbox for the current cache location",
-		})
-		return &result
-	}
 	setupConfig := sandbox.WindowsSandboxSetupConfig{
-		SandboxHome:    sandboxHome,
-		CommandCWD:     workspaceRoot,
-		WorkspaceRoots: []string{workspaceRoot},
-		// The same augmentation setup and the command plan apply, so doctor
-		// fingerprints what a real command fingerprints. Checking the bare profile
-		// made doctor call a correctly prepared machine "out of date", which is the
-		// mismatch this pairing exists to close. Safe to resolve in this process:
-		// doctor runs in the operator's shell, not behind the sandbox TEMP
-		// redirection that stops the runner deriving these for itself.
-		//
-		// The RECORDED runtime root goes on the profile as well, because that is
-		// what makes the stamp check run at all. Without it profile.Runtime is nil,
-		// validateWindowsSandboxRuntimeStamp returns nil early, and doctor reported
-		// a healthy sandbox on a machine whose runtime tree had been evicted and
-		// silently recreated without the capability ACE -- precisely the state the
-		// stamp exists to catch, and the state where every sandboxed command then
-		// fails with nothing explaining why.
-		//
-		// Read from the marker rather than selected, so doctor takes no lease and
-		// creates nothing, and read ONCE: this is the root whose currentness was
-		// checked above, not a second read that a rewritten marker could change. A
-		// marker from an older schema records no root, which leaves this exactly as
-		// it was.
-		PermissionProfile: sandbox.WindowsSandboxProfileWithRuntimeRoots(
-			sandbox.PermissionProfileWithRuntimeRoot(profile, recorded),
-			[]string{workspaceRoot},
-		),
+		SandboxHome:       sandboxHome,
+		CommandCWD:        workspaceRoot,
+		WorkspaceRoots:    []string{workspaceRoot},
+		PermissionProfile: profile,
 	}
 	if err := sandbox.ValidateWindowsSandboxSetupMarker(setupConfig); err != nil {
 		result := check("sandbox.backend", "Sandbox backend", StatusWarn, fmt.Sprintf("Native sandbox backend %s is installed, but Windows sandbox setup is missing or out of date: %v.", backend.Name, err), map[string]any{
@@ -1,247 +0,0 @@
-package doctor
-
-import (
-	"os"
-	"runtime"
-	"strings"
-	"testing"
-
-	"github.com/Twigpine/zero/internal/config"
-	"github.com/Twigpine/zero/internal/sandbox"
-)
-
-// DOCTOR HAS TO ASK THE ONE QUESTION THE MARKER CANNOT ANSWER.
-//
-// The marker hashes ACL-plan entries, which are pathnames. Whether the directory
-// those pathnames resolve to is still the tree setup provisioned is a different
-// question, and the runtime stamp is what answers it. validateWindowsSandboxRuntimeStamp
-// returns nil early when profile.Runtime is nil, which is correct for the setup
-// side and for unrestricted profiles, and was wrong here: doctor built its
-// profile with PermissionProfileFromPolicy, which never sets Runtime, so the
-// check was skipped and `zero doctor` reported a healthy sandbox on exactly the
-// state the stamp was added to detect -- an evicted runtime tree, silently
-// recreated with ordinary permissions and no capability ACE, where every
-// sandboxed command then fails with nothing explaining why.
-// doctorRuntimeCandidate returns a runtime root this workspace would really
-// select, taken from the candidate set the Windows plan folds in.
-//
-// It has to be a REAL candidate. An arbitrary path is already in the ACL plan on
-// one side and not the other, so the plan hashes diverge and validation fails
-// before the stamp is ever consulted -- which would make this test fail without
-// the fix for a reason that has nothing to do with the stamp.
-// redirectUserCache points os.UserCacheDir at test-owned storage.
-//
-// WITHOUT THIS THE TEST WRITES INTO THE DEVELOPER'S REAL CACHE. The runtime
-// candidate is derived from the process's actual user cache directory, so the
-// test was creating, recursively removing, recreating and then cleanup-removing
-// a directory under the real %LocalAppData%zerountime (or ~/.cache/zero on
-// Unix). The workspace digest made a collision with a live runtime tree
-// unlikely rather than impossible, and "unlikely" is not the standard for a
-// test that calls RemoveAll.
-//
-// All three variables, because os.UserCacheDir reads a different one per
-// platform: %LocalAppData% on Windows, $XDG_CACHE_HOME or $HOME on Unix, and
-// $HOME on macOS.
-func redirectUserCache(t *testing.T) {
-	t.Helper()
-	cache := t.TempDir()
-	t.Setenv("LOCALAPPDATA", cache)
-	t.Setenv("XDG_CACHE_HOME", cache)
-	t.Setenv("HOME", cache)
-}
-
-func doctorRuntimeCandidate(t *testing.T, workspace string) string {
-	t.Helper()
-	bare := doctorProfile(t, workspace)
-	augmented := sandbox.WindowsSandboxProfileWithRuntimeRoots(bare, []string{workspace})
-	existing := map[string]bool{}
-	for _, root := range bare.FileSystem.WriteRoots {
-		existing[root.Root] = true
-	}
-	for _, root := range augmented.FileSystem.WriteRoots {
-		if !existing[root.Root] {
-			return root.Root
-		}
-	}
-	t.Skip("no runtime candidate is derivable in this environment")
-	return ""
-}
-
-func doctorProfile(t *testing.T, workspace string) sandbox.PermissionProfile {
-	t.Helper()
-	scope, err := sandbox.NewScope(workspace, nil)
-	if err != nil {
-		t.Fatalf("NewScope: %v", err)
-	}
-	return sandbox.PermissionProfileFromPolicy(workspace, doctorSandboxPolicy(config.SandboxConfig{}), scope)
-}
-
-func writeDoctorSetupMarker(t *testing.T, home, workspace, runtimeRoot string) {
-	t.Helper()
-	profile := doctorProfile(t, workspace)
-	setup := sandbox.WindowsSandboxSetupConfig{
-		SandboxHome:    home,
-		CommandCWD:     workspace,
-		WorkspaceRoots: []string{workspace},
-		PermissionProfile: sandbox.WindowsSandboxProfileWithRuntimeRoots(
-			sandbox.PermissionProfileWithRuntimeRoot(profile, runtimeRoot),
-			[]string{workspace},
-		),
-	}
-	if _, err := sandbox.WriteWindowsSandboxSetupMarker(setup); err != nil {
-		t.Fatalf("WriteWindowsSandboxSetupMarker: %v", err)
-	}
-}
-
-func TestDoctorReportsAnEvictedRuntimeTree(t *testing.T) {
-	home := t.TempDir()
-	workspace := t.TempDir()
-	t.Setenv("ZERO_WINDOWS_SANDBOX_HOME", home)
-	redirectUserCache(t)
-
-	runtimeRoot := doctorRuntimeCandidate(t, workspace)
-	// Belt and braces: if the redirection above ever stops working, fail loudly
-	// rather than quietly operating on the developer's real cache.
-	if !strings.HasPrefix(runtimeRoot, os.TempDir()) && !strings.Contains(runtimeRoot, t.Name()) {
-		t.Fatalf("the runtime candidate %q is outside test-owned storage; this test creates and removes that path", runtimeRoot)
-	}
-	if err := os.MkdirAll(runtimeRoot, 0o700); err != nil {
-		t.Fatalf("create the runtime root: %v", err)
-	}
-	t.Cleanup(func() { _ = os.RemoveAll(runtimeRoot) })
-	writeDoctorSetupMarker(t, home, workspace, runtimeRoot)
-
-	backend := sandbox.Backend{Name: sandbox.BackendWindowsRestrictedToken}
-
-	// A healthy machine first, or the eviction assertion below would be satisfied
-	// by a check that warns unconditionally.
-	if result := windowsSandboxSetupCheck("windows", backend, workspace, config.SandboxConfig{}); result != nil {
-		t.Fatalf("a freshly set-up machine was reported unhealthy: %s", result.Message)
-	}
-
-	// cleanupSandboxRuntimeRoots evicts inactive roots on an age and count policy,
-	// and the next run recreates the pathname with inherited permissions. The plan
-	// hash never moves, so only the stamp can tell.
-	if err := os.RemoveAll(runtimeRoot); err != nil {
-		t.Fatalf("evict the runtime root: %v", err)
-	}
-	if err := os.MkdirAll(runtimeRoot, 0o700); err != nil {
-		t.Fatalf("recreate the pathname the way an ordinary run would: %v", err)
-	}
-
-	result := windowsSandboxSetupCheck("windows", backend, workspace, config.SandboxConfig{})
-	if result == nil {
-		t.Fatal("doctor reported a healthy sandbox while the provisioned runtime tree was gone; every sandboxed command on this machine would fail with nothing explaining why")
-	}
-	if !strings.Contains(strings.ToLower(result.Message), "setup") {
-		t.Errorf("the warning does not point at setup: %s", result.Message)
-	}
-}
-
-// UNKNOWN IS NOT CURRENT.
-//
-// WindowsSandboxRecordedRuntimeRootIsCurrent answers three ways: the recorded
-// root is still one a command would select, it is not, or the inputs a command
-// selects from cannot be resolved at all. The third used to fall through to the
-// pinned branch, so a valid old marker and stamp made doctor report healthy
-// while selectSandboxRuntimeRoot — reached by BuildCommandPlan — propagated the
-// same resolution failure and stopped every command from launching. Doctor and
-// the command disagreed about the same machine.
-//
-// Stale and unknown are different states, so this is deliberately separate from
-// the cache-A-to-cache-B test rather than another assertion on the same boolean.
-func TestDoctorWarnsWhenTheRuntimeRootCannotBeResolved(t *testing.T) {
-	home := t.TempDir()
-	workspace := t.TempDir()
-	t.Setenv("ZERO_WINDOWS_SANDBOX_HOME", home)
-	redirectUserCache(t)
-
-	runtimeRoot := doctorRuntimeCandidate(t, workspace)
-	if !strings.HasPrefix(runtimeRoot, os.TempDir()) && !strings.Contains(runtimeRoot, t.Name()) {
-		t.Fatalf("the runtime candidate %q is outside test-owned storage", runtimeRoot)
-	}
-	if err := os.MkdirAll(runtimeRoot, 0o700); err != nil {
-		t.Fatalf("create the runtime root: %v", err)
-	}
-	t.Cleanup(func() { _ = os.RemoveAll(runtimeRoot) })
-	writeDoctorSetupMarker(t, home, workspace, runtimeRoot)
-
-	backend := sandbox.Backend{Name: sandbox.BackendWindowsRestrictedToken}
-
-	// Healthy first, so the assertion below cannot be satisfied by a check that
-	// warns unconditionally.
-	if result := windowsSandboxSetupCheck("windows", backend, workspace, config.SandboxConfig{}); result != nil {
-		t.Fatalf("SETUP INVALID: a freshly set-up machine was reported unhealthy: %s", result.Message)
-	}
-
-	// Now take away the inputs the selection is derived from, leaving the marker
-	// and the provisioned tree exactly as they were. os.UserCacheDir fails with no
-	// location to read, which is the same error a command would hit.
-	t.Setenv("LOCALAPPDATA", "")
-	t.Setenv("XDG_CACHE_HOME", "")
-	t.Setenv("HOME", "")
-
-	result := windowsSandboxSetupCheck("windows", backend, workspace, config.SandboxConfig{})
-	if result == nil {
-		t.Fatal("doctor reported a healthy sandbox while the runtime root a command selects could not be resolved; every command would fail to launch with doctor saying nothing")
-	}
-	if status, _ := result.Details["setupStatus"].(string); status != "runtime-root-unresolved" {
-		t.Errorf("setupStatus = %q, want runtime-root-unresolved so the unknown state is not reported as staleness: %s", status, result.Message)
-	}
-	if _, ok := result.Details["error"]; !ok {
-		t.Errorf("the warning does not carry the resolver cause, so an operator cannot act on it: %+v", result.Details)
-	}
-}
-
-// A CACHE DIRECTORY THAT RESOLVES TO NOTHING IS UNRESOLVED TOO, not only one
-// that fails to resolve. os.UserCacheDir succeeds on a value like "." and the
-// selection a command makes refuses it after canonicalizing, with "user cache
-// directory is unavailable". The diagnostic behind this check did not, so doctor
-// went on to compare the recorded root against candidates derived from an empty
-// cache root and reported staleness, which sends an operator to re-run setup on
-// a machine where setup cannot select a root either.
-//
-// The sandbox package pins the two entry points to one error through the
-// resolver seam on every platform. This is the consumer's half, and it is
-// Windows-only because that is where the environment can produce the value:
-// %LocalAppData% is returned as it is, while a relative XDG_CACHE_HOME is an
-// error on Linux and a relative HOME still joins to a usable path on macOS.
-// Reported by @jatmn.
-func TestDoctorReportsAnEmptyCacheRootAsUnresolvedNotStale(t *testing.T) {
-	if runtime.GOOS != "windows" {
-		t.Skip("only %LocalAppData% hands os.UserCacheDir a value that canonicalizes to nothing")
-	}
-	home := t.TempDir()
-	workspace := t.TempDir()
-	t.Setenv("ZERO_WINDOWS_SANDBOX_HOME", home)
-	redirectUserCache(t)
-
-	runtimeRoot := doctorRuntimeCandidate(t, workspace)
-	if !strings.HasPrefix(runtimeRoot, os.TempDir()) && !strings.Contains(runtimeRoot, t.Name()) {
-		t.Fatalf("the runtime candidate %q is outside test-owned storage", runtimeRoot)
-	}
-	if err := os.MkdirAll(runtimeRoot, 0o700); err != nil {
-		t.Fatalf("create the runtime root: %v", err)
-	}
-	t.Cleanup(func() { _ = os.RemoveAll(runtimeRoot) })
-	writeDoctorSetupMarker(t, home, workspace, runtimeRoot)
-
-	backend := sandbox.Backend{Name: sandbox.BackendWindowsRestrictedToken}
-	if result := windowsSandboxSetupCheck("windows", backend, workspace, config.SandboxConfig{}); result != nil {
-		t.Fatalf("SETUP INVALID: a freshly set-up machine was reported unhealthy: %s", result.Message)
-	}
-
-	for _, value := range []string{".", " . "} {
-		t.Setenv("LOCALAPPDATA", value)
-		result := windowsSandboxSetupCheck("windows", backend, workspace, config.SandboxConfig{})
-		if result == nil {
-			t.Fatalf("LOCALAPPDATA=%q: doctor reported a healthy sandbox while no command can select a runtime root", value)
-		}
-		if status, _ := result.Details["setupStatus"].(string); status != "runtime-root-unresolved" {
-			t.Errorf("LOCALAPPDATA=%q: setupStatus = %q, want runtime-root-unresolved: %s", value, status, result.Message)
-		}
-		if cause, _ := result.Details["error"].(string); !strings.Contains(cause, "user cache directory is unavailable") {
-			t.Errorf("LOCALAPPDATA=%q: the warning does not carry the resolver cause a command would report: %+v", value, result.Details)
-		}
-	}
-}
@@ -374,7 +374,7 @@ func (tool skillTool) Run(_ context.Context, args map[string]any) tools.Result {
 	available := make([]string, 0, len(merged))
 	for _, skill := range merged {
 		if skill.Name == name {
-			return tools.Result{Status: tools.StatusOK, Output: skill.Content}
+			return tools.Result{Status: tools.StatusOK, Output: skills.ToolOutput(skill)}
 		}
 		available = append(available, skill.Name)
 	}
@@ -673,3 +673,28 @@ func TestExpandPluginRootPathRelative(t *testing.T) {
 		}
 	}
 }
+
+// The multi-root skill tool is the one the CLI registers, so it has to point at
+// a skill's bundled files the same way the core tool does (#584).
+func TestNewSkillToolPointsAtTheSkillsBundledFiles(t *testing.T) {
+	home := t.TempDir()
+	t.Setenv("HOME", home)
+	t.Setenv("USERPROFILE", home)
+	dir := t.TempDir()
+	writeTestSkill(t, dir, "pdf", "Run scripts/fill.py.")
+	script := filepath.Join(dir, "pdf", "scripts", "fill.py")
+	if err := os.MkdirAll(filepath.Dir(script), 0o755); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(script, []byte("print('fill')\n"), 0o755); err != nil {
+		t.Fatal(err)
+	}
+
+	got := NewSkillTool(dir, nil).Run(context.Background(), map[string]any{"name": "pdf"})
+	if got.Status != tools.StatusOK {
+		t.Fatalf("Status = %s (output: %s)", got.Status, got.Output)
+	}
+	if !strings.Contains(got.Output, "- scripts/fill.py") || !strings.Contains(got.Output, filepath.Join(dir, "pdf")) {
+		t.Fatalf("output does not point at the bundled script:\n%s", got.Output)
+	}
+}
@@ -132,7 +132,9 @@ var descriptors = []Descriptor{
 	// way in; RequiresAuth is forced true so catalog consumers know the provider
 	// needs an interactive login before a request will succeed.
 	func() Descriptor {
-		d := openAICompat("chatgpt", "ChatGPT", "https://chatgpt.com/backend-api/codex", "gpt-5.5", nil)
+		// New ChatGPT profiles follow the vendor's GPT-5.5 replacement guidance:
+		// https://learn.chatgpt.com/docs/models#gpt-55-retirement
+		d := openAICompat("chatgpt", "ChatGPT", "https://chatgpt.com/backend-api/codex", "gpt-6.1-sol", nil)
 		d.RequiresAuth = true
 		return oauthProvider(d, false, false)
 	}(),
@@ -44,6 +44,20 @@ var minimaxCuratedModels = []Model{
 		OutputModalities: []string{"text"},
 		Source:           minimaxModelSource,
 	},
+	{
+		ID:                     "MiniMax-M3.1-Flash",
+		Description:            "fast reasoning model",
+		ContextWindow:          1_000_000,
+		ToolCall:               true,
+		Reasoning:              true,
+		ReasoningEfforts:       []string{"low", "medium", "high", "xhigh", "max"},
+		DefaultReasoningEffort: "max",
+		InputModalities:        []string{"text", "image", "video"},
+		OutputModalities:       []string{"text"},
+		InputCost:              0.15,
+		OutputCost:             0.60,
+		Source:                 minimaxModelSource,
+	},
 	{
 		ID:               "MiniMax-M2.7",
 		Description:      "agentic coding model",
@@ -113,10 +127,12 @@ var curatedModels = map[string][]Model{
 		{ID: "deepseek/deepseek-chat", Description: "coding model"},
 	},
 	"chatgpt": {
-		{ID: "gpt-5.5", Description: "recommended Codex model"},
-		{ID: "gpt-5.4", Description: "strong Codex model"},
-		{ID: "gpt-5.4-mini", Description: "fast Codex model"},
-		{ID: "gpt-5.3-codex-spark", Description: "research preview fast Codex model"},
+		// Suggestions only: availability comes from the authenticated Codex catalog.
+		// https://learn.chatgpt.com/docs/models
+		{ID: "gpt-6.1-sol", Description: "balanced Codex model"},
+		{ID: "gpt-6.1-terra", Description: "fast model for writing, visual understanding, and computer use"},
+		{ID: "gpt-6-astra", Description: "advanced reasoning model"},
+		{ID: "gpt-6-luna", Description: "efficient coding model"},
 	},
 	"deepseek": {
 		{ID: "deepseek-chat", Description: "catalog default"},
@@ -35,8 +35,8 @@ func TestModelsAreProviderScoped(t *testing.T) {
 		},
 		{
 			provider: "chatgpt",
-			want:     []string{"gpt-5.5", "gpt-5.4", "gpt-5.4-mini", "gpt-5.3-codex-spark"},
-			notWant:  []string{"gpt-5", "gpt-4.1", "openai/gpt-4.1"},
+			want:     []string{"gpt-6.1-sol", "gpt-6.1-terra", "gpt-6-astra", "gpt-6-luna"},
+			notWant:  []string{"gpt-5.5", "gpt-5.4", "gpt-5.4-mini", "gpt-5.3-codex-spark", "openai/gpt-4.1"},
 		},
 		{
 			provider: "aimlapi",
@@ -65,7 +65,7 @@ func TestModelsAreProviderScoped(t *testing.T) {
 		},
 		{
 			provider: "minimaxi-cn",
-			want:     []string{"MiniMax-M3", "MiniMax-M2.7", "MiniMax-M2.1"},
+			want:     []string{"MiniMax-M3", "MiniMax-M3.1-Flash", "MiniMax-M2.7", "MiniMax-M2.1"},
 			notWant:  []string{"gpt-4.1", "claude-sonnet-4.5"},
 		},
 		{
@@ -165,8 +165,8 @@ func TestMiniMaxModelsExposeCurrentCapabilities(t *testing.T) {
 		t.Fatal("provider minimax missing from catalog")
 	}
 	models := Models(descriptor)
-	if len(models) < 2 || models[0].ID != "MiniMax-M3" || models[1].ID != "MiniMax-M2.7" {
-		t.Fatalf("MiniMax current models = %#v, want MiniMax-M3 followed by MiniMax-M2.7", modelIDs(models))
+	if len(models) < 3 || models[0].ID != "MiniMax-M3" || models[1].ID != "MiniMax-M3.1-Flash" || models[2].ID != "MiniMax-M2.7" {
+		t.Fatalf("MiniMax current models = %#v, want MiniMax-M3, MiniMax-M3.1-Flash, then MiniMax-M2.7", modelIDs(models))
 	}

 	m3 := models[0]
@@ -178,7 +178,23 @@ func TestMiniMaxModelsExposeCurrentCapabilities(t *testing.T) {
 		t.Fatalf("MiniMax-M3 modalities = input:%#v output:%#v", m3.InputModalities, m3.OutputModalities)
 	}

-	m27 := models[1]
+	m31Flash := models[1]
+	if m31Flash.ContextWindow != 1_000_000 || !m31Flash.ToolCall || !m31Flash.Reasoning {
+		t.Fatalf("MiniMax-M3.1-Flash capabilities = %#v, want 1M context with tools and reasoning", m31Flash)
+	}
+	if !reflect.DeepEqual(m31Flash.ReasoningEfforts, []string{"low", "medium", "high", "xhigh", "max"}) ||
+		m31Flash.DefaultReasoningEffort != "max" {
+		t.Fatalf("MiniMax-M3.1-Flash reasoning efforts = %#v default %q", m31Flash.ReasoningEfforts, m31Flash.DefaultReasoningEffort)
+	}
+	if !reflect.DeepEqual(m31Flash.InputModalities, []string{"text", "image", "video"}) ||
+		!reflect.DeepEqual(m31Flash.OutputModalities, []string{"text"}) {
+		t.Fatalf("MiniMax-M3.1-Flash modalities = input:%#v output:%#v", m31Flash.InputModalities, m31Flash.OutputModalities)
+	}
+	if m31Flash.InputCost != 0.15 || m31Flash.OutputCost != 0.60 {
+		t.Fatalf("MiniMax-M3.1-Flash costs = %f/%f, want 0.15/0.60", m31Flash.InputCost, m31Flash.OutputCost)
+	}
+
+	m27 := models[2]
 	if m27.ContextWindow != 204_800 || !m27.ToolCall || !m27.Reasoning {
 		t.Fatalf("MiniMax-M2.7 capabilities = %#v, want 204.8K context with tools and reasoning", m27)
 	}
@@ -0,0 +1,31 @@
+package providermodeldiscovery
+
+import (
+	"context"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/Twigpine/zero/internal/config"
+)
+
+func TestChatGPTDiscoveryUsesCurrentCompatibilityVersion(t *testing.T) {
+	server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if got := r.URL.Query().Get("client_version"); got != "0.159.0" {
+			t.Errorf("client_version = %q, want documented GPT-6.1 Terra compatibility floor 0.159.0", got)
+		}
+		_, _ = w.Write([]byte(`{"models":[{"slug":"gpt-future-coder"}]}`))
+	}))
+	defer server.Close()
+	profile := config.ProviderProfile{
+		CatalogID: "chatgpt", ProviderKind: config.ProviderKindOpenAICompatible,
+		BaseURL: server.URL + "/backend-api/codex", APIKey: "test-bearer",
+	}
+	models, err := Discover(context.Background(), profile, Options{HTTPClient: server.Client()})
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(models) != 1 || models[0].ID != "gpt-future-coder" {
+		t.Fatalf("models = %#v, want only gpt-future-coder", models)
+	}
+}
@@ -20,8 +20,9 @@ import (
 )

 const (
-	anthropicVersion             = "2023-06-01"
-	chatGPTModelsProtocolVersion = "0.146.0"
+	anthropicVersion = "2023-06-01"
+	// GPT-6.1 Terra's documented Codex client floor: https://learn.chatgpt.com/docs/models.
+	chatGPTModelsProtocolVersion = "0.159.0"
 )

 type Model struct {
@@ -49,6 +49,19 @@ func thinkingBudgetForEffort(effort string) int {
 	}
 }

+func miniMaxM31FlashEffort(model, effort string) string {
+	if !strings.EqualFold(strings.TrimSpace(model), "MiniMax-M3.1-Flash") {
+		return ""
+	}
+	effort = strings.ToLower(strings.TrimSpace(effort))
+	switch effort {
+	case "low", "medium", "high", "xhigh", "max":
+		return effort
+	default:
+		return ""
+	}
+}
+
 // resolveThinking returns the thinking budget and the max_tokens to send. When a
 // budget is requested, max_tokens is raised if needed so the budget plus a
 // minimum response both fit (Anthropic rejects budget >= max_tokens). enabled is
@@ -377,11 +390,15 @@ func (provider *Provider) anthropicRequest(request zeroruntime.CompletionRequest
 		Messages:  messages,
 		Stream:    true,
 	}
-	// Extended thinking: enable when a reasoning effort was requested. The budget
-	// is counted against max_tokens, so raise max_tokens to leave room for the
-	// response. Temperature is intentionally left unset (Anthropic requires the
-	// default when thinking is on).
-	if budget, effectiveMax, enabled := resolveThinking(request.ReasoningEffort, provider.maxTokens); enabled {
+	// MiniMax-M3.1-Flash uses adaptive thinking with output_config.effort instead
+	// of Anthropic's token budget. For other models, the budget is counted against
+	// max_tokens, so raise max_tokens to leave room for the response. Temperature
+	// is intentionally left unset (Anthropic requires the default when thinking
+	// is on).
+	if effort := miniMaxM31FlashEffort(provider.model, request.ReasoningEffort); effort != "" {
+		mapped.Thinking = &thinkingConfig{Type: "adaptive"}
+		mapped.OutputConfig = &outputConfig{Effort: effort}
+	} else if budget, effectiveMax, enabled := resolveThinking(request.ReasoningEffort, provider.maxTokens); enabled {
 		mapped.MaxTokens = effectiveMax
 		mapped.Thinking = &thinkingConfig{Type: "enabled", BudgetTokens: budget}
 	}
@@ -266,6 +266,53 @@ func TestStreamCompletionEnablesThinkingWhenEffortRequested(t *testing.T) {
 	}
 }

+func TestStreamCompletionUsesMiniMaxAdaptiveEffort(t *testing.T) {
+	t.Parallel()
+
+	for _, effort := range []string{"low", "xhigh"} {
+		effort := effort
+		t.Run(effort, func(t *testing.T) {
+			t.Parallel()
+			var gotBody map[string]any
+			server := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+				if err := json.NewDecoder(r.Body).Decode(&gotBody); err != nil {
+					t.Fatalf("decode request body: %v", err)
+				}
+				writeSSEEvent(w, "message_stop", `{"type":"message_stop"}`)
+			}))
+			defer server.Close()
+
+			provider, err := New(Options{BaseURL: server.URL + "/", Model: "MiniMax-M3.1-Flash", MaxTokens: 64_000})
+			if err != nil {
+				t.Fatalf("New returned error: %v", err)
+			}
+			stream, err := provider.StreamCompletion(context.Background(), zeroruntime.CompletionRequest{
+				Messages:        []zeroruntime.Message{{Role: zeroruntime.MessageRoleUser, Content: "hi"}},
+				ReasoningEffort: effort,
+			})
+			if err != nil {
+				t.Fatalf("StreamCompletion returned error: %v", err)
+			}
+			drain(stream)
+
+			thinking, ok := gotBody["thinking"].(map[string]any)
+			if !ok || thinking["type"] != "adaptive" {
+				t.Fatalf("thinking = %#v, want adaptive", gotBody["thinking"])
+			}
+			if _, ok := thinking["budget_tokens"]; ok {
+				t.Fatalf("MiniMax thinking must not include budget_tokens: %#v", thinking)
+			}
+			outputConfig, ok := gotBody["output_config"].(map[string]any)
+			if !ok || outputConfig["effort"] != effort {
+				t.Fatalf("output_config = %#v, want effort %q", gotBody["output_config"], effort)
+			}
+			if mt, _ := gotBody["max_tokens"].(float64); int(mt) != 64_000 {
+				t.Fatalf("max_tokens = %#v, want unchanged 64000", gotBody["max_tokens"])
+			}
+		})
+	}
+}
+
 func TestStreamCompletionOmitsThinkingWithoutEffort(t *testing.T) {
 	t.Parallel()
 	var gotBody map[string]any
@@ -10,17 +10,21 @@ type messagesRequest struct {
 	System any `json:"system,omitempty"`
 	// Thinking enables extended thinking when a reasoning effort was requested.
 	// Omitted (nil) for normal requests so behavior is unchanged.
-	Thinking *thinkingConfig `json:"thinking,omitempty"`
-	Tools    []anthropicTool `json:"tools,omitempty"`
-	Stream   bool            `json:"stream"`
+	Thinking     *thinkingConfig `json:"thinking,omitempty"`
+	OutputConfig *outputConfig   `json:"output_config,omitempty"`
+	Tools        []anthropicTool `json:"tools,omitempty"`
+	Stream       bool            `json:"stream"`
 }

-// thinkingConfig requests extended thinking with a token budget. Type is always
-// "enabled"; BudgetTokens is how many tokens the model may spend thinking (it is
-// counted against max_tokens).
+// thinkingConfig requests extended thinking. Anthropic uses "enabled" with a
+// token budget; MiniMax-M3.1-Flash uses "adaptive" with output_config.effort.
 type thinkingConfig struct {
 	Type         string `json:"type"`
-	BudgetTokens int    `json:"budget_tokens"`
+	BudgetTokens int    `json:"budget_tokens,omitempty"`
+}
+
+type outputConfig struct {
+	Effort string `json:"effort"`
 }

 type anthropicMessage struct {
@@ -288,10 +288,11 @@ func TestScanSSEDataWithContextContentResetsOnData(t *testing.T) {
 	pr, pw := io.Pipe()

 	go func() {
-		// One data line every 30ms for ~300ms (well past the 120ms content window,
-		// with comfortable slack under the 100ms idle timeout), so the content
-		// watchdog keeps resetting, then close cleanly.
-		for i := 0; i < 10; i++ {
+		// One data line every 30ms for ~750ms (well past the 600ms content window,
+		// with ~470ms of slack under the 500ms idle timeout, so a scheduling stall
+		// on a busy shared runner does not trip it), so the content watchdog keeps
+		// resetting, then close cleanly.
+		for i := 0; i < 25; i++ {
 			if _, err := io.WriteString(pw, "data: chunk\n\n"); err != nil {
 				return
 			}
@@ -303,7 +304,7 @@ func TestScanSSEDataWithContextContentResetsOnData(t *testing.T) {
 	n := 0
 	done := make(chan error, 1)
 	go func() {
-		done <- ScanSSEDataWithContext(context.Background(), func() {}, pr, 100*time.Millisecond, func(string) bool { n++; return true })
+		done <- ScanSSEDataWithContext(context.Background(), func() {}, pr, 500*time.Millisecond, func(string) bool { n++; return true })
 	}()

 	select {
@@ -314,8 +315,8 @@ func TestScanSSEDataWithContextContentResetsOnData(t *testing.T) {
 	case <-time.After(3 * time.Second):
 		t.Fatal("ScanSSEDataWithContext hung on a producing stream")
 	}
-	if n != 10 {
-		t.Fatalf("handled %d data lines, want 10", n)
+	if n != 25 {
+		t.Fatalf("handled %d data lines, want 25", n)
 	}
 }

@@ -1,35 +0,0 @@
-package sandbox
-
-import (
-	"os"
-	"testing"
-)
-
-// TestMain points the sandbox runtime's user-cache root at test-owned storage
-// for the whole package.
-//
-// PLAN CONSTRUCTION CREATES DIRECTORIES, which is easy to miss because it reads
-// like naming. windowsSandboxProfileWithProvisionedRuntime provisions the root
-// it selects, and the simulated-Windows tests run on every platform, so a test
-// that supplies an explicit child environment but leaves the cache alone writes
-// into the developer's real one. Not hypothetical: the machine this was found on
-// had accumulated thousands of entries under the real runtime root from exactly
-// these runs.
-//
-// Done once for the package rather than per test, because the leak is in the
-// DEFAULT: any new test that builds a Windows plan is affected unless its author
-// remembers, and remembering is what failed here. A test that needs a specific
-// cache root still overrides sandboxUserCacheDir itself.
-func TestMain(m *testing.M) {
-	root, err := os.MkdirTemp("", "zero-sandbox-testcache-")
-	if err != nil {
-		// Fail loudly rather than silently falling back to the real cache.
-		panic("sandbox tests: create the test cache root: " + err.Error())
-	}
-	sandboxUserCacheDir = func() (string, error) { return root, nil }
-
-	code := m.Run()
-
-	_ = os.RemoveAll(root)
-	os.Exit(code)
-}
@@ -189,18 +189,7 @@ func (engine *Engine) BuildCommandPlan(spec CommandSpec) (CommandPlan, error) {
 	}
 	var runtimeCleanup func()
 	if preference != SandboxPreferenceForbid && policy.Mode != ModeDisabled {
-		// The home THIS command asked for. Windows planning resolves
-		// ZERO_WINDOWS_SANDBOX_HOME out of spec.Env and hands it to the runner for
-		// marker validation, so selection has to read the same environment or the
-		// two disagree about which marker describes the tree. See
-		// pinnedSandboxRuntimeRoot.
-		commandSandboxHome := ""
-		if spec.Env != nil {
-			if resolved, err := ResolveWindowsSandboxHome(envListToMap(spec.Env)); err == nil {
-				commandSandboxHome = resolved
-			}
-		}
-		runtimeState, cleanup, runtimeErr := prepareSandboxRuntime(workspaceRoot, commandSandboxHome)
+		runtimeState, cleanup, runtimeErr := prepareSandboxRuntime(workspaceRoot)
 		if runtimeErr != nil {
 			return CommandPlan{}, runtimeErr
 		}
@@ -393,11 +393,10 @@ func realSmokeExecutable(t *testing.T, envKey string, fallbackName string) strin

 func runWindowsRealSmokeSetup(t *testing.T, setupExe string, options WindowsSandboxSetupArgsOptions) {
 	t.Helper()
-	setupPlan, err := BuildWindowsSandboxSetupArgs(options)
+	args, err := BuildWindowsSandboxSetupArgs(options)
 	if err != nil {
 		t.Fatalf("BuildWindowsSandboxSetupArgs: %v", err)
 	}
-	args := setupPlan.Args
 	ctx, cancel := context.WithTimeout(context.Background(), 20*time.Second)
 	defer cancel()
 	cmd := exec.CommandContext(ctx, setupExe, args...)
@@ -407,30 +406,8 @@ func runWindowsRealSmokeSetup(t *testing.T, setupExe string, options WindowsSand
 	}
 }

-// prepareWindowsRealSmokeCommand gives an elevated-tier smoke command the profile
-// the planner would have given it. The elevated runner validates the command
-// against the setup marker, and setup fingerprinted a profile with the selected
-// runtime folded in, so a command built from the bare profile can only be
-// refused. The unelevated tier applies its own plan and has no marker to agree
-// with, so it is left exactly as the test wrote it.
-func prepareWindowsRealSmokeCommand(t *testing.T, base WindowsSandboxCommandArgsOptions) (WindowsSandboxCommandArgsOptions, func()) {
-	t.Helper()
-	if base.SandboxLevel != WindowsSandboxLevelRestrictedToken || base.PermissionProfile.Runtime != nil {
-		return base, func() {}
-	}
-	workspace := firstNonEmpty(base.WorkspaceRoots...)
-	prepared, release, err := plannerPreparedWindowsProfile(workspace, base.SandboxHome, base.PermissionProfile)
-	if err != nil {
-		t.Fatalf("prepare the smoke command the way the planner does: %v", err)
-	}
-	base.PermissionProfile = prepared
-	return base, release
-}
-
 func runWindowsRealSmokeCommand(t *testing.T, runnerExe string, base WindowsSandboxCommandArgsOptions, command []string, wantCode int) {
 	t.Helper()
-	base, release := prepareWindowsRealSmokeCommand(t, base)
-	defer release()
 	base.Command = command
 	args, err := BuildWindowsSandboxCommandArgs(base)
 	if err != nil {
@@ -456,8 +433,6 @@ func runWindowsRealSmokeCommand(t *testing.T, runnerExe string, base WindowsSand
 // explicit unsupported-mode rejections rather than sandboxed command failures).
 func runWindowsRealSmokeCommandExpectError(t *testing.T, runnerExe string, base WindowsSandboxCommandArgsOptions, command []string, wantSubstr ...string) {
 	t.Helper()
-	base, release := prepareWindowsRealSmokeCommand(t, base)
-	defer release()
 	base.Command = command
 	args, err := BuildWindowsSandboxCommandArgs(base)
 	if err != nil {
@@ -1,88 +0,0 @@
-package sandbox
-
-import (
-	"os"
-	"path/filepath"
-	"testing"
-)
-
-// THE TWO HALVES OF A ROLLBACK RECORD MUST DESCRIBE ONE OBJECT.
-//
-// The snapshot used to read the identity through a handle, close it, and then
-// re-resolve the pathname to read the stamp. A rename between those pairs one
-// directory's identity with another's bytes, and a rollback that correctly
-// proves it holds the first then writes the second's contents into it.
-//
-// The window itself needs an elevated installer racing an unelevated renamer and
-// is not reproducible here, so this pins the contract the binding provides: both
-// facts come back together, and they agree with the object actually at the path.
-func TestStampSnapshotPairsIdentityWithItsOwnBytes(t *testing.T) {
-	root := filepath.Join(t.TempDir(), "root")
-	if err := os.MkdirAll(root, 0o700); err != nil {
-		t.Fatal(err)
-	}
-	const contents = "the stamp that belongs to this directory"
-	if err := os.WriteFile(windowsSandboxRuntimeStampPath(root, testStampPlanHash), []byte(contents), 0o600); err != nil {
-		t.Fatal(err)
-	}
-
-	identity, identified, prior, state, err := snapshotRuntimeStampBound(root, windowsSandboxRuntimeStampName(testStampPlanHash))
-	if err != nil {
-		t.Fatalf("snapshot: %v", err)
-	}
-	existed := state == runtimeStampPresent
-	if !identified {
-		t.Fatal("the snapshot established no identity for a directory that exists")
-	}
-	if !existed || string(prior) != contents {
-		t.Fatalf("prior stamp = %q existed=%v, want %q", string(prior), existed, contents)
-	}
-	if direct, ok := runtimeDirIdentity(root); !ok || direct != identity {
-		t.Errorf("snapshot identity %q does not describe the directory at the path (%q)", identity, direct)
-	}
-}
-
-// An absent stamp still establishes the identity, because that came from the
-// directory handle and not from the stamp read.
-func TestStampSnapshotIdentifiesARootWithNoStamp(t *testing.T) {
-	root := filepath.Join(t.TempDir(), "root")
-	if err := os.MkdirAll(root, 0o700); err != nil {
-		t.Fatal(err)
-	}
-	identity, identified, _, state, err := snapshotRuntimeStampBound(root, windowsSandboxRuntimeStampName(testStampPlanHash))
-	if err != nil {
-		t.Fatalf("snapshot: %v", err)
-	}
-	existed := state == runtimeStampPresent
-	if !identified || identity == "" {
-		t.Error("a root with no stamp established no identity")
-	}
-	if existed {
-		t.Error("reported a prior stamp that does not exist")
-	}
-}
-
-// A created directory's identity must come from the creation, so the ledger
-// cannot record an object this run did not make.
-func TestCreatedDirectoryIdentityDescribesWhatWasCreated(t *testing.T) {
-	path := filepath.Join(t.TempDir(), "created")
-
-	identity, identified, err := createRuntimeDirIdentified(path)
-	if err != nil {
-		t.Fatalf("create: %v", err)
-	}
-	if !identified || identity == "" {
-		t.Fatal("creation established no identity")
-	}
-	if info, statErr := os.Stat(path); statErr != nil || !info.IsDir() {
-		t.Fatalf("the directory was not created: %v", statErr)
-	}
-	if direct, ok := runtimeDirIdentity(path); !ok || direct != identity {
-		t.Errorf("creation identity %q does not describe the directory now at the path (%q)", identity, direct)
-	}
-
-	// Creating over something that exists is the caller's already-handled signal.
-	if _, _, again := createRuntimeDirIdentified(path); !os.IsExist(again) {
-		t.Errorf("creating over an existing directory returned %v, want an IsExist error", again)
-	}
-}
@@ -1,97 +0,0 @@
-//go:build windows
-
-package sandbox
-
-import (
-	"errors"
-	"os"
-	"path/filepath"
-	"strings"
-	"testing"
-)
-
-// ABSENCE IS CLASSIFIED IN THE PRODUCER'S DOMAIN.
-//
-// The stamp is opened through NtCreateFile, so a stamp that is not there comes
-// back as STATUS_OBJECT_NAME_NOT_FOUND. Compensation compared it with the Win32
-// codes only. An NTStatus is a different type and errors.Is does not convert it,
-// so removing a stamp that never existed was reported as a rollback failure.
-//
-// Native, against real directories: the question is what the API returns at this
-// boundary, and a synthetic error value would only test the comparison.
-
-func runtimeRootForCompensation(t *testing.T) (string, string) {
-	t.Helper()
-	root := filepath.Join(t.TempDir(), "zero", "runtime", "v1", "abcd")
-	if err := os.MkdirAll(root, 0o700); err != nil {
-		t.Fatalf("create the runtime root: %v", err)
-	}
-	identity, ok := runtimeDirIdentity(root)
-	if !ok {
-		t.Fatal("identify the runtime root")
-	}
-	return root, identity
-}
-
-// A runtime directory that exists and holds no stamp: removal has nothing to do
-// and says so by succeeding.
-func TestRemovingAStampThatIsNotThereSucceeds(t *testing.T) {
-	root, identity := runtimeRootForCompensation(t)
-	name := windowsSandboxRuntimeStampName(testStampPlanHash)
-
-	if err := compensateRuntimeStampBound(root, identity, name, nil, false); err != nil {
-		t.Fatalf("removing a stamp that was never written reported a failure: %v", err)
-	}
-	if _, err := os.Stat(root); err != nil {
-		t.Errorf("the runtime directory itself was disturbed: %v", err)
-	}
-}
-
-// A fresh setup that fails BEFORE its stamp is written, composed the way the
-// setup transaction composes it: a real snapshot of the untouched tree, and the
-// compensation run with the original failure as its cause. The cause has to come
-// back alone. A second error about the stamp sends the operator looking for
-// residue that does not exist.
-func TestAFreshSetupFailureBeforeTheStampReportsOnlyItsCause(t *testing.T) {
-	root, _ := runtimeRootForCompensation(t)
-	snapshot, err := snapshotWindowsSandboxRuntimeStamp(root, testStampPlanHash)
-	if err != nil {
-		t.Fatalf("snapshot: %v", err)
-	}
-	if snapshot.priorState != runtimeStampAbsent || !snapshot.rootIdentified {
-		t.Fatalf("SETUP INVALID: the snapshot did not record an identified root with no stamp: %+v", snapshot)
-	}
-
-	cause := errors.New("windows ACL target does not exist: an earlier entry in the plan")
-	reported := runWindowsSandboxSetupCompensations(cause, nil, windowsRuntimeRootRollback{stamp: snapshot})
-
-	if reported == nil || !errors.Is(reported, cause) {
-		t.Fatalf("the original failure is no longer visible: %v", reported)
-	}
-	if text := reported.Error(); strings.Contains(text, "rollback failed") || strings.Contains(text, "setup stamp") {
-		t.Errorf("compensation added a failure of its own for a stamp that never existed:\n%s", text)
-	}
-}
-
-// AND ONLY ABSENCE. Something at the stamp's name that cannot be opened as the
-// stamp is not "already removed", and treating every failed open as success
-// would report a clean undo over residue nobody looked at. A directory at that
-// name is a failure the real API produces on demand.
-func TestAnUninspectableStampIsStillReported(t *testing.T) {
-	root, identity := runtimeRootForCompensation(t)
-	name := windowsSandboxRuntimeStampName(testStampPlanHash)
-	if err := os.Mkdir(filepath.Join(root, name), 0o700); err != nil {
-		t.Fatalf("put a directory where the stamp would be: %v", err)
-	}
-
-	err := compensateRuntimeStampBound(root, identity, name, nil, false)
-	if err == nil {
-		t.Fatal("compensation reported success over something it could not open as the stamp")
-	}
-	if isWindowsNotFound(err) {
-		t.Errorf("the failure classifies as not-found, so it would be swallowed: %v", err)
-	}
-	if _, statErr := os.Stat(filepath.Join(root, name)); statErr != nil {
-		t.Errorf("the object it could not inspect was removed anyway: %v", statErr)
-	}
-}
@@ -1,112 +0,0 @@
-package sandbox
-
-import (
-	"os"
-	"path/filepath"
-	"strings"
-	"testing"
-)
-
-// COMPENSATION MUST PROVE IT HOLDS THE OBJECT IT CHANGED.
-//
-// The forward apply and its stamp go through one handle, so they are provably
-// about one object. Compensation runs later, after those handles have closed,
-// and used to resolve the pathname again. Rename the original aside, put an
-// ordinary directory at the name, and a pathname-only undo strips a stamp from
-// the substitute, or writes bytes snapshotted from another object onto it, and
-// then removes it as though this run had created it, while the moved original
-// keeps this run's grant and stamp.
-func TestStampCompensationRefusesAReplacementDirectory(t *testing.T) {
-	parent := t.TempDir()
-	root := filepath.Join(parent, "runtime-root")
-	if err := os.MkdirAll(root, 0o700); err != nil {
-		t.Fatal(err)
-	}
-	stampPath := windowsSandboxRuntimeStampPath(root, testStampPlanHash)
-	if err := os.WriteFile(stampPath, []byte("previous-plan-hash"), 0o600); err != nil {
-		t.Fatal(err)
-	}
-
-	snapshot, err := snapshotWindowsSandboxRuntimeStamp(root, testStampPlanHash)
-	if err != nil {
-		t.Fatalf("snapshot: %v", err)
-	}
-	if snapshot.priorState != runtimeStampPresent {
-		t.Fatal("SETUP INVALID: the snapshot did not record the pre-existing stamp")
-	}
-
-	// The original is moved aside and an ordinary directory takes the name.
-	moved := filepath.Join(parent, "moved-aside")
-	if err := os.Rename(root, moved); err != nil {
-		t.Skipf("cannot rename the runtime root on this filesystem: %v", err)
-	}
-	if err := os.MkdirAll(root, 0o700); err != nil {
-		t.Fatal(err)
-	}
-	substituteStamp := filepath.Join(root, filepath.Base(stampPath))
-	if err := os.WriteFile(substituteStamp, []byte("not-ours"), 0o600); err != nil {
-		t.Fatal(err)
-	}
-
-	err = snapshot.restore()
-	if err == nil {
-		t.Fatal("compensation mutated a directory it never touched and reported success")
-	}
-	for _, want := range []string{"no longer the directory", "replacement"} {
-		if !strings.Contains(err.Error(), want) {
-			t.Errorf("the error does not say what was left behind (%q): %v", want, err)
-		}
-	}
-	if body, readErr := os.ReadFile(substituteStamp); readErr != nil || string(body) != "not-ours" {
-		t.Errorf("the substitute's stamp was overwritten: %q %v", string(body), readErr)
-	}
-	if body, readErr := os.ReadFile(filepath.Join(moved, filepath.Base(stampPath))); readErr != nil || string(body) != "previous-plan-hash" {
-		t.Errorf("the original lost its stamp: %q %v", string(body), readErr)
-	}
-}
-
-// And the created-directory ledger applies the same rule, or the stamp check
-// just moves the damage one line down.
-func TestCreatedDirectoryCompensationRefusesAReplacement(t *testing.T) {
-	parent := t.TempDir()
-	root := filepath.Join(parent, "created-root")
-	if err := os.MkdirAll(root, 0o700); err != nil {
-		t.Fatal(err)
-	}
-	rollback := windowsRuntimeRootRollback{created: createdRuntimeDirsForTest(root)}
-
-	moved := filepath.Join(parent, "moved")
-	if err := os.Rename(root, moved); err != nil {
-		t.Skipf("cannot rename on this filesystem: %v", err)
-	}
-	if err := os.MkdirAll(root, 0o700); err != nil {
-		t.Fatal(err)
-	}
-
-	err := rollback.run()
-	if err == nil {
-		t.Fatal("a substitute directory was removed as though this run had created it")
-	}
-	if !strings.Contains(err.Error(), "no longer the directory this run created") {
-		t.Errorf("the error does not name the replacement: %v", err)
-	}
-	if _, statErr := os.Stat(root); statErr != nil {
-		t.Errorf("the substitute was removed: %v", statErr)
-	}
-}
-
-// The ordinary case still cleans up, or the guard would be refusing everything.
-func TestCompensationStillRemovesWhatThisRunCreated(t *testing.T) {
-	parent := t.TempDir()
-	root := filepath.Join(parent, "mine")
-	if err := os.MkdirAll(root, 0o700); err != nil {
-		t.Fatal(err)
-	}
-	rollback := windowsRuntimeRootRollback{created: createdRuntimeDirsForTest(root)}
-	if err := rollback.run(); err != nil {
-		t.Fatalf("compensation refused a directory it really did create: %v", err)
-	}
-	if _, err := os.Stat(root); !os.IsNotExist(err) {
-		t.Errorf("the directory this run created survived compensation: %v", err)
-	}
-}
@@ -1,67 +0,0 @@
-//go:build !windows
-
-package sandbox
-
-import (
-	"errors"
-	"fmt"
-	"os"
-	"path/filepath"
-)
-
-// runtimeCompensationSwapSeam exists so the shared compensation code compiles
-// everywhere. Only the Windows build closes a check-then-mutate window, because
-// only there does setup run elevated against a tree an unelevated process can
-// rename.
-var runtimeCompensationSwapSeam func()
-
-func compensateRuntimeStampBound(root string, identity string, name string, prior []byte, existed bool) error {
-	current, ok := runtimeDirIdentity(root)
-	if !ok {
-		return fmt.Errorf("identify the sandbox runtime root %s for stamp compensation", root)
-	}
-	if current != identity {
-		return fmt.Errorf("sandbox runtime root %s is no longer the directory this setup stamped; "+
-			"leaving the replacement untouched, and the original still carries this run's stamp", root)
-	}
-	if runtimeCompensationSwapSeam != nil {
-		runtimeCompensationSwapSeam()
-	}
-	path := filepath.Join(root, name)
-	if !existed {
-		if err := os.Remove(path); err != nil && !os.IsNotExist(err) {
-			return fmt.Errorf("remove sandbox runtime setup stamp written by this run: %w", err)
-		}
-		return nil
-	}
-	if err := os.WriteFile(path, prior, 0o600); err != nil {
-		return fmt.Errorf("restore the previous sandbox runtime setup stamp: %w", err)
-	}
-	return nil
-}
-
-func removeCreatedRuntimeDirBound(path string, identity string) error {
-	current, ok := runtimeDirIdentity(path)
-	if !ok {
-		// ONLY ABSENCE MEANS THERE IS NOTHING TO UNDO. Any lookup failure used to
-		// return here, so a directory that exists but could not be identified was
-		// reported as cleanly removed while it stayed on disk. The Windows build
-		// already draws this line, and the rollback it serves promises to report
-		// what it could not remove. Reported by CodeRabbit.
-		if _, err := os.Lstat(path); errors.Is(err, os.ErrNotExist) {
-			return nil
-		}
-		return fmt.Errorf("sandbox runtime root %s was created by this run but could not be identified for removal; leaving it in place", path)
-	}
-	if current != identity {
-		return fmt.Errorf("sandbox runtime root %s is no longer the directory this run created; "+
-			"leaving the replacement in place", path)
-	}
-	if runtimeCompensationSwapSeam != nil {
-		runtimeCompensationSwapSeam()
-	}
-	if err := os.Remove(path); err != nil && !os.IsNotExist(err) {
-		return fmt.Errorf("remove sandbox runtime root %s created by this run: %w", path, err)
-	}
-	return nil
-}
@@ -1,74 +0,0 @@
-//go:build !windows
-
-package sandbox
-
-import (
-	"errors"
-	"os"
-	"path/filepath"
-	"strings"
-	"testing"
-)
-
-// ONLY ABSENCE IS NOTHING TO UNDO.
-//
-// removeCreatedRuntimeDirBound returned success on any failure to identify the
-// directory, so one that was still there, but could not be looked at, was
-// reported as removed. Here the parent loses search permission, which makes the
-// directory exist and be unidentifiable at the same time.
-func TestRemovingAnUnidentifiableCreatedDirectoryIsReported(t *testing.T) {
-	if os.Geteuid() == 0 {
-		t.Skip("root bypasses directory search permission, so the lookup cannot be made to fail")
-	}
-	parent := filepath.Join(t.TempDir(), "zero")
-	created := filepath.Join(parent, "runtime")
-	if err := os.MkdirAll(created, 0o700); err != nil {
-		t.Fatalf("create the directory: %v", err)
-	}
-	identity, ok := runtimeDirIdentity(created)
-	if !ok {
-		t.Fatal("identify the created directory")
-	}
-	if err := os.Chmod(parent, 0o600); err != nil {
-		t.Fatalf("withdraw search permission: %v", err)
-	}
-	t.Cleanup(func() { _ = os.Chmod(parent, 0o700) })
-	if _, err := os.Lstat(created); err == nil || errors.Is(err, os.ErrNotExist) {
-		t.Fatalf("SETUP INVALID: the directory should exist and be unidentifiable, Lstat said %v", err)
-	}
-
-	err := removeCreatedRuntimeDirBound(created, identity)
-	if err == nil {
-		t.Fatal("a created directory that could not be identified was reported as removed")
-	}
-	if !strings.Contains(err.Error(), "could not be identified") {
-		t.Errorf("the error does not say why the directory was left: %v", err)
-	}
-
-	// And it is still there, which is why the error is owed.
-	if err := os.Chmod(parent, 0o700); err != nil {
-		t.Fatalf("restore search permission: %v", err)
-	}
-	if _, err := os.Lstat(created); err != nil {
-		t.Fatalf("the directory was gone after all, so the error was not owed: %v", err)
-	}
-}
-
-// Absence stays a clean undo, or the test above would be satisfied by a function
-// that reports every lookup failure.
-func TestRemovingACreatedDirectoryThatIsAlreadyGoneSucceeds(t *testing.T) {
-	created := filepath.Join(t.TempDir(), "runtime")
-	if err := os.Mkdir(created, 0o700); err != nil {
-		t.Fatalf("create the directory: %v", err)
-	}
-	identity, ok := runtimeDirIdentity(created)
-	if !ok {
-		t.Fatal("identify the created directory")
-	}
-	if err := os.Remove(created); err != nil {
-		t.Fatalf("remove the directory: %v", err)
-	}
-	if err := removeCreatedRuntimeDirBound(created, identity); err != nil {
-		t.Fatalf("a created directory that is already gone was reported as a failed undo: %v", err)
-	}
-}
@@ -1,100 +0,0 @@
-//go:build windows
-
-package sandbox
-
-import (
-	"os"
-	"path/filepath"
-	"testing"
-)
-
-// swapDuringCompensation installs a seam that runs once, between the identity
-// check and the mutation, and replaces root with a fresh directory carrying a
-// file of its own.
-func swapDuringCompensation(t *testing.T, root string, aside string) *string {
-	t.Helper()
-	fired := false
-	previous := runtimeCompensationSwapSeam
-	t.Cleanup(func() { runtimeCompensationSwapSeam = previous })
-	runtimeCompensationSwapSeam = func() {
-		if fired {
-			return
-		}
-		fired = true
-		if err := os.Rename(root, aside); err != nil {
-			t.Logf("could not rename the original aside: %v", err)
-			return
-		}
-		if err := os.MkdirAll(root, 0o700); err != nil {
-			t.Fatalf("create the substitute: %v", err)
-		}
-		if err := os.WriteFile(filepath.Join(root, "substitute.txt"), []byte("belongs to whoever put it here"), 0o600); err != nil {
-			t.Fatalf("seed the substitute: %v", err)
-		}
-	}
-	return &aside
-}
-
-// A REPLACEMENT AFTER THE CHECK MUST NOT REACH THE SUBSTITUTE.
-//
-// Compensation used to read the identity through a handle, close it, and then
-// resolve the pathname again for the write or the delete. A rename plus a
-// replacement in that interval made the comparison true about one object while
-// the mutation landed on another, under elevation. Holding the handle across
-// both means the mutation follows the object that was verified, and whatever now
-// answers to the name is untouched.
-func TestStampCompensationDoesNotReachASubstitute(t *testing.T) {
-	parent := t.TempDir()
-	root := filepath.Join(parent, "zero", "runtime", "v1", "abcd")
-	if err := os.MkdirAll(root, 0o700); err != nil {
-		t.Fatalf("create the runtime root: %v", err)
-	}
-	if err := writeWindowsSandboxRuntimeStamp(root, "planhash"); err != nil {
-		t.Fatalf("write the stamp: %v", err)
-	}
-	identity, ok := runtimeDirIdentity(root)
-	if !ok {
-		t.Fatal("identify the runtime root")
-	}
-	aside := filepath.Join(parent, "renamed-aside")
-	swapDuringCompensation(t, root, aside)
-
-	// Removal of a stamp this run wrote, i.e. the fresh-setup rollback.
-	_ = compensateRuntimeStampBound(root, identity, windowsSandboxRuntimeStampName(testStampPlanHash), nil, false)
-
-	// Whatever the outcome, the substitute is not this run's business.
-	substitute := filepath.Join(root, "substitute.txt")
-	if _, err := os.Stat(substitute); err != nil {
-		t.Errorf("compensation removed a file from the substitute directory: %v", err)
-	}
-	if _, err := os.Stat(windowsSandboxRuntimeStampPath(root, testStampPlanHash)); err == nil {
-		t.Error("compensation created a stamp inside the substitute directory")
-	}
-	// And the original, which is the object that was verified, is the one that
-	// lost the stamp this run wrote.
-	if _, err := os.Stat(windowsSandboxRuntimeStampPath(aside, testStampPlanHash)); err == nil {
-		t.Error("the stamp this run wrote is still on the original object, so compensation followed the name instead")
-	}
-}
-
-// The same for the delete, where following the name would remove a directory
-// this run never created.
-func TestDirectoryCompensationDoesNotRemoveASubstitute(t *testing.T) {
-	parent := t.TempDir()
-	created := filepath.Join(parent, "created")
-	if err := os.MkdirAll(created, 0o700); err != nil {
-		t.Fatalf("create the directory: %v", err)
-	}
-	identity, ok := runtimeDirIdentity(created)
-	if !ok {
-		t.Fatal("identify the created directory")
-	}
-	aside := filepath.Join(parent, "renamed-aside")
-	swapDuringCompensation(t, created, aside)
-
-	_ = removeCreatedRuntimeDirBound(created, identity)
-
-	if _, err := os.Stat(filepath.Join(created, "substitute.txt")); err != nil {
-		t.Errorf("compensation removed the substitute directory's contents: %v", err)
-	}
-}
@@ -1,78 +0,0 @@
-//go:build windows
-
-package sandbox
-
-import (
-	"errors"
-	"io/fs"
-	"os"
-	"path/filepath"
-	"strings"
-	"testing"
-)
-
-// ONLY NOT-FOUND IS EVIDENCE OF REMOVAL.
-//
-// The verification probe used to read any Lstat error as absence, so a sharing
-// violation, an access denial, or an entry left delete-pending by another
-// process that holds a share-delete handle all reported complete compensation
-// for a directory that is still there. A holder able to clear the disposition
-// could then make the "removed" object visible again, and setup would already
-// have said the rollback finished.
-//
-// The three outcomes are distinguished with a probe seam, because a real
-// filesystem will not produce the third on demand.
-func TestDeletionVerificationDistinguishesAllThreeOutcomes(t *testing.T) {
-	previous := runtimeCompensationStat
-	t.Cleanup(func() { runtimeCompensationStat = previous })
-
-	newDir := func() (string, string) {
-		t.Helper()
-		dir := filepath.Join(t.TempDir(), "created")
-		if err := os.MkdirAll(dir, 0o700); err != nil {
-			t.Fatal(err)
-		}
-		identity, ok := runtimeDirIdentity(dir)
-		if !ok {
-			t.Fatal("identify the created directory")
-		}
-		return dir, identity
-	}
-
-	t.Run("absence is proven and reported as success", func(t *testing.T) {
-		dir, identity := newDir()
-		runtimeCompensationStat = func(string) (fs.FileInfo, error) {
-			return nil, &os.PathError{Op: "lstat", Path: dir, Err: os.ErrNotExist}
-		}
-		if err := removeCreatedRuntimeDirBound(dir, identity); err != nil {
-			t.Errorf("a proven-absent directory was reported as a failure: %v", err)
-		}
-	})
-
-	t.Run("still present is reported", func(t *testing.T) {
-		dir, identity := newDir()
-		runtimeCompensationStat = func(path string) (fs.FileInfo, error) { return os.Stat(".") }
-		err := removeCreatedRuntimeDirBound(dir, identity)
-		if err == nil {
-			t.Fatal("a directory still present after deletion was reported as removed")
-		}
-		if !strings.Contains(err.Error(), "still present") {
-			t.Errorf("unexpected error: %v", err)
-		}
-	})
-
-	t.Run("an unverifiable probe is residue, not success", func(t *testing.T) {
-		dir, identity := newDir()
-		// What a share-delete holder produces: neither absence nor presence.
-		runtimeCompensationStat = func(path string) (fs.FileInfo, error) {
-			return nil, &os.PathError{Op: "lstat", Path: path, Err: errors.New("Access is denied.")}
-		}
-		err := removeCreatedRuntimeDirBound(dir, identity)
-		if err == nil {
-			t.Fatal("an unverifiable removal was reported as complete compensation")
-		}
-		if !strings.Contains(err.Error(), "could not be verified") {
-			t.Errorf("the error does not say the removal is unproven: %v", err)
-		}
-	})
-}
@@ -1,195 +0,0 @@
-//go:build windows
-
-package sandbox
-
-import (
-	"errors"
-	"fmt"
-	"os"
-	"unsafe"
-
-	"golang.org/x/sys/windows"
-)
-
-// THE HANDLE IS THE OBJECT. A pathname is not.
-//
-// Compensation used to read the directory identity through a handle, close it,
-// and then resolve the pathname again for the mutation. A rename followed by a
-// replacement between those two steps makes the comparison true about one
-// object while the write or delete lands on another, and this runs elevated, so
-// a redirected pathname gives the mutation reach the replacer does not have
-// directly.
-//
-// Opening once and keeping the handle open across BOTH the identity check and
-// the mutation removes the interval. The child operations are relative to that
-// handle, so no ancestor is re-resolved either.
-
-// runtimeCompensationSwapSeam runs between the identity check and the mutation.
-// Nil in production; a test installs one to replace the directory in exactly the
-// window this design closes.
-var runtimeCompensationSwapSeam func()
-
-// runtimeCompensationStat is the post-deletion existence probe. A var so a test
-// can produce the third outcome, an inspection that neither proves absence nor
-// presence, which no real filesystem produces on demand.
-var runtimeCompensationStat = os.Lstat
-
-type fileDispositionInfo struct {
-	DeleteFile byte
-}
-
-func handleRuntimeIdentity(handle windows.Handle) (string, error) {
-	var info windows.ByHandleFileInformation
-	if err := windows.GetFileInformationByHandle(handle, &info); err != nil {
-		return "", err
-	}
-	return fmt.Sprintf("%d:%d:%d", info.VolumeSerialNumber, info.FileIndexHigh, info.FileIndexLow), nil
-}
-
-// openVerifiedRuntimeDirectory opens path without following a link at the final
-// component and returns it only if it is still the object identity names.
-func openVerifiedRuntimeDirectory(path string, identity string, access uint32, what string) (windows.Handle, error) {
-	utf16Path, err := windows.UTF16PtrFromString(path)
-	if err != nil {
-		return 0, err
-	}
-	handle, err := windows.CreateFile(
-		utf16Path,
-		access|windows.FILE_READ_ATTRIBUTES,
-		windows.FILE_SHARE_READ|windows.FILE_SHARE_WRITE|windows.FILE_SHARE_DELETE,
-		nil,
-		windows.OPEN_EXISTING,
-		windows.FILE_FLAG_BACKUP_SEMANTICS|windows.FILE_FLAG_OPEN_REPARSE_POINT,
-		0,
-	)
-	if err != nil {
-		return 0, err
-	}
-	current, err := handleRuntimeIdentity(handle)
-	if err != nil {
-		_ = windows.CloseHandle(handle)
-		return 0, fmt.Errorf("identify the sandbox runtime directory %s for compensation: %w", path, err)
-	}
-	if current != identity {
-		_ = windows.CloseHandle(handle)
-		return 0, fmt.Errorf("sandbox runtime root %s is no longer the directory this run %s; "+
-			"leaving the replacement in place, and the original still carries this run's changes", path, what)
-	}
-	return handle, nil
-}
-
-// markForDeletion queues the object the handle names for removal. It never
-// resolves a pathname, so it can only reach what this process already holds.
-func markForDeletion(handle windows.Handle) error {
-	info := fileDispositionInfo{DeleteFile: 1}
-	return windows.SetFileInformationByHandle(
-		handle,
-		windows.FileDispositionInfo,
-		(*byte)(unsafe.Pointer(&info)),
-		uint32(unsafe.Sizeof(info)),
-	)
-}
-
-// compensateRuntimeStampBound restores or removes the stamp through a handle on
-// the directory whose identity still matches.
-//
-// RESTORED WITHOUT A GAP. This used to delete the stamp and then recreate the
-// previous one, so a command validating in between, or a process stopping
-// there, found no stamp at all where the previous setup had left a valid one.
-// The ordinary writer replaces the stamp in one step and puts the protected
-// DACL back, which a raw write would not, so a restore is just a publish of the
-// previous bytes. Only the branch that had no stamp before deletes, because
-// absent is the state it restores. Reported by @jatmn.
-func compensateRuntimeStampBound(root string, identity string, name string, prior []byte, existed bool) error {
-	directory, err := openVerifiedRuntimeDirectory(root, identity,
-		windows.FILE_TRAVERSE|windowsFileAddFile|windows.READ_CONTROL|windows.SYNCHRONIZE, "stamped")
-	if err != nil {
-		return err
-	}
-	defer windows.CloseHandle(directory)
-
-	if runtimeCompensationSwapSeam != nil {
-		runtimeCompensationSwapSeam()
-	}
-
-	if !existed {
-		return deleteRuntimeStampChild(directory, name)
-	}
-	// Through the ordinary writer so it is protected again, and so the reader ACE
-	// is resolved the same way a fresh setup resolves it.
-	if err := writeWindowsRuntimeStampToDirectoryHandle(directory, name, string(prior)); err != nil {
-		return fmt.Errorf("restore the previous sandbox runtime setup stamp: %w", err)
-	}
-	return nil
-}
-
-// deleteRuntimeStampChild removes the stamp relative to an already verified
-// directory handle. A stamp that is not there is the desired end state; anything
-// else is reported rather than swallowed.
-func deleteRuntimeStampChild(directory windows.Handle, name string) error {
-	stamp, err := openWindowsChildNoFollow(directory, name,
-		windows.DELETE|windows.FILE_READ_ATTRIBUTES, windows.FILE_NON_DIRECTORY_FILE)
-	if err != nil {
-		// ABSENCE IS CLASSIFIED BY THE PRODUCER'S DOMAIN. openWindowsChildNoFollow
-		// goes through NtCreateFile, so a stamp that is not there arrives as
-		// STATUS_OBJECT_NAME_NOT_FOUND. This compared against the Win32 codes only,
-		// and an NTStatus is a different type that errors.Is does not convert, so a
-		// fresh setup that failed before writing any stamp reported a second,
-		// spurious rollback failure for removing a file that never existed. The
-		// snapshot path already answers this question for both domains, and using
-		// the same answer keeps snapshot and compensation agreeing about what
-		// absence means. Reported by @jatmn.
-		if isWindowsNotFound(err) {
-			return nil
-		}
-		return fmt.Errorf("remove sandbox runtime setup stamp written by this run: %w", err)
-	}
-	if err := markForDeletion(stamp); err != nil {
-		_ = windows.CloseHandle(stamp)
-		return fmt.Errorf("remove sandbox runtime setup stamp written by this run: %w", err)
-	}
-	// The entry goes when the last handle closes.
-	_ = windows.CloseHandle(stamp)
-	return nil
-}
-
-// removeCreatedRuntimeDirBound removes a directory this run created, through a
-// handle on the object identity names.
-func removeCreatedRuntimeDirBound(path string, identity string) error {
-	handle, err := openVerifiedRuntimeDirectory(path, identity, windows.DELETE, "created")
-	if err != nil {
-		if isWindowsNotFound(err) {
-			return nil
-		}
-		return err
-	}
-	if runtimeCompensationSwapSeam != nil {
-		runtimeCompensationSwapSeam()
-	}
-	if err := markForDeletion(handle); err != nil {
-		_ = windows.CloseHandle(handle)
-		return fmt.Errorf("remove sandbox runtime root %s created by this run: %w", path, err)
-	}
-	// The entry goes when the last handle closes, so releasing ours is what makes
-	// the removal observable.
-	_ = windows.CloseHandle(handle)
-	// REPORTED SUCCESS IS NOT PROOF. A handle-bound operation accepting the call
-	// has been seen not to take effect (PR #751, the promote rename), so the
-	// outcome is checked rather than assumed.
-	//
-	// THREE OUTCOMES, NOT TWO. This used to read any Lstat error as absence, so a
-	// sharing violation, an access denial, or a delete-pending entry held open by
-	// another process all reported complete compensation for a directory that is
-	// still there. A holder that clears the disposition can then make the
-	// "removed" object visible again. Only not-found is evidence of removal;
-	// everything else is at best unproven and has to be reported as residue.
-	_, statErr := runtimeCompensationStat(path)
-	switch {
-	case errors.Is(statErr, os.ErrNotExist):
-		return nil
-	case statErr == nil:
-		return fmt.Errorf("remove sandbox runtime root %s created by this run: it is still present after the deletion was accepted", path)
-	default:
-		return fmt.Errorf("remove sandbox runtime root %s created by this run: its removal could not be verified, so it may still be present: %w", path, statErr)
-	}
-}
@@ -1,15 +0,0 @@
-package sandbox
-
-// runtimeIdentityAfterCreate resolves a just-created runtime directory's
-// identity BY PATHNAME, which is a second resolution of the same name and
-// therefore a window: the directory created can be renamed away and an ordinary
-// one substituted before this runs, and the ledger then records the substitute
-// for a directory the run never made.
-//
-// It is the honest implementation off Windows, where the elevated-installer
-// versus unelevated-renamer split this closes does not apply. On Windows the
-// creation returns the handle and identity is read from that, so nothing there
-// may call this: a Windows test asserts the count is zero, which is what makes
-// "the creation establishes the identity" a checked property rather than a
-// comment.
-var runtimeIdentityAfterCreate = runtimeDirIdentity
@@ -1,15 +0,0 @@
-//go:build !windows
-
-package sandbox
-
-import "os"
-
-// createRuntimeDirIdentified keeps the pathname form off Windows, where the
-// elevated-installer-versus-unelevated-renamer split this closes does not apply.
-func createRuntimeDirIdentified(path string) (string, bool, error) {
-	if err := os.Mkdir(path, 0o700); err != nil {
-		return "", false, err
-	}
-	identity, ok := runtimeIdentityAfterCreate(path)
-	return identity, ok, nil
-}
@@ -1,131 +0,0 @@
-//go:build windows
-
-package sandbox
-
-import (
-	"errors"
-	"fmt"
-	"os"
-	"path/filepath"
-	"unsafe"
-
-	"golang.org/x/sys/windows"
-)
-
-// createRuntimeDirIdentified creates path and returns the identity of the
-// directory it created, read from the handle the creation itself returned.
-//
-// THE CREATION MUST BE THE THING THAT ESTABLISHES IDENTITY. os.Mkdir followed by
-// runtimeDirIdentity(path) is two resolutions of one name: Mkdir creates A, and
-// the reopen can land on a B substituted in between, so the rollback ledger
-// records B's identity for a directory this run never made. Compensation then
-// correctly proves it holds B and deletes it, while A keeps this run's ACL and
-// stamp under a name nothing is tracking. The runtime parent belongs to the
-// ordinary user, so that substitution needs no privilege.
-//
-// Win32 CreateFile cannot create a directory at all, whatever disposition it is
-// given, so an earlier attempt to do this with CREATE_NEW fell through to the
-// Mkdir-plus-reopen path on EVERY real creation and the documented handle
-// contract never once held. NtCreateFile with FILE_CREATE and
-// FILE_DIRECTORY_FILE does create one, and returns the handle to it.
-//
-// If the atomic create cannot be completed, this returns an error rather than
-// manufacturing an ownership record from a reopen: an unidentified create must
-// stop setup before privileged state is applied, not enter the ledger as though
-// it were proven.
-func createRuntimeDirIdentified(path string) (string, bool, error) {
-	clean := filepath.Clean(path)
-	parentPath, leaf := filepath.Split(clean)
-	leaf = filepath.Clean(leaf)
-	parentPath = filepath.Clean(parentPath)
-	if leaf == "" || leaf == "." || parentPath == clean {
-		return "", false, fmt.Errorf("sandbox runtime path %s has no component to create", path)
-	}
-
-	// The parent is either a directory that already existed when the missing
-	// components were computed, or one this same loop created a moment ago.
-	parent, err := openWindowsDirectoryByName(parentPath)
-	if err != nil {
-		return "", false, fmt.Errorf("open sandbox runtime parent %s: %w", parentPath, err)
-	}
-	defer windows.CloseHandle(parent)
-
-	handle, err := createWindowsChildDirectory(parent, leaf)
-	if err != nil {
-		// A collision is the same signal os.Mkdir gives for a component another
-		// process won the race to create, and the caller already treats that as
-		// "not ours" (windows_setup.go). Returned as a *PathError so BOTH
-		// os.IsExist, which the caller uses and which does not unwrap %w, and
-		// errors.Is recognize it; wrapping with %w alone would have turned a
-		// benign race into a hard setup failure.
-		if errors.Is(err, windows.STATUS_OBJECT_NAME_COLLISION) {
-			return "", false, &os.PathError{Op: "mkdir", Path: clean, Err: os.ErrExist}
-		}
-		return "", false, fmt.Errorf("create sandbox runtime directory %s: %w", clean, err)
-	}
-	defer windows.CloseHandle(handle)
-
-	identity, idErr := handleRuntimeIdentity(handle)
-	if idErr != nil {
-		return "", false, fmt.Errorf("identify the sandbox runtime directory created at %s: %w", clean, idErr)
-	}
-	return identity, true, nil
-}
-
-// removeWindowsObjectByHandle deletes the object this handle is open on.
-//
-// THROUGH THE HANDLE, NOT THE NAME. Every caller is on a failure path partway
-// through a creation, and the one object it may remove is the one it just made
-// and is still holding. Re-resolving the name to delete it is the substitution
-// hole this whole descent exists to close, and here it would be a pathname
-// authoritative delete of somebody else's directory.
-//
-// The handle needs DELETE access, which the creating opens above ask for.
-func removeWindowsObjectByHandle(handle windows.Handle) error {
-	var iosb windows.IO_STATUS_BLOCK
-	deleteIt := byte(1)
-	return windows.NtSetInformationFile(handle, &iosb, &deleteIt, 1, windows.FileDispositionInformation)
-}
-
-// createWindowsChildDirectory creates exactly one directory component beneath
-// parent and returns the handle to the object it created.
-//
-// Relative to a handle, so no ancestor is re-resolved and there is no interval
-// for a swap to land in. FILE_CREATE fails rather than opening anything that is
-// already there, so the returned handle cannot describe a pre-existing object,
-// and FILE_OPEN_REPARSE_POINT keeps the failure honest if one is.
-func createWindowsChildDirectory(parent windows.Handle, name string) (windows.Handle, error) {
-	objectName, err := windows.NewNTUnicodeString(name)
-	if err != nil {
-		return 0, fmt.Errorf("encode sandbox runtime component %s: %w", name, err)
-	}
-	attributes := windows.OBJECT_ATTRIBUTES{
-		RootDirectory: parent,
-		ObjectName:    objectName,
-		Attributes:    windows.OBJ_CASE_INSENSITIVE,
-	}
-	attributes.Length = uint32(unsafe.Sizeof(attributes))
-
-	var handle windows.Handle
-	var iosb windows.IO_STATUS_BLOCK
-	// DELETE because the caller has to be able to undo this creation through the
-	// handle it just got, without going back through the name. See
-	// removeWindowsObjectByHandle.
-	err = windows.NtCreateFile(
-		&handle,
-		windows.DELETE|windows.FILE_READ_ATTRIBUTES|windows.FILE_TRAVERSE|windows.SYNCHRONIZE,
-		&attributes,
-		&iosb,
-		nil,
-		windows.FILE_ATTRIBUTE_DIRECTORY,
-		windows.FILE_SHARE_READ|windows.FILE_SHARE_WRITE|windows.FILE_SHARE_DELETE,
-		windows.FILE_CREATE,
-		windows.FILE_DIRECTORY_FILE|windows.FILE_SYNCHRONOUS_IO_NONALERT|windows.FILE_OPEN_REPARSE_POINT,
-		0,
-		0,
-	)
-	if err != nil {
-		return 0, err
-	}
-	return handle, nil
-}
@@ -1,158 +0,0 @@
-package sandbox
-
-import (
-	"errors"
-	"os"
-	"path/filepath"
-	"strings"
-	"testing"
-)
-
-// CREATION IS THE OWNERSHIP BOUNDARY, NOT THE STEP AFTER IT.
-//
-// Acquisition learns it created a directory or a lease file at the create
-// itself, but used to publish that fact only once every later step had also
-// succeeded: the identity read for a directory, and the inspection, wrapping and
-// locking for a lease. A failure in between returned an error while leaving the
-// object on disk with nothing above holding a record of it. Setup then reported
-// failure and left invocation-owned state behind, and where an ancestor HAD been
-// recorded, that ancestor's compensation failed afterwards on a child it could
-// not account for.
-//
-// Both platforms are driven through acquireRuntimeLeaseForPlatform, the call
-// production makes, rather than through either descent directly.
-
-var errInjectedRuntimeFailure = errors.New("injected failure after the create")
-
-// failRuntimeCreationAt makes the step after the creation of the named object
-// fail, which is the only way to reach the paths under test.
-func failRuntimeCreationAt(t *testing.T, match string) {
-	t.Helper()
-	runtimeCreationFailure = func(created string) error {
-		if filepath.Base(created) == match {
-			return errInjectedRuntimeFailure
-		}
-		return nil
-	}
-	t.Cleanup(func() { runtimeCreationFailure = nil })
-}
-
-// runtimeRootForAcquisition returns a root whose owned tail does not exist yet,
-// so acquisition has to create it.
-func runtimeRootForAcquisition(t *testing.T) string {
-	t.Helper()
-	base := t.TempDir()
-	if err := os.MkdirAll(filepath.Join(base, "zero", "runtime"), 0o700); err != nil {
-		t.Fatal(err)
-	}
-	return filepath.Join(base, "zero", "runtime", "v1", "digest")
-}
-
-// A directory whose identity could not be read is removed, not abandoned.
-func TestAcquisitionUndoesADirectoryItCreatedButCouldNotIdentify(t *testing.T) {
-	root := runtimeRootForAcquisition(t)
-	version := filepath.Dir(root)
-	failRuntimeCreationAt(t, filepath.Base(version))
-
-	lease, created, err := acquireRuntimeLeaseForPlatform(root)
-	if err == nil {
-		lease.release()
-		t.Fatal("SETUP INVALID: acquisition succeeded, so the injected failure never fired")
-	}
-	if !errors.Is(err, errInjectedRuntimeFailure) {
-		t.Fatalf("acquisition failed for some other reason: %v", err)
-	}
-	if len(created) != 0 {
-		t.Errorf("acquisition returned a ledger for a failed create: %v", created)
-	}
-	if _, statErr := os.Stat(version); !os.IsNotExist(statErr) {
-		t.Errorf("%s was created by this run and left behind after the failure, with nothing recording it (stat: %v)", version, statErr)
-	}
-	// And what was already there is untouched.
-	if _, statErr := os.Stat(filepath.Dir(version)); statErr != nil {
-		t.Errorf("a directory that existed before the run was removed: %v", statErr)
-	}
-}
-
-// So is a lease file this run created but never got to lock.
-func TestAcquisitionUndoesALeaseFileItCreatedButCouldNotLock(t *testing.T) {
-	root := runtimeRootForAcquisition(t)
-	leasePath := sandboxRuntimeLeasePath(root)
-	failRuntimeCreationAt(t, filepath.Base(leasePath))
-
-	lease, _, err := acquireRuntimeLeaseForPlatform(root)
-	if err == nil {
-		lease.release()
-		t.Fatal("SETUP INVALID: acquisition succeeded, so the injected failure never fired")
-	}
-	if !errors.Is(err, errInjectedRuntimeFailure) {
-		t.Fatalf("acquisition failed for some other reason: %v", err)
-	}
-	if _, statErr := os.Stat(leasePath); !os.IsNotExist(statErr) {
-		t.Errorf("%s was created by this run and left behind after the failure, so its directory can never be compensated (stat: %v)", leasePath, statErr)
-	}
-}
-
-// A LEASE FILE THAT WAS ALREADY THERE IS SOMEBODY ELSE'S.
-//
-// The undo is keyed on this call having created the object, so an acquisition
-// that merely opened an existing lease must not remove it on its way out.
-func TestAcquisitionLeavesALeaseFileItDidNotCreate(t *testing.T) {
-	root := runtimeRootForAcquisition(t)
-	if err := os.MkdirAll(filepath.Dir(root), 0o700); err != nil {
-		t.Fatal(err)
-	}
-	leasePath := sandboxRuntimeLeasePath(root)
-	if err := os.WriteFile(leasePath, nil, 0o600); err != nil {
-		t.Fatal(err)
-	}
-	failRuntimeCreationAt(t, filepath.Base(leasePath))
-
-	lease, _, err := acquireRuntimeLeaseForPlatform(root)
-	if err == nil {
-		lease.release()
-		t.Fatal("SETUP INVALID: acquisition succeeded, so the injected failure never fired")
-	}
-	if _, statErr := os.Stat(leasePath); statErr != nil {
-		t.Errorf("acquisition removed a lease file it did not create: %v", statErr)
-	}
-}
-
-// AND THE LEDGER STILL COVERS WHAT SURVIVES A LATER FAILURE.
-//
-// Undoing the component that failed must not undo the ones before it: those are
-// recorded, and compensation is what removes them.
-func TestAcquisitionKeepsTheLedgerForComponentsItFinished(t *testing.T) {
-	base := t.TempDir()
-	root := filepath.Join(base, "zero", "runtime", "v1", "digest")
-	failRuntimeCreationAt(t, "v1")
-
-	lease, created, err := acquireRuntimeLeaseForPlatform(root)
-	if err == nil {
-		lease.release()
-		t.Fatal("SETUP INVALID: acquisition succeeded, so the injected failure never fired")
-	}
-	var recorded []string
-	for _, entry := range created {
-		recorded = append(recorded, filepath.Base(entry.path))
-	}
-	// "zero" and "runtime" were created and identified before the injected
-	// failure, so they belong to the caller to compensate.
-	if len(recorded) != 2 || recorded[0] != "zero" || recorded[1] != "runtime" {
-		t.Errorf("the completed components are not on the ledger the caller compensates from: %v", recorded)
-	}
-	if _, statErr := os.Stat(filepath.Join(base, "zero", "runtime")); statErr != nil {
-		t.Errorf("a component that was created and recorded was removed anyway: %v", statErr)
-	}
-	if failed := filepath.Join(base, "zero", "runtime", "v1"); !isMissing(failed) {
-		t.Errorf("%s failed after being created and was left behind", failed)
-	}
-	if !strings.Contains(err.Error(), "v1") {
-		t.Errorf("the error does not name the component that failed: %v", err)
-	}
-}
-
-func isMissing(path string) bool {
-	_, err := os.Stat(path)
-	return os.IsNotExist(err)
-}
@@ -1,24 +0,0 @@
-//go:build !windows
-
-package sandbox
-
-import "golang.org/x/sys/unix"
-
-// createRuntimeTailHandleRelative creates the owned tail through the same
-// no-follow boundary the lease uses.
-//
-// This was an os.Mkdir loop over full pathnames, on the reasoning that the
-// elevated-installer-versus-unelevated-renamer split the Windows descent closes
-// does not apply here. That is true of the elevation asymmetry and false of the
-// substitution: the fallback root is a predictable name under a temp directory
-// every local account can write to, so another user can put a link at an owned
-// component and have this loop create the rest of the tree inside it. Same
-// descent, same refusal, one implementation per platform rather than one
-// protected platform.
-func createRuntimeTailHandleRelative(base string, tail []string) ([]windowsCreatedRuntimeDir, error) {
-	created, parent, err := createRuntimeTailRetainingFD(base, tail)
-	if parent >= 0 {
-		_ = unix.Close(parent)
-	}
-	return created, err
-}
@@ -1,40 +0,0 @@
-package sandbox
-
-// The two seams the rooted descent exposes, shared by both platform
-// implementations so a test asserts the same property on either.
-//
-// They live apart from the Windows descent they started in because the POSIX
-// fallback now takes the same shape, and a seam defined next to one of two
-// implementations is how the other one quietly ends up untested.
-
-// runtimeDescentBarrier, when set, runs after the base directory has been opened
-// and before the first owned component is touched. It exists so a test can swap
-// an owned component for a link at exactly the point the old pathname walk was
-// vulnerable, and prove the redirected target is never created or granted. Nil in
-// production.
-var runtimeDescentBarrier func()
-
-// runtimeCreationFailure, when set, is consulted immediately after this run
-// creates a runtime object and before the step that would publish it: a
-// directory component's identity read, or the lease file's inspection, wrapping
-// and locking. Returning an error stands in for that step failing.
-//
-// It is the only way to reach those paths, and they are the ones where the
-// transaction knows it created something that nothing above it knows about yet.
-// The argument is the created component's path, or the lease file's name. Nil in
-// production.
-var runtimeCreationFailure func(string) error
-
-// runtimeCleanupExclusivityBarrier, when set, runs during compensation while the
-// exclusive cleanup lease is held: after it has been taken and before the first
-// thing is removed. It exists so a test can put a contender into that window and
-// prove the exclusion still covers it, which is precisely where the old shape had
-// already handed the lease back. Nil in production.
-var runtimeCleanupExclusivityBarrier func()
-
-// runtimeBaseOpenedByName, when set, receives the ONE path this descent opens by
-// name. The whole security property is which path that is: the fixed cache or
-// temp directory above the owned tail, never a predictable component Zero owns. A
-// test can assert it directly instead of inferring it from whether a swap
-// happened to be caught, which is not discriminating. Nil in production.
-var runtimeBaseOpenedByName func(string)
@@ -1,171 +0,0 @@
-//go:build !windows
-
-package sandbox
-
-import (
-	"errors"
-	"fmt"
-	"path/filepath"
-	"strconv"
-
-	"golang.org/x/sys/unix"
-)
-
-// createRuntimeTailRetainingFD creates the owned tail of the runtime root
-// beneath base, one component at a time, from retained descriptors.
-//
-// THE NAME IS NOT AUTHORIZATION, ON THIS SIDE EITHER.
-//
-// The fallback root moved from an atomically minted os.MkdirTemp parent to a
-// predictable /tmp/zero-u<uid>/runtime/v1/<digest>. The stable name is required,
-// because setup and every later process have to agree on one root without
-// talking to each other, but it also means another local account can name the
-// first owned component before this one does. refuseAliasedRuntimeComponents
-// answers about a component that is ABSENT by saying there is nothing to alias,
-// and the code then called os.MkdirAll on the parent and opened <root>.lease by
-// full pathname. Both follow a link planted after that answer, so the guard was
-// authorizing writes it could not see the destination of.
-//
-// So the base is opened by name exactly once, and every component below it is
-// created or opened relative to a retained descriptor with O_NOFOLLOW. A link at
-// an owned component is then an ELOOP or ENOTDIR from the kernel rather than a
-// redirection nobody noticed, and no owned name is ever resolved twice.
-//
-// The base itself is opened WITHOUT O_NOFOLLOW. It belongs to the operator and is
-// legitimately a link on macOS, where /tmp resolves through /private. Only the
-// tail Zero creates is held to the stricter rule.
-func createRuntimeTailRetainingFD(base string, tail []string) ([]windowsCreatedRuntimeDir, int, error) {
-	if runtimeBaseOpenedByName != nil {
-		runtimeBaseOpenedByName(base)
-	}
-	parent, err := unix.Open(base, unix.O_RDONLY|unix.O_DIRECTORY|unix.O_CLOEXEC, 0)
-	if err != nil {
-		return nil, -1, fmt.Errorf("open sandbox runtime base %s: %w", base, err)
-	}
-
-	if runtimeDescentBarrier != nil {
-		runtimeDescentBarrier()
-	}
-
-	var created []windowsCreatedRuntimeDir
-	for _, path := range tail {
-		name := filepath.Base(path)
-		child, madeIt, openErr := openOrCreateRuntimeChildNoFollow(parent, name)
-		if openErr != nil {
-			_ = unix.Close(parent)
-			return created, -1, fmt.Errorf("open sandbox runtime component %s: %w", path, openErr)
-		}
-		// CREATED, AND ABOUT TO BE FORGOTTEN. Both checks below run before the
-		// ledger entry is appended, so failing either used to return a directory
-		// this run had made with nothing above holding a record of it: setup
-		// reported failure and left it behind, and a recorded ancestor's
-		// compensation then failed on a child it could not explain.
-		//
-		// Undone relative to the parent descriptor rather than published to the
-		// ledger, because on the identity path the identity the ledger needs is the
-		// thing that just failed, and the descriptor is the only authority here
-		// that is not a name.
-		undoCreation := func(err error) error {
-			if !madeIt {
-				return err
-			}
-			if undoErr := unix.Unlinkat(parent, name, unix.AT_REMOVEDIR); undoErr != nil {
-				return fmt.Errorf("%w; and the directory this run created there could not be removed: %w", err, undoErr)
-			}
-			return err
-		}
-		if err := refuseForeignRuntimeDirectory(child, path); err != nil {
-			err = undoCreation(err)
-			_ = unix.Close(child)
-			_ = unix.Close(parent)
-			return created, -1, err
-		}
-		if madeIt {
-			identity, idErr := runtimeDirectoryIdentity(child)
-			if idErr == nil && runtimeCreationFailure != nil {
-				idErr = runtimeCreationFailure(path)
-			}
-			if idErr != nil {
-				err := undoCreation(fmt.Errorf("identify the sandbox runtime directory created at %s: %w", path, idErr))
-				_ = unix.Close(child)
-				_ = unix.Close(parent)
-				return created, -1, err
-			}
-			created = append(created, windowsCreatedRuntimeDir{path: path, identity: identity, identified: true})
-		}
-		_ = unix.Close(parent)
-		parent = child
-	}
-	return created, parent, nil
-}
-
-// openOrCreateRuntimeChildNoFollow opens name under parent, creating it if it is
-// not there, and never follows a link at that name.
-//
-// madeIt distinguishes the directory this process created from one that was
-// already there, because only the former belongs on the rollback ledger.
-func openOrCreateRuntimeChildNoFollow(parent int, name string) (fd int, madeIt bool, err error) {
-	const flags = unix.O_RDONLY | unix.O_DIRECTORY | unix.O_NOFOLLOW | unix.O_CLOEXEC
-	child, openErr := unix.Openat(parent, name, flags, 0)
-	if openErr == nil {
-		return child, false, nil
-	}
-	if !errors.Is(openErr, unix.ENOENT) {
-		// ELOOP or ENOTDIR here IS the finding: a link sits where a directory of
-		// ours should be. Returned as-is rather than translated, because the two
-		// platforms spell it differently and the caller only needs it to fail.
-		return -1, false, openErr
-	}
-	if mkErr := unix.Mkdirat(parent, name, 0o700); mkErr != nil {
-		if !errors.Is(mkErr, unix.EEXIST) {
-			return -1, false, mkErr
-		}
-		// Somebody created it between the open and the create. That is an ordinary
-		// race with another Zero process, and the reopen below is still no-follow,
-		// so a link that arrived in the same window is refused rather than used.
-		child, openErr = unix.Openat(parent, name, flags, 0)
-		if openErr != nil {
-			return -1, false, openErr
-		}
-		return child, false, nil
-	}
-	child, openErr = unix.Openat(parent, name, flags, 0)
-	if openErr != nil {
-		return -1, false, openErr
-	}
-	return child, true, nil
-}
-
-// refuseForeignRuntimeDirectory proves the directory reached belongs to this
-// user and is not group- or world-writable.
-//
-// Asked of the DESCRIPTOR, so it describes the object the descent is holding
-// rather than whatever the name resolves to now. Ownership is the part that
-// matters on a shared temp root: a component another account created first is
-// theirs, and continuing into it would put the sandbox runtime inside a tree they
-// control even though nothing about it is a link.
-func refuseForeignRuntimeDirectory(fd int, path string) error {
-	var stat unix.Stat_t
-	if err := unix.Fstat(fd, &stat); err != nil {
-		return fmt.Errorf("inspect sandbox runtime component %s: %w", path, err)
-	}
-	if uid := unix.Getuid(); uid >= 0 && stat.Uid != uint32(uid) {
-		return fmt.Errorf("refusing to use the sandbox runtime component %s: it is owned by uid %d, not %d, so another account chose what the sandbox writes into",
-			path, stat.Uid, uid)
-	}
-	if stat.Mode&(unix.S_IWGRP|unix.S_IWOTH) != 0 {
-		return fmt.Errorf("refusing to use the sandbox runtime component %s: mode %#o lets another account replace what is inside it",
-			path, stat.Mode&0o7777)
-	}
-	return nil
-}
-
-// runtimeDirectoryIdentity names the object behind a descriptor, so rollback can
-// tell the directory it created from one that replaced it under the same name.
-func runtimeDirectoryIdentity(fd int) (string, error) {
-	var stat unix.Stat_t
-	if err := unix.Fstat(fd, &stat); err != nil {
-		return "", err
-	}
-	return strconv.FormatUint(uint64(stat.Dev), 10) + ":" + strconv.FormatUint(uint64(stat.Ino), 10), nil
-}
@@ -1,124 +0,0 @@
-//go:build windows
-
-package sandbox
-
-import (
-	"errors"
-	"fmt"
-	"path/filepath"
-
-	"golang.org/x/sys/windows"
-)
-
-// createRuntimeTailHandleRelative creates the owned tail of the runtime root
-// beneath base, one component at a time, from retained handles.
-//
-// THE NAME IS NOT AUTHORIZATION. The previous shape found the deepest existing
-// ancestor with os.Stat and then created each missing component by opening its
-// parent BY NAME. Both follow a junction, so a local user could replace the
-// predictable owned "zero" component with a junction after the pre-check,
-// point it anywhere, let elevated setup create runtime\v1\<hash> beneath that
-// target, and put the original back before the post-check. The post-check then
-// saw an ordinary path, the plan applied normally, and rollback reported the
-// separately created target only as identity-mismatched residue.
-//
-// So the base is opened by name exactly once, and everything below it is
-// addressed relative to a handle with FILE_OPEN_REPARSE_POINT: an existing
-// component is opened no-follow and refused if it is a link, and a missing one
-// is created relative to its parent's handle and identified from the handle the
-// create returned. No component name is resolved twice, so there is no interval
-// for a swap to land in, and a junction placed at any owned component is seen
-// as the link it is rather than followed.
-//
-// Redirected cache and TEMP locations ABOVE the owned tail are still allowed:
-// they are part of base, which is the operator's business. The restriction is
-// on the zero/runtime/v1/<hash> components Zero itself owns.
-func createRuntimeTailHandleRelative(base string, tail []string) ([]windowsCreatedRuntimeDir, error) {
-	created, parent, err := createRuntimeTailRetainingHandle(base, tail)
-	if parent != 0 {
-		_ = windows.CloseHandle(parent)
-	}
-	return created, err
-}
-
-// createRuntimeTailRetainingHandle is the same descent, but it hands the caller
-// the retained handle to the deepest component instead of closing it.
-//
-// Lease acquisition needs that handle: the lease file is a sibling of the
-// runtime root, so creating it by pathname would re-resolve the owned components
-// the descent just verified, which is the whole interval this design removes.
-// The caller owns the returned handle and must close it, including when an error
-// is returned alongside a non-zero handle.
-func createRuntimeTailRetainingHandle(base string, tail []string) ([]windowsCreatedRuntimeDir, windows.Handle, error) {
-	if runtimeBaseOpenedByName != nil {
-		runtimeBaseOpenedByName(base)
-	}
-	parent, err := openWindowsDirectoryByName(base)
-	if err != nil {
-		return nil, 0, fmt.Errorf("open sandbox runtime base %s: %w", base, err)
-	}
-
-	if runtimeDescentBarrier != nil {
-		runtimeDescentBarrier()
-	}
-
-	var created []windowsCreatedRuntimeDir
-	for _, path := range tail {
-		name := filepath.Base(path)
-		// Exists already: another process won the race, or it was there before
-		// setup began. Opened no-follow, so a junction here is refused rather than
-		// descended, and it is not ours to record.
-		existing, openErr := openWindowsChildNoFollow(parent, name,
-			windows.FILE_READ_ATTRIBUTES|windows.FILE_TRAVERSE, windows.FILE_DIRECTORY_FILE)
-		if openErr == nil {
-			_ = windows.CloseHandle(parent)
-			parent = existing
-			continue
-		}
-		if !isWindowsNotFound(openErr) {
-			return created, parent, fmt.Errorf("inspect sandbox runtime component %s: %w", path, openErr)
-		}
-		handle, createErr := createWindowsChildDirectory(parent, name)
-		if createErr != nil {
-			if errors.Is(createErr, windows.STATUS_OBJECT_NAME_COLLISION) {
-				// Created between the open and the create by someone else. Same
-				// answer as the os.IsExist branch the caller already had: not ours,
-				// but not a failure either. Reopen no-follow so the descent
-				// continues through a verified object rather than a name.
-				existing, reopenErr := openWindowsChildNoFollow(parent, name,
-					windows.FILE_READ_ATTRIBUTES|windows.FILE_TRAVERSE, windows.FILE_DIRECTORY_FILE)
-				if reopenErr != nil {
-					return created, parent, fmt.Errorf("inspect sandbox runtime component %s after a concurrent create: %w", path, reopenErr)
-				}
-				_ = windows.CloseHandle(parent)
-				parent = existing
-				continue
-			}
-			return created, parent, fmt.Errorf("create sandbox runtime root %s: %w", path, createErr)
-		}
-		identity, idErr := handleRuntimeIdentity(handle)
-		if idErr == nil && runtimeCreationFailure != nil {
-			idErr = runtimeCreationFailure(path)
-		}
-		if idErr != nil {
-			// CREATED, AND ABOUT TO BE FORGOTTEN. The ledger entry is appended on
-			// the next line, so returning here left a directory this run had made
-			// with nothing above holding a record of it: setup reports failure and
-			// leaves it behind, and if an ancestor WAS recorded, that ancestor's
-			// compensation then fails on a directory it cannot explain.
-			//
-			// Undone through the handle rather than published to the ledger,
-			// because the identity the ledger needs is the thing that just failed.
-			err := fmt.Errorf("identify the sandbox runtime directory created at %s: %w", path, idErr)
-			if undoErr := removeWindowsObjectByHandle(handle); undoErr != nil {
-				err = fmt.Errorf("%w; and the directory this run created there could not be removed: %w", err, undoErr)
-			}
-			_ = windows.CloseHandle(handle)
-			return created, parent, err
-		}
-		created = append(created, windowsCreatedRuntimeDir{path: path, identity: identity, identified: true})
-		_ = windows.CloseHandle(parent)
-		parent = handle
-	}
-	return created, parent, nil
-}
@@ -1,93 +0,0 @@
-//go:build windows
-
-package sandbox
-
-import (
-	"os"
-	"os/exec"
-	"path/filepath"
-	"testing"
-)
-
-// THE SWAP THE OLD WALK COULD NOT SEE.
-//
-// createRuntimeDirRecording used to find the deepest existing ancestor with
-// os.Stat and then create each missing component by opening its parent BY
-// NAME. Both follow a junction. So between the pre-check and the create, a
-// local user could replace the predictable owned "zero" component with a
-// junction into a directory of their choosing, and elevated setup would create
-// runtime\v1\<hash> beneath that target, apply its ACL plan there, and then
-// find an ordinary path at the post-check once the original was put back.
-//
-// This plants the junction at the deterministic barrier between opening the
-// base and touching the first owned component, which is that exact interval,
-// and proves two things: nothing is created beneath the redirected target, and
-// the descent refuses rather than continuing through the link. A junction
-// needs no privilege on Windows, which is why it is the shape used here rather
-// than a symlink that would skip on an unelevated box.
-func TestRuntimeDescentRefusesAJunctionSwappedIntoAnOwnedComponent(t *testing.T) {
-	base := t.TempDir()
-	target := t.TempDir()
-
-	zero := filepath.Join(base, "zero")
-	tail := []string{zero, filepath.Join(zero, "runtime")}
-
-	previous := runtimeDescentBarrier
-	runtimeDescentBarrier = func() {
-		out, err := exec.Command("cmd", "/c", "mklink", "/J", zero, target).CombinedOutput()
-		if err != nil {
-			t.Fatalf("mklink /J: %v\n%s", err, out)
-		}
-	}
-	t.Cleanup(func() { runtimeDescentBarrier = previous })
-
-	created, err := createRuntimeTailHandleRelative(base, tail)
-	if err == nil {
-		t.Fatalf("the descent continued through a junction and reported success: created=%v", created)
-	}
-	if len(created) != 0 {
-		t.Errorf("a junction-swapped component still produced ownership records: %v", created)
-	}
-
-	// The whole point: the redirected target must be untouched.
-	entries, readErr := os.ReadDir(target)
-	if readErr != nil {
-		t.Fatal(readErr)
-	}
-	if len(entries) != 0 {
-		names := make([]string, 0, len(entries))
-		for _, entry := range entries {
-			names = append(names, entry.Name())
-		}
-		t.Errorf("setup created %v beneath the junction target, which is somebody else's directory", names)
-	}
-}
-
-// And the honest control: with no swap, the same descent creates the tail,
-// records an identity for each component from the creation handle, and the
-// result matches what the name resolves to afterwards.
-func TestRuntimeDescentCreatesTheOwnedTailFromHandles(t *testing.T) {
-	base := t.TempDir()
-	zero := filepath.Join(base, "zero")
-	tail := []string{zero, filepath.Join(zero, "runtime"), filepath.Join(zero, "runtime", "v1")}
-
-	created, err := createRuntimeTailHandleRelative(base, tail)
-	if err != nil {
-		t.Fatalf("descent: %v", err)
-	}
-	if len(created) != len(tail) {
-		t.Fatalf("created %d components, want %d: %v", len(created), len(tail), created)
-	}
-	for i, record := range created {
-		if record.path != tail[i] {
-			t.Errorf("record %d path = %q, want %q", i, record.path, tail[i])
-		}
-		if !record.identified {
-			t.Errorf("record %d for %s carries no identity", i, record.path)
-			continue
-		}
-		if now, ok := runtimeDirIdentity(record.path); !ok || now != record.identity {
-			t.Errorf("record %d identity %q does not describe the directory at %s (%q)", i, record.identity, record.path, now)
-		}
-	}
-}
@@ -1,27 +0,0 @@
-//go:build !windows
-
-package sandbox
-
-import (
-	"fmt"
-	"os"
-	"syscall"
-)
-
-// runtimeDirIdentity identifies the directory currently at path.
-//
-// device plus inode, read with Lstat so a link substituted at the final
-// component is identified as the link rather than followed. The Windows build
-// cannot use os.SameFile for this and explains why; here the same eager capture
-// keeps both platforms on one rule.
-func runtimeDirIdentity(path string) (string, bool) {
-	info, err := os.Lstat(path)
-	if err != nil {
-		return "", false
-	}
-	stat, ok := info.Sys().(*syscall.Stat_t)
-	if !ok {
-		return "", false
-	}
-	return fmt.Sprintf("%d:%d", stat.Dev, stat.Ino), true
-}
@@ -1,45 +0,0 @@
-//go:build windows
-
-package sandbox
-
-import (
-	"fmt"
-
-	"golang.org/x/sys/windows"
-)
-
-// runtimeDirIdentity identifies the directory currently at path, EAGERLY.
-//
-// os.SameFile cannot be used for this on Windows. A Windows fileStat loads its
-// volume serial and file index lazily, by PATHNAME, at comparison time, so an
-// identity captured before a replacement and compared afterwards reports the
-// substitute as the same object and the original as a different one: exactly
-// backwards, and silently. Measured, not reasoned about.
-//
-// Reading the identity through a handle at capture time removes the lazy step.
-// Opened with FILE_FLAG_OPEN_REPARSE_POINT so a link substituted at the final
-// component is identified as the link it is rather than followed.
-func runtimeDirIdentity(path string) (string, bool) {
-	utf16Path, err := windows.UTF16PtrFromString(path)
-	if err != nil {
-		return "", false
-	}
-	handle, err := windows.CreateFile(
-		utf16Path,
-		windows.FILE_READ_ATTRIBUTES,
-		windows.FILE_SHARE_READ|windows.FILE_SHARE_WRITE|windows.FILE_SHARE_DELETE,
-		nil,
-		windows.OPEN_EXISTING,
-		windows.FILE_FLAG_BACKUP_SEMANTICS|windows.FILE_FLAG_OPEN_REPARSE_POINT,
-		0,
-	)
-	if err != nil {
-		return "", false
-	}
-	defer windows.CloseHandle(handle)
-	var info windows.ByHandleFileInformation
-	if err := windows.GetFileInformationByHandle(handle, &info); err != nil {
-		return "", false
-	}
-	return fmt.Sprintf("%d:%d:%d", info.VolumeSerialNumber, info.FileIndexHigh, info.FileIndexLow), true
-}
@@ -1,44 +0,0 @@
-//go:build windows
-
-package sandbox
-
-import "testing"
-
-// THE COMMAND HAS TO RECOGNISE THE ROOT SETUP RECORDED, WHATEVER ITS
-// ENVIRONMENT SAYS.
-//
-// Setup derives the fallback root in an elevated terminal and records it; a
-// command re-derives it in an ordinary one to decide whether the record belongs
-// to this workspace. The user scope in that derivation used to come from
-// USERNAME, so a value that differed between the two terminals made the command
-// reject the record and select a root with no capability ACE on it.
-func TestRecordedFallbackRootSurvivesADifferentUsernameVariable(t *testing.T) {
-	// Canonical, as selection hands it over. The record check canonicalizes on
-	// its own, so a raw t.TempDir() spelling digests differently wherever TEMP is
-	// an 8.3 short path, as it is on the CI runners.
-	workspace := canonicalSandboxWorkspaceRoot(t.TempDir())
-	tempHome := t.TempDir()
-	t.Setenv("TMP", tempHome)
-	t.Setenv("TEMP", tempHome)
-
-	t.Setenv("USERNAME", "setup-terminal")
-	recorded, err := fallbackSandboxRuntimeRoot(workspace)
-	if err != nil {
-		t.Fatalf("derive the fallback root as setup: %v", err)
-	}
-	if !ownedFallbackRuntimeRecord(workspace, recorded) {
-		t.Fatalf("SETUP INVALID: the fallback root is not recognised even in the environment that derived it: %s", recorded)
-	}
-
-	t.Setenv("USERNAME", "command-terminal")
-	if !ownedFallbackRuntimeRecord(workspace, recorded) {
-		t.Fatalf("once USERNAME changed, the command no longer recognises the fallback root setup recorded: %s", recorded)
-	}
-	derived, err := fallbackSandboxRuntimeRoot(workspace)
-	if err != nil {
-		t.Fatalf("derive the fallback root as the command: %v", err)
-	}
-	if !sameWindowsRuntimeRootPath(derived, recorded) {
-		t.Fatalf("setup and the command derived different fallback roots for one workspace:\n  setup:   %s\n  command: %s", recorded, derived)
-	}
-}
@@ -1,106 +0,0 @@
-package sandbox
-
-import (
-	"path/filepath"
-	"runtime"
-	"strings"
-	"testing"
-)
-
-// EVERY OWNERSHIP-CHECKED ANCESTOR MUST ALREADY BE INSIDE ONE USER'S NAMESPACE.
-//
-// The temp-derived fallback lives under a directory that is shared on Unix
-// whenever TMPDIR is unset. Runtime preparation creates and ownership-checks
-// each component of the tail at 0700, so a fixed first component meant the
-// first account to use the fallback created a private directory every other
-// account was then refused at: traversal fails on the mode, and relaxing the
-// mode fails the ownership guard instead. The per-workspace digest is the leaf,
-// so it never got the chance to separate them.
-//
-// The invariant is therefore about the SHALLOWEST checked component, not the
-// leaf: it has to carry the user scope, or the guards below it are guarding a
-// namespace two users share.
-func TestFallbackRuntimeRootScopesItsShallowestOwnedComponent(t *testing.T) {
-	if runtime.GOOS == "windows" {
-		t.Skip("os.TempDir() resolves inside the user profile on Windows, so the shared-temp collision cannot arise")
-	}
-	workspace := t.TempDir()
-	tempHome := t.TempDir()
-	t.Setenv("TMPDIR", tempHome)
-
-	root, err := fallbackSandboxRuntimeRoot(workspace)
-	if err != nil {
-		t.Fatalf("fallbackSandboxRuntimeRoot: %v", err)
-	}
-	components := ownedRuntimeComponents(root)
-	if len(components) == 0 {
-		t.Fatal("no ownership-checked components derived for the fallback root")
-	}
-	// ownedRuntimeComponents walks upward, so the last entry is the shallowest
-	// directory the guards create and validate.
-	shallowest := filepath.Base(components[len(components)-1])
-	scope := sandboxRuntimeUserScope()
-	if !strings.Contains(shallowest, scope) {
-		t.Errorf("the shallowest ownership-checked component is %q and does not carry the user scope %q; "+
-			"two accounts on a shared temp would contend for it", shallowest, scope)
-	}
-}
-
-// The workspace still decides the leaf, so setup and the command derive the same
-// root for the same workspace. Scoping the top must not have moved that.
-func TestFallbackRuntimeRootStaysStableForOneWorkspace(t *testing.T) {
-	workspace := t.TempDir()
-	tempHome := t.TempDir()
-	t.Setenv("TMPDIR", tempHome)
-	t.Setenv("TMP", tempHome)
-	t.Setenv("TEMP", tempHome)
-
-	first, err := fallbackSandboxRuntimeRoot(workspace)
-	if err != nil {
-		t.Fatal(err)
-	}
-	second, err := fallbackSandboxRuntimeRoot(workspace)
-	if err != nil {
-		t.Fatal(err)
-	}
-	if first != second {
-		t.Errorf("the fallback root is not stable for one workspace:\n  %s\n  %s", first, second)
-	}
-
-	other, err := fallbackSandboxRuntimeRoot(t.TempDir())
-	if err != nil {
-		t.Fatal(err)
-	}
-	if other == first {
-		t.Error("two different workspaces derived the same fallback root")
-	}
-	if filepath.Dir(other) != filepath.Dir(first) {
-		t.Errorf("two workspaces for one user should differ only in the leaf:\n  %s\n  %s", first, other)
-	}
-}
-
-// And the Windows side is a deliberate choice, not an omission. Its temp root
-// is already inside the user profile, and the names have to stay fixed so
-// windowsSandboxRuntimeOwnedTail can still recognise a root that an elevated
-// setup running as a different account built.
-func TestFallbackOwnedNamesAreUnscopedOnWindowsOnly(t *testing.T) {
-	names := sandboxRuntimeFallbackOwnedNames()
-	if len(names) != len(windowsSandboxRuntimeOwnedNames) {
-		t.Fatalf("fallback names = %v, want the same depth as %v", names, windowsSandboxRuntimeOwnedNames)
-	}
-	// The components below the first are shared by both platforms, so a change
-	// to them would break the tail matcher on Windows.
-	for index := 1; index < len(names); index++ {
-		if names[index] != windowsSandboxRuntimeOwnedNames[index] {
-			t.Errorf("component %d = %q, want %q", index, names[index], windowsSandboxRuntimeOwnedNames[index])
-		}
-	}
-	scoped := names[0] != windowsSandboxRuntimeOwnedNames[0]
-	if runtime.GOOS == "windows" && scoped {
-		t.Errorf("the Windows fallback scoped its first component to %q; the tail matcher compares fixed names", names[0])
-	}
-	if runtime.GOOS != "windows" && !scoped {
-		t.Errorf("the first component is %q on %s, where the temp root can be shared between accounts",
-			names[0], runtime.GOOS)
-	}
-}
@@ -1,74 +0,0 @@
-package sandbox
-
-import (
-	"encoding/json"
-	"os"
-	"path/filepath"
-	"testing"
-)
-
-// writeRecordedRoot puts a setup marker naming root under sandboxHome.
-func writeRecordedRoot(t *testing.T, sandboxHome, root string) {
-	t.Helper()
-	path := WindowsSandboxSetupMarkerPath(sandboxHome)
-	if err := os.MkdirAll(filepath.Dir(path), 0o700); err != nil {
-		t.Fatal(err)
-	}
-	blob, err := json.Marshal(WindowsSandboxSetupMarker{SchemaVersion: windowsSandboxSetupMarkerSchemaVersion, RuntimeRoot: root})
-	if err != nil {
-		t.Fatal(err)
-	}
-	if err := os.WriteFile(path, blob, 0o600); err != nil {
-		t.Fatal(err)
-	}
-}
-
-// ONE COMMAND, ONE SANDBOX HOME.
-//
-// Runtime preparation happens before Windows platform planning, and it used to
-// resolve the sandbox home from the AMBIENT environment while the planner
-// resolves ZERO_WINDOWS_SANDBOX_HOME out of the command's own spec.Env and hands
-// that one to the runner for marker validation. So a command that explicitly
-// selects home B, while the parent process still points at home A, pinned A's
-// recorded root into the profile; the runner then loaded B's marker, saw a
-// different root, and rejected the command as out of date even though setup for
-// B was valid. The two homes need no different derivation rules to disagree,
-// only different valid selections from the same preferred/fallback pair.
-func TestThePinnedRootComesFromTheCommandsOwnSandboxHome(t *testing.T) {
-	homeA := t.TempDir()
-	homeB := t.TempDir()
-	preferred := filepath.Join(t.TempDir(), "preferred")
-	fallback := filepath.Join(t.TempDir(), "fallback")
-
-	writeRecordedRoot(t, homeA, preferred)
-	writeRecordedRoot(t, homeB, fallback)
-
-	if got := pinnedSandboxRuntimeRoot(t.TempDir(), preferred, fallback, homeB); got != fallback {
-		t.Errorf("pinned %q for a command that selected home B, want %q: the ambient home decided instead of the command's", got, fallback)
-	}
-	if got := pinnedSandboxRuntimeRoot(t.TempDir(), preferred, fallback, homeA); got != preferred {
-		t.Errorf("pinned %q for home A, want %q", got, preferred)
-	}
-}
-
-// A recorded root that this workspace could not select is still refused, which
-// is the protection that stops one workspace's tree being pinned into another.
-func TestARecordedRootFromAnotherWorkspaceIsStillRefused(t *testing.T) {
-	home := t.TempDir()
-	writeRecordedRoot(t, home, filepath.Join(t.TempDir(), "someone-elses-tree"))
-	if got := pinnedSandboxRuntimeRoot(t.TempDir(), filepath.Join(t.TempDir(), "preferred"), filepath.Join(t.TempDir(), "fallback"), home); got != "" {
-		t.Errorf("pinned %q, want none: it matches neither candidate this workspace derives", got)
-	}
-}
-
-// No command context means the ambient environment is the only authority there
-// is, so an empty home must still resolve rather than refusing outright.
-func TestAnEmptyCommandHomeFallsBackToTheAmbientEnvironment(t *testing.T) {
-	home := t.TempDir()
-	t.Setenv("ZERO_WINDOWS_SANDBOX_HOME", home)
-	preferred := filepath.Join(t.TempDir(), "preferred")
-	writeRecordedRoot(t, home, preferred)
-	if got := pinnedSandboxRuntimeRoot(t.TempDir(), preferred, filepath.Join(t.TempDir(), "fallback"), ""); got != preferred {
-		t.Errorf("pinned %q with no command home, want the ambient one to decide (%q)", got, preferred)
-	}
-}
@@ -1,88 +1,31 @@
 package sandbox

 import (
-	"errors"
+	"fmt"
 	"sync"
 )

 const sandboxRuntimeLeaseSuffix = ".lease"

-// errRuntimeLeaseReplaced says a lock was taken on a lease object that the lease
-// name no longer resolves to: cleanup unlinked or replaced it while this call was
-// waiting. Both acquisitions retry from their open on it, and surface it only
-// once the retries are spent.
-var errRuntimeLeaseReplaced = errors.New("the sandbox runtime lease was removed or replaced while it was being taken")
-
-// runtimeLeaseAcquireAttempts bounds those retries. Each stale lock means one
-// cleanup ran to completion in the window, and cleanups do not chain without
-// limit; a third stale object in a row is reported rather than chased.
-const runtimeLeaseAcquireAttempts = 3
-
-// sandboxRuntimeLease is one holder's grip on a runtime root.
-//
-// createdFile says this acquisition is the one that brought the lease file into
-// existence. Compensation needs it: rollback refuses a non-empty directory, and
-// the lease sits beside the leaf inside a directory a failed setup has to be able
-// to remove, so the file has to go with it. It may only go if it was ours, which
-// is a fact only the create knows.
 type sandboxRuntimeLease struct {
 	handle runtimeLeaseHandle
 	once   sync.Once
-	// root is the runtime root this lease is for, kept so compensation can name
-	// the lease file without deriving it again.
-	root string
-	// createdFile says THIS acquisition brought the lease file into existence.
-	createdFile bool
-}
-
-// createdLeaseFile reports whether compensation may remove the lease file.
-//
-// Only the acquisition that created it may, and only while nothing else holds
-// it. A lease that was already there belongs to whoever made it, and removing it
-// would take the coordination object out from under a live command.
-func (lease *sandboxRuntimeLease) createdLeaseFile() (string, bool) {
-	if lease == nil || !lease.createdFile {
-		return "", false
-	}
-	return sandboxRuntimeLeasePath(lease.root), true
 }

 func sandboxRuntimeLeasePath(root string) string {
 	return root + sandboxRuntimeLeaseSuffix
 }

-// The pathname-based shared acquisition that used to live here is gone with its
-// two platform halves. It opened the lease by full pathname, so it followed a
-// link planted at that name and, on Windows, opened without FILE_SHARE_DELETE,
-// which alone stops cleanup from ever removing the lease it is holding. Every
-// caller now goes through acquireRuntimeLeaseForPlatform, which descends from a
-// retained parent handle no-follow. Leaving the weaker door defined next to the
-// stronger one is how a later change quietly takes it.
-
-// appendCreatedRuntimeDirs merges one acquisition attempt's ledger into the
-// running one. A path seen again was re-created after cleanup removed the
-// earlier object, so the later identity supersedes; rollback verifies identity
-// before it removes anything, and a stale one would make it refuse the directory
-// this run actually made.
-func appendCreatedRuntimeDirs(existing, made []windowsCreatedRuntimeDir) []windowsCreatedRuntimeDir {
-	for _, record := range made {
-		replaced := false
-		for index := range existing {
-			if existing[index].path == record.path {
-				existing[index] = record
-				replaced = true
-				break
-			}
-		}
-		if !replaced {
-			existing = append(existing, record)
-		}
+func acquireSandboxRuntimeLease(root string) (*sandboxRuntimeLease, error) {
+	handle, err := acquireSharedRuntimeLease(sandboxRuntimeLeasePath(root))
+	if err != nil {
+		return nil, fmt.Errorf("acquire sandbox runtime lease: %w", err)
 	}
-	return existing
+	return &sandboxRuntimeLease{handle: handle}, nil
 }

 func tryAcquireSandboxRuntimeCleanupLease(root string) (*sandboxRuntimeLease, bool, error) {
-	handle, inUse, err := tryAcquireExclusiveRuntimeLease(root)
+	handle, inUse, err := tryAcquireExclusiveRuntimeLease(sandboxRuntimeLeasePath(root))
 	if err != nil || inUse {
 		return nil, inUse, err
 	}
@@ -1,15 +0,0 @@
-package sandbox
-
-// runtimeLeasePreCreateBarrier, when set, runs after lease acquisition has
-// decided what it is going to do and before it creates anything.
-//
-// It exists so a test can reproduce the CHECK-THEN-USE RACE rather than a
-// junction that was already there when acquisition started. The two are not the
-// same finding: a junction present from the outset was already refused by the
-// alias pre-check, while one planted in this window defeated it, because
-// os.MkdirAll and the lease open both resolved the component again and followed
-// it. A test that only plants the junction up front passes against the defective
-// implementation and proves nothing.
-//
-// Nil in production.
-var runtimeLeasePreCreateBarrier func()
@@ -1,150 +0,0 @@
-//go:build windows
-
-package sandbox
-
-import (
-	"os"
-	"os/exec"
-	"path/filepath"
-	"strings"
-	"testing"
-)
-
-// leaseRootUnder builds a production-shaped runtime root beneath a test-owned
-// cache directory, so the owned tail is real rather than assumed.
-func leaseRootUnder(t *testing.T, cacheRoot string) string {
-	t.Helper()
-	workspace := t.TempDir()
-	root, ok := deterministicSandboxRuntimeRoot(canonicalSandboxWorkspaceRoot(workspace), canonicalSandboxWorkspaceRoot(cacheRoot))
-	if !ok {
-		t.Fatalf("SETUP INVALID: no deterministic runtime root under %s", cacheRoot)
-	}
-	return root
-}
-
-// THE LEASE IS THE FIRST WRITE SETUP MAKES, SO IT IS THE ONE THAT MATTERS MOST.
-//
-// prepareSandboxRuntimeLease checked the owned components for aliases and then
-// created the parent with os.MkdirAll and opened "<root>.lease" by pathname.
-// Both follow. An ordinary same-account process could replace "zero", "runtime"
-// or "v1" with a junction after the check and before either call, and elevated
-// setup would then build the tree and the lease file inside somebody else's
-// target. Putting the component back afterwards left the later handle-relative
-// provisioning working on the legitimate tree, so no post-check ever saw it, and
-// the rollback could not compensate what it had no record of.
-//
-// A junction needs no privilege on Windows, so the caller's half of this runs on
-// an ordinary unelevated box.
-func TestRuntimeLeaseRefusesAJunctionPlantedAfterTheCheck(t *testing.T) {
-	cacheRoot := t.TempDir()
-	target := t.TempDir()
-	root := leaseRootUnder(t, cacheRoot)
-
-	owned := filepath.Join(canonicalSandboxWorkspaceRoot(cacheRoot), "zero")
-	if !strings.HasPrefix(strings.ToLower(root), strings.ToLower(owned)) {
-		t.Fatalf("SETUP INVALID: %s does not sit under the owned component %s", root, owned)
-	}
-
-	// THE JUNCTION ARRIVES IN THE WINDOW, WHICH IS THE WHOLE POINT.
-	//
-	// One that is already there when acquisition starts was refused by the old
-	// alias pre-check too, so a test that plants it up front passes against the
-	// defective implementation and proves nothing. This one lands after the
-	// decision and before the first create, which is exactly where os.MkdirAll and
-	// the pathname lease open used to follow it.
-	previous := runtimeLeasePreCreateBarrier
-	t.Cleanup(func() { runtimeLeasePreCreateBarrier = previous })
-	attempted, planted := false, false
-	runtimeLeasePreCreateBarrier = func() {
-		if attempted {
-			return
-		}
-		attempted = true
-		if out, err := exec.Command("cmd", "/c", "mklink", "/J", owned, target).CombinedOutput(); err != nil {
-			t.Logf("mklink /J unavailable: %v: %s", err, out)
-			return
-		}
-		planted = true
-	}
-
-	lease, _, err := prepareSandboxRuntimeLeaseRecording(root)
-	if lease != nil {
-		lease.release()
-	}
-	if !attempted {
-		t.Skip("the pre-create barrier never ran, so the race was not reproduced")
-	}
-	// Bound to mklink's result, not to the attempt. When it fails, acquisition
-	// goes on to create an ordinary directory at the same name, the check below
-	// finds it, and a correct acquisition would be failed as a junction bypass.
-	if !planted {
-		t.Skip("mklink /J could not create the junction here, so the race was not reproduced")
-	}
-	if _, statErr := os.Lstat(owned); statErr != nil {
-		t.Skipf("the junction could not be created here: %v", statErr)
-	}
-
-	entries, readErr := os.ReadDir(target)
-	if readErr != nil {
-		t.Fatal(readErr)
-	}
-	if len(entries) != 0 {
-		names := make([]string, 0, len(entries))
-		for _, entry := range entries {
-			names = append(names, entry.Name())
-		}
-		t.Fatalf("lease acquisition wrote %v beneath a junction planted after its own check; err=%v", names, err)
-	}
-	if err == nil {
-		t.Fatal("lease acquisition reported success through a junction planted after its check")
-	}
-}
-
-// And an ordinary cache tree still gets its lease, or the refusal above would be
-// satisfied by an acquirer that refuses everything.
-func TestRuntimeLeaseStillAcquiresOnAnOrdinaryTree(t *testing.T) {
-	cacheRoot := t.TempDir()
-	root := leaseRootUnder(t, cacheRoot)
-
-	lease, created, err := prepareSandboxRuntimeLeaseRecording(root)
-	if err != nil {
-		t.Fatalf("an ordinary cache tree was refused: %v", err)
-	}
-	defer lease.release()
-
-	if _, statErr := os.Stat(sandboxRuntimeLeasePath(root)); statErr != nil {
-		t.Fatalf("the lease file was not created: %v", statErr)
-	}
-	// The components above the leaf are recorded, so a later failure can undo
-	// them. The leaf itself belongs to provisioning and must NOT appear here.
-	for _, entry := range created {
-		if strings.EqualFold(filepath.Clean(entry.path), filepath.Clean(root)) {
-			t.Fatalf("lease acquisition claimed the runtime leaf %s, which provisioning owns", root)
-		}
-	}
-	if len(created) == 0 {
-		t.Fatal("SETUP INVALID: nothing was recorded as created, so the accounting assertion above is vacuous")
-	}
-}
-
-// A second acquisition over an existing tree records nothing: it created nothing,
-// so it owns nothing, and rollback must not remove a tree it found.
-func TestRuntimeLeaseRecordsOnlyWhatItCreated(t *testing.T) {
-	cacheRoot := t.TempDir()
-	root := leaseRootUnder(t, cacheRoot)
-
-	first, _, err := prepareSandboxRuntimeLeaseRecording(root)
-	if err != nil {
-		t.Fatalf("first acquisition: %v", err)
-	}
-	first.release()
-
-	second, created, err := prepareSandboxRuntimeLeaseRecording(root)
-	if err != nil {
-		t.Fatalf("second acquisition: %v", err)
-	}
-	defer second.release()
-	if len(created) != 0 {
-		t.Fatalf("the second acquisition claimed %d directories it did not create; rollback would delete a tree it found", len(created))
-	}
-}
@@ -1,216 +0,0 @@
-//go:build !windows
-
-package sandbox
-
-import (
-	"errors"
-	"os"
-	"path/filepath"
-	"testing"
-
-	"golang.org/x/sys/unix"
-)
-
-// unixLeaseRootUnder builds a production-shaped runtime root beneath a
-// test-owned base, so the owned tail is real rather than assumed.
-func unixLeaseRootUnder(t *testing.T, base string) string {
-	t.Helper()
-	workspace := t.TempDir()
-	root, ok := deterministicSandboxRuntimeRoot(canonicalSandboxWorkspaceRoot(workspace), canonicalSandboxWorkspaceRoot(base))
-	if !ok {
-		t.Fatalf("SETUP INVALID: no deterministic runtime root under %s", base)
-	}
-	return root
-}
-
-// countEntriesUnder reports how many entries exist anywhere beneath dir, so a
-// test can say "nothing was written here" rather than checking one name.
-func countEntriesUnder(t *testing.T, dir string) []string {
-	t.Helper()
-	var found []string
-	err := filepath.Walk(dir, func(path string, info os.FileInfo, err error) error {
-		if err != nil {
-			return err
-		}
-		if path == dir {
-			return nil
-		}
-		found = append(found, path)
-		return nil
-	})
-	if err != nil {
-		t.Fatalf("walk %s: %v", dir, err)
-	}
-	return found
-}
-
-// A PRE-CHECK CANNOT AUTHORIZE A PATHNAME WRITE THAT COMES AFTER IT.
-//
-// The fallback root moved from an atomically minted MkdirTemp parent to a
-// predictable /tmp/zero-u<uid>/runtime/v1/<digest>. The stable name is required,
-// because setup and every later process have to agree on one root without talking
-// to each other, but it also means another local account can name the first owned
-// component first. refuseAliasedRuntimeComponents answers about an ABSENT
-// component by saying there is nothing to alias, which is the state a fresh
-// fallback is in, and os.MkdirAll plus the pathname lease open then followed a
-// link planted after that answer.
-//
-// The link arrives after the last validation and before the first create, which
-// is the only window that matters: one planted earlier was refused by the old
-// pre-check too, so a test that plants it up front passes against the defect and
-// proves nothing.
-func TestRuntimeLeaseRefusesALinkPlantedAfterTheCheck(t *testing.T) {
-	base := t.TempDir()
-	target := t.TempDir()
-	root := unixLeaseRootUnder(t, base)
-
-	owned := filepath.Join(canonicalSandboxWorkspaceRoot(base), "zero")
-	if !pathWithinRoot(owned, root) {
-		t.Fatalf("SETUP INVALID: %s does not sit under the owned component %s", root, owned)
-	}
-
-	previous := runtimeDescentBarrier
-	t.Cleanup(func() { runtimeDescentBarrier = previous })
-	planted := false
-	runtimeDescentBarrier = func() {
-		if planted {
-			return
-		}
-		planted = true
-		if err := os.Symlink(target, owned); err != nil {
-			t.Errorf("plant the link: %v", err)
-		}
-	}
-
-	lease, _, err := prepareSandboxRuntimeLeaseRecording(root)
-	if lease != nil {
-		lease.release()
-	}
-
-	// SETUP: the swap really happened in the window, or nothing was under test.
-	if !planted {
-		t.Fatal("SETUP INVALID: the descent never reached the barrier, so no link was planted")
-	}
-	if err == nil {
-		t.Fatal("lease acquisition succeeded through a link planted at the first owned component")
-	}
-	// ELOOP on Linux, and either that or ENOTDIR elsewhere. Asserted as a class
-	// rather than one kernel's spelling.
-	if !errors.Is(err, unix.ELOOP) && !errors.Is(err, unix.ENOTDIR) {
-		t.Errorf("refused for the wrong reason, want a no-follow refusal: %v", err)
-	}
-
-	// THE POINT IS WHERE THE WRITES DID NOT GO. An error alone would also be
-	// produced by a later guard noticing the link after the tree was built inside
-	// somebody else's directory.
-	if leaked := countEntriesUnder(t, target); len(leaked) != 0 {
-		t.Fatalf("the redirected target received %v; the refusal happened after the writes rather than instead of them", leaked)
-	}
-}
-
-// CONTROL: an ordinary tree still gets its lease, and the one path opened by name
-// is the operator's base rather than a component Zero owns.
-//
-// Refusing everything would satisfy the test above. Asserting the base directly
-// is what makes this discriminating: inferring it from whether a swap was caught
-// says nothing about which name was trusted.
-func TestRuntimeLeaseOpensOnlyTheBaseByName(t *testing.T) {
-	base := t.TempDir()
-	root := unixLeaseRootUnder(t, base)
-
-	previousBase := runtimeBaseOpenedByName
-	t.Cleanup(func() { runtimeBaseOpenedByName = previousBase })
-	var byName []string
-	runtimeBaseOpenedByName = func(path string) { byName = append(byName, path) }
-
-	lease, created, err := prepareSandboxRuntimeLeaseRecording(root)
-	if err != nil {
-		t.Fatalf("lease acquisition failed on an ordinary tree: %v", err)
-	}
-	lease.release()
-
-	if len(byName) != 1 {
-		t.Fatalf("opened %v by name, want exactly the base", byName)
-	}
-	if got := byName[0]; got != canonicalSandboxWorkspaceRoot(base) && got != base {
-		t.Fatalf("opened %q by name, want the base %q", got, base)
-	}
-	if len(created) == 0 {
-		t.Fatal("the descent created the owned components but recorded none of them, so rollback has nothing to undo")
-	}
-	if _, err := os.Lstat(sandboxRuntimeLeasePath(root)); err != nil {
-		t.Fatalf("no lease file was created on an ordinary tree: %v", err)
-	}
-}
-
-// And a link at the LEASE NAME itself is refused by both holders, so the shared
-// and exclusive sides cannot end up locking different objects.
-func TestRuntimeLeaseRefusesALinkAtTheLeaseName(t *testing.T) {
-	base := t.TempDir()
-	root := unixLeaseRootUnder(t, base)
-
-	lease, _, err := prepareSandboxRuntimeLeaseRecording(root)
-	if err != nil {
-		t.Fatalf("SETUP: cannot seed a lease: %v", err)
-	}
-	lease.release()
-	leasePath := sandboxRuntimeLeasePath(root)
-	if err := os.Remove(leasePath); err != nil {
-		t.Fatalf("SETUP INVALID: cannot clear the seeded lease: %v", err)
-	}
-	target := filepath.Join(t.TempDir(), "target.lease")
-	if err := os.WriteFile(target, nil, 0o600); err != nil {
-		t.Fatal(err)
-	}
-	if err := os.Symlink(target, leasePath); err != nil {
-		t.Fatalf("SETUP: cannot plant the link: %v", err)
-	}
-
-	shared, _, sharedErr := prepareSandboxRuntimeLeaseRecording(root)
-	if shared != nil {
-		shared.release()
-	}
-	if sharedErr == nil {
-		t.Error("shared acquisition accepted a link at the lease name")
-	}
-
-	cleanup, inUse, cleanupErr := tryAcquireSandboxRuntimeCleanupLease(root)
-	if cleanup != nil {
-		cleanup.release()
-	}
-	if cleanupErr == nil {
-		t.Errorf("cleanup accepted a link at the lease name (inUse=%t); it would treat a lock on %s as proof the root is free to delete", inUse, target)
-	}
-}
-
-// CONTROL: the two sides still coordinate on an ordinary lease.
-func TestUnixSharedLeaseMakesCleanupReportInUse(t *testing.T) {
-	base := t.TempDir()
-	root := unixLeaseRootUnder(t, base)
-
-	held, _, err := prepareSandboxRuntimeLeaseRecording(root)
-	if err != nil {
-		t.Fatalf("cannot acquire a runtime lease: %v", err)
-	}
-
-	lease, inUse, err := tryAcquireSandboxRuntimeCleanupLease(root)
-	if lease != nil {
-		lease.release()
-	}
-	if err != nil {
-		t.Fatalf("cleanup failed against a legitimately held lease: %v", err)
-	}
-	if !inUse {
-		t.Fatal("cleanup reported the runtime root free while a shared lease was held; it would delete a tree a live command is using")
-	}
-
-	held.release()
-	lease, inUse, err = tryAcquireSandboxRuntimeCleanupLease(root)
-	if err != nil {
-		t.Fatalf("cleanup failed after the shared lease was released: %v", err)
-	}
-	if inUse {
-		t.Fatal("cleanup still reported the root in use after the only holder released it, so reclamation never happens")
-	}
-	lease.release()
-}
@@ -1,93 +0,0 @@
-package sandbox
-
-import (
-	"os"
-	"path/filepath"
-	"testing"
-)
-
-// runtimeRootUnderTest builds a root with the shape production uses, so the
-// components refuseAliasedRuntimeComponents walks stay inside test-owned
-// storage.
-//
-// Owned depth is the fixed names plus the digest, so a root placed directly in
-// t.TempDir() puts an owned component on /tmp, which the Unix ownership guard
-// correctly refuses because /tmp belongs to root. That is the guard working, not
-// a test environment problem, and it only shows up off Windows.
-func runtimeRootUnderTest(t *testing.T, leaf string) string {
-	t.Helper()
-	root := filepath.Join(t.TempDir(), "zero", "runtime", "v1", leaf)
-	if err := os.MkdirAll(root, 0o700); err != nil {
-		t.Fatal(err)
-	}
-	return root
-}
-
-// A LEASE IS OWNERSHIP FOR A TRANSACTION, NOT A SELECTION PROBE.
-//
-// Setup took a lease only to learn which root won and released it at once, so
-// nothing owned the selected root while the elevated helper provisioned the
-// tree, applied the ACL and stamp, installed network state and wrote the marker.
-// A command for another workspace scans the same runtime parent and excludes
-// only its own current root, so it can take this root's cleanup lease and
-// RemoveAll it mid-transaction; setup then publishes success for a pathname that
-// is gone.
-//
-// This pins the mechanism the fix relies on: a held lease is what makes the
-// cleanup's exclusive acquire fail.
-func TestAHeldRuntimeLeaseStopsConcurrentCleanup(t *testing.T) {
-	root := runtimeRootUnderTest(t, "deadbeefdeadbeef")
-
-	lease, err := prepareSandboxRuntimeLease(root)
-	if err != nil {
-		t.Fatalf("acquire the runtime lease: %v", err)
-	}
-	removeSandboxRuntimeRootIfUnused(root)
-	if _, err := os.Stat(root); err != nil {
-		t.Fatalf("cleanup removed a root that setup was holding: %v", err)
-	}
-	lease.release()
-
-	// And once nothing holds it, cleanup does its job: without this half the test
-	// would pass against a cleanup that never removes anything.
-	removeSandboxRuntimeRootIfUnused(root)
-	if _, err := os.Stat(root); !os.IsNotExist(err) {
-		t.Fatalf("an unheld root survived cleanup: %v", err)
-	}
-}
-
-// The lease is on the root the caller names, so holding one root does not
-// protect a sibling: setup reserving its own selection must not stop the
-// cleanup doing its work elsewhere.
-func TestAHeldLeaseDoesNotProtectASiblingRoot(t *testing.T) {
-	held := runtimeRootUnderTest(t, "aaaaaaaaaaaaaaaa")
-	sibling := filepath.Join(filepath.Dir(held), "bbbbbbbbbbbbbbbb")
-	if err := os.MkdirAll(sibling, 0o700); err != nil {
-		t.Fatal(err)
-	}
-	lease, err := prepareSandboxRuntimeLease(held)
-	if err != nil {
-		t.Fatal(err)
-	}
-	defer lease.release()
-
-	removeSandboxRuntimeRootIfUnused(sibling)
-	if _, err := os.Stat(sibling); !os.IsNotExist(err) {
-		t.Errorf("holding one root blocked cleanup of an unrelated sibling: %v", err)
-	}
-	if _, err := os.Stat(held); err != nil {
-		t.Errorf("the held root was removed: %v", err)
-	}
-}
-
-// createdRuntimeDirsForTest builds identity-bound rollback records for paths a
-// test made itself, so a test can express "these are the directories this run
-// created" without restating the identity capture.
-func createdRuntimeDirsForTest(paths ...string) []windowsCreatedRuntimeDir {
-	records := make([]windowsCreatedRuntimeDir, 0, len(paths))
-	for _, path := range paths {
-		identity, identified := runtimeDirIdentity(path)
-		records = append(records, windowsCreatedRuntimeDir{path: path, identity: identity, identified: identified})
-	}
-	return records
-}
@@ -1,21 +0,0 @@
-//go:build !windows
-
-package sandbox
-
-// acquireRuntimeLeaseForPlatform creates and locks the lease through the same
-// rooted no-follow boundary the Windows side uses.
-//
-// A PRE-CHECK IS NOT A BOUNDARY. This used to call refuseAliasedRuntimeComponents
-// and then os.MkdirAll plus a pathname lease open. The guard answers about an
-// ABSENT component by saying there is nothing to alias, which is exactly the
-// state a fresh fallback root is in, and both calls that followed resolve the
-// name again. A link planted in between was therefore created through and
-// written into, and the next check could only report it after the writes.
-//
-// The alias guard is gone from this path rather than kept alongside: retaining it
-// would suggest the two together are the protection, when the descent is the
-// protection and the guard is the thing that could not be one. Other callers
-// still use it where a pathname really is all there is.
-func acquireRuntimeLeaseForPlatform(root string) (*sandboxRuntimeLease, []windowsCreatedRuntimeDir, error) {
-	return acquireRuntimeLeaseRootedUnix(root)
-}
@@ -1,10 +0,0 @@
-//go:build windows
-
-package sandbox
-
-// acquireRuntimeLeaseForPlatform routes Windows through the rooted, no-follow
-// acquisition. See acquireRuntimeLeaseRooted for why the pathname walk it
-// replaces was unsafe.
-func acquireRuntimeLeaseForPlatform(root string) (*sandboxRuntimeLease, []windowsCreatedRuntimeDir, error) {
-	return acquireRuntimeLeaseRooted(root)
-}
@@ -1,299 +0,0 @@
-//go:build windows
-
-package sandbox
-
-import (
-	"os"
-	"path/filepath"
-	"strings"
-	"testing"
-	"unsafe"
-
-	"golang.org/x/sys/windows"
-)
-
-// seedRuntimeLeaseTree creates the owned tail and the lease the way a first run
-// does, then removes just the lease file so a test can put something else at that
-// name. The tree has to exist, or cleanup's descent has nothing to open and the
-// test would pass for the wrong reason.
-func seedRuntimeLeaseTree(t *testing.T, root string) string {
-	t.Helper()
-	lease, _, err := prepareSandboxRuntimeLeaseRecording(root)
-	if err != nil {
-		t.Skipf("cannot seed a runtime lease here: %v", err)
-	}
-	lease.release()
-	leasePath := sandboxRuntimeLeasePath(root)
-	if err := os.Remove(leasePath); err != nil {
-		t.Fatalf("SETUP INVALID: cannot clear the seeded lease: %v", err)
-	}
-	return leasePath
-}
-
-// plantFileLink puts a FILE symbolic link at leasePath pointing at an ordinary
-// file, and returns the target.
-//
-// A file link, not a junction: a junction is a directory and FILE_NON_DIRECTORY_FILE
-// already refuses it, so a junction here would pass against the defect and prove
-// nothing. Creating one needs SeCreateSymbolicLinkPrivilege, which an ordinary
-// unelevated account does not hold, so this skips there and runs on CI.
-func plantFileLink(t *testing.T, leasePath string) string {
-	t.Helper()
-	target := filepath.Join(t.TempDir(), "target.lease")
-	if err := os.WriteFile(target, nil, 0o600); err != nil {
-		t.Fatal(err)
-	}
-	if err := os.Symlink(target, leasePath); err != nil {
-		t.Skipf("cannot create a file symbolic link here, which needs SeCreateSymbolicLinkPrivilege: %v", err)
-	}
-	// SETUP: it really is a reparse point, or nothing below is about links.
-	info, err := os.Lstat(leasePath)
-	if err != nil || info.Mode()&os.ModeSymlink == 0 {
-		t.Fatalf("SETUP INVALID: %s is not a link (%v, %v)", leasePath, info, err)
-	}
-	return target
-}
-
-// assertTargetUnlocked proves nobody holds a lock on the link's target, which is
-// the object the defect made the two sides fight over.
-func assertTargetUnlocked(t *testing.T, target string) {
-	t.Helper()
-	file, err := os.OpenFile(target, os.O_RDWR, 0o600)
-	if err != nil {
-		t.Fatalf("open the link target: %v", err)
-	}
-	defer func() { _ = file.Close() }()
-	var overlapped windows.Overlapped
-	flags := uint32(windows.LOCKFILE_EXCLUSIVE_LOCK | windows.LOCKFILE_FAIL_IMMEDIATELY)
-	if err := windows.LockFileEx(windows.Handle(file.Fd()), flags, 0, 1, 0, &overlapped); err != nil {
-		t.Fatalf("the link target is locked, so the refusal happened after the lease had already been taken on it: %v", err)
-	}
-	_ = windows.UnlockFileEx(windows.Handle(file.Fd()), 0, 1, 0, &overlapped)
-}
-
-// FILE_OPEN_REPARSE_POINT SAYS DO NOT FOLLOW, NOT REFUSE.
-//
-// The rooted opener passed that flag and FILE_NON_DIRECTORY_FILE and treated the
-// pair as a guarantee. It is not: the flag returns a handle to the LINK, and
-// FILE_NON_DIRECTORY_FILE excludes directories rather than non-directory reparse
-// objects. A file symbolic link at <digest>.lease was opened, wrapped and locked
-// as though it were the lease.
-//
-// That matters because cleanup opened the same name by pathname with no
-// no-follow flag at all, so it locked the link's TARGET. Two holders, two
-// objects, both calls succeeding, and cleanup free to RemoveAll a runtime root a
-// live command was using.
-func TestSharedLeaseRefusesAFileLinkAtTheLeaseName(t *testing.T) {
-	cacheRoot := t.TempDir()
-	root := leaseRootUnder(t, cacheRoot)
-	leasePath := seedRuntimeLeaseTree(t, root)
-	target := plantFileLink(t, leasePath)
-
-	lease, _, err := prepareSandboxRuntimeLeaseRecording(root)
-	if lease != nil {
-		lease.release()
-	}
-	if err == nil {
-		t.Fatal("shared acquisition accepted a file link at the lease name, so it is holding a lock on an object cleanup does not see")
-	}
-	if !strings.Contains(err.Error(), "reparse point") {
-		t.Errorf("refused for the wrong reason: %v", err)
-	}
-	assertTargetUnlocked(t, target)
-}
-
-// And cleanup refuses the same object, so it cannot decide the root is free by
-// locking something else.
-func TestCleanupLeaseRefusesAFileLinkAtTheLeaseName(t *testing.T) {
-	cacheRoot := t.TempDir()
-	root := leaseRootUnder(t, cacheRoot)
-	leasePath := seedRuntimeLeaseTree(t, root)
-	target := plantFileLink(t, leasePath)
-
-	lease, inUse, err := tryAcquireSandboxRuntimeCleanupLease(root)
-	if lease != nil {
-		lease.release()
-	}
-	if err == nil {
-		t.Fatalf("cleanup accepted a file link at the lease name (inUse=%t); it would treat a lock on %s as proof the runtime root is free to delete", inUse, target)
-	}
-	if !strings.Contains(err.Error(), "reparse point") {
-		t.Errorf("refused for the wrong reason: %v", err)
-	}
-	assertTargetUnlocked(t, target)
-}
-
-// CONTROL: THE TWO SIDES REALLY DO COORDINATE ON AN ORDINARY LEASE.
-//
-// Without this, refusing everything would satisfy the two tests above. This one
-// runs everywhere, including on an unelevated box with no symbolic-link
-// privilege, so the mutual-exclusion contract stays pinned even where the link
-// cases skip.
-func TestASharedLeaseMakesCleanupReportInUse(t *testing.T) {
-	cacheRoot := t.TempDir()
-	root := leaseRootUnder(t, cacheRoot)
-
-	held, _, err := prepareSandboxRuntimeLeaseRecording(root)
-	if err != nil {
-		t.Skipf("cannot acquire a runtime lease here: %v", err)
-	}
-
-	lease, inUse, err := tryAcquireSandboxRuntimeCleanupLease(root)
-	if lease != nil {
-		lease.release()
-	}
-	if err != nil {
-		t.Fatalf("cleanup failed against a legitimately held lease: %v", err)
-	}
-	if !inUse {
-		t.Fatal("cleanup reported the runtime root free while a shared lease was held; it would delete a tree a live command is using")
-	}
-
-	held.release()
-
-	lease, inUse, err = tryAcquireSandboxRuntimeCleanupLease(root)
-	if err != nil {
-		t.Fatalf("cleanup failed after the shared lease was released: %v", err)
-	}
-	if inUse {
-		t.Fatal("cleanup still reported the runtime root in use after the only holder released it, so reclamation never happens")
-	}
-	if lease == nil {
-		t.Fatal("cleanup reported the root free but handed back no lease")
-	}
-	lease.release()
-}
-
-// CONTROL: a legitimate lease is still SHARED between processes.
-//
-// The classification must not turn the shared lease into an exclusive one, or
-// two concurrent commands on one workspace would stop working.
-func TestAnOrdinaryLeaseIsStillSharedByTwoHolders(t *testing.T) {
-	cacheRoot := t.TempDir()
-	root := leaseRootUnder(t, cacheRoot)
-
-	first, _, err := prepareSandboxRuntimeLeaseRecording(root)
-	if err != nil {
-		t.Skipf("cannot acquire a runtime lease here: %v", err)
-	}
-	defer first.release()
-
-	second, _, err := prepareSandboxRuntimeLeaseRecording(root)
-	if err != nil {
-		t.Fatalf("a second holder was refused an existing ordinary lease: %v", err)
-	}
-	second.release()
-}
-
-// reparseGUIDDataBuffer is the non-Microsoft form of REPARSE_GUID_DATA_BUFFER.
-//
-// Setting a tag with the Microsoft bit clear needs only write access to the file,
-// unlike a symbolic link, so this runs on an ordinary unelevated account and the
-// classification stays pinned where the link cases skip.
-type reparseGUIDDataBuffer struct {
-	ReparseTag        uint32
-	ReparseDataLength uint16
-	Reserved          uint16
-	ReparseGUID       windows.GUID
-	Data              [16]byte
-}
-
-// plantGenericReparsePoint turns an ordinary file at path into a reparse object.
-func plantGenericReparsePoint(t *testing.T, path string) {
-	t.Helper()
-	if err := os.WriteFile(path, nil, 0o600); err != nil {
-		t.Fatal(err)
-	}
-	name, err := windows.UTF16PtrFromString(path)
-	if err != nil {
-		t.Fatal(err)
-	}
-	handle, err := windows.CreateFile(name,
-		windows.GENERIC_WRITE|windows.FILE_WRITE_ATTRIBUTES, 0, nil,
-		windows.OPEN_EXISTING,
-		windows.FILE_FLAG_OPEN_REPARSE_POINT|windows.FILE_FLAG_BACKUP_SEMANTICS, 0)
-	if err != nil {
-		t.Skipf("cannot open %s to set a reparse tag: %v", path, err)
-	}
-	defer func() { _ = windows.CloseHandle(handle) }()
-
-	buffer := reparseGUIDDataBuffer{
-		// Bit 31 clear, so this is a third-party tag and needs no privilege.
-		ReparseTag:        0x00000042,
-		ReparseDataLength: 16,
-		ReparseGUID:       windows.GUID{Data1: 0x5ee0e5f1, Data2: 0x1a11, Data3: 0x4d3b, Data4: [8]byte{0x9a, 0x77, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06}},
-	}
-	size := uint32(unsafe.Sizeof(buffer))
-	var returned uint32
-	if err := windows.DeviceIoControl(handle, windowsFSCTLSetReparsePoint,
-		(*byte)(unsafe.Pointer(&buffer)), size, nil, 0, &returned, nil); err != nil {
-		t.Skipf("cannot set a generic reparse tag here: %v", err)
-	}
-}
-
-const windowsFSCTLSetReparsePoint = 0x000900A4
-
-// ANY REPARSE OBJECT AT THE LEASE NAME IS REFUSED, NOT ONLY A SYMBOLIC LINK.
-//
-// The requirement is that the name denotes an ordinary file, because that is what
-// makes the shared holder and cleanup lock the same thing. Keying on the link
-// shape instead would leave every other reparse tag accepted, and it would leave
-// this untested on any machine without the symbolic-link privilege.
-//
-// WHAT THIS CASE CANNOT SHOW, so nobody reads a local pass as more than it is:
-// an unknown third-party tag is unresolvable, so the OLD pathname cleanup fails
-// on it too, with ERROR_CANT_ACCESS_FILE rather than by classifying anything. It
-// therefore pins that both sites refuse, and the reason check below is what
-// separates a refusal from an accident. Only the symbolic-link cases above show
-// the half that matters most, a pathname open SUCCEEDING on the target, and those
-// need the privilege. Read their result in CI, not here.
-func TestLeaseRefusesAnyReparseObjectAtTheLeaseName(t *testing.T) {
-	for name, acquire := range map[string]func(string) error{
-		"shared": func(root string) error {
-			lease, _, err := prepareSandboxRuntimeLeaseRecording(root)
-			if lease != nil {
-				lease.release()
-			}
-			return err
-		},
-		"cleanup": func(root string) error {
-			lease, _, err := tryAcquireSandboxRuntimeCleanupLease(root)
-			if lease != nil {
-				lease.release()
-			}
-			return err
-		},
-	} {
-		t.Run(name, func(t *testing.T) {
-			cacheRoot := t.TempDir()
-			root := leaseRootUnder(t, cacheRoot)
-			leasePath := seedRuntimeLeaseTree(t, root)
-			plantGenericReparsePoint(t, leasePath)
-
-			// SETUP: the attribute really is set, or this asserts nothing.
-			info, err := os.Lstat(leasePath)
-			if err != nil || info.Mode()&os.ModeIrregular == 0 && info.Mode()&os.ModeSymlink == 0 {
-				attrs, statErr := windowsFileAttributes(leasePath)
-				if statErr != nil || attrs&windows.FILE_ATTRIBUTE_REPARSE_POINT == 0 {
-					t.Fatalf("SETUP INVALID: %s is not a reparse object (attrs=%#x err=%v)", leasePath, attrs, statErr)
-				}
-			}
-
-			err = acquire(root)
-			if err == nil {
-				t.Fatalf("%s acquisition accepted a reparse object at the lease name", name)
-			}
-			if !strings.Contains(err.Error(), "reparse point") {
-				t.Errorf("refused for the wrong reason: %v", err)
-			}
-		})
-	}
-}
-
-func windowsFileAttributes(path string) (uint32, error) {
-	name, err := windows.UTF16PtrFromString(path)
-	if err != nil {
-		return 0, err
-	}
-	return windows.GetFileAttributes(name)
-}
@@ -1,282 +0,0 @@
-//go:build !windows
-
-package sandbox
-
-import (
-	"errors"
-	"fmt"
-	"os"
-	"path/filepath"
-
-	"golang.org/x/sys/unix"
-)
-
-// acquireRuntimeLeaseRootedUnix creates and locks the lease for root without
-// resolving an owned component by name.
-//
-// The lease is the FIRST thing setup writes, so it is the write a redirection has
-// to be caught before, not after. It is a sibling of the runtime root rather than
-// one of its owned components, which is why the alias guard never looked at it at
-// all: it walks the components, and the lease is not one of them.
-//
-// Only the components above the leaf are created here. The leaf belongs to
-// provisioning, which records it for rollback, and creating it here would mean
-// two owners for one directory.
-func acquireRuntimeLeaseRootedUnix(root string) (*sandboxRuntimeLease, []windowsCreatedRuntimeDir, error) {
-	base, components, owned := windowsSandboxRuntimeOwnedTail(root)
-	if !owned || len(components) == 0 {
-		// Fail rather than fall back to the pathname walk: the walk is the defect.
-		return nil, nil, fmt.Errorf("acquire sandbox runtime lease for %s: %w", root, errRuntimeTailNotOwned)
-	}
-	if runtimeLeasePreCreateBarrier != nil {
-		runtimeLeasePreCreateBarrier()
-	}
-	// The base is the operator's, and may legitimately be a redirected or linked
-	// temp location, so it is created and opened by name exactly as before.
-	// Everything below it is Zero's and is addressed by descriptor.
-	if err := os.MkdirAll(base, 0o700); err != nil {
-		return nil, nil, fmt.Errorf("create sandbox runtime base: %w", err)
-	}
-
-	// The lease sits beside the leaf, so the deepest directory needed here is the
-	// leaf's parent.
-	tail := make([]string, 0, len(components)-1)
-	current := base
-	for _, component := range components[:len(components)-1] {
-		current = filepath.Join(current, component)
-		tail = append(tail, current)
-	}
-
-	// THE WHOLE ACQUISITION RETRIES, NOT JUST THE LOCK. A contender that waited
-	// on the lease while cleanup held it exclusively can wake on an object cleanup
-	// has since unlinked, and by then cleanup has compensated the tail this run
-	// created as well. Re-taking the lock alone would lock a fresh file beside a
-	// leaf that is gone, so a stale lock sends the caller back through tail
-	// creation. The ledger accumulates across attempts: a directory this run made
-	// on an earlier pass and cleanup did not remove is still this run's to
-	// compensate.
-	var created []windowsCreatedRuntimeDir
-	for attempt := 1; ; attempt++ {
-		made, parent, err := createRuntimeTailRetainingFD(base, tail)
-		created = appendCreatedRuntimeDirs(created, made)
-		if err != nil {
-			return nil, created, err
-		}
-		handle, madeLease, err := acquireSharedRuntimeLeaseAtFD(parent, filepath.Base(sandboxRuntimeLeasePath(root)))
-		_ = unix.Close(parent)
-		if err == nil {
-			return &sandboxRuntimeLease{handle: handle, root: root, createdFile: madeLease}, created, nil
-		}
-		if errors.Is(err, errRuntimeLeaseReplaced) && attempt < runtimeLeaseAcquireAttempts {
-			continue
-		}
-		return nil, created, fmt.Errorf("acquire sandbox runtime lease: %w", err)
-	}
-}
-
-// acquireSharedRuntimeLeaseAtFD opens the lease relative to a verified parent and
-// takes the shared lock on it.
-func acquireSharedRuntimeLeaseAtFD(parent int, name string) (runtimeLeaseHandle, bool, error) {
-	file, created, err := openRuntimeLeaseAtFD(parent, name)
-	if err != nil {
-		return runtimeLeaseHandle{}, false, err
-	}
-	if err := unix.Flock(int(file.Fd()), unix.LOCK_SH); err != nil {
-		// The lock is the last step, and until now the created fact went out only
-		// alongside a lease object. Failing here returned no object, so this
-		// invocation's own lease file stayed on disk with nothing recording it,
-		// and its directory could never be compensated afterwards.
-		err = undoCreatedRuntimeLeaseAtFD(created, parent, name, err)
-		_ = file.Close()
-		return runtimeLeaseHandle{}, false, err
-	}
-	// THE LOCK IS ON AN INODE, THE COORDINATION IS ON A NAME. Flock blocks while
-	// cleanup holds the exclusive lock, and cleanup unlinks the lease name before
-	// it lets go. A waiter then acquires the old, unlinked inode and returns a
-	// lease that coordinates with nobody: the next cleanup creates a fresh file at
-	// the name, locks it unopposed, and reports the root unused while this holder
-	// is still running in it. So the lock is only a lease once the locked object
-	// is still what the name resolves to. Reported by @gnanam1990.
-	current, err := runtimeLeaseIsCurrentAtFD(parent, name, int(file.Fd()))
-	if err != nil {
-		_ = file.Close()
-		return runtimeLeaseHandle{}, false, err
-	}
-	if !current {
-		// Not undone by name, whatever created says: the name is no longer this
-		// object's, so an unlink there would take somebody else's lease.
-		_ = file.Close()
-		return runtimeLeaseHandle{}, false, errRuntimeLeaseReplaced
-	}
-	return runtimeLeaseHandle{file: file}, created, nil
-}
-
-// runtimeLeaseIsCurrentAtFD reports whether the locked lease is still the object
-// the lease name resolves to under parent.
-//
-// Both answers come from the descriptor side: the locked file's own identity and
-// the directory entry read relative to the retained parent, no-follow. A missing
-// entry, a different inode at the name, or a locked inode with no links left all
-// mean the coordination moved on while this call was waiting.
-func runtimeLeaseIsCurrentAtFD(parent int, name string, fd int) (bool, error) {
-	var locked, entry unix.Stat_t
-	if err := unix.Fstat(fd, &locked); err != nil {
-		return false, fmt.Errorf("inspect the locked sandbox runtime lease %s: %w", name, err)
-	}
-	if locked.Nlink == 0 {
-		return false, nil
-	}
-	if err := unix.Fstatat(parent, name, &entry, unix.AT_SYMLINK_NOFOLLOW); err != nil {
-		if errors.Is(err, unix.ENOENT) {
-			return false, nil
-		}
-		return false, fmt.Errorf("inspect the sandbox runtime lease entry %s: %w", name, err)
-	}
-	return locked.Dev == entry.Dev && locked.Ino == entry.Ino, nil
-}
-
-// undoCreatedRuntimeLeaseAtFD removes a lease file this call created, when a
-// later step means no lease object will be returned to carry that fact.
-//
-// Relative to the verified parent descriptor, and only when this call is the one
-// that created the file: a lease that was already there belongs to whoever is
-// holding it.
-func undoCreatedRuntimeLeaseAtFD(created bool, parent int, name string, cause error) error {
-	if !created {
-		return cause
-	}
-	if undoErr := unix.Unlinkat(parent, name, 0); undoErr != nil {
-		return fmt.Errorf("%w; and the lease file this run created could not be removed: %w", cause, undoErr)
-	}
-	return cause
-}
-
-// openRuntimeLeaseAtFD opens or creates the lease under parent and proves it is
-// an ordinary file.
-//
-// O_NOFOLLOW so a symlink planted at the lease name is an ELOOP rather than an
-// open of its target, and a regular-file check besides, because O_NOFOLLOW says
-// nothing about a fifo or a device a local user can also create there. Both
-// holders have to end up on the same object or the lock protects nothing.
-func openRuntimeLeaseAtFD(parent int, name string) (*os.File, bool, error) {
-	const flags = unix.O_RDWR | unix.O_NOFOLLOW | unix.O_CLOEXEC
-	// O_EXCL FIRST, so the create is what reports the create. A Stat beforehand
-	// answers about a moment that has passed by the time the open runs, and
-	// compensation would then delete a lease another process had just made.
-	created := true
-	fd, err := unix.Openat(parent, name, flags|unix.O_CREAT|unix.O_EXCL, 0o600)
-	if errors.Is(err, unix.EEXIST) {
-		created = false
-		fd, err = unix.Openat(parent, name, flags, 0)
-	}
-	if err != nil {
-		if errors.Is(err, unix.ELOOP) {
-			return nil, false, fmt.Errorf("refusing to use the sandbox runtime lease at %s: it is a link, so the holders would lock different objects: %w", name, err)
-		}
-		return nil, false, err
-	}
-	// Every failure from here on has already created the file when created is
-	// true, and returns no lease object to carry that fact, so each one undoes
-	// its own creation rather than leaving an unrecorded artifact behind.
-	if runtimeCreationFailure != nil {
-		if injected := runtimeCreationFailure(name); injected != nil {
-			injected = undoCreatedRuntimeLeaseAtFD(created, parent, name, injected)
-			_ = unix.Close(fd)
-			return nil, false, injected
-		}
-	}
-	var stat unix.Stat_t
-	if err := unix.Fstat(fd, &stat); err != nil {
-		err = undoCreatedRuntimeLeaseAtFD(created, parent, name, fmt.Errorf("inspect the sandbox runtime lease %s: %w", name, err))
-		_ = unix.Close(fd)
-		return nil, false, err
-	}
-	if stat.Mode&unix.S_IFMT != unix.S_IFREG {
-		err := undoCreatedRuntimeLeaseAtFD(created, parent, name, fmt.Errorf("refusing to use the sandbox runtime lease at %s: it is not an ordinary file (mode %#o)", name, stat.Mode&unix.S_IFMT))
-		_ = unix.Close(fd)
-		return nil, false, err
-	}
-	if uid := unix.Getuid(); uid >= 0 && stat.Uid != uint32(uid) {
-		err := undoCreatedRuntimeLeaseAtFD(created, parent, name, fmt.Errorf("refusing to use the sandbox runtime lease at %s: it is owned by uid %d, not %d", name, stat.Uid, uid))
-		_ = unix.Close(fd)
-		return nil, false, err
-	}
-	file := os.NewFile(uintptr(fd), name)
-	if file == nil {
-		err := undoCreatedRuntimeLeaseAtFD(created, parent, name, fmt.Errorf("wrap the sandbox runtime lease handle for %s", name))
-		_ = unix.Close(fd)
-		return nil, false, err
-	}
-	return file, created, nil
-}
-
-// tryAcquireExclusiveRuntimeLeaseRootedUnix is cleanup's acquisition, resolved
-// the way acquisition resolves it.
-//
-// It opens and never creates the tree: a runtime root that is not there has no
-// lease to take, and cleanup rebuilding it in order to lock it would be inventing
-// the thing it is about to remove.
-func tryAcquireExclusiveRuntimeLeaseRootedUnix(root string) (runtimeLeaseHandle, bool, error) {
-	base, components, owned := windowsSandboxRuntimeOwnedTail(root)
-	if !owned || len(components) == 0 {
-		return runtimeLeaseHandle{}, false, fmt.Errorf("open the sandbox runtime lease parent for %s: %w", root, errRuntimeTailNotOwned)
-	}
-	parent, err := unix.Open(base, unix.O_RDONLY|unix.O_DIRECTORY|unix.O_CLOEXEC, 0)
-	if err != nil {
-		return runtimeLeaseHandle{}, false, fmt.Errorf("open sandbox runtime base %s: %w", base, err)
-	}
-	path := base
-	for _, name := range components[:len(components)-1] {
-		path = filepath.Join(path, name)
-		child, openErr := unix.Openat(parent, name, unix.O_RDONLY|unix.O_DIRECTORY|unix.O_NOFOLLOW|unix.O_CLOEXEC, 0)
-		if openErr != nil {
-			_ = unix.Close(parent)
-			return runtimeLeaseHandle{}, false, fmt.Errorf("open sandbox runtime component %s: %w", path, openErr)
-		}
-		if err := refuseForeignRuntimeDirectory(child, path); err != nil {
-			_ = unix.Close(child)
-			_ = unix.Close(parent)
-			return runtimeLeaseHandle{}, false, err
-		}
-		_ = unix.Close(parent)
-		parent = child
-	}
-	defer func() { _ = unix.Close(parent) }()
-
-	name := filepath.Base(sandboxRuntimeLeasePath(root))
-	for attempt := 1; ; attempt++ {
-		file, created, err := openRuntimeLeaseAtFD(parent, name)
-		if err != nil {
-			return runtimeLeaseHandle{}, false, err
-		}
-		if err := unix.Flock(int(file.Fd()), unix.LOCK_EX|unix.LOCK_NB); err != nil {
-			if errors.Is(err, unix.EWOULDBLOCK) || errors.Is(err, unix.EAGAIN) {
-				// Somebody holds it, so it is not this call's to remove, whatever the
-				// open reported. A file this call created cannot reach here anyway.
-				_ = file.Close()
-				return runtimeLeaseHandle{}, true, nil
-			}
-			err = undoCreatedRuntimeLeaseAtFD(created, parent, name, err)
-			_ = file.Close()
-			return runtimeLeaseHandle{}, false, err
-		}
-		// The exclusive side has the same window in miniature: between its open
-		// and its lock another cleanup can unlink and replace the name, and an
-		// exclusive lock on the old inode would then remove a tree a holder of the
-		// new one is entering. Same check, same answer: only the current object
-		// counts, and a stale one is reopened rather than reasoned about.
-		current, err := runtimeLeaseIsCurrentAtFD(parent, name, int(file.Fd()))
-		if err != nil {
-			_ = file.Close()
-			return runtimeLeaseHandle{}, false, err
-		}
-		if !current {
-			_ = file.Close()
-			if attempt < runtimeLeaseAcquireAttempts {
-				continue
-			}
-			return runtimeLeaseHandle{}, false, errRuntimeLeaseReplaced
-		}
-		return runtimeLeaseHandle{file: file}, false, nil
-	}
-}
@@ -1,408 +0,0 @@
-//go:build windows
-
-package sandbox
-
-import (
-	"errors"
-	"fmt"
-	"os"
-	"path/filepath"
-	"unsafe"
-
-	"golang.org/x/sys/windows"
-)
-
-// acquireRuntimeLeaseRooted creates and locks the lease file for root without
-// ever resolving an owned component by name.
-//
-// THE LEASE IS ACQUIRED BEFORE ANYTHING ELSE, SO IT CANNOT BE THE WEAK LINK.
-//
-// Provisioning already descends from the fixed cache or TEMP base through
-// retained no-follow handles, because a predictable owned component is exactly
-// what an ordinary same-account process can replace with a junction. Lease
-// acquisition ran first and did neither: it checked the components for aliases,
-// then called os.MkdirAll on the parent by pathname and opened
-// "<root>.lease" by pathname, both of which follow. A junction dropped on
-// "zero", "runtime" or "v1" between the check and either call put elevated
-// setup's first writes inside somebody else's tree, and restoring the component
-// afterwards left the later handle-relative provisioning working on the
-// legitimate tree so no post-check ever saw it.
-//
-// The lease file is a SIBLING of the runtime root rather than one of its owned
-// components, so refuseAliasedRuntimeComponents never inspected it at all. Here
-// it is created relative to the retained handle for the directory that contains
-// it, which is the deepest owned component, so its name is resolved exactly once
-// and relative to a verified object.
-//
-// Only the components above the leaf are created. The leaf itself belongs to
-// provisioning, which records it for rollback; creating it here would mean two
-// owners for one directory.
-func acquireRuntimeLeaseRooted(root string) (*sandboxRuntimeLease, []windowsCreatedRuntimeDir, error) {
-	base, components, owned := windowsSandboxRuntimeOwnedTail(root)
-	if !owned {
-		// Fail rather than fall back to the pathname walk: the walk is the defect.
-		return nil, nil, fmt.Errorf("acquire sandbox runtime lease for %s: %w", root, errRuntimeTailNotOwned)
-	}
-	if len(components) == 0 {
-		return nil, nil, fmt.Errorf("acquire sandbox runtime lease for %s: %w", root, errRuntimeTailNotOwned)
-	}
-	// The base is the operator's, and may legitimately be a redirected cache or
-	// TEMP location, so it is created and opened by name exactly as provisioning
-	// does. Everything below it is Zero's and is addressed by handle.
-	if runtimeLeasePreCreateBarrier != nil {
-		runtimeLeasePreCreateBarrier()
-	}
-	if err := os.MkdirAll(base, 0o700); err != nil {
-		return nil, nil, fmt.Errorf("create sandbox runtime base: %w", err)
-	}
-
-	// The lease sits beside the leaf, so the deepest directory needed here is the
-	// leaf's parent.
-	parents := components[:len(components)-1]
-	tail := make([]string, 0, len(parents))
-	current := base
-	for _, component := range parents {
-		current = filepath.Join(current, component)
-		tail = append(tail, current)
-	}
-
-	// THE WHOLE ACQUISITION RETRIES, NOT JUST THE LOCK. A contender that waited
-	// on the lease while cleanup held it exclusively can wake on an object cleanup
-	// has since marked for deletion, and by then cleanup has compensated the tail
-	// this run created as well. Re-taking the lock alone would lock a fresh file
-	// beside a leaf that is gone, so a stale lock sends the caller back through
-	// tail creation. The ledger accumulates across attempts: a directory this run
-	// made on an earlier pass and cleanup did not remove is still this run's to
-	// compensate.
-	var created []windowsCreatedRuntimeDir
-	for attempt := 1; ; attempt++ {
-		made, parent, err := createRuntimeTailRetainingHandle(base, tail)
-		created = appendCreatedRuntimeDirs(created, made)
-		if err != nil {
-			if parent != 0 {
-				_ = windows.CloseHandle(parent)
-			}
-			return nil, created, err
-		}
-		handle, madeLease, err := acquireSharedRuntimeLeaseAt(parent, filepath.Base(sandboxRuntimeLeasePath(root)))
-		_ = windows.CloseHandle(parent)
-		if err == nil {
-			return &sandboxRuntimeLease{handle: handle, root: root, createdFile: madeLease}, created, nil
-		}
-		if errors.Is(err, errRuntimeLeaseReplaced) && attempt < runtimeLeaseAcquireAttempts {
-			continue
-		}
-		return nil, created, fmt.Errorf("acquire sandbox runtime lease: %w", err)
-	}
-}
-
-// acquireSharedRuntimeLeaseAt is acquireSharedRuntimeLease with the file named
-// relative to a directory handle instead of by full path.
-func acquireSharedRuntimeLeaseAt(parent windows.Handle, name string) (runtimeLeaseHandle, bool, error) {
-	objectName, err := windows.NewNTUnicodeString(name)
-	if err != nil {
-		return runtimeLeaseHandle{}, false, fmt.Errorf("encode sandbox runtime lease name %s: %w", name, err)
-	}
-	attributes := windows.OBJECT_ATTRIBUTES{
-		RootDirectory: parent,
-		ObjectName:    objectName,
-		Attributes:    windows.OBJ_CASE_INSENSITIVE,
-	}
-	attributes.Length = uint32(unsafe.Sizeof(attributes))
-
-	var handle windows.Handle
-	var iosb windows.IO_STATUS_BLOCK
-	// FILE_OPEN_IF because the lease is shared: whoever gets there first creates
-	// it. FILE_NON_DIRECTORY_FILE so a directory under that name is refused.
-	//
-	// FILE_OPEN_REPARSE_POINT IS NOT A REFUSAL. It says do not follow, so the call
-	// returns a handle to the LINK, and FILE_NON_DIRECTORY_FILE excludes
-	// directories rather than non-directory reparse objects. A file symbolic link
-	// planted at <digest>.lease was therefore opened and locked as if it were the
-	// lease. No-follow and classification are two requirements; the flag is only
-	// the first, and the handle is asked for the second below.
-	err = windows.NtCreateFile(
-		&handle,
-		windows.DELETE|windows.GENERIC_READ|windows.GENERIC_WRITE|windows.SYNCHRONIZE,
-		&attributes,
-		&iosb,
-		nil,
-		windows.FILE_ATTRIBUTE_NORMAL,
-		windows.FILE_SHARE_READ|windows.FILE_SHARE_WRITE|windows.FILE_SHARE_DELETE,
-		windows.FILE_OPEN_IF,
-		windows.FILE_NON_DIRECTORY_FILE|windows.FILE_SYNCHRONOUS_IO_NONALERT|windows.FILE_OPEN_REPARSE_POINT,
-		0,
-		0,
-	)
-	if err != nil {
-		return runtimeLeaseHandle{}, false, err
-	}
-	// FILE_CREATED here, rather than a Stat before the call, because only the
-	// create itself can distinguish the file it made from one that arrived a
-	// moment earlier.
-	created := iosb.Information == windowsFileCreatedDisposition
-	// And every failure below has already created the file when that is true,
-	// while returning no lease object to carry the fact, so each undoes its own
-	// creation instead of leaving an unrecorded artifact in a directory
-	// compensation then cannot empty.
-	if runtimeCreationFailure != nil {
-		if injected := runtimeCreationFailure(name); injected != nil {
-			injected = undoCreatedRuntimeLease(created, handle, injected)
-			_ = windows.CloseHandle(handle)
-			return runtimeLeaseHandle{}, false, injected
-		}
-	}
-	if err := refuseReparseRuntimeLeaseHandle(handle, name); err != nil {
-		err = undoCreatedRuntimeLease(created, handle, err)
-		_ = windows.CloseHandle(handle)
-		return runtimeLeaseHandle{}, false, err
-	}
-	file := os.NewFile(uintptr(handle), name)
-	if file == nil {
-		err := undoCreatedRuntimeLease(created, handle, fmt.Errorf("wrap the sandbox runtime lease handle for %s", name))
-		_ = windows.CloseHandle(handle)
-		return runtimeLeaseHandle{}, false, err
-	}
-	lease := runtimeLeaseHandle{file: file}
-	if err := windows.LockFileEx(windows.Handle(file.Fd()), 0, 0, 1, 0, &lease.overlapped); err != nil {
-		err = undoCreatedRuntimeLease(created, windows.Handle(file.Fd()), err)
-		_ = file.Close()
-		return runtimeLeaseHandle{}, false, err
-	}
-	// THE LOCK IS ON AN OBJECT, THE COORDINATION IS ON A NAME. LockFileEx waits
-	// while cleanup holds the exclusive lock, and cleanup marks the lease for
-	// deletion before it lets go; this handle was opened with FILE_SHARE_DELETE
-	// precisely so cleanup could. A waiter then locks a delete-pending object and
-	// returns a lease that coordinates with nobody: once this handle closes the
-	// name is free, the next cleanup creates a fresh file there, locks it
-	// unopposed, and reports the root unused while this holder is still running
-	// in it. So the lock is only a lease once the locked object is still what the
-	// name resolves to. Reported by @gnanam1990.
-	current, err := runtimeLeaseIsCurrentAt(parent, name, windows.Handle(file.Fd()))
-	if err != nil {
-		_ = file.Close()
-		return runtimeLeaseHandle{}, false, err
-	}
-	if !current {
-		// Not undone, whatever created says: the object is already going, and the
-		// name now belongs to whoever comes next. Closing this handle is what lets
-		// the name go so the retry can take a current one.
-		_ = file.Close()
-		return runtimeLeaseHandle{}, false, errRuntimeLeaseReplaced
-	}
-	return lease, created, nil
-}
-
-// runtimeLeaseIsCurrentAt reports whether the locked lease is still the object
-// the lease name resolves to under parent.
-//
-// Both answers come from handles: the locked file's own identity, and a fresh
-// no-follow open of the name relative to the retained parent, asked for nothing
-// but attributes. A name that is gone or delete-pending, or that resolves to a
-// different volume and file index, means the coordination moved on while this
-// call was waiting.
-func runtimeLeaseIsCurrentAt(parent windows.Handle, name string, locked windows.Handle) (bool, error) {
-	var lockedInfo windows.ByHandleFileInformation
-	if err := windows.GetFileInformationByHandle(locked, &lockedInfo); err != nil {
-		return false, fmt.Errorf("inspect the locked sandbox runtime lease %s: %w", name, err)
-	}
-	objectName, err := windows.NewNTUnicodeString(name)
-	if err != nil {
-		return false, fmt.Errorf("encode sandbox runtime lease name %s: %w", name, err)
-	}
-	attributes := windows.OBJECT_ATTRIBUTES{
-		RootDirectory: parent,
-		ObjectName:    objectName,
-		Attributes:    windows.OBJ_CASE_INSENSITIVE,
-	}
-	attributes.Length = uint32(unsafe.Sizeof(attributes))
-	var probe windows.Handle
-	var iosb windows.IO_STATUS_BLOCK
-	err = windows.NtCreateFile(
-		&probe,
-		windows.FILE_READ_ATTRIBUTES|windows.SYNCHRONIZE,
-		&attributes,
-		&iosb,
-		nil,
-		0,
-		windows.FILE_SHARE_READ|windows.FILE_SHARE_WRITE|windows.FILE_SHARE_DELETE,
-		windows.FILE_OPEN,
-		windows.FILE_NON_DIRECTORY_FILE|windows.FILE_SYNCHRONOUS_IO_NONALERT|windows.FILE_OPEN_REPARSE_POINT,
-		0,
-		0,
-	)
-	if err != nil {
-		if errors.Is(err, windows.STATUS_OBJECT_NAME_NOT_FOUND) || errors.Is(err, windows.STATUS_DELETE_PENDING) {
-			return false, nil
-		}
-		return false, fmt.Errorf("inspect the sandbox runtime lease entry %s: %w", name, err)
-	}
-	defer func() { _ = windows.CloseHandle(probe) }()
-	var entryInfo windows.ByHandleFileInformation
-	if err := windows.GetFileInformationByHandle(probe, &entryInfo); err != nil {
-		return false, fmt.Errorf("inspect the sandbox runtime lease entry %s: %w", name, err)
-	}
-	return lockedInfo.VolumeSerialNumber == entryInfo.VolumeSerialNumber &&
-		lockedInfo.FileIndexHigh == entryInfo.FileIndexHigh &&
-		lockedInfo.FileIndexLow == entryInfo.FileIndexLow, nil
-}
-
-// undoCreatedRuntimeLease removes a lease file this call created, when a later
-// step means no lease object will be returned to carry that fact.
-//
-// Through the handle, and only when this call is the one that created the file:
-// a lease that was already there belongs to whoever is holding it.
-func undoCreatedRuntimeLease(created bool, handle windows.Handle, cause error) error {
-	if !created {
-		return cause
-	}
-	if undoErr := removeWindowsObjectByHandle(handle); undoErr != nil {
-		return fmt.Errorf("%w; and the lease file this run created could not be removed: %w", cause, undoErr)
-	}
-	return cause
-}
-
-// refuseReparseRuntimeLeaseHandle proves the opened lease is an ordinary file.
-//
-// Asked of the HANDLE, not the name, so there is no second resolution for a
-// substitution to land in. This is the check the directory descent already makes
-// in openWindowsChildNoFollow; the lease carried the no-follow flag and not the
-// classification that has to go with it.
-func refuseReparseRuntimeLeaseHandle(handle windows.Handle, name string) error {
-	var info windows.ByHandleFileInformation
-	if err := windows.GetFileInformationByHandle(handle, &info); err != nil {
-		return fmt.Errorf("inspect the sandbox runtime lease %s: %w", name, err)
-	}
-	if info.FileAttributes&windows.FILE_ATTRIBUTE_REPARSE_POINT != 0 {
-		return fmt.Errorf("refusing to use the sandbox runtime lease at %s: a reparse point here means the shared and exclusive holders would lock different objects", name)
-	}
-	return nil
-}
-
-// openRuntimeLeaseParentRooted opens, and never creates, the directory holding
-// the lease for root.
-//
-// Cleanup's side of the contract. It descends the same owned components from the
-// same base with the same no-follow opens acquisition uses, so both sides resolve
-// the lease name relative to a verified directory rather than by pathname.
-//
-// It creates nothing. A runtime tree that is not there has no lease to take, and
-// cleanup rebuilding the tree in order to lock it would be inventing the thing it
-// is about to remove.
-func openRuntimeLeaseParentRooted(root string) (windows.Handle, error) {
-	base, components, owned := windowsSandboxRuntimeOwnedTail(root)
-	if !owned || len(components) == 0 {
-		return 0, fmt.Errorf("open the sandbox runtime lease parent for %s: %w", root, errRuntimeTailNotOwned)
-	}
-	parent, err := openWindowsDirectoryByName(base)
-	if err != nil {
-		return 0, fmt.Errorf("open sandbox runtime base %s: %w", base, err)
-	}
-	// The lease is a SIBLING of the leaf, so the deepest directory needed here is
-	// the leaf's parent, exactly as in acquisition.
-	for _, name := range components[:len(components)-1] {
-		child, openErr := openWindowsChildNoFollow(parent, name,
-			windows.FILE_READ_ATTRIBUTES|windows.FILE_TRAVERSE, windows.FILE_DIRECTORY_FILE)
-		if openErr != nil {
-			_ = windows.CloseHandle(parent)
-			return 0, openErr
-		}
-		_ = windows.CloseHandle(parent)
-		parent = child
-	}
-	return parent, nil
-}
-
-// tryAcquireExclusiveRuntimeLeaseRooted is cleanup's acquisition, resolved the
-// way acquisition resolves it.
-func tryAcquireExclusiveRuntimeLeaseRooted(root string) (runtimeLeaseHandle, bool, error) {
-	parent, err := openRuntimeLeaseParentRooted(root)
-	if err != nil {
-		return runtimeLeaseHandle{}, false, err
-	}
-	defer func() { _ = windows.CloseHandle(parent) }()
-
-	name := filepath.Base(sandboxRuntimeLeasePath(root))
-	objectName, err := windows.NewNTUnicodeString(name)
-	if err != nil {
-		return runtimeLeaseHandle{}, false, fmt.Errorf("encode sandbox runtime lease name %s: %w", name, err)
-	}
-	attributes := windows.OBJECT_ATTRIBUTES{
-		RootDirectory: parent,
-		ObjectName:    objectName,
-		Attributes:    windows.OBJ_CASE_INSENSITIVE,
-	}
-	attributes.Length = uint32(unsafe.Sizeof(attributes))
-
-	for attempt := 1; ; attempt++ {
-		var handle windows.Handle
-		var iosb windows.IO_STATUS_BLOCK
-		// FILE_OPEN_IF matches what cleanup did by pathname with O_CREATE: a
-		// runtime root whose lease file is gone is held by nobody, and creating the
-		// empty lease is how that is expressed.
-		err = windows.NtCreateFile(
-			&handle,
-			windows.DELETE|windows.GENERIC_READ|windows.GENERIC_WRITE|windows.SYNCHRONIZE,
-			&attributes,
-			&iosb,
-			nil,
-			windows.FILE_ATTRIBUTE_NORMAL,
-			windows.FILE_SHARE_READ|windows.FILE_SHARE_WRITE|windows.FILE_SHARE_DELETE,
-			windows.FILE_OPEN_IF,
-			windows.FILE_NON_DIRECTORY_FILE|windows.FILE_SYNCHRONOUS_IO_NONALERT|windows.FILE_OPEN_REPARSE_POINT,
-			0,
-			0,
-		)
-		if err != nil {
-			return runtimeLeaseHandle{}, false, err
-		}
-		created := iosb.Information == windowsFileCreatedDisposition
-		if err := refuseReparseRuntimeLeaseHandle(handle, name); err != nil {
-			err = undoCreatedRuntimeLease(created, handle, err)
-			_ = windows.CloseHandle(handle)
-			return runtimeLeaseHandle{}, false, err
-		}
-		file := os.NewFile(uintptr(handle), name)
-		if file == nil {
-			err := undoCreatedRuntimeLease(created, handle, fmt.Errorf("wrap the sandbox runtime lease handle for %s", name))
-			_ = windows.CloseHandle(handle)
-			return runtimeLeaseHandle{}, false, err
-		}
-		lease := runtimeLeaseHandle{file: file}
-		flags := uint32(windows.LOCKFILE_EXCLUSIVE_LOCK | windows.LOCKFILE_FAIL_IMMEDIATELY)
-		if err := windows.LockFileEx(windows.Handle(file.Fd()), flags, 0, 1, 0, &lease.overlapped); err != nil {
-			if errors.Is(err, windows.ERROR_LOCK_VIOLATION) {
-				// Somebody holds it, so it is not this call's to remove, whatever the
-				// open reported. A file this call created cannot reach here anyway.
-				_ = file.Close()
-				return runtimeLeaseHandle{}, true, nil
-			}
-			err = undoCreatedRuntimeLease(created, windows.Handle(file.Fd()), err)
-			_ = file.Close()
-			return runtimeLeaseHandle{}, false, err
-		}
-		// The exclusive side has the same window in miniature: between its open
-		// and its lock another cleanup can mark the object for deletion, and an
-		// exclusive lock on that object would then remove a tree a holder of the
-		// replacement is entering. Same check, same answer: only the current
-		// object counts, and a stale one is reopened rather than reasoned about.
-		current, err := runtimeLeaseIsCurrentAt(parent, name, windows.Handle(file.Fd()))
-		if err != nil {
-			_ = file.Close()
-			return runtimeLeaseHandle{}, false, err
-		}
-		if !current {
-			_ = file.Close()
-			if attempt < runtimeLeaseAcquireAttempts {
-				continue
-			}
-			return runtimeLeaseHandle{}, false, errRuntimeLeaseReplaced
-		}
-		return lease, false, nil
-	}
-}
-
-// windowsFileCreatedDisposition is the IO_STATUS_BLOCK Information value that
-// says NtCreateFile made the file rather than opening one that was there.
-// x/sys/windows does not export it.
-const windowsFileCreatedDisposition = 2
@@ -3,6 +3,7 @@
 package sandbox

 import (
+	"errors"
 	"os"

 	"golang.org/x/sys/unix"
@@ -12,15 +13,31 @@ type runtimeLeaseHandle struct {
 	file *os.File
 }

-// BOTH SIDES HAVE TO MEAN THE SAME OBJECT.
-//
-// Cleanup opened the lease by full pathname while acquisition opened it relative
-// to a verified parent, so a symlink at <digest>.lease gave the two of them
-// different files: a live command held a shared lock on one, cleanup took an
-// exclusive lock on the other, both succeeded, and cleanup went on to remove a
-// runtime root that was still in use.
-func tryAcquireExclusiveRuntimeLease(root string) (runtimeLeaseHandle, bool, error) {
-	return tryAcquireExclusiveRuntimeLeaseRootedUnix(root)
+func acquireSharedRuntimeLease(path string) (runtimeLeaseHandle, error) {
+	file, err := os.OpenFile(path, os.O_CREATE|os.O_RDWR, 0o600)
+	if err != nil {
+		return runtimeLeaseHandle{}, err
+	}
+	if err := unix.Flock(int(file.Fd()), unix.LOCK_SH); err != nil {
+		_ = file.Close()
+		return runtimeLeaseHandle{}, err
+	}
+	return runtimeLeaseHandle{file: file}, nil
+}
+
+func tryAcquireExclusiveRuntimeLease(path string) (runtimeLeaseHandle, bool, error) {
+	file, err := os.OpenFile(path, os.O_CREATE|os.O_RDWR, 0o600)
+	if err != nil {
+		return runtimeLeaseHandle{}, false, err
+	}
+	if err := unix.Flock(int(file.Fd()), unix.LOCK_EX|unix.LOCK_NB); err != nil {
+		_ = file.Close()
+		if errors.Is(err, unix.EWOULDBLOCK) || errors.Is(err, unix.EAGAIN) {
+			return runtimeLeaseHandle{}, true, nil
+		}
+		return runtimeLeaseHandle{}, false, err
+	}
+	return runtimeLeaseHandle{file: file}, false, nil
 }

 func (lease runtimeLeaseHandle) release() {
@@ -1,95 +0,0 @@
-package sandbox
-
-import (
-	"os"
-	"testing"
-)
-
-// A CONTENDER THAT WAITED OUT A CLEANUP HOLDS THE CURRENT LEASE, NOT THE OLD ONE.
-//
-// The schedule the exclusivity test does not reach. A command opens the lease
-// while cleanup holds it exclusively and waits for the shared lock. Cleanup
-// removes the lease name and the tree before it lets go. The waiter then wakes on
-// the old object: unlinked on POSIX, delete-pending on Windows. Locking it
-// succeeds, and the acquisition used to return that as a lease. Nothing else
-// could ever see it: the next cleanup created a fresh lease at the name, locked
-// it unopposed, and reported the root unused while the command was running in
-// it. The two locks each succeeded and coordinated nothing.
-//
-// Both seams are used as barriers, not as failures. The cleanup barrier is
-// where the contender is started, so its open lands while cleanup owns the lease;
-// the creation seam is installed there too, rather than up front, because
-// cleanup's own open consults it on POSIX and would otherwise signal before the
-// contender exists. Reported by @gnanam1990.
-func TestAContenderThatWaitedOutACleanupHoldsTheCurrentLease(t *testing.T) {
-	root, _, leasePath := runtimeTreeAwaitingCompensation(t)
-	// Only the leaf is this run's, the same shape as the exclusivity test: the
-	// contender's retained parent handle must survive compensation, which is a
-	// different race from the one under test.
-	created := createdRuntimeDirsForTest(root)
-
-	opened := make(chan struct{})
-	entered := make(chan struct{})
-	var contender *sandboxRuntimeLease
-	var contenderErr error
-	runtimeCleanupExclusivityBarrier = func() {
-		runtimeCreationFailure = func(string) error {
-			select {
-			case <-opened:
-			default:
-				close(opened)
-			}
-			return nil
-		}
-		go func() {
-			defer close(entered)
-			contender, _, contenderErr = acquireRuntimeLeaseForPlatform(root)
-		}()
-		// The contender now holds a handle on the lease cleanup is about to
-		// remove, and is waiting for the lock cleanup holds.
-		<-opened
-	}
-	t.Cleanup(func() {
-		runtimeCleanupExclusivityBarrier = nil
-		runtimeCreationFailure = nil
-	})
-
-	if err := undoWindowsSetupRuntimeCreation(root, created, leasePath, true); err != nil {
-		t.Fatalf("compensation failed: %v", err)
-	}
-	<-entered
-	runtimeCreationFailure = nil
-	if contenderErr != nil {
-		t.Fatalf("the contender could not take the lease after cleanup finished: %v", contenderErr)
-	}
-	if contender == nil {
-		t.Fatal("SETUP INVALID: the contender returned neither a lease nor an error")
-	}
-	defer contender.release()
-
-	// The proof is the next cleanup: if the contender holds the current object,
-	// cleanup finds the root in use. If it holds the object cleanup already
-	// removed, cleanup locks a fresh one and calls the root free.
-	second, inUse, err := tryAcquireSandboxRuntimeCleanupLease(root)
-	if err != nil {
-		t.Fatalf("a second cleanup could not take the lease at all, so the contender is holding an object the name no longer resolves to: %v", err)
-	}
-	if !inUse {
-		second.release()
-		t.Fatal("a second cleanup found the root unused while the contender holds a lease on it: the contender locked the object cleanup had already removed")
-	}
-	if _, statErr := os.Stat(leasePath); statErr != nil {
-		t.Errorf("the contender's lease is not at the lease name: %v", statErr)
-	}
-
-	// CONTROL: once the contender lets go, the root really is free.
-	contender.release()
-	third, inUse, err := tryAcquireSandboxRuntimeCleanupLease(root)
-	if err != nil {
-		t.Fatalf("cleanup after the contender released: %v", err)
-	}
-	if inUse {
-		t.Fatal("the root is still reported in use after the contender released")
-	}
-	third.release()
-}
@@ -3,6 +3,7 @@
 package sandbox

 import (
+	"errors"
 	"os"

 	"golang.org/x/sys/windows"
@@ -13,19 +14,34 @@ type runtimeLeaseHandle struct {
 	overlapped windows.Overlapped
 }

-// BOTH SIDES HAVE TO MEAN THE SAME OBJECT.
-//
-// This opened the lease by full pathname with os.OpenFile and no no-follow flag,
-// while shared acquisition opened it relative to a retained parent handle. A file
-// symbolic link at <digest>.lease therefore handed the two sides different
-// objects: setup or a running command held a shared lock on the link, cleanup
-// took an exclusive lock on its target, both calls succeeded, and cleanup went on
-// to RemoveAll a root somebody was still using.
-//
-// Mutual exclusion is a property of the OBJECT, so cleanup resolves the name the
-// way acquisition does and refuses the same reparse objects.
-func tryAcquireExclusiveRuntimeLease(root string) (runtimeLeaseHandle, bool, error) {
-	return tryAcquireExclusiveRuntimeLeaseRooted(root)
+func acquireSharedRuntimeLease(path string) (runtimeLeaseHandle, error) {
+	file, err := os.OpenFile(path, os.O_CREATE|os.O_RDWR, 0o600)
+	if err != nil {
+		return runtimeLeaseHandle{}, err
+	}
+	handle := runtimeLeaseHandle{file: file}
+	if err := windows.LockFileEx(windows.Handle(file.Fd()), 0, 0, 1, 0, &handle.overlapped); err != nil {
+		_ = file.Close()
+		return runtimeLeaseHandle{}, err
+	}
+	return handle, nil
+}
+
+func tryAcquireExclusiveRuntimeLease(path string) (runtimeLeaseHandle, bool, error) {
+	file, err := os.OpenFile(path, os.O_CREATE|os.O_RDWR, 0o600)
+	if err != nil {
+		return runtimeLeaseHandle{}, false, err
+	}
+	handle := runtimeLeaseHandle{file: file}
+	flags := uint32(windows.LOCKFILE_EXCLUSIVE_LOCK | windows.LOCKFILE_FAIL_IMMEDIATELY)
+	if err := windows.LockFileEx(windows.Handle(file.Fd()), flags, 0, 1, 0, &handle.overlapped); err != nil {
+		_ = file.Close()
+		if errors.Is(err, windows.ERROR_LOCK_VIOLATION) {
+			return runtimeLeaseHandle{}, true, nil
+		}
+		return runtimeLeaseHandle{}, false, err
+	}
+	return handle, false, nil
 }

 func (lease runtimeLeaseHandle) release() {
@@ -1,49 +0,0 @@
-package sandbox
-
-import (
-	"path/filepath"
-	"testing"
-)
-
-// THE SHAPE HAS TO HOLD FOR BOTH SPELLINGS OF THE FIRST COMPONENT.
-//
-// The cache-derived root uses the fixed name; the temp-derived fallback scopes
-// that component to the user wherever the temp root is shared. A tail that
-// stops matching loses the rooted no-follow traversal AND the owned-component
-// guard, and neither failure is visible where it happens.
-//
-// The scoped spelling does not exist on Windows, so without the seam this
-// branch could only ever be exercised by another platform's CI.
-func TestOwnedTailAcceptsBothFirstComponentSpellings(t *testing.T) {
-	previous := fallbackOwnedNamesForMatch
-	t.Cleanup(func() { fallbackOwnedNamesForMatch = previous })
-	fallbackOwnedNamesForMatch = func() []string {
-		return []string{windowsSandboxRuntimeOwnedNames[0] + "-u1001", "runtime", "v1"}
-	}
-
-	base := filepath.Join("C:", "shared")
-	for _, testCase := range []struct {
-		name  string
-		first string
-		want  bool
-	}{
-		{"fixed cache spelling", windowsSandboxRuntimeOwnedNames[0], true},
-		{"user-scoped fallback spelling", windowsSandboxRuntimeOwnedNames[0] + "-u1001", true},
-		{"a different user's scope", windowsSandboxRuntimeOwnedNames[0] + "-u2002", false},
-		{"an unrelated directory", "notzero", false},
-	} {
-		t.Run(testCase.name, func(t *testing.T) {
-			root := filepath.Join(base, testCase.first, "runtime", "v1", "abcdef0123456789")
-			if _, _, ok := windowsSandboxRuntimeOwnedTail(root); ok != testCase.want {
-				t.Errorf("owned tail for %s = %v, want %v", root, ok, testCase.want)
-			}
-		})
-	}
-
-	// The components below the first stay fixed for both spellings, or the
-	// traversal would accept a tree Zero does not own.
-	wrong := filepath.Join(base, windowsSandboxRuntimeOwnedNames[0]+"-u1001", "elsewhere", "v1", "abcdef0123456789")
-	if _, _, ok := windowsSandboxRuntimeOwnedTail(wrong); ok {
-		t.Errorf("owned tail accepted %s, whose middle component is not one Zero owns", wrong)
-	}
-}
@@ -1,15 +0,0 @@
-//go:build !windows
-
-package sandbox
-
-// physicalSandboxPath resolves path to the spelling the filesystem itself uses.
-//
-// Off Windows that is what canonicalSandboxWorkspaceRoot already does:
-// filepath.EvalSymlinks follows every symlink, and there is no junction to
-// follow. Two aliases remain unresolved and are handled by the identity walk in
-// runtimeRootWithinWorkspace instead: a differing case on a case-insensitive
-// volume, which EvalSymlinks preserves, and a bind mount, which no userspace
-// path API resolves because the kernel deliberately presents it as a real path.
-func physicalSandboxPath(path string) string {
-	return canonicalSandboxWorkspaceRoot(path)
-}
@@ -1,116 +0,0 @@
-//go:build windows
-
-package sandbox
-
-import (
-	"path/filepath"
-	"strings"
-
-	"golang.org/x/sys/windows"
-)
-
-const (
-	// GetFinalPathNameByHandle flags. golang.org/x/sys/windows does not export
-	// these; both are zero, and naming them keeps the call site readable.
-	fileNameNormalized = 0x0
-	volumeNameDOS      = 0x0
-)
-
-// physicalSandboxPath resolves path to the spelling the filesystem itself uses.
-//
-// canonicalSandboxWorkspaceRoot cannot do this on Windows. filepath.EvalSymlinks
-// returns a directory JUNCTION unchanged, so a TEMP that reaches into the
-// workspace through one still measures as outside it, and that is how a runtime
-// tree ends up inside the very workspace the sandbox exists to confine.
-// GetFinalPathNameByHandle answers with the target's real path, junctions and
-// mount points followed and casing as stored on disk.
-//
-// This deliberately opens WITHOUT FILE_FLAG_OPEN_REPARSE_POINT, the opposite of
-// openWindowsACLTarget. That helper must refuse to follow a reparse point,
-// because following one is the path-swap it guards against. Here the whole
-// question is where the reparse point leads, and the answer is only ever used to
-// decide that a runtime root is contained, never that it is safe.
-func physicalSandboxPath(path string) string {
-	cleaned := canonicalSandboxWorkspaceRoot(path)
-	if cleaned == "" || cleaned == "." {
-		return cleaned
-	}
-	// The runtime root does not exist yet at derivation time, which is the point.
-	// Resolve the deepest ancestor that does exist and re-append the rest, the
-	// same shape canonicalSandboxWorkspaceRoot uses for EvalSymlinks.
-	remainder := ""
-	current := cleaned
-	for {
-		if resolved, ok := finalWindowsPathName(current); ok {
-			if remainder == "" {
-				return resolved
-			}
-			return filepath.Join(resolved, remainder)
-		}
-		parent := filepath.Dir(current)
-		if parent == current {
-			// Nothing along the path could be opened. The cleaned form is the best
-			// answer available, and the caller's spelling comparison already ran.
-			return cleaned
-		}
-		remainder = filepath.Join(filepath.Base(current), remainder)
-		current = parent
-	}
-}
-
-func finalWindowsPathName(path string) (string, bool) {
-	utf16Path, err := windows.UTF16PtrFromString(path)
-	if err != nil {
-		return "", false
-	}
-	// Zero desired access is enough: GetFinalPathNameByHandle reads metadata, so
-	// this cannot be refused for lack of read rights on the directory contents.
-	handle, err := windows.CreateFile(
-		utf16Path,
-		0,
-		windows.FILE_SHARE_READ|windows.FILE_SHARE_WRITE|windows.FILE_SHARE_DELETE,
-		nil,
-		windows.OPEN_EXISTING,
-		windows.FILE_FLAG_BACKUP_SEMANTICS,
-		0,
-	)
-	if err != nil {
-		return "", false
-	}
-	defer func() { _ = windows.CloseHandle(handle) }()
-
-	buffer := make([]uint16, windows.MAX_PATH)
-	for attempt := 0; attempt < 2; attempt++ {
-		n, err := windows.GetFinalPathNameByHandle(handle, &buffer[0], uint32(len(buffer)), fileNameNormalized|volumeNameDOS)
-		if err != nil {
-			return "", false
-		}
-		if int(n) >= len(buffer) {
-			// TWO DIFFERENT CONVENTIONS, and the boundary is where they meet. On
-			// success the return value EXCLUDES the terminating null; on an
-			// insufficient buffer it INCLUDES it. So n == len(buffer) cannot be
-			// read as a complete path: a success that large would not have fitted
-			// its own terminator, which means the only reading left is a required
-			// size. Retrying on >= costs one extra call in a case that may not be
-			// reachable and removes the need to be right about which it was.
-			buffer = make([]uint16, int(n)+1)
-			continue
-		}
-		if n == 0 {
-			return "", false
-		}
-		return trimWindowsExtendedPrefix(windows.UTF16ToString(buffer[:n])), true
-	}
-	return "", false
-}
-
-// trimWindowsExtendedPrefix converts the \\?\ form GetFinalPathNameByHandle
-// returns into an ordinary path, so it compares against paths the rest of this
-// package builds with filepath.Join. \\?\UNC\server\share becomes
-// \\server\share; anything else loses the \\?\ and keeps its drive letter.
-func trimWindowsExtendedPrefix(path string) string {
-	if rest, ok := strings.CutPrefix(path, `\\?\UNC\`); ok {
-		return `\\` + rest
-	}
-	return strings.TrimPrefix(path, `\\?\`)
-}
@@ -1,208 +0,0 @@
-//go:build windows
-
-package sandbox
-
-import (
-	"os"
-	"path/filepath"
-	"testing"
-)
-
-// THE CREATION ITSELF HAS TO ESTABLISH THE IDENTITY.
-//
-// Win32 CreateFile cannot create a directory whatever disposition it is given,
-// so the CREATE_NEW attempt fell through to os.Mkdir plus a separate reopen on
-// EVERY real creation and the documented creation-handle contract never once
-// held. The runtime parent belongs to the ordinary user, who can rename the new
-// directory A away and drop an ordinary directory B at the predictable name in
-// between, and the ledger then records B for a directory this run never made.
-//
-// Driving the interleaving is not the point, and a barrier there would only
-// prove the window was still measurable. The property is that no reopen exists
-// to be raced: whatever the name resolves to afterwards, the recorded identity
-// is the object the create returned.
-func TestCreatedRuntimeDirIdentityComesFromTheCreation(t *testing.T) {
-	root := t.TempDir()
-	path := filepath.Join(root, "created")
-
-	// THE DETERMINISTIC BARRIER. The old shape resolved the pathname a second
-	// time after os.Mkdir, and that reopen is the whole window. Counting the
-	// resolutions turns "the creation establishes the identity" into something
-	// checked: on Windows the create returns the handle, so this must be zero.
-	resolutions := 0
-	restore := runtimeIdentityAfterCreate
-	runtimeIdentityAfterCreate = func(p string) (string, bool) {
-		resolutions++
-		return restore(p)
-	}
-	t.Cleanup(func() { runtimeIdentityAfterCreate = restore })
-
-	identity, identified, err := createRuntimeDirIdentified(path)
-	if err != nil || !identified {
-		t.Fatalf("create: identity=%q identified=%v err=%v", identity, identified, err)
-	}
-	if resolutions != 0 {
-		t.Errorf("the creation resolved the pathname again %d time(s); identity must come from the creation handle", resolutions)
-	}
-	info, statErr := os.Stat(path)
-	if statErr != nil || !info.IsDir() {
-		t.Fatalf("no directory was created: err=%v", statErr)
-	}
-
-	// Substitute the whole directory the way the parent's owner could, then ask
-	// what the name says now. The record must still describe what was created.
-	aside := filepath.Join(root, "moved-aside")
-	if err := os.Rename(path, aside); err != nil {
-		t.Skipf("cannot rename the runtime directory on this filesystem: %v", err)
-	}
-	if err := os.Mkdir(path, 0o700); err != nil {
-		t.Fatal(err)
-	}
-
-	substitute, ok := runtimeDirIdentity(path)
-	if !ok {
-		t.Fatal("the substitute could not be identified, so the comparison proves nothing")
-	}
-	if identity == substitute {
-		t.Fatal("SETUP INVALID: the substitute has the same identity as the created directory")
-	}
-	moved, ok := runtimeDirIdentity(aside)
-	if !ok {
-		t.Fatal("the created directory could not be identified after the rename")
-	}
-	if identity != moved {
-		t.Errorf("the recorded identity %q describes neither the created directory (%q) nor anything this run owns", identity, moved)
-	}
-}
-
-// A component another process created first must stay "not ours", and it must
-// keep saying so through os.IsExist, which is what the caller asks and which
-// does not unwrap a %w chain.
-func TestCreatedRuntimeDirRefusesAnExistingName(t *testing.T) {
-	path := filepath.Join(t.TempDir(), "created")
-	if _, _, err := createRuntimeDirIdentified(path); err != nil {
-		t.Fatalf("first create: %v", err)
-	}
-	identity, identified, err := createRuntimeDirIdentified(path)
-	if !os.IsExist(err) {
-		t.Errorf("creating over an existing directory returned %v, want an IsExist error", err)
-	}
-	if identified || identity != "" {
-		t.Errorf("a refused create still produced an ownership record: identity=%q identified=%v", identity, identified)
-	}
-}
-
-// THE THREE STATES ARE DIFFERENT FACTS.
-//
-// "Read it and there was nothing" and "could not read it" both used to arrive as
-// existed=false. The stamp writer uses FILE_OVERWRITE_IF and can replace an
-// existing stamp even where the read was denied, so that lie let a setup which
-// then FAILED delete an attestation it had no record of, leaving the previous
-// run's marker pointing at a runtime root it can no longer prove.
-func TestRuntimeStampSnapshotSeparatesAbsentPresentAndUnknown(t *testing.T) {
-	t.Run("absent", func(t *testing.T) {
-		root := t.TempDir()
-		_, _, prior, state, err := snapshotRuntimeStampBound(root, windowsSandboxRuntimeStampName(testStampPlanHash))
-		if err != nil {
-			t.Fatalf("a readable root with no stamp is not an error: %v", err)
-		}
-		if state != runtimeStampAbsent {
-			t.Errorf("state = %v, want absent", state)
-		}
-		if prior != nil {
-			t.Errorf("absent produced prior bytes %q", prior)
-		}
-	})
-
-	t.Run("present", func(t *testing.T) {
-		root := t.TempDir()
-		want := []byte("prior-attestation")
-		if err := os.WriteFile(filepath.Join(root, windowsSandboxRuntimeStampName(testStampPlanHash)), want, 0o600); err != nil {
-			t.Fatal(err)
-		}
-		_, _, prior, state, err := snapshotRuntimeStampBound(root, windowsSandboxRuntimeStampName(testStampPlanHash))
-		if err != nil {
-			t.Fatalf("snapshot: %v", err)
-		}
-		if state != runtimeStampPresent {
-			t.Errorf("state = %v, want present", state)
-		}
-		if string(prior) != string(want) {
-			t.Errorf("prior = %q, want %q", prior, want)
-		}
-	})
-
-	// A stamp NAME that cannot be read as a file. The child open is refused for a
-	// reason that is emphatically not "not found", which is the whole distinction.
-	t.Run("unknown", func(t *testing.T) {
-		root := t.TempDir()
-		if err := os.Mkdir(filepath.Join(root, windowsSandboxRuntimeStampName(testStampPlanHash)), 0o700); err != nil {
-			t.Fatal(err)
-		}
-		_, _, prior, state, err := snapshotRuntimeStampBound(root, windowsSandboxRuntimeStampName(testStampPlanHash))
-		if err == nil {
-			t.Fatal("an unreadable stamp was reported as a successful snapshot")
-		}
-		if state != runtimeStampUnknown {
-			t.Errorf("state = %v, want unknown", state)
-		}
-		if prior != nil {
-			t.Errorf("unknown produced prior bytes %q", prior)
-		}
-	})
-}
-
-// AND UNKNOWN MUST NOT AUTHORIZE COMPENSATION.
-//
-// The forward mutation is refused before it begins, so this asserts the second
-// half: a record that somehow reached compensation with an unproven prior state
-// leaves the current stamp alone and says so, rather than deleting it and
-// returning with nothing to put back.
-func TestUnknownPriorStampIsNeverCompensated(t *testing.T) {
-	root := t.TempDir()
-	stampPath := filepath.Join(root, windowsSandboxRuntimeStampName(testStampPlanHash))
-	current := []byte("the-attestation-of-the-previous-successful-setup")
-	if err := os.WriteFile(stampPath, current, 0o600); err != nil {
-		t.Fatal(err)
-	}
-	identity, identified := runtimeDirIdentity(root)
-	if !identified {
-		t.Fatal("SETUP INVALID: the runtime root could not be identified")
-	}
-
-	snapshot := windowsSandboxStampSnapshot{
-		path:           stampPath,
-		priorState:     runtimeStampUnknown,
-		root:           root,
-		rootIdentity:   identity,
-		rootIdentified: true,
-	}
-	err := snapshot.restore()
-	if err == nil {
-		t.Fatal("compensation acted on a prior state it never established, and reported success")
-	}
-
-	after, readErr := os.ReadFile(stampPath)
-	if readErr != nil {
-		t.Fatalf("the existing stamp was destroyed by a rollback that had nothing to restore: %v", readErr)
-	}
-	if string(after) != string(current) {
-		t.Errorf("the existing stamp was rewritten: got %q, want %q", after, current)
-	}
-}
-
-// And setup refuses BEFORE the ACL and stamp are applied, which is the half that
-// keeps the writer from running at all.
-func TestSetupRefusesAnUnreadablePriorStamp(t *testing.T) {
-	root := t.TempDir()
-	if err := os.Mkdir(filepath.Join(root, windowsSandboxRuntimeStampName(testStampPlanHash)), 0o700); err != nil {
-		t.Fatal(err)
-	}
-	snapshot, err := snapshotWindowsSandboxRuntimeStamp(root, testStampPlanHash)
-	if err == nil {
-		t.Fatal("setup accepted a snapshot that could not read the prior stamp")
-	}
-	if snapshot.priorState != runtimeStampUnknown {
-		t.Errorf("the refused snapshot carried state %v, want unknown", snapshot.priorState)
-	}
-}
@@ -1,134 +0,0 @@
-package sandbox
-
-import (
-	"os"
-	"path/filepath"
-	"testing"
-)
-
-// A RECORDED ROOT IS HISTORY. IT MUST NOT BE RE-DERIVED FROM TODAY'S TEMP.
-//
-// Setup reaches the temp fallback when the preferred cache-derived root cannot
-// be leased, and it records that concrete root in the marker. Command selection
-// then recognised the record only by deriving today's fallback from this
-// process's TEMP and comparing. A later IDE, service or terminal running with a
-// different TEMP therefore matched neither of today's candidates: the record was
-// ignored, selection produced a tree setup never provisioned, and the runner
-// rejected the marker as out of date. Re-running setup from the original
-// environment repeats the original answer and does not make the other
-// environment converge.
-//
-// The record is now recognised by its own shape instead, which is the question
-// that was actually being asked: does this belong to this workspace and home.
-//
-// Not gated on Windows. The marker only exists where setup wrote one, so this is
-// Windows-only in practice, but keeping it platform-neutral means the
-// setup-to-command contract runs on every CI runner rather than only one.
-func TestARecordedFallbackSurvivesATempChange(t *testing.T) {
-	home := t.TempDir()
-	workspace := canonicalSandboxWorkspaceRoot(t.TempDir())
-	// The cache is an input of the preferred candidate, so it is owned as well.
-	// Left alone it made the result depend on the machine, and gave this test two
-	// more ways to skip instead of asserting.
-	cacheRoot := t.TempDir()
-	originalCacheDir := sandboxUserCacheDir
-	sandboxUserCacheDir = func() (string, error) { return cacheRoot, nil }
-	t.Cleanup(func() { sandboxUserCacheDir = originalCacheDir })
-
-	// EVERY temp variable, in both phases. With only TMP and TEMP set this
-	// changed nothing on Unix: the fallback derived from the ambient TMPDIR, the
-	// MkdirAll below created a workspace-digest tree outside anything this test
-	// owns, the second derivation came back identical, and the test skipped
-	// having asserted nothing and left the directory behind.
-	tempA := t.TempDir()
-	redirectSandboxTestTemp(t, tempA)
-	recorded, err := fallbackSandboxRuntimeRoot(workspace)
-	if err != nil {
-		t.Fatalf("SETUP INVALID: no fallback runtime root under a temp this test owns: %v", err)
-	}
-	// Containment BEFORE the tree exists, so cleanup belongs to tempA and happens
-	// even when an assertion below fails.
-	requireWithinTestOwned(t, recorded, tempA)
-	if err := os.MkdirAll(recorded, 0o700); err != nil {
-		t.Fatal(err)
-	}
-	writeRecordedRoot(t, home, recorded)
-
-	// SETUP: with the original TEMP the record is honoured, so a failure after the
-	// flip is about the flip and not about the record being unusable.
-	preferred := preferredRuntimeRootFor(t, workspace)
-	if got := pinnedSandboxRuntimeRoot(workspace, preferred, recorded, home); got != recorded {
-		t.Fatalf("SETUP INVALID: the record is not honoured even before the temp change: got %q, want %q", got, recorded)
-	}
-
-	// The same command, run later from an environment with a different TEMP.
-	tempB := t.TempDir()
-	redirectSandboxTestTemp(t, tempB)
-	derivedNow, err := fallbackSandboxRuntimeRoot(workspace)
-	if err != nil {
-		t.Fatalf("SETUP INVALID: no fallback runtime root under the second temp: %v", err)
-	}
-	requireWithinTestOwned(t, derivedNow, tempB)
-	// A FAILURE, NOT A SKIP. The redirect owns every input the derivation reads,
-	// so an unmoved fallback means the premise of this test is broken, and a skip
-	// here is how it passed on Unix without ever reaching the assertion below.
-	if sameWindowsRuntimeRootPath(derivedNow, recorded) {
-		t.Fatalf("SETUP INVALID: the temp change did not move the derived fallback (%s), so the assertion below would prove nothing", derivedNow)
-	}
-
-	if got := pinnedSandboxRuntimeRoot(workspace, preferred, derivedNow, home); got != recorded {
-		t.Fatalf("selection abandoned the root setup provisioned after only TEMP changed: got %q, want the recorded %q", got, recorded)
-	}
-}
-
-// preferredRuntimeRootFor is the cache-derived candidate, which must stay a
-// candidate: this change is about the fallback only.
-func preferredRuntimeRootFor(t *testing.T, workspace string) string {
-	t.Helper()
-	cacheRoot, err := sandboxUserCacheDir()
-	if err != nil {
-		t.Fatalf("SETUP INVALID: no user cache directory: %v", err)
-	}
-	root, err := sandboxRuntimeRootFor(workspace, canonicalSandboxWorkspaceRoot(cacheRoot))
-	if err != nil {
-		t.Fatalf("SETUP INVALID: no preferred runtime root under a cache this test owns: %v", err)
-	}
-	return root
-}
-
-// AND THE PROTECTION THAT MADE THE OLD COMPARISON WORTH HAVING STAYS.
-//
-// Recognising the record by shape must not turn into recognising anybody's
-// record. A fallback root belonging to a DIFFERENT workspace has the same owned
-// shape and a different digest, so it is still refused.
-func TestARecordedFallbackForAnotherWorkspaceIsStillRefused(t *testing.T) {
-	home := t.TempDir()
-	mine := canonicalSandboxWorkspaceRoot(t.TempDir())
-	theirs := canonicalSandboxWorkspaceRoot(t.TempDir())
-
-	temp := t.TempDir()
-	redirectSandboxTestTemp(t, temp)
-
-	theirRoot, err := fallbackSandboxRuntimeRoot(theirs)
-	if err != nil {
-		t.Fatalf("SETUP INVALID: no fallback runtime root under a temp this test owns: %v", err)
-	}
-	requireWithinTestOwned(t, theirRoot, temp)
-	writeRecordedRoot(t, home, theirRoot)
-
-	if got := pinnedSandboxRuntimeRoot(mine, filepath.Join(t.TempDir(), "preferred"), filepath.Join(t.TempDir(), "fallback"), home); got != "" {
-		t.Fatalf("pinned %q, which was provisioned for another workspace", got)
-	}
-}
-
-// A record with no owned runtime shape at all is refused too, so the shape test
-// is doing work rather than waving anything through.
-func TestARecordedRootWithoutTheOwnedShapeIsRefused(t *testing.T) {
-	home := t.TempDir()
-	workspace := canonicalSandboxWorkspaceRoot(t.TempDir())
-	writeRecordedRoot(t, home, filepath.Join(t.TempDir(), "not-a-runtime-root"))
-
-	if got := pinnedSandboxRuntimeRoot(workspace, filepath.Join(t.TempDir(), "preferred"), filepath.Join(t.TempDir(), "fallback"), home); got != "" {
-		t.Fatalf("pinned %q, which has none of the owned runtime shape", got)
-	}
-}
@@ -1,115 +0,0 @@
-//go:build windows
-
-package sandbox
-
-import (
-	"os"
-	"os/exec"
-	"path/filepath"
-	"strings"
-	"testing"
-)
-
-// EXISTENCE MUST NOT CHOOSE THE TRUST BOUNDARY.
-//
-// createRuntimeDirRecording used to os.Stat its way to the deepest component
-// that already existed and open THAT by name. On the ordinary second-workspace
-// shape <base>\zero\runtime\v1 is already there and only the digest is missing,
-// so the single by-name open was v1 — one of the predictable, user-owned
-// components the rooted traversal exists to protect, not the cache directory
-// above the tail.
-//
-// The race that opens: the pre-check accepts an ordinary v1, os.Stat selects it,
-// the local owner replaces it with a junction, the by-name open follows it, and
-// elevated setup creates the digest beneath the redirected target. Putting the
-// original back makes the post-check see an ordinary pathname again, so the
-// creation record names the redirected object under the original path and
-// compensation leaves a privileged creation as residue.
-//
-// This drives the PRODUCTION entry point rather than the descent helper, because
-// the helper is handed a base that has already been chosen — which is precisely
-// the decision under test. A junction needs no privilege on Windows, so this
-// runs unelevated.
-func TestRuntimeRecordingOpensTheFixedBaseNotTheDeepestExistingComponent(t *testing.T) {
-	cacheBase := t.TempDir()
-	target := t.TempDir()
-
-	// The shape a second workspace finds: everything above the digest already
-	// exists, so the old walk would have picked v1 as its by-name base.
-	v1 := filepath.Join(cacheBase, "zero", "runtime", "v1")
-	if err := os.MkdirAll(v1, 0o755); err != nil {
-		t.Fatal(err)
-	}
-	root := filepath.Join(v1, "deadbeefdeadbeef")
-
-	// Sanity: the helper agrees the fixed base is the cache directory, not v1.
-	base, components, owned := windowsSandboxRuntimeOwnedTail(root)
-	if !owned || base != cacheBase {
-		t.Fatalf("SETUP INVALID: owned tail resolved base=%q owned=%v, want %q", base, owned, cacheBase)
-	}
-	if len(components) == 0 {
-		t.Fatal("SETUP INVALID: the owned tail has no components")
-	}
-
-	// THE PROPERTY, OBSERVED DIRECTLY. Whether a particular swap is caught depends
-	// on where the barrier sits; which path is opened by name does not, and that is
-	// the finding. Recorded here so the assertion cannot pass for an unrelated
-	// reason.
-	var openedByName []string
-	previousOpen := runtimeBaseOpenedByName
-	runtimeBaseOpenedByName = func(path string) { openedByName = append(openedByName, path) }
-	t.Cleanup(func() { runtimeBaseOpenedByName = previousOpen })
-
-	swapped := false
-	previous := runtimeDescentBarrier
-	runtimeDescentBarrier = func() {
-		// Fires after the base has been opened and before the first owned
-		// component is touched: exactly the interval the old walk was vulnerable
-		// in. Replace v1 with a junction into a directory the test watches.
-		if err := os.Remove(v1); err != nil {
-			t.Fatalf("SETUP INVALID: could not clear v1 to plant the junction: %v", err)
-		}
-		out, err := exec.Command("cmd", "/c", "mklink", "/J", v1, target).CombinedOutput()
-		if err != nil {
-			t.Fatalf("SETUP INVALID: mklink /J: %v\n%s", err, out)
-		}
-		swapped = true
-	}
-	t.Cleanup(func() { runtimeDescentBarrier = previous })
-
-	created, err := createRuntimeDirRecording(root)
-
-	if !swapped {
-		t.Fatal("SETUP INVALID: the barrier never ran, so no swap was attempted")
-	}
-	// Exactly one by-name open, and it is the cache directory above the tail.
-	// Under the old walk this was v1, a component the local user controls.
-	if len(openedByName) != 1 {
-		t.Fatalf("the descent opened %d paths by name, want exactly 1: %v", len(openedByName), openedByName)
-	}
-	if !strings.EqualFold(openedByName[0], cacheBase) {
-		t.Fatalf("opened %q by name, want the fixed base %q: existence must not choose the trust boundary", openedByName[0], cacheBase)
-	}
-	if err == nil {
-		t.Fatalf("the recording walked through a junction at an owned component and reported success: created=%v", created)
-	}
-	// The redirected target must be untouched: no digest created beneath it.
-	entries, readErr := os.ReadDir(target)
-	if readErr != nil {
-		t.Fatal(readErr)
-	}
-	if len(entries) != 0 {
-		names := make([]string, 0, len(entries))
-		for _, entry := range entries {
-			names = append(names, entry.Name())
-		}
-		t.Errorf("elevated setup created %v beneath the redirected target", names)
-	}
-	// And nothing may be recorded as ours under a pathname that now names
-	// somebody else's object, since that is what compensation acts on.
-	for _, record := range created {
-		if strings.HasPrefix(strings.ToLower(record.path), strings.ToLower(v1)) {
-			t.Errorf("a redirected creation was recorded as ours: %+v", record)
-		}
-	}
-}
@@ -1,180 +0,0 @@
-package sandbox
-
-import (
-	"os"
-	"os/exec"
-	"path/filepath"
-	"runtime"
-	"strings"
-	"testing"
-)
-
-// aliasTo returns a second path that IS target, spelled in a way
-// canonicalSandboxWorkspaceRoot does not fold, or "" when this platform offers
-// none.
-//
-// A plain symlink is no good: EvalSymlinks folds it, so the spelling comparison
-// already wins and nothing downstream is exercised. The two aliases that survive
-// canonicalization are a Windows directory junction, which needs no privilege,
-// and an upper-cased spelling on a case-insensitive volume, which is the macOS
-// default.
-func aliasTo(t *testing.T, target string) string {
-	t.Helper()
-
-	if runtime.GOOS == "windows" {
-		link := filepath.Join(t.TempDir(), "alias")
-		if out, err := exec.Command("cmd", "/c", "mklink", "/J", link, target).CombinedOutput(); err != nil {
-			t.Logf("mklink /J unavailable: %v %s", err, out)
-			return ""
-		}
-		// A junction that did not actually land on the target proves nothing.
-		targetInfo, err := os.Stat(target)
-		if err != nil {
-			t.Fatalf("stat target: %v", err)
-		}
-		linkInfo, err := os.Stat(link)
-		if err != nil || !os.SameFile(targetInfo, linkInfo) {
-			t.Fatalf("mklink reported success but %s is not %s", link, target)
-		}
-		return link
-	}
-
-	upper := strings.ToUpper(target)
-	if upper == target {
-		return ""
-	}
-	targetInfo, err := os.Stat(target)
-	if err != nil {
-		t.Fatalf("stat target: %v", err)
-	}
-	upperInfo, err := os.Stat(upper)
-	if err != nil || !os.SameFile(targetInfo, upperInfo) {
-		// Case-sensitive volume: the upper-cased name is a different directory, or
-		// none at all. Correctly not an alias.
-		return ""
-	}
-	return upper
-}
-
-// TestRuntimeRootRefusesAWorkspaceReachedByAnAlias is the regression for the gap
-// the macOS Smoke run exposed and the junction gap found alongside it.
-// canonicalSandboxWorkspaceRoot folds the aliases EvalSymlinks folds and no
-// others, so a runtime root that reaches the workspace under a junction, or under
-// a different casing on a case-insensitive volume, measured as OUTSIDE the
-// workspace and the runtime tree was allowed to live inside the tree the sandbox
-// exists to confine.
-//
-// Both alias shapes are covered on purpose. An alias whose target IS the
-// workspace root is caught by the identity walk; an alias into a SUBDIRECTORY is
-// not, because the walk climbs a spelling and a junction has no spelling chain
-// back into its target's parent. Only the physical-path resolution catches that
-// one, and a test that exercised the root shape alone reported green while it was
-// broken.
-func TestRuntimeRootRefusesAWorkspaceReachedByAnAlias(t *testing.T) {
-	for _, shape := range []struct {
-		name   string
-		suffix []string
-	}{
-		{name: "alias to the workspace root"},
-		{name: "alias into a workspace subdirectory", suffix: []string{"build", "tmp"}},
-	} {
-		t.Run(shape.name, func(t *testing.T) {
-			workspaceRoot := canonicalSandboxWorkspaceRoot(filepath.Join(t.TempDir(), "workspace"))
-			target := filepath.Join(append([]string{workspaceRoot}, shape.suffix...)...)
-			if err := os.MkdirAll(target, 0o700); err != nil {
-				t.Fatalf("create target: %v", err)
-			}
-
-			alias := aliasTo(t, target)
-			if alias == "" {
-				t.Skip("no alias spelling is constructible here")
-			}
-
-			// Precondition, asserted rather than assumed: the plain spelling
-			// comparison has to MISS. If canonicalization folds this alias the old
-			// code already handled it and the test proves nothing.
-			probe := filepath.Join(canonicalSandboxWorkspaceRoot(alias), "zero", "runtime", "v1", "0123456789abcdef")
-			if pathWithinRoot(workspaceRoot, probe) {
-				t.Skipf("canonicalization already folds %s into %s", alias, workspaceRoot)
-			}
-
-			t.Setenv("TMP", alias)
-			t.Setenv("TEMP", alias)
-			t.Setenv("TMPDIR", alias)
-
-			root, err := fallbackSandboxRuntimeRoot(workspaceRoot)
-			if err == nil {
-				t.Fatalf("fallback returned runtime root %s for a TEMP that reaches %s through %s; the sandbox would keep its own cache inside the tree it is confining", root, target, alias)
-			}
-			if !strings.Contains(err.Error(), "inside workspace") {
-				t.Fatalf("fallback refused for the wrong reason: %v", err)
-			}
-		})
-	}
-}
-
-// TestDeterministicRuntimeRootRejectsAnAliasedCache covers the other call site.
-// Reverting only deterministicSandboxRuntimeRoot left the whole package green
-// before this existed, so the cache-derived root had no alias coverage at all.
-func TestDeterministicRuntimeRootRejectsAnAliasedCache(t *testing.T) {
-	workspaceRoot := canonicalSandboxWorkspaceRoot(filepath.Join(t.TempDir(), "workspace"))
-	inner := filepath.Join(workspaceRoot, "cachehome")
-	if err := os.MkdirAll(inner, 0o700); err != nil {
-		t.Fatalf("create target: %v", err)
-	}
-
-	// The derived root is <cache>/zero/runtime/v1/<hash>, so aliasing <cache>/zero
-	// is what puts the whole tree inside the workspace.
-	cacheRoot := filepath.Join(t.TempDir(), "cache")
-	if err := os.MkdirAll(cacheRoot, 0o700); err != nil {
-		t.Fatalf("create cache: %v", err)
-	}
-	// A junction on Windows, a symlink elsewhere. Both point at inner by its REAL
-	// spelling on purpose. Stacking a case alias on top would build the one shape
-	// the doc on runtimeRootWithinWorkspace says stays open off Windows, an alias
-	// into a workspace SUBDIRECTORY that no spelling chain reaches, and the test
-	// would then be asserting a guarantee macOS does not make.
-	link := filepath.Join(cacheRoot, "zero")
-	if runtime.GOOS == "windows" {
-		if out, err := exec.Command("cmd", "/c", "mklink", "/J", link, inner).CombinedOutput(); err != nil {
-			t.Skipf("mklink /J unavailable: %v %s", err, out)
-		}
-	} else if err := os.Symlink(inner, link); err != nil {
-		t.Skipf("symlink unavailable: %v", err)
-	}
-	innerInfo, err := os.Stat(inner)
-	if err != nil {
-		t.Fatalf("stat target: %v", err)
-	}
-	linkInfo, err := os.Stat(link)
-	if err != nil || !os.SameFile(innerInfo, linkInfo) {
-		t.Fatalf("%s is not an alias of %s", link, inner)
-	}
-
-	root, usableOutside := deterministicSandboxRuntimeRoot(workspaceRoot, cacheRoot)
-	if usableOutside {
-		t.Fatalf("cache-derived runtime root %s reported as outside %s while it resolves inside it", root, workspaceRoot)
-	}
-}
-
-// TestRuntimeRootWithinWorkspaceKeepsAGenuinelyOutsideRootUsable is the other
-// half. The three checks only ever ADD containment answers, so the one thing that
-// could go wrong is reporting containment for a root that is merely adjacent.
-func TestRuntimeRootWithinWorkspaceKeepsAGenuinelyOutsideRootUsable(t *testing.T) {
-	parent := t.TempDir()
-	workspaceRoot := canonicalSandboxWorkspaceRoot(filepath.Join(parent, "workspace"))
-	sibling := filepath.Join(parent, "workspace-runtime", "zero", "runtime", "v1", "0123456789abcdef")
-	if err := os.MkdirAll(workspaceRoot, 0o700); err != nil {
-		t.Fatalf("create workspace: %v", err)
-	}
-	if err := os.MkdirAll(sibling, 0o700); err != nil {
-		t.Fatalf("create sibling: %v", err)
-	}
-
-	if runtimeRootWithinWorkspace(workspaceRoot, sibling) {
-		t.Fatalf("%s reported as inside %s; a sibling sharing a name prefix is not contained", sibling, workspaceRoot)
-	}
-	if _, usableOutside := deterministicSandboxRuntimeRoot(workspaceRoot, filepath.Join(parent, "cache")); !usableOutside {
-		t.Fatalf("a cache root outside the workspace was reported unusable")
-	}
-}
@@ -1,100 +0,0 @@
-package sandbox
-
-import (
-	"errors"
-	"fmt"
-	"os"
-	"path/filepath"
-)
-
-// errRuntimeComponentAliased marks a refusal that must NOT be treated as a
-// reason to relocate.
-//
-// selectSandboxRuntimeRoot falls back to the temp root when the preferred one
-// cannot be leased, which is right for an unusable directory and wrong for a
-// hostile one: relocating would leave the attacker's link in place, say nothing,
-// and simply move to the next predictable name for them to take as well. A
-// machine in this state needs an operator, not a retry.
-var errRuntimeComponentAliased = errors.New("sandbox runtime component is aliased")
-
-// THE RUNTIME TREE IS CREATED IN A DIRECTORY OTHER PEOPLE CAN WRITE TO.
-//
-// The fallback root used to come from os.MkdirTemp, which mints a random name
-// atomically at mode 0700, so no other local user could name the directory, let
-// alone pre-create it. Deriving it from a digest of the workspace path bought
-// stable cache reuse across runs and gave that name away: every component is
-// computable by anyone who can guess the workspace path, and on Linux
-// os.TempDir() is the shared, world-writable /tmp whenever TMPDIR is unset.
-//
-// What follows from a name another user can create is not subtle.
-// os.MkdirAll returns nil when Stat says the path is already a directory, and
-// Stat FOLLOWS LINKS, so a link planted at the leaf is silently accepted; the
-// cache, data and tmp directories are then created inside whatever it points at,
-// os.Chmod and os.Chtimes follow it too, and the root is handed to the platform
-// backend as a WRITE ROOT (a read-write bind under bwrap) with TMPDIR, GOCACHE,
-// GOMODCACHE and the package-manager caches all pointed inside it. The sandbox
-// would be granting the confined command write access to a directory an
-// attacker chose.
-//
-// Two things close it, and both are needed. The path carries a per-user
-// component so ordinary users are not sharing one tree, and every component Zero
-// owns is verified to be a real directory belonging to this user before anything
-// is created through it. The name alone is not enough: /tmp is world-writable,
-// so another user can create the per-user directory FIRST and wait.
-//
-// This is the same rule refuseReparsedRuntimeAncestors applies during elevated
-// Windows setup. That guard was never on this path, which is the shared one
-// every platform takes for every command.
-func refuseAliasedRuntimeComponents(root string) error {
-	for _, component := range ownedRuntimeComponents(root) {
-		info, err := os.Lstat(component)
-		if err != nil {
-			if os.IsNotExist(err) {
-				// Not there yet, so there is nothing to alias. The caller re-checks
-				// after creation, because this alone is a check-then-use.
-				continue
-			}
-			return fmt.Errorf("inspect sandbox runtime component %s: %w", component, err)
-		}
-		// ModeIrregular as well as ModeSymlink: a Windows junction is reported as
-		// irregular, needs no privilege to create, and a guard written against
-		// symlinks alone is inert against it.
-		if info.Mode()&(os.ModeSymlink|os.ModeIrregular) != 0 {
-			return fmt.Errorf("%w: refusing to use the sandbox runtime through a link at %s: "+
-				"a link here redirects the directory the sandbox is granted write access to", errRuntimeComponentAliased, component)
-		}
-		if !info.IsDir() {
-			// NOT tagged as aliased, deliberately. An ordinary file sitting where a
-			// runtime component belongs is a broken machine rather than a hostile
-			// one, and relocating to the other candidate is the sane recovery that
-			// was already there. Only a link or a directory belonging to somebody
-			// else says an attacker chose this path, and those are the two the
-			// caller must refuse outright rather than route around.
-			return fmt.Errorf("sandbox runtime component %s exists and is not a directory", component)
-		}
-		if err := refuseForeignRuntimeComponent(component, info); err != nil {
-			return err
-		}
-	}
-	return nil
-}
-
-// ownedRuntimeComponents lists the trailing components Zero creates, deepest
-// first. Anything above them belongs to the user or the machine.
-func ownedRuntimeComponents(root string) []string {
-	cleaned := filepath.Clean(root)
-	if cleaned == "" || cleaned == "." {
-		return nil
-	}
-	components := make([]string, 0, windowsSandboxRuntimeOwnedDepth)
-	current := cleaned
-	for range windowsSandboxRuntimeOwnedDepth {
-		components = append(components, current)
-		parent := filepath.Dir(current)
-		if parent == current {
-			break
-		}
-		current = parent
-	}
-	return components
-}
@@ -1,16 +0,0 @@
-package sandbox
-
-import (
-	"crypto/sha256"
-	"encoding/hex"
-	"testing"
-)
-
-// digestFor recomputes the fallback leaf name the way fallbackSandboxRuntimeRoot
-// does, so the test moves with the implementation rather than pinning a literal.
-func digestFor(workspaceRoot string, scope string) string {
-	digest := sha256.Sum256([]byte(canonicalSandboxWorkspaceRoot(workspaceRoot) + "\x00" + scope))
-	return hex.EncodeToString(digest[:8])
-}
-
-var _ = testing.Verbose
@@ -1,16 +0,0 @@
-//go:build !windows
-
-package sandbox
-
-import (
-	"os"
-	"testing"
-)
-
-// A POSIX symlink is the reachable alias off Windows.
-func linkRuntimeComponent(t *testing.T, link, target string) {
-	t.Helper()
-	if err := os.Symlink(target, link); err != nil {
-		t.Skipf("cannot create a symlink in this environment: %v", err)
-	}
-}
@@ -1,19 +0,0 @@
-//go:build windows
-
-package sandbox
-
-import (
-	"os/exec"
-	"testing"
-)
-
-// A JUNCTION, not a symlink: it needs no privilege, which is what makes it the
-// alias an ordinary local user can actually plant, and os.Lstat reports it as
-// ModeIrregular rather than ModeSymlink.
-func linkRuntimeComponent(t *testing.T, link, target string) {
-	t.Helper()
-	output, err := exec.Command("cmd", "/c", "mklink", "/J", link, target).CombinedOutput()
-	if err != nil {
-		t.Skipf("cannot create a junction in this environment: %v (%s)", err, output)
-	}
-}
@@ -1,208 +0,0 @@
-package sandbox
-
-import (
-	"errors"
-	"os"
-	"path/filepath"
-	"strings"
-	"testing"
-)
-
-// A LINK AT AN OWNED COMPONENT MUST NOT BE BUILT THROUGH.
-//
-// This is the shared path every platform takes for every command, and until now
-// it had no guard at all: the only link refusal in this package was wired into
-// elevated Windows setup. os.MkdirAll returns nil when Stat says the path is
-// already a directory, and Stat FOLLOWS links, so a link planted at an owned
-// component is accepted silently. The cache, data and tmp directories are then
-// created inside whatever it points at, Chmod and Chtimes follow it too, and the
-// root is handed to the backend as a WRITE ROOT with TMPDIR, GOCACHE and the
-// package-manager caches pointed inside it. The sandbox would be granting the
-// confined command write access to a directory somebody else chose.
-func TestTheRuntimeGuardRefusesALinkAtEveryOwnedComponent(t *testing.T) {
-	for depth := range windowsSandboxRuntimeOwnedDepth {
-		t.Run("replaced "+string(rune('0'+depth))+" levels above the leaf", func(t *testing.T) {
-			base := t.TempDir()
-			root := filepath.Join(append([]string{base}, append(append([]string{}, windowsSandboxRuntimeOwnedNames...), "abcdef0123456789")...)...)
-			if err := os.MkdirAll(root, 0o700); err != nil {
-				t.Fatalf("create the runtime tree: %v", err)
-			}
-
-			swapped := root
-			for range depth {
-				swapped = filepath.Dir(swapped)
-			}
-			tail, err := filepath.Rel(swapped, root)
-			if err != nil {
-				t.Fatalf("relate the swapped component to the root: %v", err)
-			}
-			if err := os.RemoveAll(swapped); err != nil {
-				t.Fatalf("clear the component to replace: %v", err)
-			}
-			target := filepath.Join(t.TempDir(), "somewhere-else")
-			if err := os.MkdirAll(target, 0o700); err != nil {
-				t.Fatalf("create the link target: %v", err)
-			}
-			linkRuntimeComponent(t, swapped, target)
-			if tail != "." {
-				if err := os.MkdirAll(filepath.Join(target, tail), 0o700); err != nil {
-					t.Fatalf("recreate the components below the link: %v", err)
-				}
-			}
-
-			err = refuseAliasedRuntimeComponents(root)
-			if err == nil {
-				t.Fatalf("a link at an owned component was accepted; the runtime tree would have been built inside %s and granted to the sandbox as a write root", target)
-			}
-			// ASSERTED ON THE SENTINEL, not on a word in the message.
-			//
-			// This read strings.Contains(err.Error(), "link") and was vacuous:
-			// t.TempDir() names its directory after the test, the subtest was called
-			// "link N levels above the leaf", and every component path therefore
-			// contained "link". Deleting the reparse refusal left a different error
-			// ("exists and is not a directory") whose PATH still satisfied the
-			// assertion, so all four subtests passed with the fix removed.
-			//
-			// The sentinel is also the real contract: selectSandboxRuntimeRoot
-			// branches on errors.Is to decide refuse-versus-relocate.
-			if !errors.Is(err, errRuntimeComponentAliased) {
-				t.Errorf("a link was not reported as a hostile alias, so selection would relocate around it instead of refusing: %v", err)
-			}
-			if !strings.Contains(err.Error(), "redirects the directory") {
-				t.Errorf("the refusal does not name the reason: %v", err)
-			}
-		})
-	}
-}
-
-// An ordinary tree passes, or the guard above would be satisfied by one that
-// refuses everything.
-func TestTheRuntimeGuardAcceptsAnOrdinaryTree(t *testing.T) {
-	base := t.TempDir()
-	root := filepath.Join(append([]string{base}, append(append([]string{}, windowsSandboxRuntimeOwnedNames...), "abcdef0123456789")...)...)
-	if err := os.MkdirAll(root, 0o700); err != nil {
-		t.Fatalf("create the runtime tree: %v", err)
-	}
-	if err := refuseAliasedRuntimeComponents(root); err != nil {
-		t.Fatalf("an ordinary runtime tree was refused: %v", err)
-	}
-}
-
-// A tree that does not exist yet is fine: there is nothing to alias, and this is
-// the ordinary first-run case.
-func TestTheRuntimeGuardAcceptsATreeThatDoesNotExistYet(t *testing.T) {
-	base := t.TempDir()
-	root := filepath.Join(append([]string{base}, append(append([]string{}, windowsSandboxRuntimeOwnedNames...), "abcdef0123456789")...)...)
-	if err := refuseAliasedRuntimeComponents(root); err != nil {
-		t.Fatalf("an absent runtime tree was refused: %v", err)
-	}
-}
-
-// A FILE where an owned component belongs is refused too, rather than producing
-// a confusing failure deeper in.
-func TestTheRuntimeGuardRefusesAFileWhereADirectoryBelongs(t *testing.T) {
-	base := t.TempDir()
-	root := filepath.Join(append([]string{base}, append(append([]string{}, windowsSandboxRuntimeOwnedNames...), "abcdef0123456789")...)...)
-	if err := os.MkdirAll(filepath.Dir(root), 0o700); err != nil {
-		t.Fatalf("create the runtime parents: %v", err)
-	}
-	if err := os.WriteFile(root, []byte("not a directory"), 0o600); err != nil {
-		t.Fatalf("seed the file: %v", err)
-	}
-	if err := refuseAliasedRuntimeComponents(root); err == nil {
-		t.Fatal("a file standing where the runtime root belongs was accepted")
-	}
-}
-
-// THE SHARED-TEMP FALLBACK IS SCOPED TO THIS USER.
-//
-// It replaced os.MkdirTemp, which minted a random 0700 directory atomically, so
-// no other local user could name it. A digest of the workspace path alone is the
-// same string for every account on the host, and on Linux os.TempDir() is the
-// world-writable /tmp whenever TMPDIR is unset: two users on the same path would
-// name one directory and the first one there would own it.
-func TestTheTempFallbackRootIsScopedToTheUser(t *testing.T) {
-	workspace := t.TempDir()
-	root, err := fallbackSandboxRuntimeRoot(canonicalSandboxWorkspaceRoot(workspace))
-	if err != nil {
-		t.Fatalf("fallbackSandboxRuntimeRoot: %v", err)
-	}
-	scope := sandboxRuntimeUserScope()
-	if strings.TrimSpace(scope) == "" {
-		t.Fatal("the user scope is empty, so the digest is the same for every account")
-	}
-
-	// The leaf must move when the scope does. Recomputed the way the function
-	// does, rather than asserting on a hardcoded digest.
-	same := digestFor(workspace, scope)
-	other := digestFor(workspace, scope+"-someone-else")
-	if same == other {
-		t.Fatal("the user scope does not reach the digest")
-	}
-	if !strings.HasSuffix(filepath.Clean(root), same) {
-		t.Errorf("the fallback root %s does not end in the user-scoped digest %s", root, same)
-	}
-}
-
-// And the fallback still has the shape the owned-component guard and the Windows
-// rooted traversal both recognize. A path that stops matching silently loses
-// BOTH protections, which is the expensive direction.
-func TestTheTempFallbackRootKeepsTheOwnedShape(t *testing.T) {
-	workspace := t.TempDir()
-	root, err := fallbackSandboxRuntimeRoot(canonicalSandboxWorkspaceRoot(workspace))
-	if err != nil {
-		t.Fatalf("fallbackSandboxRuntimeRoot: %v", err)
-	}
-	if _, _, ok := windowsSandboxRuntimeOwnedTail(root); !ok {
-		t.Fatalf("the fallback root %s is not recognized as an owned runtime tail, so the rooted traversal falls back to opening it by name", root)
-	}
-	components := ownedRuntimeComponents(root)
-	if len(components) != windowsSandboxRuntimeOwnedDepth {
-		t.Fatalf("the guard walks %d components of %s, want %d", len(components), root, windowsSandboxRuntimeOwnedDepth)
-	}
-}
-
-// THROUGH prepareSandboxRuntime, not the helper.
-//
-// The helper being correct proves nothing on its own: the guard has to be on the
-// path runner.go actually calls, before the MkdirAll that would build the tree
-// through the link and before the root is handed back as a write root.
-func TestPreparingTheRuntimeRefusesALinkedRoot(t *testing.T) {
-	workspace := t.TempDir()
-	cacheRoot := t.TempDir()
-	original := sandboxUserCacheDir
-	sandboxUserCacheDir = func() (string, error) { return cacheRoot, nil }
-	t.Cleanup(func() { sandboxUserCacheDir = original })
-
-	canonical := canonicalSandboxWorkspaceRoot(workspace)
-	root, err := sandboxRuntimeRootFor(canonical, canonicalSandboxWorkspaceRoot(cacheRoot))
-	if err != nil {
-		t.Skipf("no cache-derived runtime root in this environment: %v", err)
-	}
-
-	// Somebody else got to the predictable name first and pointed it elsewhere.
-	if err := os.MkdirAll(filepath.Dir(root), 0o700); err != nil {
-		t.Fatalf("create the runtime parents: %v", err)
-	}
-	target := filepath.Join(t.TempDir(), "somewhere-else")
-	if err := os.MkdirAll(target, 0o700); err != nil {
-		t.Fatalf("create the link target: %v", err)
-	}
-	linkRuntimeComponent(t, root, target)
-
-	runtimeState, cleanup, err := prepareSandboxRuntime(canonical, "")
-	if cleanup != nil {
-		cleanup()
-	}
-	if err == nil {
-		t.Fatalf("the runtime was prepared at %s through a link; %s would have been bound read-write into the sandbox with TMPDIR and the build caches inside it", runtimeState.Root, target)
-	}
-	if !errors.Is(err, errRuntimeComponentAliased) {
-		t.Errorf("the linked root was not reported as a hostile alias: %v", err)
-	}
-	for _, name := range []string{"cache", "data", "tmp"} {
-		if _, statErr := os.Stat(filepath.Join(target, name)); statErr == nil {
-			t.Errorf("the runtime tree was created inside the link target at %s", filepath.Join(target, name))
-		}
-	}
-}
@@ -1,67 +0,0 @@
-//go:build !windows
-
-package sandbox
-
-import (
-	"fmt"
-	"os"
-	"syscall"
-)
-
-// refuseForeignRuntimeComponent rejects a component owned by somebody else.
-//
-// The link check above stops the redirection; this stops the quieter half. /tmp
-// is world-writable and sticky, so another local user can create the components
-// Zero owns BEFORE Zero ever runs. A directory they own but Zero writes into is
-// a place they can read the sandbox's caches out of, and the sticky bit does not
-// help because they own it. os.MkdirAll accepts it silently, since the path
-// already exists as a directory.
-//
-// Ownership rather than mode, because a 0777 directory belonging to this user is
-// the user's own business while a 0700 directory belonging to another user is
-// not something Zero should adopt.
-func refuseForeignRuntimeComponent(component string, info os.FileInfo) error {
-	stat, ok := info.Sys().(*syscall.Stat_t)
-	if !ok {
-		// No ownership information on this filesystem. The link check already ran.
-		return nil
-	}
-	if uid := os.Getuid(); uid >= 0 && int(stat.Uid) != uid {
-		return fmt.Errorf("%w: refusing to use the sandbox runtime directory %s: it belongs to uid %d, not to this user (uid %d)",
-			errRuntimeComponentAliased, component, stat.Uid, uid)
-	}
-	return nil
-}
-
-// sandboxRuntimeUserScope isolates the derived runtime tree per user.
-//
-// Two users on one host derive the same digest for the same workspace path, so
-// without this they name one directory in shared temp and the first one there
-// owns it. A uid is not a secret and is not meant to be: it removes the
-// collision, and refuseAliasedRuntimeComponents handles the case where somebody
-// got there first.
-func sandboxRuntimeUserScope() string {
-	return fmt.Sprintf("u%d", os.Getuid())
-}
-
-// sandboxRuntimeFallbackOwnedNames are the components the temp-derived runtime
-// root is built from.
-//
-// THE USER BOUNDARY COMES FIRST, ABOVE EVERY PRIVATE COMPONENT. On Unix
-// os.TempDir() is a SHARED directory whenever TMPDIR is unset, and runtime
-// preparation creates and ownership-checks each of these components at 0700. A
-// fixed first component therefore meant the first account to use the fallback
-// created a private directory that every other account was then refused at:
-// traversal fails on the mode, and relaxing the mode fails the ownership guard
-// instead. The per-workspace digest is the leaf, so it never got the chance to
-// separate them, and the fallback became first-user-wins on a shared host.
-//
-// Scoping the FIRST component keeps every ownership-checked ancestor inside a
-// namespace that already belongs to one user, which is the property the guards
-// below assume. The workspace digest stays the leaf so setup and the command
-// still derive the same path for the same workspace.
-func sandboxRuntimeFallbackOwnedNames() []string {
-	names := append([]string(nil), windowsSandboxRuntimeOwnedNames...)
-	names[0] = names[0] + "-" + sandboxRuntimeUserScope()
-	return names
-}
@@ -1,51 +0,0 @@
-//go:build windows
-
-package sandbox
-
-import (
-	"os"
-	"strings"
-)
-
-// refuseForeignRuntimeComponent has no ownership check on Windows.
-//
-// The derived root lives under the per-user cache directory or the per-session
-// TEMP, both of which are already user-private, and the elevated setup path
-// applies its own capability ACL. The link refusal in the shared guard is the
-// part that matters here.
-func refuseForeignRuntimeComponent(string, os.FileInfo) error {
-	return nil
-}
-
-// sandboxRuntimeUserScope names the account the tree belongs to.
-//
-// Windows TEMP is already per-user, so this is belt and braces rather than the
-// load-bearing separation it is on Unix. Kept so the derived path has the same
-// shape on every platform and one code path builds it.
-//
-// FROM THE TOKEN, NOT THE ENVIRONMENT. Setup derives the fallback root in the
-// elevated terminal and records it; a command later re-derives it in an
-// ordinary one to decide whether that record belongs to this workspace. Those
-// are two processes with independent environments, so a USERNAME that differed
-// between them made the command reject the root setup had provisioned and
-// select one that carries no capability ACE. The token's user SID is the same
-// in both, because setup refuses to run as any account other than the one that
-// will use Zero. The Unix build reads the uid for the same reason. Reported by
-// CodeRabbit.
-func sandboxRuntimeUserScope() string {
-	sid, err := currentProcessSID()
-	if err != nil {
-		return "u"
-	}
-	return "u" + strings.ToLower(sid)
-}
-
-// sandboxRuntimeFallbackOwnedNames are the components the temp-derived runtime
-// root is built from. Unscoped on Windows: os.TempDir() already resolves inside
-// the user's own profile, so the shared-temp collision the Unix build guards
-// against cannot arise, and the fixed names keep windowsSandboxRuntimeOwnedTail
-// able to recognise a root built by an elevated setup running as another
-// account.
-func sandboxRuntimeFallbackOwnedNames() []string {
-	return windowsSandboxRuntimeOwnedNames
-}
@@ -1,80 +0,0 @@
-package sandbox
-
-import (
-	"testing"
-)
-
-// A RECORDED ROOT IS HISTORY, NOT PROOF THAT A COMMAND WOULD STILL SELECT IT.
-//
-// Doctor pins the marker's runtime root so it can check the stamp without
-// taking a lease. A command does not pin blindly: it derives the current cache
-// and fallback candidates and honours the marker only when its root is one of
-// them. Run setup with the cache at A, relocate the cache so commands derive B,
-// and the stamped A tree stays behind. Pinning A reported a healthy machine
-// immediately before every real command rejected A and failed on the marker.
-//
-// This records a marker with the cache resolver pointed at A, flips the
-// resolver to B, and asks the same question a command asks. The seam is the
-// production resolver, so the derivation under test is the command's own.
-func TestRecordedRuntimeRootIsNotCurrentOnceTheCacheMoves(t *testing.T) {
-	cacheA := t.TempDir()
-	cacheB := t.TempDir()
-	workspace := t.TempDir()
-
-	previous := sandboxUserCacheDir
-	t.Cleanup(func() { sandboxUserCacheDir = previous })
-	sandboxUserCacheDir = func() (string, error) { return cacheA, nil }
-
-	config := WindowsSandboxSetupConfig{
-		SandboxHome:    t.TempDir(),
-		CommandCWD:     workspace,
-		WorkspaceRoots: []string{workspace},
-		PermissionProfile: PermissionProfile{
-			FileSystem: FileSystemPolicy{
-				Kind:       FileSystemRestricted,
-				WriteRoots: []WritableRoot{{Root: workspace}},
-			},
-			Network: NetworkPolicy{Mode: NetworkDeny},
-		},
-	}
-	// The marker records profile.Runtime.Root, which setup sets from the root
-	// it SELECTS. Select it here the way a command under cache A would, and
-	// put it on the profile the way doctor does, so the recorded root is the
-	// genuine A-derived one rather than a value this test invented.
-	rootA, err := sandboxRuntimeRootFor(canonicalSandboxWorkspaceRoot(workspace), canonicalSandboxWorkspaceRoot(cacheA))
-	if err != nil {
-		t.Fatalf("derive the runtime root under cache A: %v", err)
-	}
-	config.PermissionProfile = PermissionProfileWithRuntimeRoot(
-		WindowsSandboxProfileWithRuntimeRoots(config.PermissionProfile, config.WorkspaceRoots),
-		rootA,
-	)
-	if _, err := WriteWindowsSandboxSetupMarker(config); err != nil {
-		t.Fatalf("WriteWindowsSandboxSetupMarker: %v", err)
-	}
-
-	recorded, current, err := WindowsSandboxRecordedRuntimeRootIsCurrent(config.SandboxHome, workspace)
-	if err != nil {
-		t.Fatalf("with the cache still at A: %v", err)
-	}
-	if recorded == "" {
-		t.Fatal("SETUP INVALID: the marker recorded no runtime root, so the comparison proves nothing")
-	}
-	if !current {
-		t.Fatalf("the root setup just recorded (%s) is reported stale while the cache has not moved", recorded)
-	}
-
-	// The cache moves. Commands now derive under B; the stamped tree is under A.
-	sandboxUserCacheDir = func() (string, error) { return cacheB, nil }
-
-	recordedAfter, currentAfter, err := WindowsSandboxRecordedRuntimeRootIsCurrent(config.SandboxHome, workspace)
-	if err != nil {
-		t.Fatalf("with the cache at B: %v", err)
-	}
-	if recordedAfter != recorded {
-		t.Errorf("the recorded root changed on read: %q then %q", recorded, recordedAfter)
-	}
-	if currentAfter {
-		t.Fatalf("the marker's root %s is under cache A, commands derive under B, and it was still reported current", recorded)
-	}
-}
@@ -1,108 +0,0 @@
-package sandbox
-
-import (
-	"errors"
-	"strings"
-	"testing"
-)
-
-// THE DIAGNOSTIC REFUSES WHAT SELECTION REFUSES, WITH THE SAME WORDS.
-//
-// os.UserCacheDir can succeed and still hand back a value that canonicalizes to
-// nothing: an empty string, ".", or only whitespace. Selection fails closed on
-// that with "user cache directory is unavailable". The diagnostic that answers
-// "would a command still select the recorded root?" spelled the same steps out
-// separately and did not have that check, so it derived candidates from the
-// empty root and returned an answer, current or stale, on a machine where every
-// command fails before it reads a marker. Doctor then reported staleness, or
-// nothing at all, instead of runtime-root-unresolved.
-//
-// Both now get their inputs from one resolver. This drives the two production
-// entry points with the same stubbed cache value and requires the same error
-// from each. The seam is the production resolver's own. Reported by @jatmn.
-func TestTheDiagnosticRefusesTheCacheRootSelectionRefuses(t *testing.T) {
-	workspace := t.TempDir()
-	usableCache := t.TempDir()
-
-	previous := sandboxUserCacheDir
-	t.Cleanup(func() { sandboxUserCacheDir = previous })
-	sandboxUserCacheDir = func() (string, error) { return usableCache, nil }
-
-	config := WindowsSandboxSetupConfig{
-		SandboxHome:    t.TempDir(),
-		CommandCWD:     workspace,
-		WorkspaceRoots: []string{workspace},
-		PermissionProfile: PermissionProfile{
-			FileSystem: FileSystemPolicy{
-				Kind:       FileSystemRestricted,
-				WriteRoots: []WritableRoot{{Root: workspace}},
-			},
-			Network: NetworkPolicy{Mode: NetworkDeny},
-		},
-	}
-	root, err := sandboxRuntimeRootFor(canonicalSandboxWorkspaceRoot(workspace), canonicalSandboxWorkspaceRoot(usableCache))
-	if err != nil {
-		t.Fatalf("derive the runtime root: %v", err)
-	}
-	config.PermissionProfile = PermissionProfileWithRuntimeRoot(
-		WindowsSandboxProfileWithRuntimeRoots(config.PermissionProfile, config.WorkspaceRoots),
-		root,
-	)
-	if _, err := WriteWindowsSandboxSetupMarker(config); err != nil {
-		t.Fatalf("WriteWindowsSandboxSetupMarker: %v", err)
-	}
-	// Healthy first: with a usable cache the diagnostic answers, and says
-	// current. Without this the legs below could pass on a marker that was
-	// never readable.
-	if recorded, current, err := WindowsSandboxRecordedRuntimeRootIsCurrent(config.SandboxHome, workspace); err != nil || recorded == "" || !current {
-		t.Fatalf("SETUP INVALID: with a usable cache the diagnostic gave (%q, %v, %v)", recorded, current, err)
-	}
-
-	for _, testCase := range []struct {
-		name  string
-		cache string
-	}{
-		{name: "empty", cache: ""},
-		{name: "dot", cache: "."},
-		{name: "whitespace", cache: "   "},
-		{name: "dot with whitespace", cache: " . "},
-	} {
-		t.Run(testCase.name, func(t *testing.T) {
-			sandboxUserCacheDir = func() (string, error) { return testCase.cache, nil }
-
-			_, lease, _, selectErr := selectSandboxRuntimeRoot(workspace, true, config.SandboxHome)
-			if lease != nil {
-				lease.release()
-			}
-			if selectErr == nil {
-				t.Fatalf("SETUP INVALID: selection accepted the cache root %q, so there is no refusal to match", testCase.cache)
-			}
-
-			recorded, current, diagnosticErr := WindowsSandboxRecordedRuntimeRootIsCurrent(config.SandboxHome, workspace)
-			if diagnosticErr == nil {
-				t.Fatalf("the diagnostic answered current=%v for a cache root %q that selection refuses with %q; doctor would report that instead of runtime-root-unresolved", current, testCase.cache, selectErr)
-			}
-			if diagnosticErr.Error() != selectErr.Error() {
-				t.Errorf("the diagnostic failed with %q and selection with %q; an operator is told two different things about one cause", diagnosticErr, selectErr)
-			}
-			if !strings.Contains(diagnosticErr.Error(), "user cache directory is unavailable") {
-				t.Errorf("the cause does not name the cache directory: %q", diagnosticErr)
-			}
-			if current {
-				t.Errorf("an unresolvable selection was still reported current")
-			}
-			if recorded == "" {
-				t.Errorf("the recorded root was dropped from the answer, so doctor cannot say which marker it was looking at")
-			}
-		})
-	}
-
-	// And a resolver error is still reported as itself, by both.
-	boom := errors.New("no cache location is defined")
-	sandboxUserCacheDir = func() (string, error) { return "", boom }
-	_, _, _, selectErr := selectSandboxRuntimeRoot(workspace, true, config.SandboxHome)
-	_, _, diagnosticErr := WindowsSandboxRecordedRuntimeRootIsCurrent(config.SandboxHome, workspace)
-	if !errors.Is(selectErr, boom) || !errors.Is(diagnosticErr, boom) || selectErr.Error() != diagnosticErr.Error() {
-		t.Errorf("a resolver error reads differently: selection %q, diagnostic %q", selectErr, diagnosticErr)
-	}
-}
@@ -1,32 +0,0 @@
-//go:build !windows
-
-package sandbox
-
-import (
-	"errors"
-	"fmt"
-	"io/fs"
-	"os"
-	"path/filepath"
-)
-
-// snapshotRuntimeStampBound keeps the pathname form off Windows. The split it
-// closes there is specific to an elevated installer racing an unelevated
-// renamer; the same eager identity capture still applies.
-//
-// The three-state result is NOT Windows-specific, though: "read it and there was
-// nothing" and "could not read it" are different facts on every platform, and
-// only the first may authorize a compensating delete. A permission or I/O error
-// here stops setup rather than being recorded as proven absence.
-func snapshotRuntimeStampBound(root string, name string) (identity string, identified bool, prior []byte, state runtimeStampState, err error) {
-	identity, identified = runtimeDirIdentity(root)
-	path := filepath.Join(root, name)
-	data, readErr := os.ReadFile(path)
-	if readErr != nil {
-		if errors.Is(readErr, fs.ErrNotExist) {
-			return identity, identified, nil, runtimeStampAbsent, nil
-		}
-		return identity, identified, nil, runtimeStampUnknown, fmt.Errorf("read the sandbox runtime stamp at %s: %w", path, readErr)
-	}
-	return identity, identified, data, runtimeStampPresent, nil
-}
@@ -1,98 +0,0 @@
-//go:build windows
-
-package sandbox
-
-import (
-	"errors"
-	"fmt"
-	"io"
-	"os"
-
-	"golang.org/x/sys/windows"
-)
-
-// snapshotRuntimeStampBound reads the runtime root's identity and its existing
-// stamp through ONE handle.
-//
-// The two used to be taken separately: runtimeDirIdentity opened the root, read
-// its volume and file ID, and closed the handle, and then os.ReadFile resolved
-// the pathname again. A rename and substitution in that interval pairs A's
-// identity with B's stamp bytes, and a rollback that correctly proves it holds A
-// then writes B's bytes into it, corrupting an attestation that predates this
-// run. The lease stops cleanup selecting the root; it does not stop the parent's
-// owner renaming it.
-//
-// Reading the child relative to the identified handle removes the second
-// resolution, so both facts describe the same object by construction.
-//
-// EVERY FAILURE IS AN ERROR, NOT AN ABSENCE. This used to collapse an encoding
-// failure, a directory that would not open, an identity that could not be read,
-// a denied child open and a short read all into the same "there was no stamp"
-// answer that a genuine ERROR_FILE_NOT_FOUND produces. The stamp writer uses
-// FILE_OVERWRITE_IF and can replace an existing stamp even where the read was
-// denied, so that lie let a FAILED setup delete an attestation it had no record
-// of, leaving the previous run's marker pointing at an unusable runtime root.
-// Only a positive not-found produces runtimeStampAbsent.
-func snapshotRuntimeStampBound(root string, name string) (identity string, identified bool, prior []byte, state runtimeStampState, err error) {
-	utf16Root, err := windows.UTF16PtrFromString(root)
-	if err != nil {
-		return "", false, nil, runtimeStampUnknown, fmt.Errorf("encode sandbox runtime root %s: %w", root, err)
-	}
-	directory, err := windows.CreateFile(
-		utf16Root,
-		windows.FILE_TRAVERSE|windows.FILE_READ_ATTRIBUTES|windows.SYNCHRONIZE,
-		windows.FILE_SHARE_READ|windows.FILE_SHARE_WRITE|windows.FILE_SHARE_DELETE,
-		nil,
-		windows.OPEN_EXISTING,
-		windows.FILE_FLAG_BACKUP_SEMANTICS|windows.FILE_FLAG_OPEN_REPARSE_POINT,
-		0,
-	)
-	if err != nil {
-		// A root that is simply not there yet is the ordinary first-run case: the
-		// created-directory rollback owns it and there is no prior stamp to lose.
-		if isWindowsNotFound(err) {
-			return "", false, nil, runtimeStampAbsent, nil
-		}
-		return "", false, nil, runtimeStampUnknown, fmt.Errorf("open sandbox runtime root %s: %w", root, err)
-	}
-	defer windows.CloseHandle(directory)
-
-	identity, idErr := handleRuntimeIdentity(directory)
-	if idErr != nil {
-		return "", false, nil, runtimeStampUnknown, fmt.Errorf("identify sandbox runtime root %s: %w", root, idErr)
-	}
-
-	stamp, err := openWindowsChildNoFollow(directory, name,
-		windows.GENERIC_READ|windows.FILE_READ_ATTRIBUTES, windows.FILE_NON_DIRECTORY_FILE)
-	if err != nil {
-		if isWindowsNotFound(err) {
-			// Proven absent. The identity still stands: it came from the handle
-			// above, not from this.
-			return identity, true, nil, runtimeStampAbsent, nil
-		}
-		return identity, true, nil, runtimeStampUnknown, fmt.Errorf("open the sandbox runtime stamp in %s: %w", root, err)
-	}
-	file := os.NewFile(uintptr(stamp), name)
-	defer file.Close()
-	data, readErr := io.ReadAll(file)
-	if readErr != nil {
-		return identity, true, nil, runtimeStampUnknown, fmt.Errorf("read the sandbox runtime stamp in %s: %w", root, readErr)
-	}
-	return identity, true, data, runtimeStampPresent, nil
-}
-
-// isWindowsNotFound reports the two statuses that mean the object genuinely is
-// not there, as opposed to the many that mean it could not be looked at.
-//
-// openWindowsChildNoFollow wraps its NTSTATUS, and CreateFile returns the Win32
-// errno, so both spellings are checked rather than assuming one layer.
-func isWindowsNotFound(err error) bool {
-	if err == nil {
-		return false
-	}
-	return errors.Is(err, os.ErrNotExist) ||
-		errors.Is(err, windows.ERROR_FILE_NOT_FOUND) ||
-		errors.Is(err, windows.ERROR_PATH_NOT_FOUND) ||
-		errors.Is(err, windows.STATUS_OBJECT_NAME_NOT_FOUND) ||
-		errors.Is(err, windows.STATUS_OBJECT_PATH_NOT_FOUND)
-}
@@ -21,6 +21,11 @@ const (
 	sandboxRuntimeMaxRoots = 64
 )

+var fallbackSandboxRuntimes = struct {
+	sync.Mutex
+	roots map[string]string
+}{roots: make(map[string]string)}
+
 type SandboxRuntime struct {
 	Root  string `json:"root,omitempty"`
 	Cache string `json:"cache,omitempty"`
@@ -28,106 +33,37 @@ type SandboxRuntime struct {
 	Temp  string `json:"temp,omitempty"`
 }

-// sandboxRuntimeRootFor derives the per-workspace runtime root. It is separated
-// from prepareSandboxRuntime because the elevated Windows setup path needs the
-// same answer WITHOUT taking a lease or creating anything: a sandbox principal
-// is a separate account with no inherited rights under the user cache, so setup
-// has to grant it write access to this tree before any command runs.
-//
-// Both callers must agree exactly. If they ever drift, setup grants the ACE on
-// one directory while commands write to another, and the failure is a bare
-// ACCESS_DENIED from npm or go build with nothing pointing at the sandbox.
-func sandboxRuntimeRootFor(workspaceRoot string, cacheRoot string) (string, error) {
-	if root, ok := deterministicSandboxRuntimeRoot(workspaceRoot, cacheRoot); ok {
-		return root, nil
+func prepareSandboxRuntime(workspaceRoot string) (SandboxRuntime, func(), error) {
+	workspaceRoot = filepath.Clean(strings.TrimSpace(workspaceRoot))
+	if workspaceRoot == "" || workspaceRoot == "." {
+		return SandboxRuntime{}, nil, errors.New("sandbox runtime requires a workspace root")
+	}
+	cacheRoot, err := sandboxUserCacheDir()
+	if err != nil {
+		return SandboxRuntime{}, nil, fmt.Errorf("resolve user cache directory: %w", err)
+	}
+	cacheRoot = filepath.Clean(strings.TrimSpace(cacheRoot))
+	if cacheRoot == "" || cacheRoot == "." {
+		return SandboxRuntime{}, nil, errors.New("user cache directory is unavailable")
 	}
-	return fallbackSandboxRuntimeRoot(workspaceRoot)
-}
-
-// deterministicSandboxRuntimeRoot returns the cache-derived runtime root and
-// whether it is usable, meaning it lands outside the workspace.
-//
-// Neither this nor the fallback creates anything now, so a caller that only
-// needs to NAME the tree can safely go through sandboxRuntimeRootFor and get the
-// answer commands will actually use. This remains separate for callers that need
-// to distinguish the cache-derived root from the temp-derived one.
-func deterministicSandboxRuntimeRoot(workspaceRoot string, cacheRoot string) (string, bool) {
 	digest := sha256.Sum256([]byte(workspaceRoot))
-	// The same inventory the rooted traversal recognizes a runtime root by. See
-	// windowsSandboxRuntimeOwnedNames: two spellings of this list is how a real
-	// runtime root stops being recognized as owned, and that failure opens by
-	// name instead of by handle.
-	root := filepath.Join(append(append([]string{cacheRoot}, windowsSandboxRuntimeOwnedNames...), hex.EncodeToString(digest[:8]))...)
-	return root, !runtimeRootWithinWorkspace(workspaceRoot, root)
-}
-
-// runtimeRootWithinWorkspace reports whether root lands inside workspaceRoot.
-//
-// pathWithinRoot compares SPELLINGS, and canonicalSandboxWorkspaceRoot folds only
-// the aliases filepath.EvalSymlinks folds. Two get through it. Case: filepath.Rel
-// folds case on Windows via sameWord but not elsewhere, so on a case-insensitive
-// macOS volume /var/folders/x and /VAR/FOLDERS/X are one directory that every
-// string comparison here keeps apart. Junctions: EvalSymlinks returns a Windows
-// directory junction unchanged, so a TEMP that reaches the workspace through one
-// measures as outside it.
-//
-// Three checks, each of which can only ADD a containment answer. That asymmetry
-// is the safety argument: the failure that matters is the runtime tree living
-// inside the workspace, so a missed alias is the expensive direction and an extra
-// relocation is the cheap one.
-//
-//  1. the spellings as given;
-//  2. the spellings resolved to physical paths, which on Windows follows
-//     junctions at any depth via GetFinalPathNameByHandle;
-//  3. filesystem identity across root's existing ancestors, which catches a case
-//     alias on a case-insensitive volume where step 2 has no API to call.
-//
-// Step 3 only sees an alias whose target IS the workspace root, because it walks
-// a SPELLING upward and a junction has no spelling chain back into its target's
-// parent. That shape is covered by step 2 on Windows. It remains open off Windows
-// for a bind mount, which the kernel presents as a real path with no way to ask
-// where it came from; closing that needs mountinfo parsing, not a path API.
-func runtimeRootWithinWorkspace(workspaceRoot string, root string) bool {
+	root := filepath.Join(cacheRoot, "zero", "runtime", "v1", hex.EncodeToString(digest[:8]))
 	if pathWithinRoot(workspaceRoot, root) {
-		return true
-	}
-	if physicalWorkspace := physicalSandboxPath(workspaceRoot); physicalWorkspace != "" {
-		if pathWithinRoot(physicalWorkspace, physicalSandboxPath(root)) {
-			return true
+		root, err = fallbackSandboxRuntimeRoot(workspaceRoot)
+		if err != nil {
+			return SandboxRuntime{}, nil, err
 		}
 	}
-	workspaceInfo, err := os.Stat(workspaceRoot)
+	lease, err := prepareSandboxRuntimeLease(root)
 	if err != nil {
-		// An unresolvable workspace leaves nothing to compare against. The
-		// spelling checks above already returned their answer.
-		return false
-	}
-	// root itself usually does not exist yet, which is the point: start at the
-	// deepest component and walk up, so the first directory that does exist gets
-	// compared and every ancestor above it after that.
-	current := filepath.Clean(root)
-	for {
-		if info, err := os.Stat(current); err == nil && os.SameFile(workspaceInfo, info) {
-			return true
+		root, err = fallbackSandboxRuntimeRoot(workspaceRoot)
+		if err != nil {
+			return SandboxRuntime{}, nil, err
 		}
-		parent := filepath.Dir(current)
-		if parent == current {
-			return false
+		lease, err = prepareSandboxRuntimeLease(root)
+		if err != nil {
+			return SandboxRuntime{}, nil, err
 		}
-		current = parent
-	}
-}
-
-func prepareSandboxRuntime(workspaceRoot string, sandboxHome string) (SandboxRuntime, func(), error) {
-	// One selection function, shared with setup. See selectSandboxRuntimeRoot.
-	//
-	// The ledger is deliberately unused here. A COMMAND is not a transaction: it
-	// does not publish a marker and has nothing to roll back to, and a component
-	// created on this path is the runtime tree the command is about to use. Only
-	// setup owns an undo.
-	root, lease, _, err := selectSandboxRuntimeRoot(workspaceRoot, true, sandboxHome)
-	if err != nil {
-		return SandboxRuntime{}, nil, err
 	}
 	prepared := false
 	defer func() {
@@ -141,29 +77,30 @@ func prepareSandboxRuntime(workspaceRoot string, sandboxHome string) (SandboxRun
 		Data:  filepath.Join(root, "data"),
 		Temp:  filepath.Join(root, "tmp"),
 	}
-	// BEFORE ANYTHING IS CREATED. os.MkdirAll returns nil when Stat says the path
-	// is already a directory, and Stat follows links, so a link planted at an
-	// owned component is silently accepted and the whole tree is built inside
-	// whatever it points at, and the root then becomes a write root the backend
-	// binds read-write with TMPDIR and the build caches pointed inside it.
-	if err := refuseAliasedRuntimeComponents(runtimeState.Root); err != nil {
-		return SandboxRuntime{}, nil, err
+	directories := []string{
+		runtimeState.Root,
+		runtimeState.Cache,
+		runtimeState.Data,
+		runtimeState.Temp,
+		filepath.Join(runtimeState.Cache, "npm"),
+		filepath.Join(runtimeState.Cache, "yarn"),
+		filepath.Join(runtimeState.Cache, "corepack"),
+		filepath.Join(runtimeState.Cache, "pip"),
+		filepath.Join(runtimeState.Cache, "go-build"),
+		filepath.Join(runtimeState.Data, "go-mod"),
+		filepath.Join(runtimeState.Data, "cargo"),
 	}
-	if err := os.MkdirAll(runtimeState.Root, 0o700); err != nil {
-		return SandboxRuntime{}, nil, fmt.Errorf("create sandbox runtime directory %s: %w", runtimeState.Root, err)
+	for _, directory := range directories {
+		if err := os.MkdirAll(directory, 0o700); err != nil {
+			return SandboxRuntime{}, nil, fmt.Errorf("create sandbox runtime directory %s: %w", directory, err)
+		}
+		if err := os.Chmod(directory, 0o700); err != nil {
+			return SandboxRuntime{}, nil, fmt.Errorf("secure sandbox runtime directory %s: %w", directory, err)
+		}
 	}
-	// AND AGAIN AFTER, because the check above is a check-then-use on its own: a
-	// component swapped during creation would still redirect the tree. Pairing the
-	// two narrows the window to the creation itself.
-	if err := refuseAliasedRuntimeComponents(runtimeState.Root); err != nil {
-		return SandboxRuntime{}, nil, err
-	}
-	// Everything below the root goes through a handle on it. The children are
-	// the one part of the tree a sandboxed command can replace; see
-	// prepareSandboxRuntimeTree.
 	now := sandboxRuntimeNow()
-	if err := prepareSandboxRuntimeTree(runtimeState.Root, now); err != nil {
-		return SandboxRuntime{}, nil, err
+	if err := os.Chtimes(runtimeState.Root, now, now); err != nil {
+		return SandboxRuntime{}, nil, fmt.Errorf("touch sandbox runtime root: %w", err)
 	}
 	cleanupSandboxRuntimeRoots(filepath.Dir(runtimeState.Root), runtimeState.Root, now)
 	prepared = true
@@ -171,19 +108,10 @@ func prepareSandboxRuntime(workspaceRoot string, sandboxHome string) (SandboxRun
 }

 func prepareSandboxRuntimeLease(root string) (*sandboxRuntimeLease, error) {
-	lease, _, err := prepareSandboxRuntimeLeaseRecording(root)
-	return lease, err
-}
-
-// prepareSandboxRuntimeLeaseRecording also reports the owned directories it
-// created, so a caller that can roll back knows what it owns.
-//
-// Nothing recorded here is created by the leaf's owner. Provisioning creates and
-// records the leaf itself; these are the components above it, which used to be
-// produced by an os.MkdirAll that nobody accounted for. Setup could therefore
-// fail after the lease and leave a tree behind with no record that it made it.
-func prepareSandboxRuntimeLeaseRecording(root string) (*sandboxRuntimeLease, []windowsCreatedRuntimeDir, error) {
-	return acquireRuntimeLeaseForPlatform(root)
+	if err := os.MkdirAll(filepath.Dir(root), 0o700); err != nil {
+		return nil, fmt.Errorf("create sandbox runtime parent: %w", err)
+	}
+	return acquireSandboxRuntimeLease(root)
 }

 // cleanupSandboxRuntimeRoots applies a conservative age/count policy. Cleanup
@@ -246,60 +174,22 @@ func combineSandboxCleanups(cleanups ...func()) func() {
 	}
 }

-// fallbackSandboxRuntimeRoot returns the runtime root for a workspace whose
-// cache-derived root would land inside itself.
-//
-// DERIVED, not minted, and that is the whole of the fix. It used to call
-// os.MkdirTemp and remember the answer in a process-global map, which made the
-// result private to whichever process asked first. Elevated Windows setup
-// granted the sandbox principal write access to the directory IT created, then
-// every later __windows-command-runner process created a DIFFERENT one and
-// pointed TMP, GOCACHE, npm and the rest at it. Those directories are created by
-// the calling user and carry no ACE for the principal, so ordinary cache and
-// temp writes failed with a bare ACCESS_DENIED and nothing naming the sandbox.
-//
-// sandboxRuntimeRootFor already documents that both callers must agree exactly.
-// Hashing the workspace, the same way the cache-derived root does, is what makes
-// that true for this branch too: every process reaches the same path without
-// having to share any state.
-//
-// It creates nothing, so deterministicSandboxRuntimeRoot's promise about naming
-// a tree without materializing it now holds for the fallback as well.
 func fallbackSandboxRuntimeRoot(workspaceRoot string) (string, error) {
-	// Canonicalized for the same reason prepareSandboxRuntime canonicalizes the
-	// workspace and cache roots: pathWithinRoot compares SPELLINGS, so a raw
-	// os.TempDir() measured against a canonical workspace root compares two
-	// different names for one directory and the containment check misses. Both
-	// callers resolve this in the operator's environment, so the derived path is
-	// identical on the setup and command sides and the plan hashes still agree.
-	//
-	// This closes the 8.3 short-name and symlink spellings, not every alias: a
-	// Windows directory JUNCTION comes back from EvalSymlinks unchanged, and case
-	// survives it on a case-insensitive macOS volume. The containment check below
-	// therefore does not rely on canonicalization alone; runtimeRootWithinWorkspace
-	// falls through to filesystem identity for exactly those aliases.
-	tempRoot := canonicalSandboxWorkspaceRoot(os.TempDir())
-	if tempRoot == "" || tempRoot == "." {
-		return "", errors.New("temp directory is unavailable")
+	fallbackSandboxRuntimes.Lock()
+	defer fallbackSandboxRuntimes.Unlock()
+	if root := fallbackSandboxRuntimes.roots[workspaceRoot]; root != "" {
+		return root, nil
 	}
-	// SCOPED TO THIS USER, unlike the cache-derived root, because this one lives
-	// in shared temp. On Linux os.TempDir() is the world-writable /tmp whenever
-	// TMPDIR is unset, and a digest of the workspace path alone is the same string
-	// for every user on the host: two accounts working on the same path would name
-	// one directory and the first one there would own it. The uid is not a secret
-	// and is not doing secrecy work; it removes the collision, and
-	// refuseAliasedRuntimeComponents handles somebody having got there first.
-	digest := sha256.Sum256([]byte(workspaceRoot + "\x00" + sandboxRuntimeUserScope()))
-	root := filepath.Join(append(append([]string{tempRoot}, sandboxRuntimeFallbackOwnedNames()...), hex.EncodeToString(digest[:8]))...)
-	if runtimeRootWithinWorkspace(workspaceRoot, root) {
-		// Both candidates land inside the workspace, so there is nowhere left to
-		// put a runtime tree the workspace's own policy does not govern. Refused
-		// rather than pointed somewhere arbitrary: a runtime root inside the
-		// workspace makes the sandbox's own cache writes indistinguishable from
-		// the work it is meant to be confining.
-		return "", fmt.Errorf("sandbox runtime root %q would fall inside workspace %q; "+
-			"open the workspace somewhere other than the cache or temp directory", root, workspaceRoot)
+	parent, err := os.MkdirTemp("", "zero-runtime-")
+	if err != nil {
+		return "", fmt.Errorf("create fallback sandbox runtime: %w", err)
 	}
+	root := filepath.Join(parent, "runtime")
+	if pathWithinRoot(workspaceRoot, root) {
+		_ = os.RemoveAll(parent)
+		return "", fmt.Errorf("fallback sandbox runtime root %q must be outside workspace %q", root, workspaceRoot)
+	}
+	fallbackSandboxRuntimes.roots[workspaceRoot] = root
 	return root, nil
 }

@@ -336,302 +226,3 @@ func permissionProfileWithRuntime(profile PermissionProfile, runtimeState Sandbo
 	profile.FileSystem.WriteRoots = append(profile.FileSystem.WriteRoots, WritableRoot{Root: runtimeState.Root})
 	return profile
 }
-
-// canonicalSandboxWorkspaceRoot normalizes a workspace root the way
-// Engine.resolveCommandDir already does — clean, absolutize, then resolve
-// symlinks — so every derivation keyed to a workspace agrees on the string.
-//
-// The runtime root is a hash of this, and the elevated Windows setup grants the
-// principal that tree while commands derive it again. Cleaning alone was not
-// enough for the two to agree, and it does not take a symlink for them to
-// differ: a path opened in different casing, or through an 8.3 short name (what
-// a Windows CI runner's TEMP looks like), resolves to a different spelling.
-// Setup then granted one tree and every command used another, so the grant that
-// makes npm/go/pip caches writable landed where nothing reads and surfaced as a
-// bare ACCESS_DENIED.
-//
-// Resolution failing is not an error: an unresolvable root still needs a stable
-// key, and falling back to the cleaned absolute path is what the command path
-// does too.
-func canonicalSandboxWorkspaceRoot(root string) string {
-	cleaned := filepath.Clean(strings.TrimSpace(root))
-	if cleaned == "" || cleaned == "." {
-		return ""
-	}
-	if !filepath.IsAbs(cleaned) {
-		if absolute, err := filepath.Abs(cleaned); err == nil {
-			cleaned = absolute
-		}
-	}
-	// EvalSymlinks fails outright when the LEAF does not exist, which is the
-	// normal case for a cache or runtime root that has not been created yet. A
-	// plain call therefore resolved an existing workspace while leaving a
-	// not-yet-created cache root unresolved, and the two were compared against
-	// each other — the containment check that decides whether the runtime tree
-	// must move out of the workspace then ran on /private/var/... versus
-	// /var/..., missed, and left the tree inside the workspace.
-	//
-	// Resolve the longest existing ancestor and re-append the rest, so a path
-	// normalizes the same way whether or not its final segments exist yet.
-	remainder := ""
-	current := cleaned
-	for {
-		if resolved, err := filepath.EvalSymlinks(current); err == nil {
-			if remainder == "" {
-				return resolved
-			}
-			return filepath.Join(resolved, remainder)
-		}
-		parent := filepath.Dir(current)
-		if parent == current {
-			// Nothing along the path resolved; the cleaned absolute form is the
-			// best stable key available.
-			return cleaned
-		}
-		remainder = filepath.Join(filepath.Base(current), remainder)
-		current = parent
-	}
-}
-
-// selectSandboxRuntimeRoot picks the runtime root a command will actually use,
-// and holds a lease on it while the caller decides what to do with it.
-//
-// SETUP AND THE COMMAND HAVE TO SELECT THE SAME WAY, or they disagree about
-// which tree exists. Setup used to derive the cache-based root and fingerprint a
-// plan naming it, while a command derived the same root, failed to lease it, and
-// silently relocated to the temp fallback. The command's plan then named the
-// fallback and the marker rejected it:
-//
-//	windows sandbox setup is out of date: permission roots or deny lists changed
-//
-// which blames permissions for a runtime-root disagreement, and re-running setup
-// could not recover because setup deterministically chose the same unleasable
-// root again. That is a permanent brick, not a retry.
-//
-// Extracted from prepareSandboxRuntime so both sides run this one function. The
-// caller must release the returned lease.
-// pinnedSandboxRuntimeRoot returns the root a previous setup recorded, when
-// that root is one this workspace could actually select.
-//
-// The candidate check is what keeps this honest. One sandbox home serves
-// whichever workspace ran setup last, so a recorded root can belong to a
-// different workspace entirely; pinning to that would point this command's
-// runtime at another workspace's tree. A recorded root is only honoured when it
-// matches one of the two roots THIS workspace derives, which is also the only
-// pair the selections could ever have disagreed about.
-// sandboxHome is the home THIS command asked for, not the one the parent
-// process happens to be pointed at.
-//
-// TWO ENVIRONMENT AUTHORITIES IS ONE TOO MANY. Runtime preparation ran before
-// Windows platform planning and resolved the home from the ambient environment,
-// while the planner resolves ZERO_WINDOWS_SANDBOX_HOME out of the command's own
-// spec.Env and hands THAT to the runner for marker validation. A request that
-// selects home B while the parent still points at home A pinned A's recorded
-// root into the profile, and the runner then loaded B's marker and rejected the
-// command as out of date even though setup for B was perfectly valid. The two
-// homes do not need different derivation rules to disagree, only different valid
-// selections from the same preferred/fallback pair.
-//
-// Empty means no command context, so the ambient environment is the only
-// authority there is and resolving it here is correct.
-func pinnedSandboxRuntimeRoot(workspaceRoot, preferred, fallback, sandboxHome string) string {
-	// No GOOS gate. The marker only exists where setup wrote one, so this is
-	// already Windows-only in practice, and keeping the code path platform-neutral
-	// means the setup-to-command contract is exercised on every CI runner instead
-	// of only the Windows one.
-	home := strings.TrimSpace(sandboxHome)
-	if home == "" {
-		resolved, err := ResolveWindowsSandboxHome(nil)
-		if err != nil {
-			return ""
-		}
-		home = resolved
-	}
-	recorded := WindowsSandboxRecordedRuntimeRoot(home)
-	if recorded == "" {
-		return ""
-	}
-	for _, candidate := range []string{preferred, fallback} {
-		if candidate != "" && sameWindowsRuntimeRootPath(recorded, candidate) {
-			return candidate
-		}
-	}
-	// A RECORD IS HISTORY, NOT SOMETHING TO RE-DERIVE.
-	//
-	// The candidates above are both computed from THIS process's environment, and
-	// the fallback one is computed from its TEMP. Setup can legitimately land on
-	// the fallback when the preferred cache root cannot be leased, and it records
-	// that concrete root. If a later IDE, service or terminal runs with a
-	// different TEMP, the recorded root matches neither of today's candidates, the
-	// record is ignored, and selection picks a tree setup never provisioned. The
-	// runner then rejects the marker as out of date, and re-running setup from the
-	// original environment does not make the other environment converge.
-	//
-	// So the recorded root is recognised by its OWN shape instead: it must be a
-	// runtime root Zero owns, its first component must be the fallback's, and its
-	// leaf must be this workspace's fallback digest. That answers "does this
-	// record belong to this workspace and home" without asking "what would I
-	// choose from scratch right now", which is the question that reintroduced the
-	// second selection.
-	//
-	// The cache-derived digest is deliberately NOT accepted here. A moved user
-	// cache stays stale, which is a different failure with a different remedy.
-	if ownedFallbackRuntimeRecord(workspaceRoot, recorded) {
-		return recorded
-	}
-	return ""
-}
-
-// ownedFallbackRuntimeRecord reports whether recorded is a fallback runtime root
-// this workspace would own, judged on the record's own shape.
-func ownedFallbackRuntimeRecord(workspaceRoot, recorded string) bool {
-	workspaceRoot = canonicalSandboxWorkspaceRoot(workspaceRoot)
-	if workspaceRoot == "" || strings.TrimSpace(recorded) == "" {
-		return false
-	}
-	base, components, owned := windowsSandboxRuntimeOwnedTail(recorded)
-	if !owned || base == "" || len(components) == 0 {
-		return false
-	}
-	names := sandboxRuntimeFallbackOwnedNames()
-	if len(names) == 0 || !strings.EqualFold(components[0], names[0]) {
-		return false
-	}
-	digest := sha256.Sum256([]byte(workspaceRoot + "\x00" + sandboxRuntimeUserScope()))
-	return strings.EqualFold(components[len(components)-1], hex.EncodeToString(digest[:8]))
-}
-
-// resolveSandboxRuntimeRootInputs is the part of runtime-root selection that
-// names things and touches nothing: the canonical workspace, and the root a
-// command prefers for it. It takes no lease and creates no directory.
-//
-// ONE RESOLVER, BECAUSE THERE WERE TWO AND THEY DISAGREED. Selection and the
-// diagnostic that asks "would a command still select the recorded root?" each
-// spelled these steps out, and the diagnostic's copy was missing one: a user
-// cache directory that resolves to nothing. os.UserCacheDir can succeed and
-// still hand back a value that canonicalizes to "" or ".", and selection fails
-// closed on that with "user cache directory is unavailable" while the
-// diagnostic carried on and derived candidates from the empty root. Doctor
-// could then report the recorded root as current, or as merely stale, on a
-// machine where every command fails before it ever looks at a marker. That is
-// the false-healthy signal this whole change exists to remove, reached through
-// a different edge of the same resolver.
-//
-// The diagnostic's doc comment already said it asked the command's question
-// through the command's function. Now it does: both callers get their inputs
-// here, so an input one of them refuses is refused by the other, with the same
-// error. Reported by @jatmn.
-func resolveSandboxRuntimeRootInputs(workspaceRoot string) (workspace string, preferred string, err error) {
-	workspace = canonicalSandboxWorkspaceRoot(workspaceRoot)
-	if workspace == "" || workspace == "." {
-		return "", "", errors.New("sandbox runtime requires a workspace root")
-	}
-	cacheRoot, err := sandboxUserCacheDir()
-	if err != nil {
-		return "", "", fmt.Errorf("resolve user cache directory: %w", err)
-	}
-	cacheRoot = canonicalSandboxWorkspaceRoot(cacheRoot)
-	if cacheRoot == "" || cacheRoot == "." {
-		return "", "", errors.New("user cache directory is unavailable")
-	}
-	preferred, err = sandboxRuntimeRootFor(workspace, cacheRoot)
-	if err != nil {
-		return "", "", err
-	}
-	return workspace, preferred, nil
-}
-
-// WindowsSandboxRecordedRuntimeRootIsCurrent answers the question a diagnostic
-// has to ask before trusting the marker: would a command run NOW still select
-// the runtime root that setup recorded?
-//
-// The marker's root is a historical fact, needed to check the stamp without
-// mutating anything. It is not proof that the root is still selectable. Setup
-// can run with the user cache at A, the cache can then move so commands derive
-// B, and the stamped A tree stays behind. A diagnostic that pins A reports a
-// healthy machine while every real command rejects A as not a current
-// candidate and fails on the out-of-date marker.
-//
-// This derives the same candidates a command derives, through the same
-// resolver, and applies the same equality, by calling the same function the
-// command path calls. It takes no lease and creates nothing.
-func WindowsSandboxRecordedRuntimeRootIsCurrent(sandboxHome, workspaceRoot string) (recorded string, current bool, err error) {
-	recorded = WindowsSandboxRecordedRuntimeRoot(sandboxHome)
-	if recorded == "" {
-		return "", false, nil
-	}
-	workspaceRoot, preferred, err := resolveSandboxRuntimeRootInputs(workspaceRoot)
-	if err != nil {
-		return recorded, false, err
-	}
-	fallback, _ := fallbackSandboxRuntimeRoot(workspaceRoot)
-	return recorded, pinnedSandboxRuntimeRoot(workspaceRoot, preferred, fallback, sandboxHome) != "", nil
-}
-
-// selectSandboxRuntimeRoot picks the runtime root and returns the lease on it
-// together with the directories acquiring that lease had to create.
-//
-// honorRecorded is true on the command side and false during setup: setup is
-// making the choice, so it must not consult a record it is about to overwrite,
-// or a single unlucky relocation to the temp fallback would pin every future
-// setup to temp.
-//
-// THE LEDGER IS PART OF THE ANSWER, NOT A DETAIL OF IT. Acquiring the lease
-// creates zero/runtime/v1 when they are not there, and this was the two-result
-// wrapper that dropped that fact on the floor. Setup then provisioned the leaf,
-// recorded only the leaf for rollback, and any failure before the marker left
-// the parents behind with nothing that knew it had made them. An error carries
-// the partial ledger out for the same reason: acquisition can create one
-// component and fail on the next.
-func selectSandboxRuntimeRoot(workspaceRoot string, honorRecorded bool, sandboxHome string) (string, *sandboxRuntimeLease, []windowsCreatedRuntimeDir, error) {
-	workspaceRoot, root, err := resolveSandboxRuntimeRootInputs(workspaceRoot)
-	if err != nil {
-		return "", nil, nil, err
-	}
-	// CONSUME SETUP'S CHOICE, do not re-make it. Everything below is a fresh
-	// selection whose answer depends on whether a lease can be taken right now,
-	// and a command reaching a different answer than setup did is the outage this
-	// contract exists to prevent: the tree the command names was never
-	// provisioned, so its ACL plan hash cannot match and no amount of re-running
-	// setup fixes it.
-	if honorRecorded {
-		fallbackRoot, _ := fallbackSandboxRuntimeRoot(workspaceRoot)
-		if pinned := pinnedSandboxRuntimeRoot(workspaceRoot, root, fallbackRoot, sandboxHome); pinned != "" {
-			lease, pinnedCreated, leaseErr := prepareSandboxRuntimeLeaseRecording(pinned)
-			if leaseErr != nil {
-				// NOT relocated. Relocating is what produced the permanent brick:
-				// the other root has no capability ACL, so the command would be
-				// rejected anyway, with a message about permissions. Failing here
-				// says the true thing and points at the action that fixes it.
-				return "", nil, pinnedCreated, fmt.Errorf("sandbox runtime root %s was provisioned by setup but cannot be used now (%w); "+
-					"re-run `zero sandbox setup` from an elevated (Administrator) terminal", pinned, leaseErr)
-			}
-			return pinned, lease, pinnedCreated, nil
-		}
-	}
-	lease, created, err := prepareSandboxRuntimeLeaseRecording(root)
-	if err == nil {
-		return root, lease, created, nil
-	}
-	// AN ALIASED COMPONENT IS NOT A REASON TO RELOCATE. Falling back here would
-	// leave the link in place, report nothing, and move to the next predictable
-	// name, which the same attacker can take as well. Relocating is for a root
-	// that is merely unusable.
-	if errors.Is(err, errRuntimeComponentAliased) {
-		return "", nil, created, err
-	}
-	// The preferred root could not be leased. Relocating is right, and it is what
-	// commands already did; the defect was that setup never learned about it.
-	root, err = fallbackSandboxRuntimeRoot(workspaceRoot)
-	if err != nil {
-		return "", nil, created, err
-	}
-	// APPENDED, not replaced. The preferred attempt may have created components
-	// before it failed to lease, and those are this invocation's too.
-	fallbackLease, fallbackCreated, err := prepareSandboxRuntimeLeaseRecording(root)
-	created = append(created, fallbackCreated...)
-	if err != nil {
-		return "", nil, created, err
-	}
-	return root, fallbackLease, created, nil
-}
@@ -16,7 +16,7 @@ func TestPrepareSandboxRuntimeStaysOutsideWorkspace(t *testing.T) {
 	sandboxUserCacheDir = func() (string, error) { return cacheRoot, nil }
 	t.Cleanup(func() { sandboxUserCacheDir = original })

-	runtimeState, release, err := prepareSandboxRuntime(workspace, "")
+	runtimeState, release, err := prepareSandboxRuntime(workspace)
 	if err != nil {
 		t.Fatalf("prepareSandboxRuntime: %v", err)
 	}
@@ -59,7 +59,7 @@ func TestPrepareSandboxRuntimeCleansExpiredSibling(t *testing.T) {
 	if err := os.Chtimes(expired, old, old); err != nil {
 		t.Fatal(err)
 	}
-	_, release, err := prepareSandboxRuntime(workspace, "")
+	_, release, err := prepareSandboxRuntime(workspace)
 	if err != nil {
 		t.Fatalf("prepareSandboxRuntime: %v", err)
 	}
@@ -71,34 +71,15 @@ func TestPrepareSandboxRuntimeCleansExpiredSibling(t *testing.T) {

 func TestPrepareSandboxRuntimeFallsBackWhenUserCacheIsInsideWorkspace(t *testing.T) {
 	workspace := t.TempDir()
-	// BOTH derivation inputs belong to the test, not just the cache one. The
-	// fallback root is derived from os.TempDir(), which reads TMPDIR on Unix and
-	// TMP/TEMP on Windows, and its name is deterministic, so stubbing only the
-	// cache left this test creating a persistent tree in the developer real temp
-	// directory that later runs then found already present.
-	tempHome := t.TempDir()
-	t.Setenv("TMPDIR", tempHome)
-	t.Setenv("TMP", tempHome)
-	t.Setenv("TEMP", tempHome)
-
 	original := sandboxUserCacheDir
 	sandboxUserCacheDir = func() (string, error) { return filepath.Join(workspace, ".cache"), nil }
 	t.Cleanup(func() { sandboxUserCacheDir = original })

-	runtimeState, release, err := prepareSandboxRuntime(workspace, "")
+	runtimeState, release, err := prepareSandboxRuntime(workspace)
 	if err != nil {
 		t.Fatalf("prepareSandboxRuntime: %v", err)
 	}
-	defer func() {
-		release()
-		// Only the root this test created.
-		_ = os.RemoveAll(runtimeState.Root)
-	}()
-	// And the redirect actually took, or the cleanup above removes one tree while
-	// the real one is still left behind somewhere else.
-	if !pathWithinRoot(canonicalSandboxWorkspaceRoot(tempHome), runtimeState.Root) {
-		t.Fatalf("fallback runtime root %q is outside the test-owned temp directory %q", runtimeState.Root, tempHome)
-	}
+	defer release()
 	if pathWithinRoot(workspace, runtimeState.Root) {
 		t.Fatalf("fallback runtime root %q must stay outside workspace %q", runtimeState.Root, workspace)
 	}
@@ -120,7 +101,7 @@ func TestCleanupSandboxRuntimeSkipsActiveLease(t *testing.T) {
 		sandboxRuntimeNow = originalNow
 	})

-	runtimeState, release, err := prepareSandboxRuntime(workspace, "")
+	runtimeState, release, err := prepareSandboxRuntime(workspace)
 	if err != nil {
 		t.Fatalf("prepareSandboxRuntime: %v", err)
 	}
@@ -1,105 +0,0 @@
-package sandbox
-
-import (
-	"errors"
-	"fmt"
-	"io/fs"
-	"os"
-	"path/filepath"
-	"runtime"
-	"time"
-)
-
-// runtimeTreeChildren are the directories prepared inside every runtime root,
-// relative to it, each listed after its parent.
-var runtimeTreeChildren = []string{
-	"cache",
-	"data",
-	"tmp",
-	filepath.Join("cache", "npm"),
-	filepath.Join("cache", "yarn"),
-	filepath.Join("cache", "corepack"),
-	filepath.Join("cache", "pip"),
-	filepath.Join("cache", "go-build"),
-	filepath.Join("data", "go-mod"),
-	filepath.Join("data", "cargo"),
-}
-
-// prepareSandboxRuntimeTree creates and secures the runtime root's own
-// directories through a handle on the root, and refuses any that is a link.
-//
-// THE CHILDREN ARE THE SANDBOX'S TO CHANGE. The root is a write root: every
-// backend grants or binds it read-write so a sandboxed command can use its TMP
-// and caches, and that same grant lets the command remove cache, data or tmp
-// and leave a link to a host directory in its place. The tree outlives the
-// command, and the fallback root is derived rather than minted, so the NEXT Zero
-// process prepares the same tree, unsandboxed. It used to MkdirAll and Chmod each
-// child by pathname after checking only the root and its ancestors, which
-// followed the planted link: a contained command could get Zero to create npm
-// inside a host directory and change that directory's mode. Reported by @jatmn.
-//
-// So nothing below the root is reached by pathname. os.Root resolves every
-// component relative to the root's handle and refuses one that leads out of it,
-// which covers the parent of every grandchild too; a child that is a link at
-// all is refused outright, because nothing but tampering puts one there; and
-// the mode is set on the opened directory rather than on a name that could be
-// swapped between the check and the change.
-func prepareSandboxRuntimeTree(root string, now time.Time) error {
-	tree, err := os.OpenRoot(root)
-	if err != nil {
-		return fmt.Errorf("open sandbox runtime root %s: %w", root, err)
-	}
-	defer func() { _ = tree.Close() }()
-	if err := secureRuntimeTreeDirectory(tree, ".", root); err != nil {
-		return err
-	}
-	for _, child := range runtimeTreeChildren {
-		path := filepath.Join(root, child)
-		if err := tree.Mkdir(child, 0o700); err != nil && !errors.Is(err, fs.ErrExist) {
-			return fmt.Errorf("create sandbox runtime directory %s: %w", path, err)
-		}
-		info, err := tree.Lstat(child)
-		if err != nil {
-			return fmt.Errorf("inspect sandbox runtime directory %s: %w", path, err)
-		}
-		// ModeIrregular as well as ModeSymlink: a Windows junction reports as
-		// irregular, and needs no privilege to create.
-		if info.Mode()&(os.ModeSymlink|os.ModeIrregular) != 0 {
-			return fmt.Errorf("%w: refusing to prepare the sandbox runtime through a link at %s, which a command run inside "+
-				"the sandbox can leave there to redirect Zero's own writes; remove %s and Zero will recreate it",
-				errRuntimeComponentAliased, path, root)
-		}
-		if !info.IsDir() {
-			return fmt.Errorf("sandbox runtime directory %s exists and is not a directory; remove %s and Zero will recreate it", path, root)
-		}
-		if err := secureRuntimeTreeDirectory(tree, child, path); err != nil {
-			return err
-		}
-	}
-	if err := tree.Chtimes(".", now, now); err != nil {
-		return fmt.Errorf("touch sandbox runtime root: %w", err)
-	}
-	return nil
-}
-
-// secureRuntimeTreeDirectory sets 0700 on the directory it opened, not on a
-// name, so a swap after the check cannot move the change onto something else.
-//
-// Not on Windows, where a directory's access is its ACL, which setup manages,
-// and chmod only toggles a read-only attribute Windows does not honour on
-// directories. Setting it there would also need write-attribute access this
-// handle does not have, for no effect.
-func secureRuntimeTreeDirectory(tree *os.Root, name string, path string) error {
-	if runtime.GOOS == "windows" {
-		return nil
-	}
-	directory, err := tree.Open(name)
-	if err != nil {
-		return fmt.Errorf("open sandbox runtime directory %s: %w", path, err)
-	}
-	defer func() { _ = directory.Close() }()
-	if err := directory.Chmod(0o700); err != nil {
-		return fmt.Errorf("secure sandbox runtime directory %s: %w", path, err)
-	}
-	return nil
-}
@@ -1,146 +0,0 @@
-package sandbox
-
-import (
-	"errors"
-	"os"
-	"os/exec"
-	"path/filepath"
-	"runtime"
-	"testing"
-)
-
-// A LINK LEFT IN THE TREE BY ONE COMMAND IS NOT FOLLOWED BY THE NEXT ZERO.
-//
-// The runtime root is a write root, so a sandboxed command can replace any
-// directory under it with a link to a host directory it cannot write. The
-// fallback root is derived rather than minted, so the next Zero process,
-// unsandboxed, prepares that same tree. It created and chmodded every child by
-// pathname and followed the link: jatmn's probe got outside/npm created and the
-// outside directory's mode changed through cache. Reported by @jatmn.
-//
-// Every child is covered, grandchildren included, because each is its own
-// pathname and closing one leaves the others open.
-
-// plantRuntimeLink replaces nothing; it makes link point at target, the way a
-// sandboxed command would: a symlink off Windows, and a junction on Windows,
-// where a symlink needs a privilege and a junction does not.
-func plantRuntimeLink(t *testing.T, link, target string) {
-	t.Helper()
-	if runtime.GOOS == "windows" {
-		if out, err := exec.Command("cmd", "/c", "mklink", "/J", link, target).CombinedOutput(); err != nil {
-			t.Fatalf("SETUP INVALID: plant a junction at %s: %v %s", link, err, out)
-		}
-	} else if err := os.Symlink(target, link); err != nil {
-		t.Fatalf("SETUP INVALID: plant a symlink at %s: %v", link, err)
-	}
-	linkInfo, err := os.Stat(link)
-	if err != nil {
-		t.Fatalf("SETUP INVALID: the planted link does not resolve: %v", err)
-	}
-	targetInfo, err := os.Stat(target)
-	if err != nil || !os.SameFile(linkInfo, targetInfo) {
-		t.Fatalf("SETUP INVALID: %s does not lead to %s, so the prepare below would prove nothing", link, target)
-	}
-}
-
-// fallbackRuntimeForTest makes the next prepareSandboxRuntime choose the temp
-// fallback, inside a temp directory this test owns.
-func fallbackRuntimeForTest(t *testing.T) string {
-	t.Helper()
-	workspace := t.TempDir()
-	redirectSandboxTestTemp(t, t.TempDir())
-	original := sandboxUserCacheDir
-	// A cache inside the workspace puts the preferred root there, which is what
-	// sends selection to the fallback.
-	sandboxUserCacheDir = func() (string, error) { return filepath.Join(workspace, ".cache"), nil }
-	t.Cleanup(func() { sandboxUserCacheDir = original })
-	return workspace
-}
-
-func TestPreparingTheRuntimeDoesNotFollowAChildLinkLeftByACommand(t *testing.T) {
-	for _, child := range runtimeTreeChildren {
-		t.Run(filepath.ToSlash(child), func(t *testing.T) {
-			workspace := fallbackRuntimeForTest(t)
-			first, release, err := prepareSandboxRuntime(workspace, "")
-			if err != nil {
-				t.Fatalf("SETUP INVALID: the first prepare failed: %v", err)
-			}
-			release()
-			requireWithinTestOwned(t, first.Root, os.TempDir())
-			if pathWithinRoot(canonicalSandboxWorkspaceRoot(workspace), canonicalSandboxWorkspaceRoot(first.Root)) {
-				t.Fatalf("SETUP INVALID: %s is the preferred root inside the workspace, not the fallback", first.Root)
-			}
-
-			// A host directory the command cannot write inside the sandbox, with a
-			// mode a chmod through the link would change.
-			outside := t.TempDir()
-			if runtime.GOOS != "windows" {
-				if err := os.Chmod(outside, 0o755); err != nil {
-					t.Fatal(err)
-				}
-			}
-			planted := filepath.Join(first.Root, child)
-			if err := os.RemoveAll(planted); err != nil {
-				t.Fatalf("SETUP INVALID: remove %s: %v", planted, err)
-			}
-			plantRuntimeLink(t, planted, outside)
-
-			second, release, err := prepareSandboxRuntime(workspace, "")
-			if err == nil {
-				release()
-				t.Fatalf("the next prepare accepted a link at %s (root %s)", child, second.Root)
-			}
-			if !errors.Is(err, errRuntimeComponentAliased) {
-				t.Fatalf("refused for a reason other than the link, so this does not pin the guard: %v", err)
-			}
-			entries, readErr := os.ReadDir(outside)
-			if readErr != nil {
-				t.Fatalf("read the outside directory: %v", readErr)
-			}
-			if len(entries) != 0 {
-				t.Errorf("the prepare created %d entries inside the directory the link pointed at, starting with %s", len(entries), entries[0].Name())
-			}
-			if runtime.GOOS != "windows" {
-				info, statErr := os.Stat(outside)
-				if statErr != nil {
-					t.Fatal(statErr)
-				}
-				if info.Mode().Perm() != 0o755 {
-					t.Errorf("the prepare changed the mode of the directory the link pointed at to %04o", info.Mode().Perm())
-				}
-			}
-		})
-	}
-}
-
-// The ordinary case still works through the handle: a second prepare of an
-// intact tree succeeds, and off Windows it puts a loosened child back to 0700.
-func TestPreparingAnIntactRuntimeTwiceStillSecuresItsChildren(t *testing.T) {
-	workspace := fallbackRuntimeForTest(t)
-	first, release, err := prepareSandboxRuntime(workspace, "")
-	if err != nil {
-		t.Fatalf("the first prepare failed: %v", err)
-	}
-	release()
-	requireWithinTestOwned(t, first.Root, os.TempDir())
-	loosened := filepath.Join(first.Root, "cache", "npm")
-	if runtime.GOOS != "windows" {
-		if err := os.Chmod(loosened, 0o755); err != nil {
-			t.Fatal(err)
-		}
-	}
-	_, release, err = prepareSandboxRuntime(workspace, "")
-	if err != nil {
-		t.Fatalf("the second prepare of an intact tree failed: %v", err)
-	}
-	release()
-	for _, child := range runtimeTreeChildren {
-		info, err := os.Lstat(filepath.Join(first.Root, child))
-		if err != nil || !info.IsDir() {
-			t.Fatalf("%s is missing or not a directory after prepare: %v", child, err)
-		}
-		if runtime.GOOS != "windows" && info.Mode().Perm() != 0o700 {
-			t.Errorf("%s is %04o after prepare, want 0700", child, info.Mode().Perm())
-		}
-	}
-}
@@ -1,9 +0,0 @@
-//go:build !windows
-
-package sandbox
-
-// currentProcessSID has no meaning off Windows. Windows sandbox setup args are
-// only ever built on Windows; this exists so the shared builder compiles.
-func currentProcessSID() (string, error) {
-	return "", nil
-}
@@ -1,30 +0,0 @@
-//go:build windows
-
-package sandbox
-
-import (
-	"fmt"
-
-	"golang.org/x/sys/windows"
-)
-
-// currentProcessSID is the SID of the token running this process.
-//
-// Called from BuildWindowsSandboxSetupArgs, which runs in the OPERATOR'S shell
-// before elevation, so it answers "who will read the stamp afterwards" rather
-// than "who is provisioning it". Those are different principals whenever setup
-// elevates, and the difference is the whole point: the elevated helper creates
-// the runtime leaf when it is absent, so anything inferred from that leaf
-// describes the installer and not the consumer.
-func currentProcessSID() (string, error) {
-	token := windows.GetCurrentProcessToken()
-	user, err := token.GetTokenUser()
-	if err != nil {
-		return "", fmt.Errorf("resolve the calling user SID for sandbox setup: %w", err)
-	}
-	sid := user.User.Sid
-	if sid == nil {
-		return "", fmt.Errorf("resolve the calling user SID for sandbox setup: the token carried none")
-	}
-	return sid.String(), nil
-}
@@ -1,54 +0,0 @@
-package sandbox
-
-import (
-	"os"
-	"testing"
-)
-
-// redirectSandboxTestTemp points EVERY variable os.TempDir reads at dir.
-//
-// os.TempDir reads TMPDIR on Unix and TMP, then TEMP, on Windows. A platform
-// neutral test that sets only the Windows pair changes nothing on Unix, where
-// fallbackSandboxRuntimeRoot then derives from the ambient temp: a path outside
-// every t.TempDir the test owns, which the test goes on to create or clear. The
-// cache resolver override does not reach this input, so a fixture that redirects
-// the cache and two of the three variables still has one production input it
-// does not own. Reported by @jatmn.
-//
-// It also verifies the redirect took, because a redirect that silently did
-// nothing is exactly how this went unnoticed: the tests passed, or skipped, and
-// the tree they made was somewhere else.
-func redirectSandboxTestTemp(t *testing.T, dir string) {
-	t.Helper()
-	t.Setenv("TMPDIR", dir)
-	t.Setenv("TMP", dir)
-	t.Setenv("TEMP", dir)
-	got := canonicalSandboxWorkspaceRoot(os.TempDir())
-	want := canonicalSandboxWorkspaceRoot(dir)
-	if !sameWindowsRuntimeRootPath(got, want) {
-		t.Fatalf("SETUP INVALID: os.TempDir() is %s after redirecting it to %s, so anything derived from temp lands outside this test", got, want)
-	}
-}
-
-// requireWithinTestOwned fails unless path is inside one of the directories the
-// test owns, compared canonically on both sides because the derivation
-// canonicalizes and a raw t.TempDir() can be a second spelling of the same place
-// (/var against /private/var, an 8.3 profile name on a Windows runner).
-//
-// Called BEFORE a derived path is created or removed. Checking afterwards
-// reports a tree in somebody else's directory once it already exists.
-func requireWithinTestOwned(t *testing.T, path string, owned ...string) {
-	t.Helper()
-	canonical := canonicalSandboxWorkspaceRoot(path)
-	for _, root := range owned {
-		if pathWithinRoot(canonicalSandboxWorkspaceRoot(root), canonical) {
-			return
-		}
-	}
-	t.Fatalf("%s (canonically %s) is outside every directory this test owns %v; refusing to create or remove it", path, canonical, owned)
-}
-
-// testStampPlanHash is the plan the stamp tests attest. The stamp is named after
-// the plan it proves, so a test that writes one and then looks for it has to use
-// the same plan on both sides; one shared value keeps that from being a typo.
-const testStampPlanHash = "planhash"
@@ -1,216 +0,0 @@
-//go:build windows
-
-package sandbox
-
-import (
-	"encoding/binary"
-	"fmt"
-	"os"
-	"os/exec"
-	"strings"
-	"testing"
-	"unsafe"
-
-	"golang.org/x/sys/windows"
-)
-
-const (
-	aceAdmissionLegEnv = "ZERO_TEST_ACE_ADMISSION_LEG"
-
-	// Not in x/sys/windows. Values from winnt.h.
-	testAccessAllowedObjectACEType   = 0x5
-	testAccessAllowedCallbackACEType = 0x9
-
-	aceAdmissionTrustee = "S-1-5-21-1111-2222-3333-4444"
-)
-
-// rawACE lays out an ACE by hand: header, mask, then whatever the leg wants to
-// sit where a plain ACE keeps its SID.
-func rawACE(aceType, aceFlags byte, mask uint32, tail []byte) []byte {
-	ace := make([]byte, windowsPlainACEHeaderSize+len(tail))
-	ace[0] = aceType
-	ace[1] = aceFlags
-	binary.LittleEndian.PutUint16(ace[2:], uint16(len(ace)))
-	binary.LittleEndian.PutUint32(ace[4:], mask)
-	copy(ace[windowsPlainACEHeaderSize:], tail)
-	return ace
-}
-
-// aclEndingAtAGuardPage builds an ACL whose last byte is the last readable byte
-// before a PAGE_NOACCESS page. A reader that trusts a length the entry did not
-// vouch for walks off the end and faults, instead of quietly reading whatever
-// the heap had next, which is what makes an over-read observable at all.
-func aclEndingAtAGuardPage(t *testing.T, aces ...[]byte) *windows.ACL {
-	t.Helper()
-	size := 8
-	for _, ace := range aces {
-		size += len(ace)
-	}
-	if size%4 != 0 {
-		t.Fatalf("SETUP INVALID: ACL size %d is not DWORD aligned", size)
-	}
-	page := os.Getpagesize()
-	base, err := windows.VirtualAlloc(0, uintptr(2*page), windows.MEM_RESERVE|windows.MEM_COMMIT, windows.PAGE_READWRITE)
-	if err != nil {
-		t.Fatalf("VirtualAlloc: %v", err)
-	}
-	var previous uint32
-	if err := windows.VirtualProtect(base+uintptr(page), uintptr(page), windows.PAGE_NOACCESS, &previous); err != nil {
-		t.Fatalf("VirtualProtect: %v", err)
-	}
-	start := base + uintptr(page) - uintptr(size)
-	// The address is not Go memory, so it is turned into a pointer without the
-	// uintptr-to-Pointer conversion vet rejects.
-	region := unsafe.Slice((*byte)(*(*unsafe.Pointer)(unsafe.Pointer(&start))), size)
-	region[0] = 4 // ACL_REVISION_DS, the revision that admits object ACEs
-	binary.LittleEndian.PutUint16(region[2:], uint16(size))
-	binary.LittleEndian.PutUint16(region[4:], uint16(len(aces)))
-	offset := 8
-	for _, ace := range aces {
-		offset += copy(region[offset:], ace)
-	}
-	return (*windows.ACL)(unsafe.Pointer(&region[0]))
-}
-
-func sidBytes(t *testing.T, sid *windows.SID) []byte {
-	t.Helper()
-	return append([]byte(nil), unsafe.Slice((*byte)(unsafe.Pointer(sid)), int(windows.GetLengthSid(sid)))...)
-}
-
-// TestACEAdmissionHelperProcess is the child. Every leg puts an ACL against a
-// guard page, so a regression is an access violation, and an access violation
-// in the test binary itself would take every other test in the package down
-// with it and could be misread as a pass.
-func TestACEAdmissionHelperProcess(t *testing.T) {
-	leg := os.Getenv(aceAdmissionLegEnv)
-	if leg == "" {
-		return
-	}
-	wanted, err := windows.StringToSid(aceAdmissionTrustee)
-	if err != nil {
-		t.Fatal(err)
-	}
-	_, required, err := windowsACLAccess(WindowsACLAllowWrite)
-	if err != nil {
-		t.Fatal(err)
-	}
-	whole := sidBytes(t, wanted)
-	// The first sub-authority and no more: revision, count and authority all
-	// match the trustee, so a comparison that believes the count keeps going.
-	prefix := whole[:12]
-	inherit := byte(windows.CONTAINER_INHERIT_ACE | windows.OBJECT_INHERIT_ACE)
-
-	var acl *windows.ACL
-	switch leg {
-	case "plain":
-		acl = aclEndingAtAGuardPage(t, rawACE(windows.ACCESS_ALLOWED_ACE_TYPE, inherit, uint32(required), whole))
-	case "object":
-		acl = aclEndingAtAGuardPage(t, rawACE(testAccessAllowedObjectACEType, inherit, uint32(required), prefix))
-	case "callback":
-		acl = aclEndingAtAGuardPage(t, rawACE(testAccessAllowedCallbackACEType, inherit, uint32(required), prefix))
-	case "truncated":
-		acl = aclEndingAtAGuardPage(t, rawACE(windows.ACCESS_ALLOWED_ACE_TYPE, inherit, uint32(required), prefix))
-	case "object then plain":
-		// An unreadable entry must not end the walk: the grant behind it counts.
-		acl = aclEndingAtAGuardPage(t,
-			rawACE(testAccessAllowedObjectACEType, inherit, uint32(required), prefix),
-			rawACE(windows.ACCESS_ALLOWED_ACE_TYPE, inherit, uint32(required), whole))
-	default:
-		t.Fatalf("unknown leg %q", leg)
-	}
-	fmt.Printf("carries=%v\n", windowsDACLCarriesGrant(acl, wanted, required, inherit))
-}
-
-// ONLY A PLAIN ALLOW OR DENY ACE IS READ AS ONE.
-//
-// The attestation walks the DACL and compared every entry's would-be SID with
-// the capability before it looked at the entry's type. Only ACCESS_ALLOWED_ACE
-// and ACCESS_DENIED_ACE keep a SID directly behind the mask; an object ACE has a
-// flags word and GUIDs there, and EqualSid is undefined for something that is
-// not a SID. The bytes below are arranged so that the misreading is not
-// hypothetical: each entry's tail spells the START of the trustee's SID, so its
-// revision and sub-authority count match and the comparison runs on for the
-// length that count claims, which is past the end of the entry and into a page
-// that cannot be read. With the type and size admitted first nothing is read
-// past the header, the legs answer false, and the plain entry still answers
-// true so the walk has not simply stopped crediting anything.
-//
-// Each leg runs in a child process. An access violation cannot be recovered in
-// Go, and one in this binary would end the package's other tests with it.
-// Reported by @jatmn.
-func TestOnlyPlainACEsAreReadDuringAttestation(t *testing.T) {
-	for _, testCase := range []struct {
-		leg  string
-		want bool
-	}{
-		{leg: "plain", want: true},
-		{leg: "object", want: false},
-		{leg: "callback", want: false},
-		{leg: "truncated", want: false},
-		{leg: "object then plain", want: true},
-	} {
-		t.Run(testCase.leg, func(t *testing.T) {
-			child := exec.Command(os.Args[0], "-test.run=^TestACEAdmissionHelperProcess$", "-test.count=1")
-			child.Env = append(os.Environ(), aceAdmissionLegEnv+"="+testCase.leg)
-			output, err := child.CombinedOutput()
-			if err != nil {
-				t.Fatalf("the attestation did not survive this entry (%v); an entry was read in a layout its type does not have:\n%s", err, lastLines(string(output), 12))
-			}
-			want := fmt.Sprintf("carries=%v", testCase.want)
-			if !strings.Contains(string(output), want) {
-				t.Fatalf("want %s, got:\n%s", want, lastLines(string(output), 12))
-			}
-		})
-	}
-}
-
-func lastLines(text string, count int) string {
-	lines := strings.Split(strings.TrimRight(text, "\r\n"), "\n")
-	if len(lines) > count {
-		lines = lines[len(lines)-count:]
-	}
-	return strings.Join(lines, "\n")
-}
-
-// THE TYPE RULE HAS TO HOLD ON ITS OWN. The guard-page legs above are also
-// stopped by the size rule, because their tails are short, so they pass with the
-// type rule removed. An object or callback entry can just as well be long enough
-// to hold the SID its would-be count claims, and then only its type says it is
-// not a plain ACE. Here every entry carries the trustee's whole SID at the plain
-// offset, which is the layout that would make a misread entry compare equal, and
-// only the two plain types may be admitted.
-func TestOnlyPlainACETypesAreAdmitted(t *testing.T) {
-	wanted, err := windows.StringToSid(aceAdmissionTrustee)
-	if err != nil {
-		t.Fatal(err)
-	}
-	whole := sidBytes(t, wanted)
-	for _, testCase := range []struct {
-		name    string
-		aceType byte
-		admit   bool
-	}{
-		{name: "allowed", aceType: windows.ACCESS_ALLOWED_ACE_TYPE, admit: true},
-		{name: "denied", aceType: windows.ACCESS_DENIED_ACE_TYPE, admit: true},
-		{name: "allowed object", aceType: 0x5, admit: false},
-		{name: "denied object", aceType: 0x6, admit: false},
-		{name: "allowed callback", aceType: 0x9, admit: false},
-		{name: "denied callback", aceType: 0xA, admit: false},
-		{name: "allowed callback object", aceType: 0xB, admit: false},
-		{name: "mandatory label", aceType: 0x11, admit: false},
-	} {
-		t.Run(testCase.name, func(t *testing.T) {
-			raw := rawACE(testCase.aceType, 0, 0x1F01FF, whole)
-			ace, sid := windowsPlainACESID((*windows.ACE_HEADER)(unsafe.Pointer(&raw[0])))
-			if (ace != nil) != testCase.admit {
-				t.Fatalf("type 0x%X admitted=%v, want %v", testCase.aceType, ace != nil, testCase.admit)
-			}
-			if testCase.admit && !sid.Equals(wanted) {
-				t.Errorf("an admitted plain entry did not yield its SID")
-			}
-		})
-	}
-	if ace, _ := windowsPlainACESID(nil); ace != nil {
-		t.Error("a nil header was admitted")
-	}
-}
@@ -6,7 +6,6 @@ import (
 	"errors"
 	"fmt"
 	"os"
-	"path/filepath"
 	"sort"
 	"strings"
 	"unsafe"
@@ -36,160 +35,13 @@ type windowsACLSnapshot struct {
 	Path         string
 	Descriptor   *windows.SECURITY_DESCRIPTOR
 	Materialized bool
-	// Identity is the object the forward apply actually modified, captured from
-	// the open handle. Compensation reopens BY NAME, and a name is not an
-	// object: see rollbackWindowsACLSnapshots.
-	Identity windowsObjectIdentity
-}
-
-// windowsObjectIdentity identifies a filesystem object independently of the
-// name it currently answers to.
-type windowsObjectIdentity struct {
-	volume uint32
-	high   uint32
-	low    uint32
-	valid  bool
-}
-
-func windowsIdentityFromHandle(handle windows.Handle) windowsObjectIdentity {
-	var info windows.ByHandleFileInformation
-	if err := windows.GetFileInformationByHandle(handle, &info); err != nil {
-		return windowsObjectIdentity{}
-	}
-	return windowsObjectIdentity{
-		volume: info.VolumeSerialNumber,
-		high:   info.FileIndexHigh,
-		low:    info.FileIndexLow,
-		valid:  true,
-	}
-}
-
-// matches is deliberately false when either side is unknown. A compensation
-// that cannot prove it is acting on the object it changed must not act.
-func (id windowsObjectIdentity) matches(other windowsObjectIdentity) bool {
-	return id.valid && other.valid &&
-		id.volume == other.volume && id.high == other.high && id.low == other.low
-}
-
-// windowsACLStampRequest asks the apply to write the runtime setup stamp THROUGH
-// THE SAME HANDLE it just applied the capability ACE through.
-//
-// The stamp exists to prove that the directory a command later uses is the
-// object setup granted the ACE to. Writing it afterwards by pathname cannot
-// prove that, however carefully the second open is done: the ACE goes on
-// through a rooted handle, that handle closes, network setup runs, and only then
-// does the marker write re-open the name. A local process that can reach the
-// user-owned runtime tree can remove the predictable root in that window and
-// put an ordinary directory in its place. The re-open correctly rejects a
-// junction, but an ordinary replacement is not a reparse point and is not the
-// ACL-bearing object either, so it collects a valid-looking stamp. Marker
-// validation then passes over a directory with no capability ACE, and the next
-// WRITE_RESTRICTED command fails its cache and TMP writes with setup insisting
-// it is current.
-//
-// Closing that means never naming the target again after the ACE lands. The
-// hash is known before the apply, so the stamp can simply ride along.
-type windowsACLStampRequest struct {
-	Root     string
-	PlanHash string
-	// RootIdentity is the file identity of the runtime directory the SNAPSHOT
-	// read, and RootIdentified says whether that capture actually succeeded.
-	//
-	// TWO OPENS OF ONE NAME ARE NOT ONE OBJECT. The snapshot proved "these prior
-	// bytes belong to B" through its own handle and then closed it; the apply
-	// resolved the same pathname again and mutated whatever answered, without
-	// either of them ever proving B and that object are the same. The root owner
-	// can rename the prior root aside, put an ordinary directory at the
-	// predictable name for the snapshot, and restore the original before the
-	// apply. The setup lease is a sibling and does not bind the root entry.
-	//
-	// The consequence was not merely a wrong ACL. On a later failure, stamp
-	// compensation compared the object it found against the snapshot's identity,
-	// refused to restore, and left this run's stamp on a directory whose marker
-	// still described the previous successful setup: a failed setup invalidating a
-	// good one.
-	RootIdentity   string
-	RootIdentified bool
-}
-
-// windowsACLStampSwapHook fires in the exact window this design closes: after
-// the capability ACE is on the object and before the stamp is written. Nil in
-// production; a test uses it to replace the runtime root with an ordinary
-// directory, which is what a local process would do.
-var windowsACLStampSwapHook func(path string)
-
-// windowsACLStampWriteHook replaces the ride-along stamp write. Nil in
-// production; a test uses it to reach the post-commit failure path, which no
-// ordinary input produces once the bound handle is already open.
-var windowsACLStampWriteHook func(path string) error
-
-// verifyStampRootIdentity refuses when the object this apply holds is not the one
-// the snapshot read.
-//
-// windowsACLStampIdentitySwapHook exists so a test can substitute the directory
-// in exactly the interval between the two opens, which is otherwise unreachable
-// from any ordinary input.
-func verifyStampRootIdentity(handle windows.Handle, path string, stamp *windowsACLStampRequest) error {
-	if !stamp.RootIdentified {
-		return fmt.Errorf("the sandbox runtime root %s could not be identified when its prior state was recorded, so this setup cannot prove it is about to change the same directory", path)
-	}
-	identity, err := handleRuntimeIdentity(handle)
-	if err != nil {
-		return fmt.Errorf("identify the sandbox runtime root %s before applying its ACL: %w", path, err)
-	}
-	if identity != stamp.RootIdentity {
-		return fmt.Errorf("the sandbox runtime root %s is no longer the directory this run recorded the prior state of, so applying the ACL and stamp here would attest to an object nobody inspected", path)
-	}
-	return nil
-}
-
-// windowsACLStampIdentitySwapHook fires in the interval between the snapshot's
-// close and the apply's open, which is where a substitution can actually land.
-// Nil in production.
-var windowsACLStampIdentitySwapHook func(path string)
-
-// windowsSameRuntimeRootPath compares two runtime roots the way the filesystem
-// does, so the stamp rides along with the right target regardless of spelling.
-// Beside its only caller: in the shared file it was dead code on every other
-// platform.
-func windowsSameRuntimeRootPath(left, right string) bool {
-	left = filepath.Clean(strings.TrimSpace(left))
-	right = filepath.Clean(strings.TrimSpace(right))
-	if left == "" || right == "" {
-		return false
-	}
-	return strings.EqualFold(left, right)
-}
-
-// writeRidingStamp writes the stamp through the handle the capability ACE was
-// applied on, or through the test hook when one is installed.
-func writeRidingStamp(handle windows.Handle, path string, planHash string) error {
-	if windowsACLStampWriteHook != nil {
-		return windowsACLStampWriteHook(path)
-	}
-	return writeWindowsRuntimeStampToDirectoryHandle(handle, windowsSandboxRuntimeStampName(planHash), planHash)
-}
-
-// restoreWindowsACLThroughHandle puts a captured DACL back on the object the
-// handle names, by handle rather than by pathname for the same reason the stamp
-// rides along: after the apply, the name is no longer proof of the object.
-func restoreWindowsACLThroughHandle(handle windows.Handle, descriptor *windows.SECURITY_DESCRIPTOR) error {
-	dacl, _, err := descriptor.DACL()
-	if err != nil {
-		return fmt.Errorf("read the captured windows DACL: %w", err)
-	}
-	return windows.SetSecurityInfo(handle, windows.SE_FILE_OBJECT, windows.DACL_SECURITY_INFORMATION, nil, nil, dacl, nil)
 }

 func applyWindowsACLPlan(plan WindowsACLPlan) (func() error, error) {
-	return applyWindowsACLPlanWithStamp(plan, nil)
-}
-
-func applyWindowsACLPlanWithStamp(plan WindowsACLPlan, stamp *windowsACLStampRequest) (func() error, error) {
 	groups := groupWindowsACLPlanByPath(plan)
 	snapshots := make([]windowsACLSnapshot, 0, len(groups))
 	for _, group := range groups {
-		snapshot, applied, err := applyWindowsACLPathGroupWithStamp(group, stamp)
+		snapshot, applied, err := applyWindowsACLPathGroup(group)
 		if err != nil {
 			rollbackErr := rollbackWindowsACLSnapshots(snapshots)
 			if rollbackErr != nil {
@@ -238,10 +90,6 @@ func groupWindowsACLPlanByPath(plan WindowsACLPlan) []windowsACLPathGroup {
 }

 func applyWindowsACLPathGroup(group windowsACLPathGroup) (windowsACLSnapshot, bool, error) {
-	return applyWindowsACLPathGroupWithStamp(group, nil)
-}
-
-func applyWindowsACLPathGroupWithStamp(group windowsACLPathGroup, stamp *windowsACLStampRequest) (windowsACLSnapshot, bool, error) {
 	path := strings.TrimSpace(group.Path)
 	if path == "" || len(group.Entries) == 0 {
 		return windowsACLSnapshot{}, false, nil
@@ -253,13 +101,6 @@ func applyWindowsACLPathGroupWithStamp(group windowsACLPathGroup, stamp *windows
 	// elevated setup a lower-privileged local user could swap the target for a
 	// symlink/junction between operations and redirect the ACL change onto a
 	// system object it never validated (issue #728, a TOCTOU privilege boundary).
-	// THE INTERVAL IS HERE, BEFORE THIS OPEN. The snapshot read its object through
-	// its own handle and closed it; a handle already open cannot be renamed out
-	// from under itself, so the only place a substitution can land is between that
-	// close and this open. The hook exists so a test can put it exactly there.
-	if windowsACLStampIdentitySwapHook != nil && stamp != nil && windowsSameRuntimeRootPath(stamp.Root, path) {
-		windowsACLStampIdentitySwapHook(path)
-	}
 	materialized := false
 	handle, isDir, err := openWindowsACLTarget(path)
 	if err != nil {
@@ -328,18 +169,6 @@ func applyWindowsACLPathGroupWithStamp(group windowsACLPathGroup, stamp *windows
 	} else {
 		nextDACL = baseDACL
 	}
-	// BEFORE THE FIRST MUTATION, NOT BEFORE THE STAMP. Checking later would leave
-	// the ACL already applied to the wrong object, which is the change that
-	// matters most.
-	//
-	// Fails CLOSED. An unestablished identity refuses rather than passing, because
-	// "we could not tell" is exactly the case this exists for; treating it as
-	// permission would make the guard a no-op precisely when it is needed.
-	if stamp != nil && windowsSameRuntimeRootPath(stamp.Root, path) {
-		if err := verifyStampRootIdentity(handle, path, stamp); err != nil {
-			return fail(err)
-		}
-	}
 	if err := windows.SetSecurityInfo(handle, windows.SE_FILE_OBJECT, windows.DACL_SECURITY_INFORMATION, nil, nil, nextDACL, nil); err != nil {
 		return fail(fmt.Errorf("apply windows ACL for %s: %w", path, err))
 	}
@@ -347,33 +176,8 @@ func applyWindowsACLPathGroupWithStamp(group windowsACLPathGroup, stamp *windows
 	// The handle has served its purpose (read+write bound to one object) and is
 	// closed now — rollback re-opens no-follow rather than holding a handle for
 	// the whole sandbox lifetime, since one caller discards the rollback closure.
-	// BEFORE THE HANDLE CLOSES, and only for the target the stamp names. This is
-	// the whole point: the ACE and the stamp land on one kernel object with no
-	// pathname resolution in between.
-	if stamp != nil && windowsSameRuntimeRootPath(stamp.Root, path) {
-		if windowsACLStampSwapHook != nil {
-			windowsACLStampSwapHook(path)
-		}
-		if err := writeRidingStamp(handle, path, stamp.PlanHash); err != nil {
-			// THE ACE AND ITS STAMP ARE ONE TRANSACTION.
-			//
-			// SetSecurityInfo above has already committed, and this function
-			// returns no rollback closure on its error paths, so the caller has
-			// nothing to compensate with. Without this restore a failed setup
-			// reports failure while leaving the capability grant on a pre-existing
-			// runtime root: the tree stays writable by the restricted token and
-			// nothing on disk records that it should not be.
-			if restoreErr := restoreWindowsACLThroughHandle(handle, descriptor); restoreErr != nil {
-				return fail(fmt.Errorf("stamp windows ACL target %s: %w (the committed ACL could not be restored either: %v)", path, err, restoreErr))
-			}
-			return fail(fmt.Errorf("stamp windows ACL target %s: %w", path, err))
-		}
-	}
-	// Captured while the handle is still open, because this is the last moment
-	// the object and the name are known to be the same thing.
-	identity := windowsIdentityFromHandle(handle)
 	_ = windows.CloseHandle(handle)
-	return windowsACLSnapshot{Path: path, Descriptor: descriptor, Materialized: materialized, Identity: identity}, true, nil
+	return windowsACLSnapshot{Path: path, Descriptor: descriptor, Materialized: materialized}, true, nil
 }

 // openWindowsACLTarget opens path for reading and rewriting its DACL without
@@ -385,24 +189,6 @@ func applyWindowsACLPathGroupWithStamp(group windowsACLPathGroup, stamp *windows
 // is exactly the redirection this guard exists to prevent. A missing target is
 // surfaced as os.ErrNotExist so the caller's materialize path still fires.
 func openWindowsACLTarget(path string) (windows.Handle, bool, error) {
-	// A RUNTIME ROOT IS OPENED BY HANDLE, NOT BY NAME.
-	//
-	// FILE_FLAG_OPEN_REPARSE_POINT below protects only the FINAL component; every
-	// ancestor in the pathname is resolved normally. The runtime tail is the one
-	// part of the tree Zero creates and therefore the one part an unprivileged
-	// local user can predict and pre-empt, and junctions need no privilege, so a
-	// swap at an owned ancestor between the last check and this open redirects the
-	// elevated capability ACL into a directory of their choosing.
-	//
-	// Everything else here is the user's own tree, where an ancestor reparse point
-	// is ordinary configuration and following it is correct.
-	if _, _, owned := windowsSandboxRuntimeOwnedTail(path); owned {
-		handle, err := openWindowsRuntimeTailDirectory(path, windows.READ_CONTROL|windows.WRITE_DAC|windows.FILE_TRAVERSE)
-		if err != nil {
-			return 0, false, err
-		}
-		return handle, true, nil
-	}
 	utf16Path, err := windows.UTF16PtrFromString(path)
 	if err != nil {
 		return 0, false, fmt.Errorf("encode windows ACL target %s: %w", path, err)
@@ -816,76 +602,27 @@ func rollbackWindowsACLSnapshots(snapshots []windowsACLSnapshot) error {
 	var errs []error
 	for index := len(snapshots) - 1; index >= 0; index-- {
 		snapshot := snapshots[index]
-		// A NAME IS NOT AN OBJECT ONCE THE APPLY HANDLE HAS CLOSED.
-		//
-		// The forward apply and its stamp go through one handle, so they are
-		// provably about one object. Compensation runs later, after a network or
-		// marker failure, and resolves these names again. Opening no-follow stops
-		// a reparse point but accepts an ORDINARY directory moved into the name
-		// since: the original is renamed aside, a substitute is created, and
-		// rollback then restores the pre-apply DACL onto the substitute, strips a
-		// stamp there, and reports success, while the moved original keeps this
-		// run's capability ACE and a valid stamp. Setup would claim a completed
-		// rollback with the modified object still reachable elsewhere, having
-		// also mutated something it never touched going forward.
-		//
-		// So every compensation proves it holds the object it changed, and
-		// otherwise leaves the substitute alone and says plainly what was left
-		// behind.
 		if snapshot.Materialized {
-			// NOT identity-checked, and the reason is a real limit rather than an
-			// oversight. A materialized target is one this run created, and its
-			// plan routinely denies Everyone read (that is what a protected
-			// metadata carve-out IS), so the attributes identity needs cannot be
-			// read back even by the owner: the check turned rollback of every
-			// materialized directory into "Access is denied". Establishing
-			// identity here would mean holding the apply handle open until the
-			// last failure point, which is a larger change than this one.
 			if err := os.RemoveAll(snapshot.Path); err != nil {
 				errs = append(errs, fmt.Errorf("remove materialized windows ACL target %s: %w", snapshot.Path, err))
 			}
 			continue
 		}
-		// ONE OPEN, AND THE IDENTITY COMES FROM THE HANDLE THAT GETS MUTATED.
-		//
-		// Checking identity through a separate open and then resolving the name
-		// again for the restore proves nothing about the second handle: the two
-		// opens are a check-then-use, and the fact established (this NAME resolved
-		// to the object we changed) is not the fact the write depends on (this
-		// HANDLE is that object). Opening once and asking the handle who it is
-		// removes the window rather than narrowing it.
+		dacl, _, err := snapshot.Descriptor.DACL()
+		if err != nil {
+			errs = append(errs, fmt.Errorf("read rollback windows DACL for %s: %w", snapshot.Path, err))
+			continue
+		}
+		// Re-open no-follow rather than restoring by pathname: the restore must
+		// land on the real object, not a reparse point swapped in since apply. The
+		// residual window is small because the target is ACL-restricted by now, but
+		// a handle keeps the restore honest. On a materialized-target rollback we
+		// remove it above, so only the restore-existing path opens here.
 		handle, _, err := openWindowsACLTarget(snapshot.Path)
 		if err != nil {
 			errs = append(errs, fmt.Errorf("re-open windows ACL target %s for rollback: %w", snapshot.Path, err))
 			continue
 		}
-		// TWO DIFFERENT REFUSALS. A snapshot whose identity was never established
-		// fails the match below exactly as a replaced target does, and both must
-		// leave the object alone, but telling the operator it was replaced when it
-		// was never identified sends them looking for a substitution that did not
-		// happen. Reported by CodeRabbit.
-		if !snapshot.Identity.valid {
-			_ = windows.CloseHandle(handle)
-			errs = append(errs, fmt.Errorf(
-				"windows ACL target %s could not be identified when this setup changed it, "+
-					"so rollback cannot prove it is the same object and leaves it with this run's grant",
-				snapshot.Path))
-			continue
-		}
-		if !snapshot.Identity.matches(windowsIdentityFromHandle(handle)) {
-			_ = windows.CloseHandle(handle)
-			errs = append(errs, fmt.Errorf(
-				"windows ACL target %s is no longer the object this setup modified; "+
-					"leaving the replacement untouched, and the original still carries this run's grant",
-				snapshot.Path))
-			continue
-		}
-		dacl, _, err := snapshot.Descriptor.DACL()
-		if err != nil {
-			_ = windows.CloseHandle(handle)
-			errs = append(errs, fmt.Errorf("read rollback windows DACL for %s: %w", snapshot.Path, err))
-			continue
-		}
 		if err := windows.SetSecurityInfo(handle, windows.SE_FILE_OBJECT, windows.DACL_SECURITY_INFORMATION, nil, nil, dacl, nil); err != nil {
 			errs = append(errs, fmt.Errorf("rollback windows ACL for %s: %w", snapshot.Path, err))
 		}
@@ -48,63 +48,6 @@ func TestApplyWindowsACLPathGroupHandleBasedRoundTrip(t *testing.T) {
 	}
 }

-// Both refusals leave the target alone, but they are different facts: one
-// target was never identified, the other was replaced. The operator has to be
-// told which, or a missing identity reads as a substitution that never happened.
-func TestRollbackTellsAnUnidentifiedTargetFromAReplacedOne(t *testing.T) {
-	dir := t.TempDir()
-	group := windowsACLPathGroup{
-		Path: dir,
-		Entries: []WindowsACLEntry{{
-			Action:     WindowsACLAllowWrite,
-			Path:       dir,
-			Capability: "S-1-1-0",
-		}},
-	}
-	snapshot, applied, err := applyWindowsACLPathGroup(group)
-	if err != nil || !applied {
-		t.Fatalf("apply: applied=%v err=%v", applied, err)
-	}
-	t.Cleanup(func() {
-		if err := rollbackWindowsACLSnapshots([]windowsACLSnapshot{snapshot}); err != nil {
-			t.Errorf("restore: %v", err)
-		}
-	})
-
-	other := t.TempDir()
-	handle, _, err := openWindowsACLTarget(other)
-	if err != nil {
-		t.Fatalf("open the other directory: %v", err)
-	}
-	otherIdentity := windowsIdentityFromHandle(handle)
-	_ = windows.CloseHandle(handle)
-	if !otherIdentity.valid {
-		t.Fatal("SETUP INVALID: could not identify the other directory")
-	}
-
-	for _, tc := range []struct {
-		name     string
-		identity windowsObjectIdentity
-		want     string
-		notWant  string
-	}{
-		{"never identified", windowsObjectIdentity{}, "could not be identified", "no longer the object"},
-		{"replaced", otherIdentity, "no longer the object", "could not be identified"},
-	} {
-		t.Run(tc.name, func(t *testing.T) {
-			changed := snapshot
-			changed.Identity = tc.identity
-			err := rollbackWindowsACLSnapshots([]windowsACLSnapshot{changed})
-			if err == nil {
-				t.Fatal("rollback acted on a target it could not prove was the one it changed")
-			}
-			if !strings.Contains(err.Error(), tc.want) || strings.Contains(err.Error(), tc.notWant) {
-				t.Fatalf("rollback error = %q, want it to say %q and not %q", err, tc.want, tc.notWant)
-			}
-		})
-	}
-}
-
 // dirDeniesReadSID reports whether path's DACL has a DENY ACE for wantSID whose
 // mask covers FILE_GENERIC_READ (DenyRead shape) without the full write-probe
 // mask of experimental DenyWrite.
Author	SHA1	Message	Date
Kevin CodexandClaude Opus 5.5	208918f699	ci(coderabbit): make Release-As releases explicit in the release check The release check claimed a Release-As footer could override the bump rule, but CodeRabbit's pre-merge checker cannot see commit footers, so a forced-version release would fail with no stated way through. It now names both versions and says a maintainer must ignore the check for such a release. Co-Authored-By: Claude Opus 5.5 <[email protected]>	2026-11-04 10:43:23 +08:00
Kevin CodexandClaude Opus 5.5	f94c9cc830	ci(coderabbit): decide the release check from the title and diff only CodeRabbit's pre-merge checker does not receive the head branch name, the PR body, or GitHub Check results. On #29 it reported the release check as inconclusive for that reason, and an inconclusive check does not block approval, so on a real release PR the gate would have done nothing. Scope is now decided from what the checker sees: a PR is a release PR when its title starts with "chore(main): release" or its diff changes the package version or the release-please manifest. Either signal is enough, so a retitled release PR still fails on its title. The body and GitHub Checks items are removed because the checker cannot evaluate them; CI stays covered by required status checks and by the release workflow's own gate. An item that still cannot be decided now fails instead of reporting inconclusive. Co-Authored-By: Claude Opus 5.5 <[email protected]>	2026-11-04 10:32:44 +08:00
Kevin CodexandClaude Opus 5.5	173fbb7073	ci(coderabbit): count every non-feature changelog section as a fix The bump-level item only mapped "Bug Fixes" and "Performance Improvements" to a fix, so a revert-only release, which release-please cuts as a patch with a "Reverts" section, would have been rejected. release-please bumps the patch for any releasable change that is not a feature or breaking, so every other section now counts as a fix. Addresses CodeRabbit's third review on #29. Co-Authored-By: Claude Opus 5.5 <[email protected]>	2026-11-04 10:21:17 +08:00
Kevin CodexandClaude Opus 5.5	fb238621bf	ci(coderabbit): validate the release PR body and every check conclusion release-please finds a merged release PR by parsing its body: the `---` delimiters and a release-notes heading that names the version. A body edited out of that layout can leave the merge without a tag, GitHub release, or npm publish, so the release check now fails it. The checks item now fails any completed check whose conclusion is not success, neutral, or skipped, which adds action_required and stale. Pending or absent checks still do not fail it. Addresses CodeRabbit's second review on #29. Co-Authored-By: Claude Opus 5.5 <[email protected]>	2026-11-04 10:09:35 +08:00
Kevin CodexandClaude Opus 5.5	3da783fa8b	ci(coderabbit): select release PRs by branch, not title The release check treated a PR as a release only when both its head branch and its title matched, so editing a release PR's title skipped every validation and the check passed. Release PRs are now selected by the release-please head branch alone, and a malformed title fails the check as its own item. NEW now comes from package.json at the PR head. Addresses CodeRabbit's review on #29. Co-Authored-By: Claude Opus 5.5 <[email protected]>	2026-11-04 09:59:39 +08:00
Kevin CodexandClaude Opus 5.5	b253c2d2aa	ci(coderabbit): let CodeRabbit approve PRs and gate release PRs Move from the deprecated reviews.request_changes_workflow to reviews.auto_approvals.enabled. CodeRabbit reads the old key as an alias, and setting both keys to different values turns approvals off. Release-please PRs publish to npm when merged, so a blocking "npm release PR validated" check runs on them: only the release files change, the version agrees everywhere, the bump fits the changelog, and no GitHub Check failed. Stop ignoring github-actions[bot], which opens those PRs and was why CodeRabbit skipped them. Co-Authored-By: Claude Opus 5.5 <[email protected]>	2026-11-04 09:47:45 +08:00
github-actions[bot]	7c55a09c07	chore(main): release 0.12.0 (#1150 ) Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>	2026-11-03 08:10:03 +08:00
Octopusandocto-patch	d99f92b81a	feat(providers): add MiniMax-M3.1-Flash to catalog (#1180 ) * feat(providers): add MiniMax-M3.1-Flash to catalog * fix(providers): forward MiniMax adaptive effort --------- Co-authored-by: octo-patch <[email protected]>	2026-11-02 18:28:17 +08:00
zzucs	5aa54fe1d2	fix(agent): mark headless guardrail stops incomplete (#1182 ) * fix(agent): mark headless guardrail stops incomplete The empty-turn and repeated-tool-failure guards returned the run as a successful final answer even with RequireCompletionSignal enabled, so zero exec reported exit 0 and a success run_end for work it had just abandoned. Set Result.Incomplete and a reason at both stops when the headless gate is on. Thresholds, stop answers, retry behavior, interactive runs, and --no-completion-gate are unchanged. Fixes #1154 * fix(agent): count consecutive empty turns in stop reason The headless no-output stop reason used result.Turns, which counts every turn in the run, so a run with an earlier tool-call turn claimed "stopped after 4 turns with no output" for a 3-turn empty streak. Use the guard's emptyTurns counter and say "consecutive". Pinned by a case with a successful tool turn before the empty streak.	2026-11-02 14:25:40 +08:00
oniisan	f912f80d00	fix(chatgpt): refresh subscription defaults and compatibility version (#1179 ) * fix(chatgpt): refresh subscription defaults and compatibility version Replace retired preset suggestions with vendor-documented Sol, Terra, Astra, and Luna IDs. Update the Codex discovery compatibility floor for Terra and verify that saved model selections are retained. Refs Twigpine/zero#1177 (defaults portion) * test(chatgpt): assert discovered model in compatibility probe Verify the live response contains exactly gpt-future-coder in addition to checking the Codex compatibility query value. Refs Twigpine/zero#1177 * docs(chatgpt): align Terra description with vendor guidance	2026-11-02 14:25:00 +08:00
ShryukGrandhi	9c4e3693e7	test(providerio): widen the idle window in the content-reset test (#1172 ) TestScanSSEDataWithContextContentResetsOnData wrote a data line every 30ms against a 100ms idle timeout, so a ~70ms scheduling stall on a shared runner tripped the idle watchdog. Use a 500ms idle timeout (600ms content window) and 25 writes (~750ms), which keeps every gap far under the idle timeout while the run still outlasts the content window. Signed-off-by: ShryukGrandhi <[email protected]>	2026-11-02 14:24:24 +08:00
zzucs	63962081cf	fix(sessions): capture freeform apply_patch checkpoints (#1183 ) * fix(sessions): capture freeform apply_patch checkpoints Freeform apply_patch arguments are raw patch text, but both checkpoint paths passed them to json.Unmarshal and dropped the error, so they found no mutation targets and recorded no checkpoint. The tool still executed, leaving rewind unable to restore the edit. Resolve targets through a shared agent.CheckpointTargets helper that prepares freeform input on the registered built-in apply_patch, exactly as executeToolCall does. This keeps absolute patch headers workspace-relative (rewriting them and setting cwd) instead of handing absolute paths to the checkpoint layer. Non-freeform calls now use the same tolerant decoder as execution, so the two cannot disagree. Covers the CLI, spec-draft, and TUI capture paths. Fixes #1162 * test(sessions): canonicalize freeform absolute-header fixtures t.TempDir returns the symlinked /var spelling on macOS and short names on Windows, while the apply_patch tool resolves the physical root. The absolute-header cases must build the header from the resolved root, as the repo's cross-platform test guidance requires.	2026-11-02 14:23:58 +08:00
Vasanth T	2e10a4b60b	test(swarm): wait for every slot to be released before checking the drain (#1181 ) A finishing member's task reads Done before its slot is freed: the supervisor completes the task, removes the member and only then frees the slot in afterExit. The test waited for Done == 5 and checked the running count straight after, so a busy runner could see one slot still held. It now waits for the drain itself.	2026-10-29 06:30:41 -07:00
Vasanth T	4247a07aba	test(tools): stop the tty session before removing its root (#1175 ) TestExecCommandTTYSkipsInteractiveBlock runs ssh to a bogus host under tty with a 250ms yield. When ssh is still resolving the host after the yield, the session keeps running in the test's directory and nothing stops it, so on Windows t.TempDir's cleanup fails with a sharing violation. It failed Smoke (windows-latest) on a README-only PR. Stop the session in a cleanup, and build the tool on execTestRoot, which retries the removal while Windows still holds the directory right after the stop, as the other exec tests that leave a session do.	2026-10-27 06:25:41 -07:00
Vasanth T	b837448dd4	docs: list Vercel under Partners (#1176 ) Adds Vercel beside GMI Cloud in the Partners table of both READMEs, with Vercel's official logotype in light and dark variants, taken unchanged from its brand page apart from a trailing newline.	2026-10-27 15:29:59 +08:00
Vasanth T	a0b036a3c7	docs: add Vercel OSS Program badge (#1174 ) * docs: add Vercel OSS Program badge * docs: add the Vercel OSS Program badge to README_ZH too The two READMEs carry the same badge row, so the Chinese one gets the badge as well. Both now list alt before src, like the other badges in the row.	2026-10-27 12:30:50 +05:30
Vasanth T	164150633a	feat(skills): install a skill's bundled files and point the skill tool at them (#1167 ) * feat(skills): install a skill's bundled files and point the skill tool at them A skill was only its SKILL.md: install copied that one file, and the skill tool returned only the markdown body, so a skill whose instructions run its own scripts was broken by installing it, and a hand-placed skill directory gave the model no way to find the files beside it. Install now copies the whole skill directory, leaving out hidden entries (.git, .env) and symlinks, which it neither follows nor copies but reports. A script stays executable; everything else is installed 0644. A tree over 1,000 files or 32 MiB is refused. skills.lock records a treeHash beside the SKILL.md hash; skill info reports drift in bundled files, and an entry from before this is still checked on SKILL.md alone. A reinstall that changed only a bundled file is reported as an update. Both skill tools end their output with the skill's directory and a list of up to 50 bundled files when there are any; a SKILL.md-only skill is returned unchanged. Closes #584 * fix(skills): compare a reinstall over an old lock entry with SKILL.md alone A lock entry written before tree hashes has no treeHash, so the reinstall compared the new tree hash with an empty string and always counted as an update. Re-adding an unchanged skill that is only a SKILL.md printed "Updated" and "the files bundled with it changed" when there were none. That entry comes from an install that copied SKILL.md alone, so compare with that: it is an update when SKILL.md changed or the reinstall now brings bundled files. InstallResult gains previousTreeHash, and the CLI uses it to say the bundled files are now installed rather than changed on the first reinstall over such an entry. * fix(skills): match the tree hash when recovering an interrupted install lockReconciler matched both trees of an interrupted workspace on the SKILL.md hash alone. A reinstall that changes only a bundled file keeps SKILL.md, so a kill between the tree swap and the lockfile publish left two trees carrying the recorded hash, and recovery kept the new one and retired the backup the lockfile still described. A tree now matches an entry with a tree hash only when both hashes match. An entry from before tree hashes is still matched on SKILL.md alone. A tree hash that cannot be computed is returned as an error, the way an unreadable SKILL.md already is, so recovery decides nothing about a tree it could not read. Also bring the package comment up to date: install copies the skill directory, not SKILL.md alone.	2026-10-25 07:24:59 -07:00
				`@@ -0,0 +1 @@`
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