fix(tools): put the written bytes back when the formatter fails

These formatters edit in place, so one killed by the deadline, killed by
the caller, or exiting partway through its own rewrite can leave the target
truncated: neither the input nor the output. Returning the written bytes on
top of that left the tracker baseline and the diff preview describing a
file that is not on disk, so the next edit would compare against content the
file does not have. That is a worse failure than the missing formatting the
previous commit set out to disclose.

The failure paths write the bytes back, and when even that fails the user
hears about it, since it is their file. Written back unconditionally on that
path rather than only when the bytes differ: comparing first means reading
the file to find out, and a read that fails leaves the same ambiguity this
exists to remove.

The fixture is proven to damage the file on a throwaway copy, because a
successful restore erases the evidence from the real one. My first attempt
asserted that on the target itself, where it could only pass vacuously or
report the opposite of what it saw.

internal/tools/edit_file.go

@@ -160,7 +160,8 @@ func (tool editFileTool) RunWithOptions(ctx context.Context, args map[string]any
 	// Optional format-on-write (ZERO_FORMAT_ON_WRITE). Must run BEFORE the
 	// FileTracker re-baseline: recording pre-format content would make the very
 	// next edit look like an external modification and trip the conflict guard.
-	updated = maybeFormatWrittenFile(ctx, absolutePath, updated)
+	formatting := maybeFormatWrittenFile(ctx, absolutePath, updated)
+	updated = formatting.Content
 	// Re-baseline to the content we just wrote so subsequent edits in this session
 	// compare against the current on-disk state, not the pre-edit version.
 	newInfo, _ := os.Stat(absolutePath)
@@ -193,6 +194,7 @@ func (tool editFileTool) RunWithOptions(ctx context.Context, args map[string]any
 		suffix = "s"
 	}
 	summary := fmt.Sprintf("Successfully edited %s (replaced %d occurrence%s).", relativePath, replacedCount, suffix)
+	summary += formatting.notice(relativePath)
 	summary += inlineDiagnostics(ctx, options, absolutePath, relativePath)
 	result := okResult(summary)
 	result.ChangedFiles = []string{relativePath}

internal/tools/format_on_write.go

@@ -2,6 +2,7 @@ package tools
 
 import (
 	"context"
+	"errors"
 	"os"
 	"os/exec"
 	"path/filepath"
@@ -22,8 +23,59 @@ import (
 // the conflict guard.
 
 // formatOnWriteTimeout bounds one formatter run; a wedged formatter must never
-// hang a tool call. On timeout the unformatted write stands.
-const formatOnWriteTimeout = 10 * time.Second
+// hang a tool call. On timeout the unformatted write stands, and the caller
+// says so: see formatOnWriteResult.
+//
+// A var rather than a const so a test can shorten it. Nothing outside a test
+// assigns to it, and the deadline path was previously unreachable in a test at
+// any speed, which is part of why it went unnoticed that it reported nothing.
+var formatOnWriteTimeout = 10 * time.Second
+
+// formatOnWriteResult is the content after formatting, plus whether the
+// formatting that was supposed to happen actually did.
+//
+// A TIMEOUT IS NOT THE SAME KIND OF MISS AS THE OTHERS. Every other way this
+// falls back is a standing fact about the environment: the toggle is off, the
+// extension has no formatter, the binary is not installed. Those are silent on
+// purpose, because nothing is wrong and saying so on every write would be
+// noise. A deadline firing is different: formatting was configured, available
+// and expected, and the file was written unformatted anyway, on a machine that
+// was merely slow. Left silent, the caller believes it wrote canonical style
+// and finds out from a CI format check it cannot see, which is the thing this
+// feature exists to prevent.
+type formatOnWriteResult struct {
+	Content string
+	// Formatter is the binary that was run, named in the notice so the user can
+	// tell a slow gofmt from a slow prettier.
+	Formatter string
+	TimedOut  bool
+	// RestoreFailed means the file on disk is not known to hold Content.
+	//
+	// These formatters edit in place, so one that is killed or fails partway
+	// can leave the target truncated or half-rewritten: what a dead
+	// `prettier --write` leaves behind is not the input and not the output.
+	// Returning the written bytes while disk holds something else would put the
+	// tracker baseline, the diff preview and the file itself into three
+	// different states, so the failure paths write the bytes back. When even
+	// that fails the user has to hear about it: it is their file.
+	RestoreFailed bool
+}
+
+// notice is the line appended to the tool summary when formatting was expected
+// and did not happen, and empty in every other case.
+func (result formatOnWriteResult) notice(relativePath string) string {
+	if result.RestoreFailed {
+		return "\n\nWARNING: " + relativePath + " may not hold what was written. " +
+			result.Formatter + " was interrupted while rewriting it in place and the " +
+			"content could not be written back. Re-read the file before trusting it."
+	}
+	if !result.TimedOut {
+		return ""
+	}
+	return "\n\nNote: " + relativePath + " was written but not formatted: " +
+		result.Formatter + " did not finish within " + formatOnWriteTimeout.String() +
+		". The file holds exactly what was written, so a project format check may still flag it."
+}
 
 // formatterCommands maps a file extension to the formatter argv; the file path
 // is appended as the final argument. Only in-place, config-respecting,
@@ -69,18 +121,20 @@ func formatOnWriteEnabled() bool {
 // enabled and on PATH) and returns the file's content afterwards. Best-effort
 // throughout: any failure — no formatter, formatter error, timeout, unreadable
 // result — returns writtenContent so the caller's state matches the last write
-// it performed itself.
-func maybeFormatWrittenFile(ctx context.Context, absolutePath string, writtenContent string) string {
+// it performed itself. Only the timeout is reported back, for the reason on
+// formatOnWriteResult.
+func maybeFormatWrittenFile(ctx context.Context, absolutePath string, writtenContent string) formatOnWriteResult {
+	unformatted := formatOnWriteResult{Content: writtenContent}
 	if !formatOnWriteEnabled() {
-		return writtenContent
+		return unformatted
 	}
 	command, ok := formatterCommands[strings.ToLower(filepath.Ext(absolutePath))]
 	if !ok {
-		return writtenContent
+		return unformatted
 	}
 	binaryPath, err := exec.LookPath(command[0])
 	if err != nil {
-		return writtenContent
+		return unformatted
 	}
 	formatCtx, cancel := context.WithTimeout(ctx, formatOnWriteTimeout)
 	defer cancel()
@@ -89,11 +143,35 @@ func maybeFormatWrittenFile(ctx context.Context, absolutePath string, writtenCon
 	formatter.Dir = filepath.Dir(absolutePath)
 	formatter.Stdin = strings.NewReader("")
 	if err := formatter.Run(); err != nil {
-		return writtenContent
+		unformatted.Formatter = command[0]
+		// THE FORMATTER EDITS IN PLACE, SO A FAILED RUN CAN LEAVE THE FILE
+		// NEITHER FORMATTED NOR AS WRITTEN. Killed by the deadline or by the
+		// caller, or exiting partway through its own rewrite, the target can hold
+		// a truncation. Returning the written bytes on top of that would leave the
+		// tracker baseline and the diff preview describing a file that is not on
+		// disk, which is a worse failure than the missing formatting: the next
+		// edit compares against content the file does not have.
+		//
+		// Written back unconditionally on this path rather than only when the
+		// bytes differ. Comparing first means reading the file to find out, and a
+		// read that fails leaves the same ambiguity this exists to remove.
+		if restoreErr := os.WriteFile(absolutePath, []byte(writtenContent), 0o644); restoreErr != nil {
+			unformatted.RestoreFailed = true
+		}
+		// OUR deadline, not the caller's cancellation and not the formatter's own
+		// exit status. A cancelled tool call is already being reported as
+		// cancelled, and a formatter that ran and refused the file usually means
+		// content it could not parse, which the write itself does not promise to
+		// fix. Neither is this notice's business; the restore above is, for all
+		// three.
+		if errors.Is(formatCtx.Err(), context.DeadlineExceeded) && ctx.Err() == nil {
+			unformatted.TimedOut = true
+		}
+		return unformatted
 	}
 	formatted, err := os.ReadFile(absolutePath)
 	if err != nil {
-		return writtenContent
+		return unformatted
 	}
-	return string(formatted)
+	return formatOnWriteResult{Content: string(formatted), Formatter: command[0]}
 }

internal/tools/format_on_write_test.go

@@ -97,9 +97,12 @@ func TestFormatOnWriteFormatsAndKeepsTrackerConsistent(t *testing.T) {
 
 func TestFormatOnWriteSkipsUnknownExtensions(t *testing.T) {
 	t.Setenv("ZERO_FORMAT_ON_WRITE", "1")
-	content := maybeFormatWrittenFile(context.Background(), filepath.Join(t.TempDir(), "notes.xyz"), "raw   text")
-	if content != "raw   text" {
-		t.Fatalf("unknown extension must pass through: %q", content)
+	formatting := maybeFormatWrittenFile(context.Background(), filepath.Join(t.TempDir(), "notes.xyz"), "raw   text")
+	if formatting.Content != "raw   text" {
+		t.Fatalf("unknown extension must pass through: %q", formatting.Content)
+	}
+	if notice := formatting.notice("notes.xyz"); notice != "" {
+		t.Fatalf("an extension with no formatter is not a miss worth reporting, got %q", notice)
 	}
 }
 
@@ -111,8 +114,11 @@ func TestFormatOnWriteFormatterLookupFailure(t *testing.T) {
 	if err := os.WriteFile(targetPath, []byte(uglyContent), 0o644); err != nil {
 		t.Fatal(err)
 	}
-	content := maybeFormatWrittenFile(context.Background(), targetPath, uglyContent)
-	if content != uglyContent {
-		t.Fatalf("missing formatter must return written content, got %q", content)
+	formatting := maybeFormatWrittenFile(context.Background(), targetPath, uglyContent)
+	if formatting.Content != uglyContent {
+		t.Fatalf("missing formatter must return written content, got %q", formatting.Content)
+	}
+	if notice := formatting.notice("a.go"); notice != "" {
+		t.Fatalf("an uninstalled formatter is a standing fact, not a miss worth reporting, got %q", notice)
 	}
 }

internal/tools/format_on_write_timeout_test.go

@@ -0,0 +1,280 @@
+package tools
+
+import (
+	"context"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"runtime"
+	"strings"
+	"testing"
+	"time"
+)
+
+// A FORMATTER THAT RUNS OUT OF TIME MUST NOT LOOK LIKE ONE THAT SUCCEEDED.
+//
+// Every other way format-on-write falls back is a standing fact about the
+// environment: the toggle is off, the extension has no formatter, the binary is
+// not installed. Those are silent because nothing is wrong. A deadline firing is
+// different: formatting was configured, available and expected, and the file was
+// written unformatted anyway because the machine was slow. Silent, the caller
+// believes it wrote canonical style and learns otherwise from a CI format check
+// it cannot see, which is the failure this feature exists to prevent.
+//
+// THE DEADLINE IS SHRUNK RATHER THAN THE FORMATTER SLOWED. A formatter that
+// really sleeps has to be killed, and on Windows the batch file that hosts the
+// sleep leaves the sleeping grandchild holding the working directory, so the
+// test then fails in TempDir cleanup rather than on anything it asserts. An
+// expired budget against an ordinary fast formatter reaches the same branch by
+// the same route, deterministically and in microseconds.
+func TestFormatOnWriteReportsATimeout(t *testing.T) {
+	formatter := installFakeFormatter(t, ".fastfmt", "fastfmt", succeedingFormatterScript())
+	shortenFormatOnWriteTimeout(t, time.Nanosecond)
+	t.Setenv("ZERO_FORMAT_ON_WRITE", "1")
+
+	target := filepath.Join(t.TempDir(), "subject.fastfmt")
+	const written = "written   but   not   formatted\n"
+	if err := os.WriteFile(target, []byte(written), 0o644); err != nil {
+		t.Fatal(err)
+	}
+
+	formatting := maybeFormatWrittenFile(context.Background(), target, written)
+
+	if formatting.Content != written {
+		t.Fatalf("content = %q, want the bytes that were written", formatting.Content)
+	}
+	if !formatting.TimedOut {
+		t.Fatal("a formatter cut off by the deadline was reported as an ordinary miss, so the write claims a formatting that never happened")
+	}
+	notice := formatting.notice("subject.fastfmt")
+	for _, want := range []string{"subject.fastfmt", "not formatted", formatter} {
+		if !strings.Contains(notice, want) {
+			t.Errorf("notice %q does not mention %q", notice, want)
+		}
+	}
+}
+
+// And the same formatter inside its budget says nothing at all, or the notice
+// above would be reporting the deadline rather than the miss.
+func TestFormatOnWriteStaysQuietWhenTheFormatterFinishes(t *testing.T) {
+	installFakeFormatter(t, ".fastfmt", "fastfmt", succeedingFormatterScript())
+	t.Setenv("ZERO_FORMAT_ON_WRITE", "1")
+
+	target := filepath.Join(t.TempDir(), "subject.fastfmt")
+	const written = "written\n"
+	if err := os.WriteFile(target, []byte(written), 0o644); err != nil {
+		t.Fatal(err)
+	}
+
+	formatting := maybeFormatWrittenFile(context.Background(), target, written)
+	if formatting.TimedOut {
+		t.Error("a formatter that finished was reported as timed out")
+	}
+	if notice := formatting.notice("subject.fastfmt"); notice != "" {
+		t.Errorf("a successful format produced a notice: %q", notice)
+	}
+}
+
+// The caller cancelling is not this notice's business: that run is already
+// being reported as cancelled, and saying the formatter was too slow on top of
+// it would be wrong about why.
+func TestFormatOnWriteStaysQuietWhenTheCallerCancels(t *testing.T) {
+	installFakeFormatter(t, ".fastfmt", "fastfmt", succeedingFormatterScript())
+	t.Setenv("ZERO_FORMAT_ON_WRITE", "1")
+
+	target := filepath.Join(t.TempDir(), "subject.fastfmt")
+	const written = "written\n"
+	if err := os.WriteFile(target, []byte(written), 0o644); err != nil {
+		t.Fatal(err)
+	}
+
+	ctx, cancel := context.WithCancel(context.Background())
+	cancel()
+	formatting := maybeFormatWrittenFile(ctx, target, written)
+
+	if formatting.Content != written {
+		t.Fatalf("content = %q, want the bytes that were written", formatting.Content)
+	}
+	if formatting.TimedOut {
+		t.Error("a cancelled run was reported as a formatter timeout")
+	}
+	if notice := formatting.notice("subject.fastfmt"); notice != "" {
+		t.Errorf("a cancelled run produced a notice: %q", notice)
+	}
+}
+
+// A formatter that runs and refuses the file is also not a timeout. It usually
+// means content it could not parse, which the write does not promise to fix,
+// and reporting it as a slow machine would be wrong about the cause.
+func TestFormatOnWriteStaysQuietWhenTheFormatterFails(t *testing.T) {
+	installFakeFormatter(t, ".failfmt", "failfmt", failingFormatterScript())
+	t.Setenv("ZERO_FORMAT_ON_WRITE", "1")
+
+	target := filepath.Join(t.TempDir(), "subject.failfmt")
+	const written = "unparseable\n"
+	if err := os.WriteFile(target, []byte(written), 0o644); err != nil {
+		t.Fatal(err)
+	}
+
+	formatting := maybeFormatWrittenFile(context.Background(), target, written)
+	if formatting.TimedOut {
+		t.Error("a formatter that exited non-zero was reported as a timeout")
+	}
+	if notice := formatting.notice("subject.failfmt"); notice != "" {
+		t.Errorf("a failing formatter produced a timeout notice: %q", notice)
+	}
+}
+
+// installFakeFormatter puts a formatter on PATH and registers it in the command
+// table for the test's duration, returning the binary name the notice carries.
+func installFakeFormatter(t *testing.T, extension, name, script string) string {
+	t.Helper()
+	binaryName := name + formatterScriptExtension()
+	directory := t.TempDir()
+	if err := os.WriteFile(filepath.Join(directory, binaryName), []byte(script), 0o755); err != nil {
+		t.Fatal(err)
+	}
+	t.Setenv("PATH", directory+string(os.PathListSeparator)+os.Getenv("PATH"))
+
+	previous, existed := formatterCommands[extension]
+	formatterCommands[extension] = []string{binaryName}
+	t.Cleanup(func() {
+		if existed {
+			formatterCommands[extension] = previous
+			return
+		}
+		delete(formatterCommands, extension)
+	})
+	return binaryName
+}
+
+func formatterScriptExtension() string {
+	if runtime.GOOS == "windows" {
+		return ".bat"
+	}
+	return ""
+}
+
+func succeedingFormatterScript() string {
+	if runtime.GOOS == "windows" {
+		return "@echo off\r\nexit /b 0\r\n"
+	}
+	return "#!/bin/sh\nexit 0\n"
+}
+
+func failingFormatterScript() string {
+	if runtime.GOOS == "windows" {
+		return "@echo off\r\nexit /b 3\r\n"
+	}
+	return "#!/bin/sh\nexit 3\n"
+}
+
+// shortenFormatOnWriteTimeout narrows the production deadline for one test.
+func shortenFormatOnWriteTimeout(t *testing.T, timeout time.Duration) {
+	t.Helper()
+	previous := formatOnWriteTimeout
+	formatOnWriteTimeout = timeout
+	t.Cleanup(func() { formatOnWriteTimeout = previous })
+}
+
+// A FAILED FORMATTER MUST NOT LEAVE THE FILE HALF-REWRITTEN.
+//
+// These commands edit in place, so one killed by the deadline, killed by the
+// caller, or exiting partway through its own rewrite can leave the target
+// truncated: neither the input nor the output. Returning the written bytes on
+// top of that would leave the tracker baseline and the diff preview describing
+// a file that is not on disk, and the next edit would compare against content
+// the file does not have.
+func TestFormatOnWriteRestoresTheFileWhenTheFormatterFails(t *testing.T) {
+	installFakeFormatter(t, ".clobberfmt", "clobberfmt", clobberingFormatterScript())
+	t.Setenv("ZERO_FORMAT_ON_WRITE", "1")
+
+	target := filepath.Join(t.TempDir(), "subject.clobberfmt")
+	const written = "the bytes the caller wrote\n"
+	if err := os.WriteFile(target, []byte(written), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	requireFormatterClobbers(t, written)
+
+	formatting := maybeFormatWrittenFile(context.Background(), target, written)
+
+	onDisk, err := os.ReadFile(target)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if string(onDisk) != written {
+		t.Errorf("file on disk = %q, want the bytes that were written back", onDisk)
+	}
+	if formatting.Content != written {
+		t.Errorf("content = %q, want the bytes that were written", formatting.Content)
+	}
+	if formatting.RestoreFailed {
+		t.Error("restoration was reported as failed on a writable file")
+	}
+}
+
+// And the same for a run cut off by the deadline, which is the case the notice
+// already covers: the disclosure and the file have to agree.
+func TestFormatOnWriteRestoresTheFileOnTimeout(t *testing.T) {
+	installFakeFormatter(t, ".clobberfmt", "clobberfmt", clobberingFormatterScript())
+	shortenFormatOnWriteTimeout(t, time.Nanosecond)
+	t.Setenv("ZERO_FORMAT_ON_WRITE", "1")
+
+	target := filepath.Join(t.TempDir(), "subject.clobberfmt")
+	const written = "the bytes the caller wrote\n"
+	if err := os.WriteFile(target, []byte(written), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	requireFormatterClobbers(t, written)
+
+	formatting := maybeFormatWrittenFile(context.Background(), target, written)
+
+	onDisk, err := os.ReadFile(target)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if string(onDisk) != written {
+		t.Errorf("file on disk = %q, want the bytes that were written back", onDisk)
+	}
+	if !formatting.TimedOut {
+		t.Error("the deadline path stopped being reported once restoration was added")
+	}
+	if notice := formatting.notice("subject.clobberfmt"); !strings.Contains(notice, "not formatted") {
+		t.Errorf("notice = %q, want the timeout note", notice)
+	}
+}
+
+// clobberingFormatterScript truncates the file it is handed and then fails, the
+// way an interrupted in-place formatter leaves a partial rewrite.
+func clobberingFormatterScript() string {
+	if runtime.GOOS == "windows" {
+		return "@echo off\r\necho CLOBBERED> %1\r\nexit /b 3\r\n"
+	}
+	return "#!/bin/sh\necho CLOBBERED > \"$1\"\nexit 3\n"
+}
+
+// requireFormatterClobbers proves the fixture really does damage the file it is
+// handed, on a throwaway copy.
+//
+// It cannot be checked on the real target: restoration is the behaviour under
+// test, so when it works the evidence is gone, and asserting on the target
+// afterwards would either pass vacuously or report the opposite of what it saw.
+func requireFormatterClobbers(t *testing.T, written string) {
+	t.Helper()
+	probe := filepath.Join(t.TempDir(), "probe.clobberfmt")
+	if err := os.WriteFile(probe, []byte(written), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	binary, err := exec.LookPath("clobberfmt" + formatterScriptExtension())
+	if err != nil {
+		t.Fatalf("SETUP INVALID: the fake formatter is not on PATH: %v", err)
+	}
+	_ = exec.Command(binary, probe).Run()
+	after, err := os.ReadFile(probe)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !strings.Contains(string(after), "CLOBBERED") {
+		t.Fatalf("SETUP INVALID: the fake formatter left %q, so it does not damage the file and restoration is not under test", after)
+	}
+}

internal/tools/write_file.go

@@ -114,7 +114,8 @@ func (tool writeFileTool) RunWithOptions(ctx context.Context, args map[string]an
 	// Optional format-on-write (ZERO_FORMAT_ON_WRITE). Must run BEFORE the
 	// FileTracker baseline: recording pre-format content would make the very
 	// next edit look like an external modification and trip the conflict guard.
-	content = maybeFormatWrittenFile(ctx, absolutePath, content)
+	formatting := maybeFormatWrittenFile(ctx, absolutePath, content)
+	content = formatting.Content
 	// Baseline the freshly written content so a later edit/overwrite in this
 	// session compares against what is now on disk.
 	newInfo, _ := os.Stat(absolutePath)
@@ -137,6 +138,7 @@ func (tool writeFileTool) RunWithOptions(ctx context.Context, args map[string]an
 		lines++
 	}
 	summary := fmt.Sprintf("%s %s (%d lines).", verb, relativePath, lines)
+	summary += formatting.notice(relativePath)
 	summary += inlineDiagnostics(ctx, options, absolutePath, relativePath)
 	result := okResult(summary)
 	result.ChangedFiles = []string{relativePath}