refactor(client): clone only changed subtrees on engine publish

chore(client): add session sync streaming benchmark
chore(client): regenerate on rebased protocol
2026-08-18 21:53:12 -04:00 · 2026-08-18 21:29:12 -04:00 · 2026-08-18 21:29:11 -04:00 · 2026-08-18 20:56:08 -04:00 · 2026-08-18 20:44:57 -04:00 · 2026-08-18 20:44:57 -04:00
29 changed files with 3539 additions and 312 deletions
@@ -0,0 +1,352 @@
+# Sync Engine Plan (draft)
+
+Redesign of client state management: consume the event-sourced server that
+already exists. Goal: **less code, more reliable** — optimism, reconnect, and
+retry become structural guarantees instead of defended special cases.
+
+Status: outline for discussion. Sections marked `TODO` get filled in as we talk.
+
+---
+
+## 0. Context ledger (receipts — do not lose these)
+
+Facts established by code audit and prototyping, with locations.
+
+### The server is already event-sourced
+
+- Durable per-session event log in SQLite: `event(aggregate_id, seq, type, data)`
+  + `event_sequence` counter, unique index `(aggregate_id, seq)`
+  (`packages/core/src/event/sql.ts`, migration `20260323234822_events`).
+- Event append, projections, and `commit` hooks all run in **one SQLite
+  transaction** (`packages/core/src/bus.ts:249-370`, `behavior: "immediate"`).
+- 40 of 46 session event types are durable; the 6 ephemeral are streaming
+  deltas/progress/usage ticks (`packages/schema/src/session-event.ts:626`).
+  Durable events carry complete facts (`text.ended` has full text).
+- All hydration tables are written **only** by projectors inside the append
+  transaction: `SessionMessageTable`, `SessionTable` → `session/projector.ts`;
+  `SessionInboxTable` → `inbox.ts` `project*` fns called only from
+  `projector.ts:521,555,571,577`. `SessionPendingTable` is dead (zero writers).
+- Message rows carry the aggregate seq of their creating event
+  (`projector.ts:386`), and `Session.messages` already paginates by it
+  (`session.ts:501`).
+- Replay+live log endpoint exists:
+  `GET /api/experimental/session/:id/log?after=seq&follow=true`
+  (`packages/protocol/src/groups/session.ts:631`). Only devtools consumes it.
+- Idempotent admission ~exists: `SessionInbox.admit` (`inbox.ts:137`) adopts an
+  existing client-supplied `SessionMessage.ID`, re-checks after conflict
+  defects. Gap: adopts without comparing payloads (no conflict on mismatch).
+- Reverts are committed server-side inside `Session.prompt`
+  (`core/src/session.ts:567` at audit time) — client-side revert.commit is
+  already redundant.
+
+### The client ignores all of it
+
+- The client data layer is `packages/client/src/solid/data.ts` (~1,530 lines;
+  `packages/tui/src/context/data.tsx` is now a 22-line wrapper) — shared by
+  TUI/desktop/web, so the engine lands once for all clients. Three racing
+  write paths into one Solid store: a ~60-case bus-event switch, fetch
+  handlers doing wholesale `reconcile()`, and (in PR #42807) an optimistic
+  ledger both must consult. `sync.complete(key)` markers dedupe fetches but
+  are not watermarks.
+- Opening a session today = four separate fetches: `session.get`,
+  `session.list({parentID})` (children), `session.inbox.list`, messages
+  (`data.ts:1084-1140`) — four sync keys, no shared consistent point.
+- Reconnect = `sync.reset()` + refetch everything; no gap proof.
+
+### Prior art
+
+- PRs #42807 (optimistic message send) and #42808 (optimistic session create):
+  the stopgap. They landed client-generated IDs (keep) and ~15 hand-placed
+  guard sites defending the optimistic ledger (delete when this plan ships).
+- Prototype: `~/code/open-source/sync-proto` — Effect v4 + Solid. Engine core
+  ~200 lines. Six laws as passing tests; seeded 2-client chaos sim (lost
+  requests, lost responses after durable write, rejections, connection cuts,
+  latency shifts) with convergence / ordering / uniqueness / no-flicker
+  invariants; sim caught a real protocol bug (validation-before-adoption) on
+  first run. Two-pane demo (await vs optimistic) with narrated scenarios.
+- Proposal doc: `~/code/open-source/opencode-sync-proposal/PROPOSAL.md`
+  ("Intents, Not Mutations").
+
+### Vocabulary
+
+- **fold** — pure reduction of durable events into state; same meaning on both
+  sides. **intent** — a user action not yet acknowledged, client ID, in the
+  **outbox**. **watermark** — the aggregate seq a snapshot reflects.
+  **echo determinism** — an intent renders byte-identical to the row its
+  admission event folds to, so acks are invisible.
+- The six laws: idempotency, echo determinism, sync opacity, ordering,
+  convergence, failure atomicity.
+
+### Ordering constraint (the one subtle thing)
+
+Watermark reads must share a transaction with row reads:
+`seq`-then-rows double-applies accumulators; rows-then-`seq` loses events.
+
+---
+
+## 1. Problem
+
+TODO — expand together. Sketch:
+
+- Optimism, reconnect, and retry are each defended point-by-point instead of
+  guaranteed structurally; every new sync path re-fights the same races.
+- The client maintains a hand-written mutable mirror of state the server can
+  already replay deterministically.
+
+## 2. The claim: less code, more reliable
+
+TODO — expand together. Sketch:
+
+- Fold cases ≈ today's event switch (that code stays, becomes pure).
+- Deleted: optimistic ledger + guards, echo replace-in-place, sync-survival
+  merges, `submitTails` throttling, `sync.complete` session keys,
+  refetch-everything reconnect.
+- Added: engine core (~200 lines proven in prototype) + watermark plumbing.
+- Reliability from structure: the six laws hold by construction, not vigilance.
+
+## 3. Design
+
+```
+state   = fold(snapshot, durable events)   server truth only
+outbox  = ordered pending intents          client IDs, idempotent, serial resend
+overlay = live ephemeral fragments         deltas/progress, superseded by durable facts
+view    = render(state ⊕ outbox ⊕ overlay) derived
+```
+
+The entire client surface for one session — two calls, one stream:
+
+```
+GET /session/:id/snapshot                  fetch → { session, children, inbox,
+                                                     messages: last(N), seq }
+GET /session/:id/log?after=seq&follow=true one ordered stream:
+    replay:  durable events seq+1..head    (deltas are live-only, absent here)
+    marker:  log.synced                    "caught up" — first-class engine signal
+    live:    durable + ephemeral session events interleaved, publish order
+```
+
+- **Decided:** the log endpoint's follow phase carries ephemeral session
+  events too (widen its union from `Durable | Synced`). One server-merged
+  stream per session; the client never merges two event feeds for one
+  aggregate, so a delta can never precede its own `Started`.
+- **Overlay semantics:** `Map<(messageID, ordinal), accumulated>` — never
+  enters the fold. The durable full-value boundary (`text.ended`, carries
+  complete text) supersedes and clears the overlay entry in the same atomic
+  update that folds it — same no-seam trick as an intent leaving the outbox
+  on ack. Dropped deltas are self-healing by construction. Same mechanism for
+  all 6 ephemeral types (`tool.progress`/`usage.updated` superseded by
+  `tool.success|failed`/`step.ended`).
+- **Decided (mid-stream reconnect):** accept today's behavior — after
+  re-hydrate, overlay accumulates from now, so in-flight text may render with
+  a missing prefix until its `Ended` supersedes. Simplest code, self-healing,
+  no protocol change. (Optional later polish: if `Started` wasn't observed
+  live, show a streaming indicator instead of partial text.)
+- Overlay entries render only once their base part exists in folded state
+  (covers the one degenerate case: a live delta arriving during the replay
+  window).
+- **Outbox ack contract: the echo is the only ack.** Intents carry a
+  client-minted message ID. The HTTP 2xx is a latency hint, never trusted;
+  the intent leaves the outbox only when the durable event carrying its ID
+  folds in (same atomic store update — no flicker), or on a typed rejection
+  (drop + surface). Timeout/disconnect = unknown fate = intent just stays.
+  On reconnect: tail first, wait for `log.synced` (replay acks anything that
+  landed), then resend unacked serially — adopt-on-same-ID (`inbox.ts:137`)
+  makes resend duplicate-free. Lost request and lost response collapse into
+  the same case ("no echo yet"), which is why no guard machinery survives.
+- **Contract: the snapshot is the only entry point.** The engine never replays
+  a session from genesis; fold's domain is "snapshot + events after its seq."
+  Forced by `session.forked` — the event carries only `{ parentID, boundary }`
+  and the projector copies parent rows, which no client fold could reproduce.
+  Bonus: licenses future log truncation (serve back to the oldest live
+  snapshot, not seq 0).
+- Compaction/revert/fork all fold cleanly under that contract: compaction only
+  adds (summary message + epoch marker); revert folds exactly what the
+  projector does (drop messages/inbox at-or-after boundary, clear marker —
+  `projector.ts:660-690`); fork needs no fold case at all (a forked session's
+  snapshot already contains the copied messages).
+- Reconnect: tail from last seq first; on typed seq-unavailable → snapshot →
+  resend unacknowledged → tail. Gapless by construction.
+- Scope boundary: session aggregate only. Ambient state (catalog, agents,
+  projects, vcs, worktrees) stays on the existing global bus as-is.
+- Render target: existing Solid store shape, so components don't change.
+
+## 4. Server changes (small)
+
+- S1: watermarked hydration — **decided (tentative, pending prior-art review):**
+  one new endpoint, `GET /api/session/:id/snapshot`, one read transaction:
+
+  ```
+  GET /api/session/:id/snapshot?recent=N
+  → { session, children, inbox, messages: last(N), seq }
+  ```
+
+  `recent` defaults to 200 — today's first page exactly (`data.ts:1136`,
+  `limit: 200`). Tuning it smaller later is a parameter change, not a design
+  change.
+
+  - Named `snapshot`, not `state`: point-in-time consistency is the contract,
+    and `state` collides with the engine's `state = fold(...)`.
+  - One transaction → one watermark. Rejected alternative: adding `seq` to
+    each existing read — three watermarks force per-slice replay guards in
+    the engine, and the session row's accumulators (usage sums) double-count
+    on overlap replay. Server savings reappear as engine complexity, worse.
+  - `children` are info rows only; child sessions are separate aggregates and
+    hydrate their own messages/inbox when opened.
+  - Older history stays on the existing seq-cursored `messages` endpoint,
+    folded in as inert backfill (IDs dedupe; revert/compaction arrive on the
+    log).
+  - Prior art for snapshot+watermark+tail: Discord (`seq`/RESUME), Telegram
+    (`pts`/`getDifference`), Linear (`lastSyncId`), Postgres logical
+    replication (exported snapshot + LSN), Kafka/EventStore catch-up
+    subscriptions, Replicache (cookie — whose launch checklist requires the
+    watermark "read in the same transaction as the client view data").
+    Slack's deprecated `rtm.start` is the cautionary tale for unbounded
+    snapshots (their edge cache cut boot payloads 7–44×; 42-message pages).
+    Details: `notes/hydration-prior-art.md`.
+  - Adopted from prior art:
+    - `log?after=seq` returns a typed **seq-unavailable error** when the seq
+      is not servable → client full-rehydrates. Never silently clamp to the
+      oldest retained event (Discord op 9 / Telegram `differenceTooLong` /
+      Postgres explicitly warns against clamping).
+    - **Cursor honesty (S1 follow-up, landed):** `events.persist` defaults to
+      off (only workerd enables it), so `event` payload rows usually don't
+      exist even though `event_sequence` always advances. Old behavior:
+      `after < head` "replayed" nothing from the empty table and marked
+      synced — silent desync. Now `openLog` proves retained rows cover
+      `(after, head]` (`Bus.retainedCount`) and returns seq-unavailable
+      otherwise; the engine re-snapshots. A cursor is a resumption claim and
+      gets the strict check; a cursorless read remains "what's retained".
+      Client side, the engine also treats a `log.synced` marker past its
+      folded seq as a gap → snapshot recovery, closing the residual
+      in-process race between the head check and the live subscription.
+      Persistence therefore stays optional: snapshot + live tail + honest
+      refusal is a complete contract. A short retention window (persist on +
+      pruning sweep) can drop in later behind the same check to make busy
+      reconnects replay instead of refetch.
+    - Resume-first discipline: reconnect tries the tail from the last seq
+      before re-snapshotting (Discord meters full rehydrates, not resumes).
+    - Keep the existing `log.synced` caught-up marker as a first-class engine
+      signal (EventStoreDB added `CaughtUp` after pain; suppress notifications
+      during replay).
+- ~~S2: adopt strictness~~ — dropped. The engine only needs adopt-on-same-ID,
+  which exists (`inbox.ts:137` + AGENTS.md reconciliation rules). Conflict on
+  same-ID/different-payload defends against a client bug we never create.
+- S3: widen `session.log` follow phase to interleave ephemeral session events,
+  **opt-in via `?ephemeral=true`** — default stays durable-only, so existing
+  log consumers (devtools) are untouched. The response union widens either way
+  (extra variants simply never appear unless requested); replay is durable-only
+  in both modes since ephemeral events aren't retained. Also add the typed
+  seq-unavailable error. Later: promote out of experimental.
+- S4: filter param on `event.subscribe` (`/api/event` today sends everything
+  to everyone and fails slow consumers by contract). The ambient stream slims
+  to: non-session events + session-lifecycle events needed by the session
+  list for unopened sessions (created/deleted/renamed/moved, usage, execution
+  status). Exact param shape TBD.
+
+## 5. Delivery: parallel build in this worktree, compare locally
+
+Not a staged production migration — this branch carries a complete alternate
+data layer next to the existing one, so both versions can be run and compared
+before any adoption decision.
+
+1. Server additions S1–S4 (additive; existing clients unaffected).
+2. Engine as a new module in `packages/client` (`solid/engine.ts` or similar)
+   beside `solid/data.ts` — same store shape and consumer-facing API, fed by
+   snapshot + session stream + outbox instead of fetch + bus + ledger.
+   `data.ts` is not modified.
+3. TUI in this worktree wired to the engine layer.
+4. Compare: run this worktree's full stack (`bun run dev`) against stock v2
+   side by side — submit latency, kill/reconnect behavior, streaming, revert,
+   forks. Port the six-laws + chaos tests as the regression net.
+5. If it wins: adoption (swap the wiring, delete `data.ts` + PR #42807/#42808
+   guards) is its own decision with two working implementations to diff.
+
+   Deletion inventory at adoption: `data.ts` (~1,530 lines — the 64-case bus
+   switch becomes the pure fold; 4-fetch session open → one snapshot; all
+   refetch-on-reconnect machinery → nothing), PRs #42807/#42808 never merge
+   (stopgap ledger avoided entirely), TUI `util/single-flight.ts` + call
+   sites (submit is idempotent by minted ID), per-callsite await-then-toast
+   action plumbing → one generic outbox-rejection path. Rendering components
+   unchanged by design (same store shape).
+
+### Commit-by-commit
+
+Status: Lane A done (`0af27771` snapshot core, `d611b39f` endpoint,
+`b21a3c7d` typed seq-unavailable, `74929dd7` ephemeral opt-in, `e6eac3a0`
+simplify — S4 pending shape decision). Lane B done through validation +
+simplify (`54a2ec54` fold, `0f8831b3` engine, `2c29a743` laws, `354ab6f9`
+chaos, `c9ec1385` simplify). Wiring done: `5d70561f` engine-backed data
+layer, `48e237d9` TUI wiring, `90760fc6`/`5f63f252` regression fixes (5 of 6
+initial TUI failures were real engine-layer gaps — reconnect refresh, queued
+lifecycle, pending refresh, synced-gated sync, family-index pollution — all
+fixed; 1 obsolete assertion updated with justification). Suites: TUI 729/0,
+client green except 3 pre-existing generated-client failures (repro on stock
+v2). Remaining: live comparison pass (B7), S4 shape decision.
+
+Two lanes, disjoint packages, parallelizable. Worker sessions execute; the
+coordinating session reviews each commit and keeps this doc current. Every
+commit typechecks and passes its package tests before landing.
+
+**Lane A — server** (`packages/core`, `packages/protocol`, generated client):
+
+- **Snapshot** (S1)
+  - `feat(core): Session.snapshot` — service method, one read transaction:
+    `{ session, children, inbox, messages: last(recent), seq }`. Core tests:
+    seq consistency under concurrent publish (a write landing between the
+    transaction's reads must be impossible), empty session, recent windowing.
+  - `feat(protocol): session.snapshot endpoint` — route + handler + `bun run
+    generate` from packages/client.
+- **Session stream** (S3)
+  - `feat(core): typed seq-unavailable on session.log` — `after > head` (or
+    below retention, future) fails typed instead of silently serving. Test
+    the `after == head` boundary explicitly (Zero's fence-post bug).
+  - `feat(core): session.log follow includes ephemeral events` — opt-in
+    `?ephemeral=true` (default durable-only, existing consumers untouched);
+    widen union (`Durable | Synced` → session events + `Synced`), interleave
+    live ephemeral session events in publish order + generate. Test: delta
+    never precedes its Started on one stream; replay phase stays durable-only
+    in both modes; default mode carries no ephemeral events.
+- **Ambient filter** (S4, can trail; nothing in Lane B blocks on it)
+  - `feat(protocol): event.subscribe filter` — ambient scope.
+
+**Lane B — engine** (`packages/client/src/solid`, `packages/client/test`):
+
+- **Fold**
+  - `feat(client): session fold` — pure module: durable session events →
+    session state (messages, inbox, markers). No transport, no store. The
+    ~60-case switch in `data.ts` is the reference for event semantics.
+- **Engine core**
+  - `feat(client): engine core` — outbox (intents, serial resend, ack-on-
+    fold-in-same-update), overlay (ephemeral fragments, superseded-and-
+    cleared by durable facts), view derivation, reconnect loop against a
+    transport interface.
+- **Validation**
+  - `test(client): six laws` — idempotency, echo determinism, sync opacity,
+    ordering, convergence, failure atomicity — fake in-process transport,
+    TestClock, no server.
+  - `test(client): seeded chaos sim` — two clients, lost requests/responses,
+    rejections, cuts, latency shifts; convergence + no-flicker invariants.
+- **Wiring** (waits on Snapshot's generated client)
+  - `feat(client): engine-backed data layer` — `data.ts`-compatible surface
+    over the engine, real transport.
+  - `feat(tui): wire TUI to engine layer` — this worktree only.
+  - Verification pass: termctrl live comparison vs stock v2; record demo.
+
+Order: Lane A (Snapshot → Session stream → Ambient filter) and Lane B
+(Fold → Engine core → Validation) run in parallel; Wiring waits on Snapshot,
+then the verification pass waits on Session stream + Wiring.
+
+## 6. Validation
+
+- Port sync-proto's six laws + seeded chaos sim to run against the real engine
+  with a faked transport. TODO: where these tests live, CI story.
+- Explicit boundary test at `seq == snapshot.seq` (empty tail vs purged log) —
+  Zero shipped a fence-post bug at exactly this boundary (rocicorp/mono#5589).
+
+## 7. Open questions (to resolve together)
+- Web app / desktop adoption order after TUI.
+- S4 filter shape: type list vs a named "ambient" scope.
+
+## 8. Non-goals
+
+- No server storage migration; no new event types; no runner changes.
+- No component/rendering changes; no changes to ambient (non-session) state.
@@ -30,6 +30,7 @@
    "generate": "bun run script/build.ts",
    "check:generated": "bun run generate && git diff --exit-code -- src/promise/generated src/effect/generated src/effect/api",
    "test": "bun test --timeout 5000",
+    "bench:sync": "bun scripts/bench-session-sync.ts",
    "typecheck": "tsgo --noEmit"
  },
  "dependencies": {
@@ -0,0 +1,61 @@
+# Session sync streaming performance
+
+## Goal
+
+Keep the engine data layer's per-token-delta cost negligible next to the
+legacy `createData` path, so the sync engine's immutable fold/render
+architecture is not a streaming CPU regression.
+
+## Benchmark
+
+```
+cd packages/client
+bun scripts/bench-session-sync.ts
+```
+
+Env knobs: `BENCH_TRANSCRIPT` (messages, default 200), `BENCH_DELTAS`
+(default 2000), `BENCH_RUNS` (default 7, median after 1 warmup).
+
+## Metrics
+
+- `engine_deltas_ms` (primary) — wall clock to stream `BENCH_DELTAS` text
+  deltas into the active assistant message.
+- `legacy_deltas_ms` — same scenario through legacy `createData`.
+- `*_hydrate_ms`, `*_retained_mb` — secondary.
+
+## Files in scope
+
+- `src/solid/engine-data.ts` — store adapter (`update`, clone boundary)
+- `src/solid/engine/engine.ts` — `render`, `applyOverlayToMessages`
+
+## Experiment log (200 messages, 2000 deltas)
+
+Baseline: engine 240µs/delta vs legacy 0.3µs/delta.
+
+1. KEEP — drop full-view `structuredClone` per publish → 41ms (11.6×).
+   Follow-up: raw pass-through was unsound (reconcile mutates the store's
+   backing tree in place, corrupting engine state / aliased clones).
+2. KEEP — identity-diff consecutive views (the fold is a persistent
+   structure) and clone only changed subtrees, each clone used at exactly
+   one store path → 67ms sound (7.1× vs baseline).
+3. KEEP — reference-preserving `render`: identity-stable `pending` when the
+   outbox is empty, skip pending-derived message work when nothing pending,
+   remap only overlay-touched messages → 16.3µs/delta.
+4. KEEP — skip `legacy.session.remember` clone when `view.session` is
+   identity-unchanged → 9.4µs/delta.
+5. KEEP — hand-rolled recursive clone instead of `structuredClone` for the
+   small per-event subtrees → 3.5µs/delta.
+
+Final: 240 → 3.5µs/delta (69×) at 200 messages; 23.6µs/delta at 2000
+messages (remaining cost is the O(n) identity walk, ~10ns/message/delta).
+
+## Dead ends
+
+- WeakMap-memoized structural-sharing clone: unsound. `reconcile` merges
+  nodes in place, so a memoized clone reachable from two store paths (or a
+  later view) gets corrupted; caught by the TUI `updates session location
+  when moved` test via fold state that aliases `previous.location`.
+- Microtask coalescing of publishes: each stream item already arrives on
+  its own microtask, so `queueMicrotask` batching collapses nothing; real
+  frame coalescing (~16ms timer) left unexplored as unnecessary at current
+  numbers.
@@ -0,0 +1,223 @@
+// Benchmark: legacy createData vs engine createEngineData session sync.
+//
+// Scenarios per layer:
+//   hydrate  — populate a session with TRANSCRIPT messages
+//   deltas   — stream DELTAS text deltas into the active assistant message
+//   retained — heap retained by the populated layer (post-GC)
+//
+// Run from packages/client: bun scripts/bench-session-sync.ts
+// Emits METRIC lines (median of RUNS after 1 warmup).
+
+import { heapStats } from "bun:jsc"
+import { createRoot } from "solid-js"
+import { createData } from "../src/solid/data"
+import type { CreateDataInput } from "../src/solid/data"
+import { createEngineData } from "../src/solid/engine-data"
+import type { OpenCodeEvent, SessionMessageInfo } from "../src/promise"
+
+const TRANSCRIPT = Number(process.env.BENCH_TRANSCRIPT ?? 200)
+const DELTAS = Number(process.env.BENCH_DELTAS ?? 2000)
+const RUNS = Number(process.env.BENCH_RUNS ?? 7)
+
+const sessionID = "ses_bench"
+const assistantID = `msg_a${TRANSCRIPT - 1}`
+
+function transcript(): SessionMessageInfo[] {
+  return Array.from({ length: TRANSCRIPT }, (_, index): SessionMessageInfo => {
+    const created = 1_700_000_000_000 + index
+    if (index % 2 === 0)
+      return { id: `msg_u${index}`, type: "user", text: `user message ${index} ${"lorem ".repeat(40)}`, time: { created } }
+    return {
+      id: `msg_a${index}`,
+      type: "assistant",
+      time: index === TRANSCRIPT - 1 ? { created } : { created, completed: created + 1 },
+      agent: "build",
+      content: [{ type: "text", text: `assistant reply ${index} ${"ipsum ".repeat(40)}` }],
+    } as SessionMessageInfo
+  })
+}
+
+function sessionInfo() {
+  return {
+    id: sessionID,
+    projectID: "proj_bench",
+    cost: 0,
+    tokens: { input: 0, output: 0, reasoning: 0, cache: { read: 0, write: 0 } },
+    time: { created: 1, updated: 2 },
+    title: "bench",
+    location: { directory: "/bench" },
+  }
+}
+
+const textStarted = (seq: number) => ({
+  id: "evt_start",
+  created: 3,
+  type: "session.text.started" as const,
+  durable: { aggregateID: sessionID, seq, version: 1 },
+  data: { sessionID, assistantMessageID: assistantID, ordinal: 0 },
+})
+const textDelta = (index: number) => ({
+  id: `evt_d${index}`,
+  created: 4,
+  type: "session.text.delta" as const,
+  data: { sessionID, assistantMessageID: assistantID, ordinal: 0, delta: "x" },
+})
+const textEnded = (seq: number) => ({
+  id: "evt_end",
+  created: 5,
+  type: "session.text.ended" as const,
+  durable: { aggregateID: sessionID, seq, version: 1 },
+  data: { sessionID, assistantMessageID: assistantID, ordinal: 0, text: "END" },
+})
+
+type Layer = {
+  hydrate: () => Promise<void>
+  dispatch: (event: Record<string, unknown>) => void
+  finalText: () => string | undefined
+  dispose: () => void
+}
+
+function legacyLayer(): Layer {
+  let handler: ((event: { name: OpenCodeEvent["type"]; details: OpenCodeEvent }) => void) | undefined
+  const messages = transcript()
+  const api = {
+    session: { get: async () => sessionInfo() },
+    message: { list: async () => ({ data: messages.toReversed(), cursor: {} }) },
+  } as unknown as ReturnType<CreateDataInput["api"]>
+  return createRoot((dispose) => {
+    const data = createData({
+      api: () => api,
+      directory: "/bench",
+      event: {
+        on: () => () => {},
+        listen(next) {
+          handler = next
+          return () => {}
+        },
+      },
+      connection: { status: () => "connected" },
+    })
+    return {
+      async hydrate() {
+        await data.session.sync(sessionID)
+        await data.session.message.sync(sessionID)
+      },
+      dispatch(event) {
+        handler?.({ name: event.type as OpenCodeEvent["type"], details: event as unknown as OpenCodeEvent })
+      },
+      finalText() {
+        const message = data.session.message.get(sessionID, assistantID)
+        const part = message?.type === "assistant" ? message.content.findLast((item) => item.type === "text") : undefined
+        return part?.type === "text" ? part.text : undefined
+      },
+      dispose,
+    }
+  })
+}
+
+function engineLayer(): Layer {
+  const queue: Array<Record<string, unknown>> = []
+  let wake: (() => void) | undefined
+  const api = {
+    session: {
+      snapshot: async () => ({
+        session: sessionInfo(),
+        children: [],
+        inbox: [],
+        messages: transcript(),
+        seq: 10,
+      }),
+      async *log() {
+        yield { type: "log.synced", aggregateID: sessionID, seq: 10 }
+        while (true) {
+          const item = queue.shift()
+          if (item) {
+            yield item
+            continue
+          }
+          await new Promise<void>((resolve) => {
+            wake = resolve
+          })
+        }
+      },
+      prompt: async () => ({}),
+    },
+  } as unknown as ReturnType<CreateDataInput["api"]>
+  return createRoot((dispose) => {
+    const data = createEngineData({
+      api: () => api,
+      directory: "/bench",
+      event: { on: () => () => {}, listen: () => () => {} },
+      connection: { status: () => "connected" },
+    })
+    return {
+      async hydrate() {
+        await data.session.sync(sessionID)
+      },
+      dispatch(event) {
+        queue.push(event)
+        wake?.()
+        wake = undefined
+      },
+      finalText() {
+        const message = data.session.message.get(sessionID, assistantID)
+        const part = message?.type === "assistant" ? message.content.findLast((item) => item.type === "text") : undefined
+        return part?.type === "text" ? part.text : undefined
+      },
+      dispose,
+    }
+  })
+}
+
+async function settle(check: () => boolean) {
+  for (let attempt = 0; attempt < 10_000; attempt++) {
+    if (check()) return
+    await new Promise((resolve) => setTimeout(resolve, 0))
+  }
+  throw new Error("scenario did not settle")
+}
+
+async function scenario(make: () => Layer) {
+  const layer = make()
+  const hydrateStart = performance.now()
+  await layer.hydrate()
+  const hydrate = performance.now() - hydrateStart
+
+  const deltaStart = performance.now()
+  layer.dispatch(textStarted(11))
+  for (let index = 0; index < DELTAS; index++) layer.dispatch(textDelta(index))
+  layer.dispatch(textEnded(12))
+  await settle(() => layer.finalText() === "END")
+  const deltas = performance.now() - deltaStart
+
+  Bun.gc(true)
+  const retained = heapStats().heapSize
+  layer.dispose()
+  return { hydrate, deltas, retained }
+}
+
+function median(values: number[]) {
+  const sorted = [...values].sort((a, b) => a - b)
+  return sorted[Math.floor(sorted.length / 2)]
+}
+
+async function measure(name: string, make: () => Layer) {
+  await scenario(make) // warmup
+  Bun.gc(true)
+  const baseline = heapStats().heapSize
+  const runs = [] as Awaited<ReturnType<typeof scenario>>[]
+  for (let run = 0; run < RUNS; run++) runs.push(await scenario(make))
+  const hydrate = median(runs.map((run) => run.hydrate))
+  const deltas = median(runs.map((run) => run.deltas))
+  const retained = median(runs.map((run) => run.retained)) - baseline
+  console.log(
+    `${name}: hydrate ${hydrate.toFixed(2)}ms  deltas ${deltas.toFixed(2)}ms (${((deltas * 1000) / DELTAS).toFixed(1)}µs/delta)  retained ${(retained / 1024 / 1024).toFixed(2)}MB`,
+  )
+  console.log(`METRIC ${name}_hydrate_ms=${hydrate.toFixed(3)}`)
+  console.log(`METRIC ${name}_deltas_ms=${deltas.toFixed(3)}`)
+  console.log(`METRIC ${name}_retained_mb=${(retained / 1024 / 1024).toFixed(3)}`)
+}
+
+console.log(`transcript=${TRANSCRIPT} deltas=${DELTAS} runs=${RUNS}`)
+await measure("legacy", legacyLayer)
+await measure("engine", engineLayer)
@@ -12,10 +12,10 @@ import type { Brand } from "effect"
 import type { Model } from "@opencode-ai/schema/model"
 import type { SessionMessage } from "@opencode-ai/schema/session-message"
 import type { SessionInbox } from "@opencode-ai/schema/session-inbox"
+import type { Event } from "@opencode-ai/schema/event"
 import type { PromptInput } from "@opencode-ai/schema/prompt-input"
 import type { AgentAttachment } from "@opencode-ai/schema/prompt"
 import type { Skill } from "@opencode-ai/schema/skill"
-import type { Event } from "@opencode-ai/schema/event"
 import type { InstructionEntry } from "@opencode-ai/schema/instruction-entry"
 import type { Schema } from "effect"
 import type { EventLog } from "@opencode-ai/schema/event-log"
@@ -139,36 +139,46 @@ export type Endpoint5_5Input = { readonly sessionID: Session.ID }
 export type Endpoint5_5Output = Session.Info
 export type SessionGetOperation<E = never> = (input: Endpoint5_5Input) => Effect.Effect<Endpoint5_5Output, E>

-export type Endpoint5_6Input = { readonly sessionID: Session.ID }
-export type Endpoint5_6Output = void
-export type SessionRemoveOperation<E = never> = (input: Endpoint5_6Input) => Effect.Effect<Endpoint5_6Output, E>
+export type Endpoint5_6Input = { readonly sessionID: Session.ID; readonly recent?: number | undefined }
+export type Endpoint5_6Output = {
+  readonly session: Session.Info
+  readonly children: ReadonlyArray<Session.Info>
+  readonly inbox: ReadonlyArray<SessionInbox.Info>
+  readonly messages: ReadonlyArray<SessionMessage.Info>
+  readonly seq: Event.Seq
+}
+export type SessionSnapshotOperation<E = never> = (input: Endpoint5_6Input) => Effect.Effect<Endpoint5_6Output, E>

-export type Endpoint5_7Input = { readonly sessionID: Session.ID; readonly boundary: Session.ForkRequestBoundary }
-export type Endpoint5_7Output = Session.Info
-export type SessionForkOperation<E = never> = (input: Endpoint5_7Input) => Effect.Effect<Endpoint5_7Output, E>
+export type Endpoint5_7Input = { readonly sessionID: Session.ID }
+export type Endpoint5_7Output = void
+export type SessionRemoveOperation<E = never> = (input: Endpoint5_7Input) => Effect.Effect<Endpoint5_7Output, E>

-export type Endpoint5_8Input = { readonly sessionID: Session.ID; readonly agent: Agent.ID }
-export type Endpoint5_8Output = void
-export type SessionSwitchAgentOperation<E = never> = (input: Endpoint5_8Input) => Effect.Effect<Endpoint5_8Output, E>
+export type Endpoint5_8Input = { readonly sessionID: Session.ID; readonly boundary: Session.ForkRequestBoundary }
+export type Endpoint5_8Output = Session.Info
+export type SessionForkOperation<E = never> = (input: Endpoint5_8Input) => Effect.Effect<Endpoint5_8Output, E>

-export type Endpoint5_9Input = { readonly sessionID: Session.ID; readonly model: Model.Ref }
+export type Endpoint5_9Input = { readonly sessionID: Session.ID; readonly agent: Agent.ID }
 export type Endpoint5_9Output = void
-export type SessionSwitchModelOperation<E = never> = (input: Endpoint5_9Input) => Effect.Effect<Endpoint5_9Output, E>
+export type SessionSwitchAgentOperation<E = never> = (input: Endpoint5_9Input) => Effect.Effect<Endpoint5_9Output, E>

-export type Endpoint5_10Input = { readonly sessionID: Session.ID; readonly title: string }
+export type Endpoint5_10Input = { readonly sessionID: Session.ID; readonly model: Model.Ref }
 export type Endpoint5_10Output = void
-export type SessionRenameOperation<E = never> = (input: Endpoint5_10Input) => Effect.Effect<Endpoint5_10Output, E>
+export type SessionSwitchModelOperation<E = never> = (input: Endpoint5_10Input) => Effect.Effect<Endpoint5_10Output, E>

-export type Endpoint5_11Input = {
+export type Endpoint5_11Input = { readonly sessionID: Session.ID; readonly title: string }
+export type Endpoint5_11Output = void
+export type SessionRenameOperation<E = never> = (input: Endpoint5_11Input) => Effect.Effect<Endpoint5_11Output, E>
+
+export type Endpoint5_12Input = {
  readonly sessionID: Session.ID
  readonly directory: AbsolutePath
  readonly workspaceID?: Workspace.ID | undefined
  readonly delivery?: SessionInbox.Delivery | undefined
 }
-export type Endpoint5_11Output = void
-export type SessionMoveOperation<E = never> = (input: Endpoint5_11Input) => Effect.Effect<Endpoint5_11Output, E>
+export type Endpoint5_12Output = void
+export type SessionMoveOperation<E = never> = (input: Endpoint5_12Input) => Effect.Effect<Endpoint5_12Output, E>

-export type Endpoint5_12Input = {
+export type Endpoint5_13Input = {
  readonly sessionID: Session.ID
  readonly id?: SessionMessage.ID | undefined
  readonly text: string
@@ -179,10 +189,10 @@ export type Endpoint5_12Input = {
  readonly delivery?: SessionInbox.Delivery | undefined
  readonly resume?: boolean | undefined
 }
-export type Endpoint5_12Output = SessionInbox.User
-export type SessionPromptOperation<E = never> = (input: Endpoint5_12Input) => Effect.Effect<Endpoint5_12Output, E>
+export type Endpoint5_13Output = SessionInbox.User
+export type SessionPromptOperation<E = never> = (input: Endpoint5_13Input) => Effect.Effect<Endpoint5_13Output, E>

-export type Endpoint5_13Input = {
+export type Endpoint5_14Input = {
  readonly sessionID: Session.ID
  readonly id?: SessionMessage.ID | undefined
  readonly command: string
@@ -195,19 +205,19 @@ export type Endpoint5_13Input = {
  readonly delivery?: SessionInbox.Delivery | undefined
  readonly resume?: boolean | undefined
 }
-export type Endpoint5_13Output = SessionInbox.User
-export type SessionCommandOperation<E = never> = (input: Endpoint5_13Input) => Effect.Effect<Endpoint5_13Output, E>
+export type Endpoint5_14Output = SessionInbox.User
+export type SessionCommandOperation<E = never> = (input: Endpoint5_14Input) => Effect.Effect<Endpoint5_14Output, E>

-export type Endpoint5_14Input = {
+export type Endpoint5_15Input = {
  readonly sessionID: Session.ID
  readonly id?: SessionMessage.ID | undefined
  readonly skill: Skill.ID
  readonly resume?: boolean | undefined
 }
-export type Endpoint5_14Output = void
-export type SessionSkillOperation<E = never> = (input: Endpoint5_14Input) => Effect.Effect<Endpoint5_14Output, E>
+export type Endpoint5_15Output = void
+export type SessionSkillOperation<E = never> = (input: Endpoint5_15Input) => Effect.Effect<Endpoint5_15Output, E>

-export type Endpoint5_15Input = {
+export type Endpoint5_16Input = {
  readonly sessionID: Session.ID
  readonly id?: SessionMessage.ID | undefined
  readonly text: string
@@ -216,97 +226,98 @@ export type Endpoint5_15Input = {
  readonly delivery?: SessionInbox.Delivery | undefined
  readonly resume?: boolean | undefined
 }
-export type Endpoint5_15Output = SessionInbox.Synthetic
-export type SessionSyntheticOperation<E = never> = (input: Endpoint5_15Input) => Effect.Effect<Endpoint5_15Output, E>
+export type Endpoint5_16Output = SessionInbox.Synthetic
+export type SessionSyntheticOperation<E = never> = (input: Endpoint5_16Input) => Effect.Effect<Endpoint5_16Output, E>

-export type Endpoint5_16Input = {
+export type Endpoint5_17Input = {
  readonly sessionID: Session.ID
  readonly id?: Event.ID | undefined
  readonly command: string
 }
-export type Endpoint5_16Output = void
-export type SessionShellOperation<E = never> = (input: Endpoint5_16Input) => Effect.Effect<Endpoint5_16Output, E>
+export type Endpoint5_17Output = void
+export type SessionShellOperation<E = never> = (input: Endpoint5_17Input) => Effect.Effect<Endpoint5_17Output, E>

-export type Endpoint5_17Input = {
+export type Endpoint5_18Input = {
  readonly sessionID: Session.ID
  readonly id?: SessionMessage.ID | undefined
  readonly delivery?: SessionInbox.Delivery | undefined
 }
-export type Endpoint5_17Output = SessionInbox.Compaction
-export type SessionCompactOperation<E = never> = (input: Endpoint5_17Input) => Effect.Effect<Endpoint5_17Output, E>
+export type Endpoint5_18Output = SessionInbox.Compaction
+export type SessionCompactOperation<E = never> = (input: Endpoint5_18Input) => Effect.Effect<Endpoint5_18Output, E>

-export type Endpoint5_18Input = { readonly sessionID: Session.ID }
-export type Endpoint5_18Output = void
-export type SessionWaitOperation<E = never> = (input: Endpoint5_18Input) => Effect.Effect<Endpoint5_18Output, E>
+export type Endpoint5_19Input = { readonly sessionID: Session.ID }
+export type Endpoint5_19Output = void
+export type SessionWaitOperation<E = never> = (input: Endpoint5_19Input) => Effect.Effect<Endpoint5_19Output, E>

-export type Endpoint5_19Input = {
+export type Endpoint5_20Input = {
  readonly sessionID: Session.ID
  readonly messageID: SessionMessage.ID
  readonly files?: boolean | undefined
 }
-export type Endpoint5_19Output = Session.Revert
-export type SessionRevertStageOperation<E = never> = (input: Endpoint5_19Input) => Effect.Effect<Endpoint5_19Output, E>
-
-export type Endpoint5_20Input = { readonly sessionID: Session.ID }
-export type Endpoint5_20Output = void
-export type SessionRevertClearOperation<E = never> = (input: Endpoint5_20Input) => Effect.Effect<Endpoint5_20Output, E>
+export type Endpoint5_20Output = Session.Revert
+export type SessionRevertStageOperation<E = never> = (input: Endpoint5_20Input) => Effect.Effect<Endpoint5_20Output, E>

 export type Endpoint5_21Input = { readonly sessionID: Session.ID }
 export type Endpoint5_21Output = void
-export type SessionRevertCommitOperation<E = never> = (input: Endpoint5_21Input) => Effect.Effect<Endpoint5_21Output, E>
+export type SessionRevertClearOperation<E = never> = (input: Endpoint5_21Input) => Effect.Effect<Endpoint5_21Output, E>

 export type Endpoint5_22Input = { readonly sessionID: Session.ID }
-export type Endpoint5_22Output = ReadonlyArray<SessionMessage.Info>
-export type SessionContextOperation<E = never> = (input: Endpoint5_22Input) => Effect.Effect<Endpoint5_22Output, E>
+export type Endpoint5_22Output = void
+export type SessionRevertCommitOperation<E = never> = (input: Endpoint5_22Input) => Effect.Effect<Endpoint5_22Output, E>

 export type Endpoint5_23Input = { readonly sessionID: Session.ID }
-export type Endpoint5_23Output = ReadonlyArray<SessionInbox.Info>
-export type SessionInboxListOperation<E = never> = (input: Endpoint5_23Input) => Effect.Effect<Endpoint5_23Output, E>
+export type Endpoint5_23Output = ReadonlyArray<SessionMessage.Info>
+export type SessionContextOperation<E = never> = (input: Endpoint5_23Input) => Effect.Effect<Endpoint5_23Output, E>

-export type Endpoint5_24Input = { readonly sessionID: Session.ID; readonly inboxID: SessionMessage.ID }
-export type Endpoint5_24Output = void
-export type SessionInboxCancelOperation<E = never> = (input: Endpoint5_24Input) => Effect.Effect<Endpoint5_24Output, E>
+export type Endpoint5_24Input = { readonly sessionID: Session.ID }
+export type Endpoint5_24Output = ReadonlyArray<SessionInbox.Info>
+export type SessionInboxListOperation<E = never> = (input: Endpoint5_24Input) => Effect.Effect<Endpoint5_24Output, E>

 export type Endpoint5_25Input = { readonly sessionID: Session.ID; readonly inboxID: SessionMessage.ID }
 export type Endpoint5_25Output = void
-export type SessionInboxSteerOperation<E = never> = (input: Endpoint5_25Input) => Effect.Effect<Endpoint5_25Output, E>
+export type SessionInboxCancelOperation<E = never> = (input: Endpoint5_25Input) => Effect.Effect<Endpoint5_25Output, E>

 export type Endpoint5_26Input = { readonly sessionID: Session.ID; readonly inboxID: SessionMessage.ID }
 export type Endpoint5_26Output = void
-export type SessionInboxQueueOperation<E = never> = (input: Endpoint5_26Input) => Effect.Effect<Endpoint5_26Output, E>
+export type SessionInboxSteerOperation<E = never> = (input: Endpoint5_26Input) => Effect.Effect<Endpoint5_26Output, E>

-export type Endpoint5_27Input = { readonly sessionID: Session.ID }
-export type Endpoint5_27Output = ReadonlyArray<InstructionEntry.Info>
+export type Endpoint5_27Input = { readonly sessionID: Session.ID; readonly inboxID: SessionMessage.ID }
+export type Endpoint5_27Output = void
+export type SessionInboxQueueOperation<E = never> = (input: Endpoint5_27Input) => Effect.Effect<Endpoint5_27Output, E>
+
+export type Endpoint5_28Input = { readonly sessionID: Session.ID }
+export type Endpoint5_28Output = ReadonlyArray<InstructionEntry.Info>
 export type SessionInstructionsEntryListOperation<E = never> = (
-  input: Endpoint5_27Input,
-) => Effect.Effect<Endpoint5_27Output, E>
+  input: Endpoint5_28Input,
+) => Effect.Effect<Endpoint5_28Output, E>

-export type Endpoint5_28Input = {
+export type Endpoint5_29Input = {
  readonly sessionID: Session.ID
  readonly key: InstructionEntry.Key
  readonly value: Schema.Json
 }
-export type Endpoint5_28Output = void
-export type SessionInstructionsEntryPutOperation<E = never> = (
-  input: Endpoint5_28Input,
-) => Effect.Effect<Endpoint5_28Output, E>
-
-export type Endpoint5_29Input = { readonly sessionID: Session.ID; readonly key: InstructionEntry.Key }
 export type Endpoint5_29Output = void
-export type SessionInstructionsEntryRemoveOperation<E = never> = (
+export type SessionInstructionsEntryPutOperation<E = never> = (
  input: Endpoint5_29Input,
 ) => Effect.Effect<Endpoint5_29Output, E>

-export type Endpoint5_30Input = { readonly sessionID: Session.ID; readonly prompt: string }
-export type Endpoint5_30Output = { readonly text: string }
-export type SessionGenerateOperation<E = never> = (input: Endpoint5_30Input) => Effect.Effect<Endpoint5_30Output, E>
+export type Endpoint5_30Input = { readonly sessionID: Session.ID; readonly key: InstructionEntry.Key }
+export type Endpoint5_30Output = void
+export type SessionInstructionsEntryRemoveOperation<E = never> = (
+  input: Endpoint5_30Input,
+) => Effect.Effect<Endpoint5_30Output, E>

-export type Endpoint5_31Input = {
+export type Endpoint5_31Input = { readonly sessionID: Session.ID; readonly prompt: string }
+export type Endpoint5_31Output = { readonly text: string }
+export type SessionGenerateOperation<E = never> = (input: Endpoint5_31Input) => Effect.Effect<Endpoint5_31Output, E>
+
+export type Endpoint5_32Input = {
  readonly sessionID: Session.ID
  readonly after?: Event.Seq | undefined
  readonly follow?: boolean | undefined
+  readonly ephemeral?: boolean | undefined
 }
-export type Endpoint5_31Output =
+export type Endpoint5_32Output =
  | (
      | {
          readonly id: Event.ID
@@ -894,24 +905,101 @@ export type Endpoint5_31Output =
          }
        }
    )
+  | {
+      readonly id: Event.ID
+      readonly created: number
+      readonly metadata?: { readonly [x: string]: unknown } | undefined
+      readonly type: "session.usage.updated"
+      readonly location?: Location.Ref | undefined
+      readonly data: {
+        readonly sessionID: Session.ID
+        readonly cost: number & Brand.Brand<"Money.USD">
+        readonly tokens: {
+          readonly input: number
+          readonly output: number
+          readonly reasoning: number
+          readonly cache: { readonly read: number; readonly write: number }
+        }
+      }
+    }
+  | {
+      readonly id: Event.ID
+      readonly created: number
+      readonly metadata?: { readonly [x: string]: unknown } | undefined
+      readonly type: "session.text.delta"
+      readonly location?: Location.Ref | undefined
+      readonly data: {
+        readonly sessionID: Session.ID
+        readonly assistantMessageID: SessionMessage.ID
+        readonly ordinal: number
+        readonly delta: string
+      }
+    }
+  | {
+      readonly id: Event.ID
+      readonly created: number
+      readonly metadata?: { readonly [x: string]: unknown } | undefined
+      readonly type: "session.reasoning.delta"
+      readonly location?: Location.Ref | undefined
+      readonly data: {
+        readonly sessionID: Session.ID
+        readonly assistantMessageID: SessionMessage.ID
+        readonly ordinal: number
+        readonly delta: string
+      }
+    }
+  | {
+      readonly id: Event.ID
+      readonly created: number
+      readonly metadata?: { readonly [x: string]: unknown } | undefined
+      readonly type: "session.tool.input.delta"
+      readonly location?: Location.Ref | undefined
+      readonly data: {
+        readonly sessionID: Session.ID
+        readonly assistantMessageID: SessionMessage.ID
+        readonly id: string
+        readonly delta: string
+      }
+    }
+  | {
+      readonly id: Event.ID
+      readonly created: number
+      readonly metadata?: { readonly [x: string]: unknown } | undefined
+      readonly type: "session.tool.progress"
+      readonly location?: Location.Ref | undefined
+      readonly data: {
+        readonly sessionID: Session.ID
+        readonly assistantMessageID: SessionMessage.ID
+        readonly id: string
+        readonly metadata: { readonly [x: string]: Schema.Json }
+      }
+    }
+  | {
+      readonly id: Event.ID
+      readonly created: number
+      readonly metadata?: { readonly [x: string]: unknown } | undefined
+      readonly type: "session.compaction.delta"
+      readonly location?: Location.Ref | undefined
+      readonly data: { readonly sessionID: Session.ID; readonly text: string }
+    }
  | EventLog.Synced
-export type SessionLogOperation<E = never> = (input: Endpoint5_31Input) => Stream.Stream<Endpoint5_31Output, E>
+export type SessionLogOperation<E = never> = (input: Endpoint5_32Input) => Stream.Stream<Endpoint5_32Output, E>

-export type Endpoint5_32Input = { readonly sessionID: Session.ID; readonly continue?: boolean | undefined }
-export type Endpoint5_32Output = void
-export type SessionInterruptOperation<E = never> = (input: Endpoint5_32Input) => Effect.Effect<Endpoint5_32Output, E>
-
-export type Endpoint5_33Input = { readonly sessionID: Session.ID }
+export type Endpoint5_33Input = { readonly sessionID: Session.ID; readonly continue?: boolean | undefined }
 export type Endpoint5_33Output = void
-export type SessionBackgroundOperation<E = never> = (input: Endpoint5_33Input) => Effect.Effect<Endpoint5_33Output, E>
+export type SessionInterruptOperation<E = never> = (input: Endpoint5_33Input) => Effect.Effect<Endpoint5_33Output, E>

-export type Endpoint5_34Input = { readonly sessionID: Session.ID; readonly messageID: SessionMessage.ID }
-export type Endpoint5_34Output = SessionMessage.Info
-export type SessionMessageOperation<E = never> = (input: Endpoint5_34Input) => Effect.Effect<Endpoint5_34Output, E>
+export type Endpoint5_34Input = { readonly sessionID: Session.ID }
+export type Endpoint5_34Output = void
+export type SessionBackgroundOperation<E = never> = (input: Endpoint5_34Input) => Effect.Effect<Endpoint5_34Output, E>

-export type Endpoint5_35Input = { readonly sessionID: Session.ID; readonly variables: { readonly [x: string]: string } }
-export type Endpoint5_35Output = void
-export type SessionEnvironmentOperation<E = never> = (input: Endpoint5_35Input) => Effect.Effect<Endpoint5_35Output, E>
+export type Endpoint5_35Input = { readonly sessionID: Session.ID; readonly messageID: SessionMessage.ID }
+export type Endpoint5_35Output = SessionMessage.Info
+export type SessionMessageOperation<E = never> = (input: Endpoint5_35Input) => Effect.Effect<Endpoint5_35Output, E>
+
+export type Endpoint5_36Input = { readonly sessionID: Session.ID; readonly variables: { readonly [x: string]: string } }
+export type Endpoint5_36Output = void
+export type SessionEnvironmentOperation<E = never> = (input: Endpoint5_36Input) => Effect.Effect<Endpoint5_36Output, E>

 export interface SessionApi<E = never> {
  readonly list: SessionListOperation<E>
@@ -920,6 +1008,7 @@ export interface SessionApi<E = never> {
  readonly export: SessionExportOperation<E>
  readonly active: SessionActiveOperation<E>
  readonly get: SessionGetOperation<E>
+  readonly snapshot: SessionSnapshotOperation<E>
  readonly remove: SessionRemoveOperation<E>
  readonly fork: SessionForkOperation<E>
  readonly switchAgent: SessionSwitchAgentOperation<E>
@@ -86,6 +86,8 @@ import type {
  Endpoint5_34Output,
  Endpoint5_35Input,
  Endpoint5_35Output,
+  Endpoint5_36Input,
+  Endpoint5_36Output,
  Endpoint6_0Input,
  Endpoint6_0Output,
  Endpoint7_0Input,
@@ -350,48 +352,56 @@ const Endpoint5_5 = (raw: RawClient["server.session"]) => (input: Endpoint5_5Inp

 const Endpoint5_6 = (raw: RawClient["server.session"]) => (input: Endpoint5_6Input) =>
  preserveEffect<Endpoint5_6Output>()(
-    raw["session.remove"]({ params: { sessionID: input["sessionID"] } }).pipe(Effect.mapError(mapClientError)),
+    raw["session.snapshot"]({ params: { sessionID: input["sessionID"] }, query: { recent: input["recent"] } }).pipe(
+      Effect.mapError(mapClientError),
+      Effect.map((value) => value.data),
+    ),
  )

 const Endpoint5_7 = (raw: RawClient["server.session"]) => (input: Endpoint5_7Input) =>
  preserveEffect<Endpoint5_7Output>()(
+    raw["session.remove"]({ params: { sessionID: input["sessionID"] } }).pipe(Effect.mapError(mapClientError)),
+  )
+
+const Endpoint5_8 = (raw: RawClient["server.session"]) => (input: Endpoint5_8Input) =>
+  preserveEffect<Endpoint5_8Output>()(
    raw["session.fork"]({ params: { sessionID: input["sessionID"] }, payload: { boundary: input["boundary"] } }).pipe(
      Effect.mapError(mapClientError),
      Effect.map((value) => value.data),
    ),
  )

-const Endpoint5_8 = (raw: RawClient["server.session"]) => (input: Endpoint5_8Input) =>
-  preserveEffect<Endpoint5_8Output>()(
-    raw["session.switchAgent"]({ params: { sessionID: input["sessionID"] }, payload: { agent: input["agent"] } }).pipe(
-      Effect.mapError(mapClientError),
-    ),
-  )
-
 const Endpoint5_9 = (raw: RawClient["server.session"]) => (input: Endpoint5_9Input) =>
  preserveEffect<Endpoint5_9Output>()(
-    raw["session.switchModel"]({ params: { sessionID: input["sessionID"] }, payload: { model: input["model"] } }).pipe(
+    raw["session.switchAgent"]({ params: { sessionID: input["sessionID"] }, payload: { agent: input["agent"] } }).pipe(
      Effect.mapError(mapClientError),
    ),
  )

 const Endpoint5_10 = (raw: RawClient["server.session"]) => (input: Endpoint5_10Input) =>
  preserveEffect<Endpoint5_10Output>()(
-    raw["session.rename"]({ params: { sessionID: input["sessionID"] }, payload: { title: input["title"] } }).pipe(
+    raw["session.switchModel"]({ params: { sessionID: input["sessionID"] }, payload: { model: input["model"] } }).pipe(
      Effect.mapError(mapClientError),
    ),
  )

 const Endpoint5_11 = (raw: RawClient["server.session"]) => (input: Endpoint5_11Input) =>
  preserveEffect<Endpoint5_11Output>()(
+    raw["session.rename"]({ params: { sessionID: input["sessionID"] }, payload: { title: input["title"] } }).pipe(
+      Effect.mapError(mapClientError),
+    ),
+  )
+
+const Endpoint5_12 = (raw: RawClient["server.session"]) => (input: Endpoint5_12Input) =>
+  preserveEffect<Endpoint5_12Output>()(
    raw["session.move"]({
      params: { sessionID: input["sessionID"] },
      payload: { directory: input["directory"], workspaceID: input["workspaceID"], delivery: input["delivery"] },
    }).pipe(Effect.mapError(mapClientError)),
  )

-const Endpoint5_12 = (raw: RawClient["server.session"]) => (input: Endpoint5_12Input) =>
-  preserveEffect<Endpoint5_12Output>()(
+const Endpoint5_13 = (raw: RawClient["server.session"]) => (input: Endpoint5_13Input) =>
+  preserveEffect<Endpoint5_13Output>()(
    raw["session.prompt"]({
      params: { sessionID: input["sessionID"] },
      payload: {
@@ -410,8 +420,8 @@ const Endpoint5_12 = (raw: RawClient["server.session"]) => (input: Endpoint5_12I
    ),
  )

-const Endpoint5_13 = (raw: RawClient["server.session"]) => (input: Endpoint5_13Input) =>
-  preserveEffect<Endpoint5_13Output>()(
+const Endpoint5_14 = (raw: RawClient["server.session"]) => (input: Endpoint5_14Input) =>
+  preserveEffect<Endpoint5_14Output>()(
    raw["session.command"]({
      params: { sessionID: input["sessionID"] },
      payload: {
@@ -432,16 +442,16 @@ const Endpoint5_13 = (raw: RawClient["server.session"]) => (input: Endpoint5_13I
    ),
  )

-const Endpoint5_14 = (raw: RawClient["server.session"]) => (input: Endpoint5_14Input) =>
-  preserveEffect<Endpoint5_14Output>()(
+const Endpoint5_15 = (raw: RawClient["server.session"]) => (input: Endpoint5_15Input) =>
+  preserveEffect<Endpoint5_15Output>()(
    raw["session.skill"]({
      params: { sessionID: input["sessionID"] },
      payload: { id: input["id"], skill: input["skill"], resume: input["resume"] },
    }).pipe(Effect.mapError(mapClientError)),
  )

-const Endpoint5_15 = (raw: RawClient["server.session"]) => (input: Endpoint5_15Input) =>
-  preserveEffect<Endpoint5_15Output>()(
+const Endpoint5_16 = (raw: RawClient["server.session"]) => (input: Endpoint5_16Input) =>
+  preserveEffect<Endpoint5_16Output>()(
    raw["session.synthetic"]({
      params: { sessionID: input["sessionID"] },
      payload: {
@@ -458,16 +468,16 @@ const Endpoint5_15 = (raw: RawClient["server.session"]) => (input: Endpoint5_15I
    ),
  )

-const Endpoint5_16 = (raw: RawClient["server.session"]) => (input: Endpoint5_16Input) =>
-  preserveEffect<Endpoint5_16Output>()(
+const Endpoint5_17 = (raw: RawClient["server.session"]) => (input: Endpoint5_17Input) =>
+  preserveEffect<Endpoint5_17Output>()(
    raw["session.shell"]({
      params: { sessionID: input["sessionID"] },
      payload: { id: input["id"], command: input["command"] },
    }).pipe(Effect.mapError(mapClientError)),
  )

-const Endpoint5_17 = (raw: RawClient["server.session"]) => (input: Endpoint5_17Input) =>
-  preserveEffect<Endpoint5_17Output>()(
+const Endpoint5_18 = (raw: RawClient["server.session"]) => (input: Endpoint5_18Input) =>
+  preserveEffect<Endpoint5_18Output>()(
    raw["session.compact"]({
      params: { sessionID: input["sessionID"] },
      payload: { id: input["id"], delivery: input["delivery"] },
@@ -477,13 +487,13 @@ const Endpoint5_17 = (raw: RawClient["server.session"]) => (input: Endpoint5_17I
    ),
  )

-const Endpoint5_18 = (raw: RawClient["server.session"]) => (input: Endpoint5_18Input) =>
-  preserveEffect<Endpoint5_18Output>()(
+const Endpoint5_19 = (raw: RawClient["server.session"]) => (input: Endpoint5_19Input) =>
+  preserveEffect<Endpoint5_19Output>()(
    raw["session.wait"]({ params: { sessionID: input["sessionID"] } }).pipe(Effect.mapError(mapClientError)),
  )

-const Endpoint5_19 = (raw: RawClient["server.session"]) => (input: Endpoint5_19Input) =>
-  preserveEffect<Endpoint5_19Output>()(
+const Endpoint5_20 = (raw: RawClient["server.session"]) => (input: Endpoint5_20Input) =>
+  preserveEffect<Endpoint5_20Output>()(
    raw["session.revert.stage"]({
      params: { sessionID: input["sessionID"] },
      payload: { messageID: input["messageID"], files: input["files"] },
@@ -493,27 +503,19 @@ const Endpoint5_19 = (raw: RawClient["server.session"]) => (input: Endpoint5_19I
    ),
  )

-const Endpoint5_20 = (raw: RawClient["server.session"]) => (input: Endpoint5_20Input) =>
-  preserveEffect<Endpoint5_20Output>()(
-    raw["session.revert.clear"]({ params: { sessionID: input["sessionID"] } }).pipe(Effect.mapError(mapClientError)),
-  )
-
 const Endpoint5_21 = (raw: RawClient["server.session"]) => (input: Endpoint5_21Input) =>
  preserveEffect<Endpoint5_21Output>()(
-    raw["session.revert.commit"]({ params: { sessionID: input["sessionID"] } }).pipe(Effect.mapError(mapClientError)),
+    raw["session.revert.clear"]({ params: { sessionID: input["sessionID"] } }).pipe(Effect.mapError(mapClientError)),
  )

 const Endpoint5_22 = (raw: RawClient["server.session"]) => (input: Endpoint5_22Input) =>
  preserveEffect<Endpoint5_22Output>()(
-    raw["session.context"]({ params: { sessionID: input["sessionID"] } }).pipe(
-      Effect.mapError(mapClientError),
-      Effect.map((value) => value.data),
-    ),
+    raw["session.revert.commit"]({ params: { sessionID: input["sessionID"] } }).pipe(Effect.mapError(mapClientError)),
  )

 const Endpoint5_23 = (raw: RawClient["server.session"]) => (input: Endpoint5_23Input) =>
  preserveEffect<Endpoint5_23Output>()(
-    raw["session.inbox.list"]({ params: { sessionID: input["sessionID"] } }).pipe(
+    raw["session.context"]({ params: { sessionID: input["sessionID"] } }).pipe(
      Effect.mapError(mapClientError),
      Effect.map((value) => value.data),
    ),
@@ -521,62 +523,70 @@ const Endpoint5_23 = (raw: RawClient["server.session"]) => (input: Endpoint5_23I

 const Endpoint5_24 = (raw: RawClient["server.session"]) => (input: Endpoint5_24Input) =>
  preserveEffect<Endpoint5_24Output>()(
-    raw["session.inbox.cancel"]({ params: { sessionID: input["sessionID"], inboxID: input["inboxID"] } }).pipe(
+    raw["session.inbox.list"]({ params: { sessionID: input["sessionID"] } }).pipe(
      Effect.mapError(mapClientError),
+      Effect.map((value) => value.data),
    ),
  )

 const Endpoint5_25 = (raw: RawClient["server.session"]) => (input: Endpoint5_25Input) =>
  preserveEffect<Endpoint5_25Output>()(
-    raw["session.inbox.steer"]({ params: { sessionID: input["sessionID"], inboxID: input["inboxID"] } }).pipe(
+    raw["session.inbox.cancel"]({ params: { sessionID: input["sessionID"], inboxID: input["inboxID"] } }).pipe(
      Effect.mapError(mapClientError),
    ),
  )

 const Endpoint5_26 = (raw: RawClient["server.session"]) => (input: Endpoint5_26Input) =>
  preserveEffect<Endpoint5_26Output>()(
-    raw["session.inbox.queue"]({ params: { sessionID: input["sessionID"], inboxID: input["inboxID"] } }).pipe(
+    raw["session.inbox.steer"]({ params: { sessionID: input["sessionID"], inboxID: input["inboxID"] } }).pipe(
      Effect.mapError(mapClientError),
    ),
  )

 const Endpoint5_27 = (raw: RawClient["server.session"]) => (input: Endpoint5_27Input) =>
  preserveEffect<Endpoint5_27Output>()(
+    raw["session.inbox.queue"]({ params: { sessionID: input["sessionID"], inboxID: input["inboxID"] } }).pipe(
+      Effect.mapError(mapClientError),
+    ),
+  )
+
+const Endpoint5_28 = (raw: RawClient["server.session"]) => (input: Endpoint5_28Input) =>
+  preserveEffect<Endpoint5_28Output>()(
    raw["session.instructions.entry.list"]({ params: { sessionID: input["sessionID"] } }).pipe(
      Effect.mapError(mapClientError),
      Effect.map((value) => value.data),
    ),
  )

-const Endpoint5_28 = (raw: RawClient["server.session"]) => (input: Endpoint5_28Input) =>
-  preserveEffect<Endpoint5_28Output>()(
+const Endpoint5_29 = (raw: RawClient["server.session"]) => (input: Endpoint5_29Input) =>
+  preserveEffect<Endpoint5_29Output>()(
    raw["session.instructions.entry.put"]({
      params: { sessionID: input["sessionID"], key: input["key"] },
      payload: { value: input["value"] },
    }).pipe(Effect.mapError(mapClientError)),
  )

-const Endpoint5_29 = (raw: RawClient["server.session"]) => (input: Endpoint5_29Input) =>
-  preserveEffect<Endpoint5_29Output>()(
+const Endpoint5_30 = (raw: RawClient["server.session"]) => (input: Endpoint5_30Input) =>
+  preserveEffect<Endpoint5_30Output>()(
    raw["session.instructions.entry.remove"]({ params: { sessionID: input["sessionID"], key: input["key"] } }).pipe(
      Effect.mapError(mapClientError),
    ),
  )

-const Endpoint5_30 = (raw: RawClient["server.session"]) => (input: Endpoint5_30Input) =>
-  preserveEffect<Endpoint5_30Output>()(
+const Endpoint5_31 = (raw: RawClient["server.session"]) => (input: Endpoint5_31Input) =>
+  preserveEffect<Endpoint5_31Output>()(
    raw["session.generate"]({ params: { sessionID: input["sessionID"] }, payload: { prompt: input["prompt"] } }).pipe(
      Effect.mapError(mapClientError),
      Effect.map((value) => value.data),
    ),
  )

-const Endpoint5_31 = (raw: RawClient["server.session"]) => (input: Endpoint5_31Input) =>
-  preserveStream<Endpoint5_31Output>()(
+const Endpoint5_32 = (raw: RawClient["server.session"]) => (input: Endpoint5_32Input) =>
+  preserveStream<Endpoint5_32Output>()(
    Stream.unwrap(
      raw["session.log"]({
        params: { sessionID: input["sessionID"] },
-        query: { after: input["after"], follow: input["follow"] },
+        query: { after: input["after"], follow: input["follow"], ephemeral: input["ephemeral"] },
      }).pipe(
        Effect.mapError(mapClientError),
        Effect.map((stream) => stream.pipe(Stream.mapError(mapClientError))),
@@ -584,29 +594,29 @@ const Endpoint5_31 = (raw: RawClient["server.session"]) => (input: Endpoint5_31I
    ),
  )

-const Endpoint5_32 = (raw: RawClient["server.session"]) => (input: Endpoint5_32Input) =>
-  preserveEffect<Endpoint5_32Output>()(
+const Endpoint5_33 = (raw: RawClient["server.session"]) => (input: Endpoint5_33Input) =>
+  preserveEffect<Endpoint5_33Output>()(
    raw["session.interrupt"]({
      params: { sessionID: input["sessionID"] },
      query: { continue: input["continue"] },
    }).pipe(Effect.mapError(mapClientError)),
  )

-const Endpoint5_33 = (raw: RawClient["server.session"]) => (input: Endpoint5_33Input) =>
-  preserveEffect<Endpoint5_33Output>()(
+const Endpoint5_34 = (raw: RawClient["server.session"]) => (input: Endpoint5_34Input) =>
+  preserveEffect<Endpoint5_34Output>()(
    raw["session.background"]({ params: { sessionID: input["sessionID"] } }).pipe(Effect.mapError(mapClientError)),
  )

-const Endpoint5_34 = (raw: RawClient["server.session"]) => (input: Endpoint5_34Input) =>
-  preserveEffect<Endpoint5_34Output>()(
+const Endpoint5_35 = (raw: RawClient["server.session"]) => (input: Endpoint5_35Input) =>
+  preserveEffect<Endpoint5_35Output>()(
    raw["session.message"]({ params: { sessionID: input["sessionID"], messageID: input["messageID"] } }).pipe(
      Effect.mapError(mapClientError),
      Effect.map((value) => value.data),
    ),
  )

-const Endpoint5_35 = (raw: RawClient["server.session"]) => (input: Endpoint5_35Input) =>
-  preserveEffect<Endpoint5_35Output>()(
+const Endpoint5_36 = (raw: RawClient["server.session"]) => (input: Endpoint5_36Input) =>
+  preserveEffect<Endpoint5_36Output>()(
    raw["session.environment"]({
      params: { sessionID: input["sessionID"] },
      payload: { variables: input["variables"] },
@@ -620,29 +630,30 @@ const adaptGroup5 = (raw: RawClient["server.session"]) => ({
  export: Endpoint5_3(raw),
  active: Endpoint5_4(raw),
  get: Endpoint5_5(raw),
-  remove: Endpoint5_6(raw),
-  fork: Endpoint5_7(raw),
-  switchAgent: Endpoint5_8(raw),
-  switchModel: Endpoint5_9(raw),
-  rename: Endpoint5_10(raw),
-  move: Endpoint5_11(raw),
-  prompt: Endpoint5_12(raw),
-  command: Endpoint5_13(raw),
-  skill: Endpoint5_14(raw),
-  synthetic: Endpoint5_15(raw),
-  shell: Endpoint5_16(raw),
-  compact: Endpoint5_17(raw),
-  wait: Endpoint5_18(raw),
-  revert: { stage: Endpoint5_19(raw), clear: Endpoint5_20(raw), commit: Endpoint5_21(raw) },
-  context: Endpoint5_22(raw),
-  inbox: { list: Endpoint5_23(raw), cancel: Endpoint5_24(raw), steer: Endpoint5_25(raw), queue: Endpoint5_26(raw) },
-  instructions: { entry: { list: Endpoint5_27(raw), put: Endpoint5_28(raw), remove: Endpoint5_29(raw) } },
-  generate: Endpoint5_30(raw),
-  log: Endpoint5_31(raw),
-  interrupt: Endpoint5_32(raw),
-  background: Endpoint5_33(raw),
-  message: Endpoint5_34(raw),
-  environment: Endpoint5_35(raw),
+  snapshot: Endpoint5_6(raw),
+  remove: Endpoint5_7(raw),
+  fork: Endpoint5_8(raw),
+  switchAgent: Endpoint5_9(raw),
+  switchModel: Endpoint5_10(raw),
+  rename: Endpoint5_11(raw),
+  move: Endpoint5_12(raw),
+  prompt: Endpoint5_13(raw),
+  command: Endpoint5_14(raw),
+  skill: Endpoint5_15(raw),
+  synthetic: Endpoint5_16(raw),
+  shell: Endpoint5_17(raw),
+  compact: Endpoint5_18(raw),
+  wait: Endpoint5_19(raw),
+  revert: { stage: Endpoint5_20(raw), clear: Endpoint5_21(raw), commit: Endpoint5_22(raw) },
+  context: Endpoint5_23(raw),
+  inbox: { list: Endpoint5_24(raw), cancel: Endpoint5_25(raw), steer: Endpoint5_26(raw), queue: Endpoint5_27(raw) },
+  instructions: { entry: { list: Endpoint5_28(raw), put: Endpoint5_29(raw), remove: Endpoint5_30(raw) } },
+  generate: Endpoint5_31(raw),
+  log: Endpoint5_32(raw),
+  interrupt: Endpoint5_33(raw),
+  background: Endpoint5_34(raw),
+  message: Endpoint5_35(raw),
+  environment: Endpoint5_36(raw),
 })

 const Endpoint6_0 = (raw: RawClient["server.message"]) => (input: Endpoint6_0Input) =>
@@ -20,6 +20,8 @@ import type {
  SessionActiveOutput,
  SessionGetInput,
  SessionGetOutput,
+  SessionSnapshotInput,
+  SessionSnapshotOutput,
  SessionRemoveInput,
  SessionRemoveOutput,
  SessionForkInput,
@@ -514,6 +516,18 @@ export function make(options: ClientOptions) {
          },
          requestOptions,
        ).then((value) => value.data),
+      snapshot: (input: SessionSnapshotInput, requestOptions?: RequestOptions) =>
+        request<{ readonly data: SessionSnapshotOutput }>(
+          {
+            method: "GET",
+            path: `/api/session/${encodeURIComponent(input.sessionID)}/snapshot`,
+            query: { recent: input["recent"] },
+            successStatus: 200,
+            declaredStatuses: [404, 500, 401, 400],
+            empty: false,
+          },
+          requestOptions,
+        ).then((value) => value.data),
      remove: (input: SessionRemoveInput, requestOptions?: RequestOptions) =>
        request<SessionRemoveOutput>(
          {
@@ -843,9 +857,9 @@ export function make(options: ClientOptions) {
          {
            method: "GET",
            path: `/api/experimental/session/${encodeURIComponent(input.sessionID)}/log`,
-            query: { after: input["after"], follow: input["follow"] },
+            query: { after: input["after"], follow: input["follow"], ephemeral: input["ephemeral"] },
            successStatus: 200,
-            declaredStatuses: [404, 401, 400],
+            declaredStatuses: [404, 409, 401, 400],
            empty: false,
          },
          requestOptions,
@@ -512,51 +512,6 @@ export type SessionRevertCommitted = {
  data: { sessionID: string; to: string }
 }

-export type ModelsDevRefreshed = {
-  id: string
-  created: number
-  metadata?: { [x: string]: any }
-  type: "models-dev.refreshed"
-  location?: LocationRef
-  data: {}
-}
-
-export type IntegrationUpdated = {
-  id: string
-  created: number
-  metadata?: { [x: string]: any }
-  type: "integration.updated"
-  location?: LocationRef
-  data: {}
-}
-
-export type IntegrationConnectionUpdated = {
-  id: string
-  created: number
-  metadata?: { [x: string]: any }
-  type: "integration.connection.updated"
-  location?: LocationRef
-  data: { integrationID: string }
-}
-
-export type CatalogUpdated = {
-  id: string
-  created: number
-  metadata?: { [x: string]: any }
-  type: "catalog.updated"
-  location?: LocationRef
-  data: {}
-}
-
-export type AgentUpdated = {
-  id: string
-  created: number
-  metadata?: { [x: string]: any }
-  type: "agent.updated"
-  location?: LocationRef
-  data: {}
-}
-
 export type SessionTextDelta = {
  id: string
  created: number
@@ -602,6 +557,51 @@ export type SessionCompactionDelta = {
  data: { sessionID: string; text: string }
 }

+export type ModelsDevRefreshed = {
+  id: string
+  created: number
+  metadata?: { [x: string]: any }
+  type: "models-dev.refreshed"
+  location?: LocationRef
+  data: {}
+}
+
+export type IntegrationUpdated = {
+  id: string
+  created: number
+  metadata?: { [x: string]: any }
+  type: "integration.updated"
+  location?: LocationRef
+  data: {}
+}
+
+export type IntegrationConnectionUpdated = {
+  id: string
+  created: number
+  metadata?: { [x: string]: any }
+  type: "integration.connection.updated"
+  location?: LocationRef
+  data: { integrationID: string }
+}
+
+export type CatalogUpdated = {
+  id: string
+  created: number
+  metadata?: { [x: string]: any }
+  type: "catalog.updated"
+  location?: LocationRef
+  data: {}
+}
+
+export type AgentUpdated = {
+  id: string
+  created: number
+  metadata?: { [x: string]: any }
+  type: "agent.updated"
+  location?: LocationRef
+  data: {}
+}
+
 export type FilesystemChanged = {
  id: string
  created: number
@@ -1994,7 +1994,15 @@ export type FormCreated = {
  data: { form: FormInfo1 }
 }

-export type SessionLogItem = SessionEventDurable | EventLogSynced
+export type SessionLogItem =
+  | SessionEventDurable
+  | SessionUsageUpdated
+  | SessionTextDelta
+  | SessionReasoningDelta
+  | SessionToolInputDelta
+  | SessionToolProgress
+  | SessionCompactionDelta
+  | EventLogSynced

 export type IntegrationOAuthMethod = { id: string; type: "oauth"; label: string; form?: FormFields }

@@ -2004,6 +2012,16 @@ export type FormInfo = { id: string; sessionID: string; title: string; metadata?

 export type SessionTransferData = { info: SessionInfo; messages: Array<SessionMessageInfo> }

+export type SessionSnapshotResponse = {
+  data: {
+    session: SessionInfo
+    children: Array<SessionInfo>
+    inbox: Array<SessionInboxInfo>
+    messages: Array<SessionMessageInfo>
+    seq: number
+  }
+}
+
 export type SessionMessagesResponse = {
  data: Array<SessionMessageInfo>
  cursor: { previous?: string | null; next?: string | null }
@@ -2218,6 +2236,16 @@ export const isInstructionEntryValueTooLargeError = (value: unknown): value is I
  "_tag" in value &&
  value["_tag"] === "InstructionEntryValueTooLargeError"

+export type SeqUnavailableError = {
+  readonly _tag: "SeqUnavailableError"
+  readonly sessionID: string
+  readonly after: number
+  readonly head?: number | undefined
+  readonly message: string
+}
+export const isSeqUnavailableError = (value: unknown): value is SeqUnavailableError =>
+  typeof value === "object" && value !== null && "_tag" in value && value["_tag"] === "SeqUnavailableError"
+
 export type ProviderNotFoundError = {
  readonly _tag: "ProviderNotFoundError"
  readonly providerID: string
@@ -3290,6 +3318,13 @@ export type SessionGetInput = { readonly sessionID: { readonly sessionID: string

 export type SessionGetOutput = { data: SessionInfo }["data"]

+export type SessionSnapshotInput = {
+  readonly sessionID: { readonly sessionID: string }["sessionID"]
+  readonly recent?: { readonly recent?: number | undefined }["recent"]
+}
+
+export type SessionSnapshotOutput = SessionSnapshotResponse["data"]
+
 export type SessionRemoveInput = { readonly sessionID: { readonly sessionID: string }["sessionID"] }

 export type SessionRemoveOutput = void
@@ -3928,8 +3963,21 @@ export type SessionGenerateOutput = SessionGenerateResponse["data"]

 export type SessionLogInput = {
  readonly sessionID: { readonly sessionID: string }["sessionID"]
-  readonly after?: { readonly after?: number | undefined; readonly follow?: boolean | undefined }["after"]
-  readonly follow?: { readonly after?: number | undefined; readonly follow?: boolean | undefined }["follow"]
+  readonly after?: {
+    readonly after?: number | undefined
+    readonly follow?: boolean | undefined
+    readonly ephemeral?: boolean | undefined
+  }["after"]
+  readonly follow?: {
+    readonly after?: number | undefined
+    readonly follow?: boolean | undefined
+    readonly ephemeral?: boolean | undefined
+  }["follow"]
+  readonly ephemeral?: {
+    readonly after?: number | undefined
+    readonly follow?: boolean | undefined
+    readonly ephemeral?: boolean | undefined
+  }["ephemeral"]
 }

 export type SessionLogOutput = SessionLogItem
@@ -0,0 +1,262 @@
+import { batch, onCleanup } from "solid-js"
+import { createStore, reconcile, unwrap } from "solid-js/store"
+import type { OpenCodeClient, OpenCodeEvent, SessionPromptInput } from "../promise"
+import { isSeqUnavailableError } from "../promise"
+import { createData } from "./data"
+import type { CreateDataInput } from "./data"
+import { Engine } from "./engine/engine"
+
+type SessionApi = Pick<OpenCodeClient["session"], "snapshot" | "log" | "prompt">
+
+// Solid's setStore path types reject readonly arrays. The store only ever
+// holds deep clones it owns, so a mutable mirror of the engine view is safe.
+type StoreSessionView = {
+  -readonly [Key in keyof Engine.SessionView]: Engine.SessionView[Key] extends ReadonlyArray<infer Item>
+    ? Item[]
+    : Engine.SessionView[Key]
+}
+
+const ambientSessionEvents = new Set<OpenCodeEvent["type"]>([
+  "session.created",
+  "session.deleted",
+  "session.renamed",
+  "session.execution.started",
+  "session.execution.succeeded",
+  "session.execution.failed",
+  "session.execution.interrupted",
+])
+
+export function createEngineTransport(api: () => SessionApi): Engine.SessionTransport {
+  return {
+    snapshot(sessionID) {
+      return api().snapshot({ sessionID, recent: 200 })
+    },
+    async *stream(sessionID, after, signal) {
+      try {
+        for await (const item of api().log(
+          { sessionID, after, follow: true, ephemeral: true },
+          signal ? { signal } : undefined,
+        )) {
+          if (item.type !== "session.forked") yield item
+        }
+      } catch (error) {
+        if (isSeqUnavailableError(error)) throw new Engine.SeqUnavailable()
+        throw error
+      }
+    },
+    async submit(input) {
+      try {
+        await api().prompt({ ...input.request, sessionID: input.sessionID, id: input.id })
+      } catch (error) {
+        if (isTypedError(error)) throw new Engine.SubmitRejected(error.message)
+        throw error
+      }
+    },
+  }
+}
+
+export function createEngineData(config: CreateDataInput) {
+  const legacy = createData({
+    ...config,
+    event: {
+      on: config.event.on,
+      listen(handler) {
+        return config.event.listen((event) => {
+          if (event.name.startsWith("session.") && !ambientSessionEvents.has(event.name)) return
+          handler(event)
+        })
+      },
+    },
+  })
+  const [views, setViews] = createStore<Record<string, StoreSessionView>>({})
+  const engines = new Map<string, Promise<Engine.SessionEngine>>()
+  const families = new Set<string>()
+  const invalidated = new Set<string>()
+  const failures = new Set<(failure: Engine.IntentFailure) => void>()
+  const cleanups = new Set<() => void>()
+  const transport = createEngineTransport(() => config.api().session)
+  let connected = false
+
+  // Reconcile mutates the store's backing tree in place, so engine state must
+  // never be aliased into it, and every clone must appear at exactly one store
+  // path (reconcile merges nodes in place, which corrupts shared clones). The
+  // fold is a persistent structure — successive views share references for
+  // everything unchanged — so diffing the previous engine view by identity and
+  // deep-cloning only the changed subtrees keeps the boundary while avoiding
+  // the full-view structuredClone per event that dominated the streaming path.
+  const rendered = new Map<string, Engine.SessionView>()
+  // Engine data is plain JSON, so a recursive copy beats structuredClone's
+  // serialization overhead on the small per-event subtrees cloned below.
+  const clone = <T,>(value: T): T => {
+    if (value === null || typeof value !== "object") return value
+    if (Array.isArray(value)) return value.map(clone) as T
+    const copy: Record<string, unknown> = {}
+    for (const key in value) copy[key] = clone(value[key as keyof T])
+    return copy as T
+  }
+  const update = (sessionID: string, view: Engine.SessionView) => {
+    const previous = rendered.get(sessionID)
+    rendered.set(sessionID, view)
+    batch(() => {
+      if (!previous || !views[sessionID]) setViews(sessionID, clone(view) as StoreSessionView)
+      else {
+        if (view.session !== previous.session) setViews(sessionID, "session", reconcile(clone(view.session)))
+        if (view.children !== previous.children)
+          setViews(sessionID, "children", reconcile(clone(view.children) as StoreSessionView["children"]))
+        if (view.inbox !== previous.inbox)
+          setViews(sessionID, "inbox", reconcile(clone(view.inbox) as StoreSessionView["inbox"]))
+        if (view.pending !== previous.pending)
+          setViews(sessionID, "pending", reconcile(clone(view.pending) as StoreSessionView["pending"]))
+        if (view.seq !== previous.seq) setViews(sessionID, "seq", view.seq)
+        if (view.active !== previous.active) setViews(sessionID, "active", view.active)
+        if (view.deleted !== previous.deleted) setViews(sessionID, "deleted", view.deleted)
+        if (view.messages.length < previous.messages.length)
+          setViews(sessionID, "messages", reconcile(clone(view.messages) as StoreSessionView["messages"]))
+        else
+          for (let index = 0; index < view.messages.length; index++)
+            if (view.messages[index] !== previous.messages[index])
+              setViews(sessionID, "messages", index, reconcile(clone(view.messages[index])))
+      }
+      if (view.session !== previous?.session) {
+        const current = legacy.session.get(sessionID)
+        if (!current || current.time.updated <= view.session.time.updated) {
+          legacy.session.remember(clone(view.session))
+        }
+      }
+      if (families.has(sessionID) && view.children !== previous?.children) {
+        view.children.forEach((child) => legacy.session.remember(clone(child)))
+      }
+    })
+  }
+
+  const ensure = (sessionID: string) => {
+    const existing = engines.get(sessionID)
+    if (existing) return existing
+    const created = Engine.createSessionEngine(sessionID, transport).then((engine) => {
+      update(sessionID, engine.view())
+      cleanups.add(engine.subscribe((view) => update(sessionID, view)))
+      cleanups.add(engine.subscribeFailures((failure) => failures.forEach((listener) => listener(failure))))
+      return engine
+    })
+    engines.set(sessionID, created)
+    void created.catch(() => engines.delete(sessionID))
+    return created
+  }
+
+  const sync = async (sessionID: string) => {
+    const engine = await ensure(sessionID)
+    if (invalidated.delete(sessionID)) await engine.refresh()
+    await engine.ready()
+  }
+
+  cleanups.add(
+    config.event.on("server.connected", () => {
+      if (!connected) {
+        connected = true
+        return
+      }
+      engines.forEach((engine) => void engine.then((handle) => handle.refresh()).catch(() => undefined))
+    }),
+  )
+
+  onCleanup(() => {
+    cleanups.forEach((cleanup) => cleanup())
+    engines.forEach((engine) => void engine.then((handle) => handle.stop()))
+  })
+
+  return {
+    ...legacy,
+    on: config.event.on,
+    listen: config.event.listen,
+    session: {
+      ...legacy.session,
+      async sync(sessionID: string, options?: { readonly children?: boolean }) {
+        if (options?.children) families.add(sessionID)
+        await sync(sessionID)
+        if (!options?.children) return
+        const view = views[sessionID]
+        view?.children.forEach((child) => legacy.session.remember(structuredClone(unwrap(child))))
+      },
+      invalidate(sessionID: string) {
+        invalidated.add(sessionID)
+      },
+      status(sessionID: string) {
+        if (views[sessionID]?.active === "running") return "running"
+        return legacy.session.status(sessionID)
+      },
+      input: {
+        list(sessionID: string) {
+          return (
+            views[sessionID]?.pending.filter((item) => item.type !== "compaction").map((item) => item.id) ??
+            legacy.session.input.list(sessionID)
+          )
+        },
+        has(sessionID: string, inboxID: string) {
+          return (
+            views[sessionID]?.pending.some((item) => item.type !== "compaction" && item.id === inboxID) ??
+            legacy.session.input.has(sessionID, inboxID)
+          )
+        },
+      },
+      pending: {
+        list(sessionID: string) {
+          void ensure(sessionID)
+          return [...(views[sessionID]?.pending ?? [])]
+        },
+        sync(sessionID: string) {
+          return sync(sessionID)
+        },
+        invalidate(sessionID: string) {
+          invalidated.add(sessionID)
+        },
+      },
+      message: {
+        list(sessionID: string) {
+          void ensure(sessionID)
+          return [...(views[sessionID]?.messages ?? [])]
+        },
+        get(sessionID: string, messageID: string) {
+          void ensure(sessionID)
+          return views[sessionID]?.messages.find((message) => message.id === messageID)
+        },
+        sync(sessionID: string) {
+          return sync(sessionID)
+        },
+        invalidate(sessionID: string) {
+          invalidated.add(sessionID)
+        },
+      },
+      async prompt(input: SessionPromptInput) {
+        return (await ensure(input.sessionID)).submit({
+          id: input.id ?? undefined,
+          text: input.text,
+          files: input.files,
+          agents: input.agents,
+          skills: input.skills,
+          metadata: input.metadata,
+          delivery: input.delivery,
+          resume: input.resume,
+        })
+      },
+      failures: {
+        listen(listener: (failure: Engine.IntentFailure) => void) {
+          failures.add(listener)
+          return () => failures.delete(listener)
+        },
+      },
+    },
+  }
+}
+
+function isTypedError(error: unknown): error is { readonly _tag: string; readonly message: string } {
+  return (
+    typeof error === "object" &&
+    error !== null &&
+    "_tag" in error &&
+    typeof error._tag === "string" &&
+    "message" in error &&
+    typeof error.message === "string"
+  )
+}
+
+export type EngineData = ReturnType<typeof createEngineData>
@@ -0,0 +1,461 @@
+import type {
+  EventLogSynced,
+  SessionCompactionDelta,
+  SessionInboxInfo,
+  SessionInboxItem,
+  SessionMessageInfo,
+  SessionPromptInput,
+  SessionReasoningDelta,
+  SessionTextDelta,
+  SessionToolInputDelta,
+  SessionToolProgress,
+  SessionUsageUpdated,
+} from "../../promise"
+import { SessionFold } from "./fold"
+import type { DurableSessionEvent, SessionFoldState, SessionSnapshot } from "./fold"
+
+export type EphemeralSessionEvent =
+  | SessionTextDelta
+  | SessionReasoningDelta
+  | SessionToolInputDelta
+  | SessionToolProgress
+  | SessionCompactionDelta
+  | SessionUsageUpdated
+
+export type SessionStreamItem = DurableSessionEvent | EphemeralSessionEvent | EventLogSynced
+
+export type Intent = {
+  readonly id: string
+  readonly item: Extract<SessionInboxItem, { readonly type: "user" | "synthetic" }>
+  readonly request: Omit<SessionPromptInput, "sessionID" | "id">
+  readonly created: number
+}
+
+export type SubmitInput = {
+  readonly id: string
+  readonly sessionID: string
+  readonly request: Intent["request"]
+}
+
+export type IntentFailure = {
+  readonly intent: Intent
+  readonly reason: string
+}
+
+export class SubmitRejected extends Error {
+  readonly _tag = "SubmitRejected"
+
+  constructor(readonly reason: string) {
+    super(reason)
+  }
+}
+
+export class SeqUnavailable extends Error {
+  readonly _tag = "SeqUnavailable"
+}
+
+export interface SessionTransport {
+  readonly snapshot: (sessionID: string) => Promise<SessionSnapshot>
+  readonly stream: (sessionID: string, after: number, signal?: AbortSignal) => AsyncIterable<SessionStreamItem>
+  readonly submit: (input: SubmitInput) => Promise<void>
+}
+
+export type SessionView = SessionFoldState & {
+  readonly pending: ReadonlyArray<SessionInboxInfo>
+}
+
+export interface SessionEngine {
+  readonly sessionID: string
+  readonly view: () => SessionView
+  readonly submit: (input: Intent["request"] & { readonly id?: string }) => Intent
+  readonly subscribe: (listener: (view: SessionView) => void) => () => void
+  readonly subscribeFailures: (listener: (failure: IntentFailure) => void) => () => void
+  readonly ready: () => Promise<void>
+  readonly refresh: () => Promise<void>
+  readonly settled: () => Promise<void>
+  readonly stop: () => void
+}
+
+export type SessionEngineOptions = {
+  readonly makeID?: () => string
+  readonly now?: () => number
+  readonly reconnect?: () => Promise<void>
+}
+
+type Overlay = ReadonlyMap<string, OverlayEntry>
+
+type OverlayEntry =
+  | { readonly type: "text"; readonly value: string }
+  | { readonly type: "reasoning"; readonly value: string }
+  | { readonly type: "tool-input"; readonly value: string }
+  | { readonly type: "tool-progress"; readonly metadata: SessionToolProgress["data"]["metadata"] }
+  | { readonly type: "compaction"; readonly value: string }
+  | { readonly type: "usage"; readonly value: SessionUsageUpdated["data"] }
+
+type EngineState = {
+  readonly folded: SessionFoldState
+  readonly outbox: ReadonlyArray<Intent>
+  readonly overlay: Overlay
+  readonly synced: boolean
+}
+
+export async function createSessionEngine(
+  sessionID: string,
+  transport: SessionTransport,
+  options: SessionEngineOptions = {},
+): Promise<SessionEngine> {
+  let counter = 0
+  const makeID = options.makeID ?? (() => `msg_${Date.now().toString(36)}_${++counter}`)
+  const now = options.now ?? Date.now
+  const reconnect = options.reconnect ?? (() => new Promise<void>((resolve) => setTimeout(resolve, 100)))
+  let state: EngineState = {
+    folded: SessionFold.fromSnapshot(await transport.snapshot(sessionID)),
+    outbox: [],
+    overlay: new Map(),
+    synced: false,
+  }
+  const listeners = new Set<(view: SessionView) => void>()
+  const failureListeners = new Set<(failure: IntentFailure) => void>()
+  const settled = new Set<() => void>()
+  const ready = Promise.withResolvers<void>()
+  let sent: string | undefined
+  let stopped = false
+  let sending = false
+  let refreshing: Promise<void> | undefined
+  const abort = new AbortController()
+
+  const publish = (next: EngineState) => {
+    state = next
+    const view = render(state)
+    listeners.forEach((listener) => listener(view))
+    if (state.outbox.length > 0) return
+    settled.forEach((resolve) => resolve())
+    settled.clear()
+  }
+
+  const applySnapshot = (snapshot: SessionSnapshot, synced = false) => {
+    const folded = SessionFold.fromSnapshot(snapshot)
+    const acknowledged = new Set([
+      ...folded.messages.map((message) => message.id),
+      ...folded.inbox.map((item) => item.id),
+    ])
+    publish({
+      folded,
+      outbox: state.outbox.filter((intent) => !acknowledged.has(intent.id)),
+      overlay: new Map(),
+      synced,
+    })
+  }
+
+  const applyDurable = (event: DurableSessionEvent) => {
+    if (event.type === "session.inbox.enqueued" && sent === event.data.inboxID) sent = undefined
+    publish({
+      folded: SessionFold.apply(state.folded, event),
+      outbox:
+        event.type === "session.inbox.enqueued"
+          ? state.outbox.filter((intent) => intent.id !== event.data.inboxID)
+          : state.outbox,
+      overlay: clearOverlay(state.overlay, event),
+      synced: state.synced,
+    })
+    send()
+  }
+
+  const reject = (intent: Intent, reason: string) => {
+    publish({ ...state, outbox: state.outbox.filter((item) => item.id !== intent.id) })
+    failureListeners.forEach((listener) => listener({ intent, reason }))
+  }
+
+  const send = () => {
+    if (!state.synced || sending || stopped) return
+    const intent = state.outbox[0]
+    if (!intent || sent === intent.id) return
+    sending = true
+    sent = intent.id
+    void (async () => {
+      try {
+        await transport.submit({ id: intent.id, sessionID, request: intent.request })
+      } catch (error) {
+        if (!(error instanceof SubmitRejected)) return
+        sent = undefined
+        reject(intent, error.reason)
+      }
+    })().finally(() => {
+      sending = false
+      send()
+    })
+  }
+
+  const sync = async () => {
+    while (!stopped) {
+      try {
+        for await (const item of transport.stream(sessionID, state.folded.seq, abort.signal)) {
+          if (stopped) return
+          if (item.type === "log.synced") {
+            // A marker past the fold means the server skipped events it could not
+            // replay for this cursor; recover through a fresh snapshot.
+            if (item.seq !== undefined && item.seq > state.folded.seq) throw new SeqUnavailable()
+            sent = undefined
+            publish({ ...state, synced: true })
+            ready.resolve()
+            send()
+            continue
+          }
+          if ("durable" in item) {
+            applyDurable(item)
+            continue
+          }
+          publish({ ...state, overlay: applyOverlay(state.overlay, item) })
+        }
+      } catch (error) {
+        if (error instanceof SeqUnavailable) {
+          try {
+            applySnapshot(await transport.snapshot(sessionID))
+          } catch {
+            await reconnect()
+          }
+          continue
+        }
+      }
+      if (stopped) return
+      publish({ ...state, synced: false })
+      await reconnect()
+    }
+  }
+
+  void sync()
+
+  return {
+    sessionID,
+    view: () => render(state),
+    submit(input) {
+      const intent: Intent = {
+        id: input.id ?? makeID(),
+        created: now(),
+        request: input,
+        item: {
+          type: "user",
+          delivery: input.delivery ?? "steer",
+          payload: {
+            text: input.text,
+            agents: input.agents?.map((agent) => ({ ...agent })),
+            metadata: input.metadata,
+          },
+        },
+      }
+      publish({ ...state, outbox: [...state.outbox, intent] })
+      send()
+      return intent
+    },
+    subscribe(listener) {
+      listeners.add(listener)
+      return () => listeners.delete(listener)
+    },
+    subscribeFailures(listener) {
+      failureListeners.add(listener)
+      return () => failureListeners.delete(listener)
+    },
+    ready: () => ready.promise,
+    refresh() {
+      if (refreshing) return refreshing
+      refreshing = transport
+        .snapshot(sessionID)
+        .then((snapshot) => {
+          if (snapshot.seq < state.folded.seq) return
+          applySnapshot(snapshot, state.synced)
+          send()
+        })
+        .finally(() => {
+          refreshing = undefined
+        })
+      return refreshing
+    },
+    settled() {
+      if (state.outbox.length === 0) return Promise.resolve()
+      return new Promise<void>((resolve) => settled.add(resolve))
+    },
+    stop() {
+      stopped = true
+      abort.abort()
+      publish({ ...state, synced: false })
+    },
+  }
+}
+
+export function render(state: Pick<EngineState, "folded" | "outbox" | "overlay">): SessionView {
+  // Render runs per ephemeral event, so preserve references for everything the
+  // event did not touch: the adapter diffs consecutive views by identity to
+  // decide what to write into the reactive store.
+  const pending =
+    state.outbox.length === 0
+      ? state.folded.inbox
+      : [
+          ...state.folded.inbox,
+          ...state.outbox.map(
+            (intent): SessionInboxInfo => ({
+              id: intent.id,
+              sessionID: state.folded.session.id,
+              timeCreated: intent.created,
+              ...intent.item,
+            }),
+          ),
+        ]
+  const appended = pendingMessages(state.folded, pending)
+  const messages = applyOverlayToMessages(
+    appended.length === 0 ? state.folded.messages : [...state.folded.messages, ...appended],
+    state.overlay,
+  )
+  const usage = state.overlay.get("usage")
+  return {
+    ...state.folded,
+    session:
+      usage?.type === "usage"
+        ? { ...state.folded.session, cost: usage.value.cost, tokens: usage.value.tokens }
+        : state.folded.session,
+    messages,
+    pending,
+  }
+}
+
+function pendingMessages(folded: SessionFoldState, pending: ReadonlyArray<SessionInboxInfo>) {
+  if (pending.length === 0) return []
+  const messageIDs = new Set(folded.messages.map((message) => message.id))
+  return pending.flatMap((item): ReadonlyArray<SessionMessageInfo> => {
+    if (item.type !== "compaction" && item.delivery === "queue") return []
+    if (messageIDs.has(item.id)) return []
+    const message = SessionFold.messageFromInbox(item)
+    return message ? [message] : []
+  })
+}
+
+function applyOverlay(overlay: Overlay, event: EphemeralSessionEvent): Overlay {
+  const next = new Map(overlay)
+  switch (event.type) {
+    case "session.text.delta": {
+      const key = partKey("text", event.data.assistantMessageID, event.data.ordinal)
+      const current = next.get(key)
+      next.set(key, {
+        type: "text",
+        value: (current?.type === "text" ? current.value : "") + event.data.delta,
+      })
+      return next
+    }
+    case "session.reasoning.delta": {
+      const key = partKey("reasoning", event.data.assistantMessageID, event.data.ordinal)
+      const current = next.get(key)
+      next.set(key, {
+        type: "reasoning",
+        value: (current?.type === "reasoning" ? current.value : "") + event.data.delta,
+      })
+      return next
+    }
+    case "session.tool.input.delta": {
+      const key = toolKey("tool-input", event.data.assistantMessageID, event.data.id)
+      const current = next.get(key)
+      next.set(key, {
+        type: "tool-input",
+        value: (current?.type === "tool-input" ? current.value : "") + event.data.delta,
+      })
+      return next
+    }
+    case "session.tool.progress":
+      next.set(toolKey("tool-progress", event.data.assistantMessageID, event.data.id), {
+        type: "tool-progress",
+        metadata: event.data.metadata,
+      })
+      return next
+    case "session.compaction.delta": {
+      const current = next.get("compaction")
+      next.set("compaction", {
+        type: "compaction",
+        value: (current?.type === "compaction" ? current.value : "") + event.data.text,
+      })
+      return next
+    }
+    case "session.usage.updated":
+      next.set("usage", { type: "usage", value: event.data })
+      return next
+  }
+}
+
+function clearOverlay(overlay: Overlay, event: DurableSessionEvent): Overlay {
+  switch (event.type) {
+    case "session.text.ended":
+      return removeOverlay(overlay, partKey("text", event.data.assistantMessageID, event.data.ordinal))
+    case "session.reasoning.ended":
+      return removeOverlay(overlay, partKey("reasoning", event.data.assistantMessageID, event.data.ordinal))
+    case "session.tool.input.ended":
+    case "session.tool.called":
+      return removeOverlay(overlay, toolKey("tool-input", event.data.assistantMessageID, event.data.id))
+    case "session.tool.success":
+    case "session.tool.failed":
+      return removeOverlay(overlay, toolKey("tool-progress", event.data.assistantMessageID, event.data.id))
+    case "session.compaction.ended":
+    case "session.compaction.failed":
+      return removeOverlay(overlay, "compaction")
+    case "session.step.ended":
+    case "session.step.failed":
+    case "session.usage.recorded":
+      return removeOverlay(overlay, "usage")
+    default:
+      return overlay
+  }
+}
+
+function removeOverlay(overlay: Overlay, key: string): Overlay {
+  if (!overlay.has(key)) return overlay
+  const next = new Map(overlay)
+  next.delete(key)
+  return next
+}
+
+function applyOverlayToMessages(messages: ReadonlyArray<SessionMessageInfo>, overlay: Overlay) {
+  if (overlay.size === 0) return messages
+  // Part keys embed the message ID as the second segment; remap only messages
+  // the overlay actually touches so everything else keeps its identity.
+  const touched = new Set<string>()
+  let compacting = false
+  overlay.forEach((_, key) => {
+    if (key === "compaction") compacting = true
+    if (key === "compaction" || key === "usage") return
+    touched.add(key.split(":")[1] ?? "")
+  })
+  if (touched.size === 0 && !compacting) return messages
+  return messages.map((message): SessionMessageInfo => {
+    if (message.type === "compaction" && message.status === "running") {
+      if (!compacting) return message
+      const entry = overlay.get("compaction")
+      return entry?.type === "compaction" ? { ...message, summary: message.summary + entry.value } : message
+    }
+    if (message.type !== "assistant" || !touched.has(message.id)) return message
+    const ordinals = { text: 0, reasoning: 0 }
+    const content = message.content.map((part) => {
+      if (part.type === "text") {
+        const entry = overlay.get(partKey("text", message.id, ordinals.text++))
+        return entry?.type === "text" ? { ...part, text: part.text + entry.value } : part
+      }
+      if (part.type === "reasoning") {
+        const entry = overlay.get(partKey("reasoning", message.id, ordinals.reasoning++))
+        return entry?.type === "reasoning" ? { ...part, text: part.text + entry.value } : part
+      }
+      const input = overlay.get(toolKey("tool-input", message.id, part.id))
+      if (input?.type === "tool-input" && part.state.status === "streaming")
+        return { ...part, state: { ...part.state, input: part.state.input + input.value } }
+      const progress = overlay.get(toolKey("tool-progress", message.id, part.id))
+      if (progress?.type === "tool-progress" && part.state.status === "running")
+        return { ...part, state: { ...part.state, metadata: progress.metadata } }
+      return part
+    })
+    return content.some((part, index) => part !== message.content[index]) ? { ...message, content } : message
+  })
+}
+
+function partKey(type: "text" | "reasoning", messageID: string, ordinal: number) {
+  return `${type}:${messageID}:${ordinal}`
+}
+
+function toolKey(type: "tool-input" | "tool-progress", messageID: string, toolID: string) {
+  return `${type}:${messageID}:${toolID}`
+}
+
+export * as Engine from "./engine"
@@ -0,0 +1,580 @@
+import type {
+  SessionEventDurable,
+  SessionInboxInfo,
+  SessionInfo,
+  SessionMessageAssistant,
+  SessionMessageAssistantTool,
+  SessionMessageInfo,
+  TokenUsageInfo,
+} from "../../promise"
+
+export type SessionFoldState = {
+  readonly session: SessionInfo
+  readonly children: ReadonlyArray<SessionInfo>
+  readonly inbox: ReadonlyArray<SessionInboxInfo>
+  readonly messages: ReadonlyArray<SessionMessageInfo>
+  readonly active: "idle" | "running"
+  readonly deleted: boolean
+  readonly seq: number
+}
+
+export type SessionSnapshot = Omit<SessionFoldState, "active" | "deleted"> & {
+  readonly active?: SessionFoldState["active"]
+}
+
+export type DurableSessionEvent = Exclude<SessionEventDurable, { readonly type: "session.forked" }>
+
+export function fromSnapshot(snapshot: SessionSnapshot): SessionFoldState {
+  return { ...snapshot, active: snapshot.active ?? "idle", deleted: false }
+}
+
+export function apply(state: SessionFoldState, event: DurableSessionEvent): SessionFoldState {
+  if (event.durable.seq <= state.seq) return state
+  const current = { ...state, seq: event.durable.seq }
+  switch (event.type) {
+    case "session.created":
+      return current
+    case "session.deleted":
+      return { ...current, deleted: true }
+    case "session.usage.recorded":
+      return { ...current, session: addUsage(state.session, event.data.cost, event.data.tokens, event.created) }
+    case "session.agent.selected":
+      return append(
+        {
+          ...current,
+          session: {
+            ...state.session,
+            agent: event.data.agent,
+            time: { ...state.session.time, updated: event.created },
+          },
+        },
+        {
+          id: messageID(event.id),
+          type: "agent-switched",
+          agent: event.data.agent,
+          previous: event.data.previous ?? state.session.agent,
+          metadata: event.metadata,
+          time: { created: event.created },
+        },
+      )
+    case "session.model.selected":
+      return append(
+        {
+          ...current,
+          session: {
+            ...state.session,
+            model: event.data.model,
+            time: { ...state.session.time, updated: event.created },
+          },
+        },
+        {
+          id: messageID(event.id),
+          type: "model-switched",
+          model: event.data.model,
+          previous: event.data.previous ?? state.session.model,
+          metadata: event.metadata,
+          time: { created: event.created },
+        },
+      )
+    case "session.moved":
+      return append(
+        {
+          ...current,
+          session: {
+            ...state.session,
+            location: event.data.location,
+            projectID: event.data.projectID,
+            subpath: event.data.subpath,
+            time: { ...state.session.time, updated: event.created },
+          },
+        },
+        {
+          id: messageID(event.id),
+          type: "location-switched",
+          location: event.data.location,
+          projectID: event.data.projectID,
+          subpath: event.data.subpath,
+          previous: {
+            location: state.session.location,
+            projectID: state.session.projectID,
+            subpath: state.session.subpath,
+          },
+          metadata: event.metadata,
+          time: { created: event.created },
+        },
+      )
+    case "session.renamed":
+      return {
+        ...current,
+        session: { ...state.session, title: event.data.title, time: { ...state.session.time, updated: event.created } },
+      }
+    case "session.inbox.enqueued":
+      return {
+        ...current,
+        session: { ...state.session, time: { ...state.session.time, updated: event.created } },
+        inbox: state.inbox.some((item) => item.id === event.data.inboxID)
+          ? state.inbox
+          : [
+              ...state.inbox,
+              {
+                id: event.data.inboxID,
+                sessionID: event.data.sessionID,
+                timeCreated: event.created,
+                ...event.data.item,
+              },
+            ],
+      }
+    case "session.inbox.delivered": {
+      const item = state.inbox.find((item) => item.id === event.data.inboxID)
+      const next = { ...current, inbox: state.inbox.filter((item) => item.id !== event.data.inboxID) }
+      if (!item) return next
+      const delivered = messageFromInbox(item, event.created)
+      return delivered ? append(next, delivered) : next
+    }
+    case "session.inbox.cancelled":
+      return { ...current, inbox: state.inbox.filter((item) => item.id !== event.data.inboxID) }
+    case "session.inbox.delivery.changed":
+      return {
+        ...current,
+        inbox: state.inbox.map((item) =>
+          item.id === event.data.inboxID ? { ...item, delivery: event.data.delivery } : item,
+        ),
+      }
+    case "session.execution.started":
+      return { ...current, active: "running" }
+    case "session.execution.succeeded":
+    case "session.execution.failed":
+    case "session.execution.interrupted":
+      return { ...updateActiveAssistant(current, (message) => ({ ...message, retry: undefined })), active: "idle" }
+    case "session.instructions.updated":
+      if (event.data.text === undefined) return current
+      return append(current, {
+        id: messageID(event.id),
+        type: "system",
+        text: event.data.text,
+        description: `Instructions updated: ${Object.keys(event.data.delta).join(", ")}`,
+        metadata: event.metadata,
+        time: { created: event.created },
+      })
+    case "session.synthetic":
+      return append(current, {
+        id: messageID(event.id),
+        type: "synthetic",
+        text: event.data.text,
+        description: event.data.description,
+        metadata: event.data.metadata,
+        time: { created: event.created },
+      })
+    case "session.skill.activated":
+      return append(current, {
+        id: messageID(event.id),
+        type: "skill",
+        skill: event.data.id,
+        name: event.data.name,
+        text: event.data.text,
+        metadata: event.metadata,
+        time: { created: event.created },
+      })
+    case "session.shell.started":
+      return append(current, {
+        id: messageID(event.id),
+        type: "shell",
+        shellID: event.data.shell.id,
+        command: event.data.shell.command,
+        status: event.data.shell.status,
+        metadata: event.metadata,
+        time: { created: event.created },
+      })
+    case "session.shell.ended":
+      return updateMessage(
+        current,
+        (message) => message.type === "shell" && message.shellID === event.data.shell.id,
+        (message) => {
+          if (message.type !== "shell") return message
+          return {
+            ...message,
+            status: event.data.shell.status,
+            exit: event.data.shell.exit,
+            output: event.data.output,
+            time: { ...message.time, completed: event.created },
+          }
+        },
+        true,
+      )
+    case "session.step.started": {
+      const existing = state.messages.some((message) => message.id === event.data.assistantMessageID)
+      if (existing)
+        return updateAssistant(current, event.data.assistantMessageID, (message) => ({
+          ...message,
+          agent: event.data.agent,
+          model: event.data.model,
+          retry: undefined,
+          error: undefined,
+          finish: undefined,
+          time: { ...message.time, completed: undefined },
+          snapshot: event.data.snapshot ? { ...message.snapshot, start: event.data.snapshot } : message.snapshot,
+        }))
+      return append(
+        updateActiveAssistant(current, (message) => ({
+          ...message,
+          retry: undefined,
+          time: { ...message.time, completed: event.created },
+        })),
+        {
+          id: event.data.assistantMessageID,
+          type: "assistant",
+          agent: event.data.agent,
+          model: event.data.model,
+          metadata: event.metadata,
+          content: [],
+          snapshot: event.data.snapshot ? { start: event.data.snapshot } : undefined,
+          time: { created: event.created },
+        },
+      )
+    }
+    case "session.step.ended":
+      return withUsage(
+        updateAssistant(current, event.data.assistantMessageID, (message) => ({
+          ...message,
+          finish: event.data.finish,
+          cost: event.data.cost,
+          tokens: event.data.tokens,
+          time: { ...message.time, completed: event.created },
+          snapshot:
+            event.data.snapshot || event.data.files
+              ? { ...message.snapshot, end: event.data.snapshot, files: event.data.files }
+              : message.snapshot,
+        })),
+        event.data.cost,
+        event.data.tokens,
+        event.created,
+      )
+    case "session.step.failed": {
+      const failed = updateAssistant(current, event.data.assistantMessageID, (message) => ({
+        ...message,
+        finish: "error",
+        error: event.data.error,
+        retry: undefined,
+        cost: event.data.cost ?? message.cost,
+        tokens: event.data.tokens ?? message.tokens,
+        time: { ...message.time, completed: event.created },
+        snapshot:
+          event.data.snapshot || event.data.files
+            ? { ...message.snapshot, end: event.data.snapshot, files: event.data.files }
+            : message.snapshot,
+      }))
+      if (event.data.cost === undefined || event.data.tokens === undefined) return failed
+      return withUsage(failed, event.data.cost, event.data.tokens, event.created)
+    }
+    case "session.text.started":
+      return updateAssistant(current, event.data.assistantMessageID, (message) => ({
+        ...message,
+        content: insertOrdinal(message.content, "text", event.data.ordinal, { type: "text", text: "" }),
+      }))
+    case "session.text.ended":
+      return updateContent(current, event.data.assistantMessageID, "text", event.data.ordinal, (part) => ({
+        ...part,
+        text: event.data.text,
+        state: event.data.state,
+      }))
+    case "session.reasoning.started":
+      return updateAssistant(current, event.data.assistantMessageID, (message) => ({
+        ...message,
+        content: insertOrdinal(message.content, "reasoning", event.data.ordinal, {
+          type: "reasoning",
+          text: "",
+          state: event.data.state,
+          time: { created: event.created },
+        }),
+      }))
+    case "session.reasoning.ended":
+      return updateContent(current, event.data.assistantMessageID, "reasoning", event.data.ordinal, (part) => ({
+        ...part,
+        text: event.data.text,
+        state: event.data.state ?? part.state,
+        time: { created: part.time?.created ?? event.created, completed: event.created },
+      }))
+    case "session.tool.input.started":
+      return updateAssistant(current, event.data.assistantMessageID, (message) => ({
+        ...message,
+        content: [
+          ...message.content,
+          {
+            type: "tool",
+            id: event.data.id,
+            name: event.data.name,
+            state: { status: "streaming", input: "" },
+            time: { created: event.created },
+          },
+        ],
+      }))
+    case "session.tool.input.ended":
+      return updateTool(current, event.data.assistantMessageID, event.data.id, (part) =>
+        part.state.status === "streaming" ? { ...part, state: { ...part.state, input: event.data.text } } : part,
+      )
+    case "session.tool.called":
+      return updateTool(current, event.data.assistantMessageID, event.data.id, (part) => ({
+        ...part,
+        executed: event.data.executed,
+        providerState: event.data.state,
+        state: { status: "running", input: event.data.input, metadata: {} },
+        time: { ...part.time, ran: event.created },
+      }))
+    case "session.tool.success":
+      return updateTool(current, event.data.assistantMessageID, event.data.id, (part) => {
+        if (part.state.status !== "running") return part
+        return {
+          ...part,
+          executed: event.data.executed || part.executed === true,
+          providerResultState: event.data.resultState,
+          state: {
+            status: "completed",
+            input: part.state.input,
+            content: event.data.content,
+            metadata: event.data.metadata,
+          },
+          time: { ...part.time, completed: event.created },
+        }
+      })
+    case "session.tool.failed":
+      return updateTool(current, event.data.assistantMessageID, event.data.id, (part) => {
+        if (part.state.status !== "streaming" && part.state.status !== "running") return part
+        return {
+          ...part,
+          executed: event.data.executed || part.executed === true,
+          providerResultState: event.data.resultState,
+          state: {
+            status: "error",
+            error: event.data.error,
+            input: typeof part.state.input === "string" ? {} : part.state.input,
+            content: event.data.content,
+            metadata: event.data.metadata,
+          },
+          time: { ...part.time, completed: event.created },
+        }
+      })
+    case "session.retry.scheduled":
+      return updateAssistant(current, event.data.assistantMessageID, (message) => ({
+        ...message,
+        retry: { attempt: event.data.attempt, at: event.data.at, error: event.data.error },
+      }))
+    case "session.compaction.started":
+      return append(
+        {
+          ...current,
+          inbox: event.data.inputID ? state.inbox.filter((item) => item.id !== event.data.inputID) : state.inbox,
+        },
+        {
+          id: event.data.inputID ?? messageID(event.id),
+          type: "compaction",
+          status: "running",
+          reason: event.data.reason,
+          summary: "",
+          recent: event.data.recent,
+          metadata: event.metadata,
+          time: { created: event.created },
+        },
+      )
+    case "session.compaction.ended": {
+      const running = state.messages.findLast(
+        (message) => message.type === "compaction" && message.status === "running",
+      )
+      if (!running)
+        return append(current, {
+          id: messageID(event.id),
+          type: "compaction",
+          status: "completed",
+          reason: event.data.reason,
+          summary: event.data.text,
+          recent: event.data.recent,
+          metadata: event.metadata,
+          time: { created: event.created },
+        })
+      return updateMessage(
+        current,
+        (message) => message.id === running.id,
+        (message) => ({
+          ...message,
+          type: "compaction",
+          status: "completed",
+          reason: event.data.reason,
+          summary: event.data.text,
+          recent: event.data.recent,
+        }),
+      )
+    }
+    case "session.compaction.failed": {
+      const running = state.messages.findLast(
+        (message) => message.type === "compaction" && message.status === "running",
+      )
+      const failed = {
+        id: running?.id ?? event.data.inputID ?? messageID(event.id),
+        type: "compaction" as const,
+        status: "failed" as const,
+        reason: event.data.reason,
+        error: event.data.error,
+        metadata: running?.metadata ?? event.metadata,
+        time: running?.time ?? { created: event.created },
+      }
+      const next = {
+        ...current,
+        inbox: event.data.inputID ? state.inbox.filter((item) => item.id !== event.data.inputID) : state.inbox,
+      }
+      return running
+        ? updateMessage(
+            next,
+            (message) => message.id === running.id,
+            () => failed,
+          )
+        : append(next, failed)
+    }
+    case "session.revert.staged":
+      return {
+        ...current,
+        session: {
+          ...state.session,
+          revert: event.data.revert,
+          time: { ...state.session.time, updated: event.created },
+        },
+      }
+    case "session.revert.cleared":
+      return {
+        ...current,
+        session: { ...state.session, revert: undefined, time: { ...state.session.time, updated: event.created } },
+      }
+    case "session.revert.committed":
+      return {
+        ...current,
+        session: { ...state.session, revert: undefined, time: { ...state.session.time, updated: event.created } },
+        messages: state.messages.filter((message) => message.id < event.data.to),
+        inbox: state.inbox.filter((item) => item.id < event.data.to),
+      }
+  }
+}
+
+function messageID(eventID: string) {
+  return eventID.replace(/^evt_/, "msg_")
+}
+
+export function messageFromInbox(item: SessionInboxInfo, created = item.timeCreated): SessionMessageInfo | undefined {
+  if (item.type === "user") return { id: item.id, type: "user", ...item.payload, time: { created } }
+  if (item.type === "synthetic") return { id: item.id, type: "synthetic", ...item.payload, time: { created } }
+}
+
+function append(state: SessionFoldState, item: SessionMessageInfo) {
+  if (state.messages.some((message) => message.id === item.id)) return state
+  return { ...state, messages: [...state.messages, item] }
+}
+
+function updateMessage(
+  state: SessionFoldState,
+  predicate: (message: SessionMessageInfo) => boolean,
+  update: (message: SessionMessageInfo) => SessionMessageInfo,
+  last = false,
+) {
+  const index = last ? state.messages.findLastIndex(predicate) : state.messages.findIndex(predicate)
+  if (index < 0) return state
+  return {
+    ...state,
+    messages: state.messages.map((message, position) => (position === index ? update(message) : message)),
+  }
+}
+
+function updateAssistant(
+  state: SessionFoldState,
+  messageID: string,
+  update: (message: SessionMessageAssistant) => SessionMessageAssistant,
+) {
+  return updateMessage(
+    state,
+    (message) => message.id === messageID && message.type === "assistant",
+    (message) => (message.type === "assistant" ? update(message) : message),
+  )
+}
+
+function updateActiveAssistant(
+  state: SessionFoldState,
+  update: (message: SessionMessageAssistant) => SessionMessageAssistant,
+) {
+  return updateMessage(
+    state,
+    (message) => message.type === "assistant" && message.time.completed === undefined,
+    (message) => (message.type === "assistant" ? update(message) : message),
+    true,
+  )
+}
+
+function updateContent<Type extends "text" | "reasoning">(
+  state: SessionFoldState,
+  messageID: string,
+  type: Type,
+  ordinal: number,
+  update: (
+    part: Extract<SessionMessageAssistant["content"][number], { readonly type: Type }>,
+  ) => Extract<SessionMessageAssistant["content"][number], { readonly type: Type }>,
+) {
+  return updateAssistant(state, messageID, (message) => {
+    const position = message.content.flatMap((part, index) => (part.type === type ? [index] : []))[ordinal]
+    const part = position === undefined ? undefined : message.content[position]
+    if (!part || part.type !== type) return message
+    return {
+      ...message,
+      content: message.content.map((item, index) =>
+        index === position
+          ? update(part as Extract<SessionMessageAssistant["content"][number], { readonly type: Type }>)
+          : item,
+      ),
+    }
+  })
+}
+
+function updateTool(
+  state: SessionFoldState,
+  messageID: string,
+  toolID: string,
+  update: (part: SessionMessageAssistantTool) => SessionMessageAssistantTool,
+) {
+  return updateAssistant(state, messageID, (message) => {
+    const index = message.content.findLastIndex((part) => part.type === "tool" && part.id === toolID)
+    if (index < 0) return message
+    return {
+      ...message,
+      content: message.content.map((part, position) =>
+        position === index && part.type === "tool" ? update(part) : part,
+      ),
+    }
+  })
+}
+
+function insertOrdinal<Type extends SessionMessageAssistant["content"][number]["type"]>(
+  content: SessionMessageAssistant["content"],
+  type: Type,
+  ordinal: number,
+  part: Extract<SessionMessageAssistant["content"][number], { readonly type: Type }>,
+) {
+  if (content.filter((item) => item.type === type)[ordinal]) return content
+  return [...content, part]
+}
+
+function addUsage(session: SessionInfo, cost: number, tokens: TokenUsageInfo, updated: number): SessionInfo {
+  return {
+    ...session,
+    cost: session.cost + cost,
+    tokens: {
+      input: session.tokens.input + tokens.input,
+      output: session.tokens.output + tokens.output,
+      reasoning: session.tokens.reasoning + tokens.reasoning,
+      cache: {
+        read: session.tokens.cache.read + tokens.cache.read,
+        write: session.tokens.cache.write + tokens.cache.write,
+      },
+    },
+    time: { ...session.time, updated },
+  }
+}
+
+function withUsage(state: SessionFoldState, cost: number, tokens: TokenUsageInfo, updated: number) {
+  return { ...state, session: addUsage(state.session, cost, tokens, updated) }
+}
+
+export * as SessionFold from "./fold"
@@ -1,2 +1,3 @@
 export * from "./data"
 export * from "./connection"
+export * from "./engine-data"
@@ -0,0 +1,108 @@
+import { describe, expect, test } from "bun:test"
+import { Engine } from "../src/solid/engine/engine"
+import { createEngineTransport } from "../src/solid/engine-data"
+import { FakeSessionServer } from "./fixture/sync-engine"
+
+describe("engine data transport", () => {
+  test("uses snapshot and ephemeral follow log contracts", async () => {
+    const server = new FakeSessionServer("session-transport")
+    const calls: Array<unknown> = []
+    const transport = createEngineTransport(() => ({
+      async snapshot(input) {
+        calls.push(input)
+        return server.snapshotValue()
+      },
+      async *log(input) {
+        calls.push(input)
+        yield { type: "log.synced" as const, aggregateID: input.sessionID, seq: 0 }
+      },
+      async prompt() {
+        throw new Error("unused")
+      },
+    }))
+
+    expect(await transport.snapshot(server.sessionID)).toEqual(server.snapshotValue())
+    const items = []
+    for await (const item of transport.stream(server.sessionID, 0)) items.push(item)
+
+    expect(items).toEqual([{ type: "log.synced", aggregateID: server.sessionID, seq: 0 }])
+    expect(calls).toEqual([
+      { sessionID: server.sessionID, recent: 200 },
+      { sessionID: server.sessionID, after: 0, follow: true, ephemeral: true },
+    ])
+  })
+
+  test("preserves the prompt request and client-minted ID", async () => {
+    const requests: Array<unknown> = []
+    const transport = createEngineTransport(() => ({
+      async snapshot() {
+        throw new Error("unused")
+      },
+      async *log() {
+        throw new Error("unused")
+      },
+      async prompt(input) {
+        requests.push(input)
+        return {
+          id: input.id!,
+          sessionID: input.sessionID,
+          timeCreated: 1,
+          type: "user",
+          payload: { text: input.text },
+          delivery: input.delivery ?? "steer",
+        }
+      },
+    }))
+
+    await transport.submit({
+      id: "msg_client",
+      sessionID: "session-submit",
+      request: {
+        id: undefined,
+        sessionID: undefined,
+        text: "hello",
+        files: [{ uri: "file:///tmp/example.txt", name: "example.txt" }],
+        delivery: "queue",
+      },
+    })
+
+    expect(requests).toEqual([
+      {
+        id: "msg_client",
+        sessionID: "session-submit",
+        text: "hello",
+        files: [{ uri: "file:///tmp/example.txt", name: "example.txt" }],
+        delivery: "queue",
+      },
+    ])
+  })
+
+  test("translates generated typed failures", async () => {
+    const transport = createEngineTransport(() => ({
+      async snapshot() {
+        throw new Error("unused")
+      },
+      async *log() {
+        throw { _tag: "SeqUnavailableError", sessionID: "session", after: 2, head: 1, message: "gone" }
+      },
+      async prompt() {
+        throw { _tag: "InvalidRequestError", message: "invalid" }
+      },
+    }))
+
+    const streamError = await collectError(transport.stream("session", 2))
+    expect(streamError).toBeInstanceOf(Engine.SeqUnavailable)
+    await expect(
+      transport.submit({ id: "msg_client", sessionID: "session", request: { text: "invalid" } }),
+    ).rejects.toEqual(new Engine.SubmitRejected("invalid"))
+  })
+})
+
+async function collectError(iterable: AsyncIterable<unknown>) {
+  try {
+    for await (const item of iterable) void item
+  } catch (error) {
+    return error
+  }
+  throw new Error("stream did not fail")
+}
@@ -0,0 +1,185 @@
+import type { SessionInfo, SessionMessageInfo } from "../../src/promise"
+import { Engine } from "../../src/solid/engine/engine"
+import type { DurableSessionEvent, SessionFoldState, SessionSnapshot } from "../../src/solid/engine/fold"
+import { SessionFold } from "../../src/solid/engine/fold"
+
+export class FakeSessionServer implements Engine.SessionTransport {
+  readonly events: Array<DurableSessionEvent> = []
+  readonly admitted: Array<string> = []
+  readonly faults = {
+    loseRequests: 0,
+    loseResponses: 0,
+    reject: 0,
+    latency: 0,
+  }
+
+  private folded: SessionFoldState
+  private readonly tails = new Set<AsyncQueue<Engine.SessionStreamItem>>()
+  private eventCounter = 0
+
+  constructor(
+    readonly sessionID: string,
+    readonly time = 1_717_171_717_000,
+  ) {
+    this.folded = SessionFold.fromSnapshot(emptySnapshot(sessionID))
+  }
+
+  async snapshot(sessionID: string) {
+    await this.pause()
+    this.assertSession(sessionID)
+    return this.snapshotValue()
+  }
+
+  async *stream(sessionID: string, after: number): AsyncIterable<Engine.SessionStreamItem> {
+    await this.pause()
+    this.assertSession(sessionID)
+    if (after > this.folded.seq) throw new Engine.SeqUnavailable()
+    const replay = this.events.filter((event) => event.durable.seq > after)
+    // Honest replay contract: a cursor is only admitted when retained events fully cover (after, seq].
+    if (replay.length < this.folded.seq - after) throw new Engine.SeqUnavailable()
+    const queue = new AsyncQueue<Engine.SessionStreamItem>()
+    this.tails.add(queue)
+    try {
+      for (const event of replay) yield event
+      yield { type: "log.synced", aggregateID: sessionID, seq: this.folded.seq }
+      while (true) yield await queue.take()
+    } finally {
+      this.tails.delete(queue)
+    }
+  }
+
+  async submit(input: Engine.SubmitInput) {
+    await this.pause()
+    this.assertSession(input.sessionID)
+    const existing = this.events.find(
+      (event) => event.type === "session.inbox.enqueued" && event.data.inboxID === input.id,
+    )
+    if (existing) return
+    if (this.faults.loseRequests > 0) {
+      this.faults.loseRequests--
+      throw new Error("request lost")
+    }
+    if (this.faults.reject > 0) {
+      this.faults.reject--
+      throw new Engine.SubmitRejected("rejected")
+    }
+    this.admitted.push(input.id)
+    this.publish({
+      id: `evt_${String(++this.eventCounter).padStart(8, "0")}`,
+      created: this.time,
+      type: "session.inbox.enqueued",
+      durable: { aggregateID: this.sessionID, seq: this.folded.seq + 1, version: 1 },
+      data: {
+        sessionID: this.sessionID,
+        inboxID: input.id,
+        item: {
+          type: "user",
+          delivery: input.request.delivery ?? "steer",
+          payload: {
+            text: input.request.text,
+            agents: input.request.agents?.map((agent) => ({ ...agent })),
+            metadata: input.request.metadata,
+          },
+        },
+      },
+    })
+    if (this.faults.loseResponses > 0) {
+      this.faults.loseResponses--
+      throw new Error("response lost")
+    }
+  }
+
+  cutConnections() {
+    this.tails.forEach((tail) => tail.fail(new Error("connection cut")))
+  }
+
+  /** Drop retained event history, simulating `events.persist` off or pruned retention. */
+  prune() {
+    this.events.length = 0
+  }
+
+  truth() {
+    return Engine.render({ folded: this.folded, outbox: [], overlay: new Map() })
+  }
+
+  snapshotValue(): SessionSnapshot {
+    return {
+      session: this.folded.session,
+      children: this.folded.children,
+      inbox: this.folded.inbox,
+      messages: this.folded.messages,
+      seq: this.folded.seq,
+      active: this.folded.active,
+    }
+  }
+
+  private publish(event: DurableSessionEvent) {
+    this.events.push(event)
+    this.folded = SessionFold.apply(this.folded, event)
+    this.tails.forEach((tail) => tail.offer(event))
+  }
+
+  private assertSession(sessionID: string) {
+    if (sessionID !== this.sessionID) throw new Error(`unknown session: ${sessionID}`)
+  }
+
+  private async pause() {
+    for (let step = 0; step < this.faults.latency; step++) await Promise.resolve()
+  }
+}
+
+export async function until(check: () => boolean, message = "condition did not become true") {
+  for (let attempt = 0; attempt < 500; attempt++) {
+    if (check()) return
+    await Bun.sleep(1)
+  }
+  throw new Error(message)
+}
+
+export function userMessages(messages: ReadonlyArray<SessionMessageInfo>) {
+  return messages.filter(
+    (message): message is Extract<SessionMessageInfo, { readonly type: "user" }> => message.type === "user",
+  )
+}
+
+function emptySnapshot(sessionID: string): SessionSnapshot {
+  const session: SessionInfo = {
+    id: sessionID,
+    projectID: "project",
+    cost: 0,
+    tokens: { input: 0, output: 0, reasoning: 0, cache: { read: 0, write: 0 } },
+    time: { created: 1_717_171_717_000, updated: 1_717_171_717_000 },
+    location: { directory: "/workspace" },
+  }
+  return { session, children: [], inbox: [], messages: [], seq: 0 }
+}
+
+class AsyncQueue<Value> {
+  private readonly values: Array<Value> = []
+  private readonly waiting: Array<{
+    readonly resolve: (value: Value) => void
+    readonly reject: (error: Error) => void
+  }> = []
+  private error?: Error
+
+  offer(value: Value) {
+    const waiter = this.waiting.shift()
+    if (waiter) {
+      waiter.resolve(value)
+      return
+    }
+    this.values.push(value)
+  }
+
+  fail(error: Error) {
+    this.error = error
+    this.waiting.splice(0).forEach((waiter) => waiter.reject(error))
+  }
+
+  take() {
+    const value = this.values.shift()
+    if (value) return Promise.resolve(value)
+    if (this.error) return Promise.reject(this.error)
+    return new Promise<Value>((resolve, reject) => this.waiting.push({ resolve, reject }))
+  }
+}
@@ -0,0 +1,167 @@
+import { describe, expect, test } from "bun:test"
+import { readFileSync } from "node:fs"
+import { Engine } from "../src/solid/engine/engine"
+import { FakeSessionServer, until, userMessages } from "./fixture/sync-engine"
+
+describe("session sync engine laws", () => {
+  test("1. idempotency: lost responses converge to one admitted message", async () => {
+    const server = new FakeSessionServer("session-idempotency")
+    server.faults.loseResponses = 1
+    const engine = await Engine.createSessionEngine(server.sessionID, server, {
+      now: () => server.time,
+      reconnect: async () => {},
+    })
+
+    engine.submit({ id: "msg_1", text: "hello" })
+    await until(() => engine.view().seq === 1)
+    server.cutConnections()
+    await engine.settled()
+
+    expect(server.admitted).toEqual(["msg_1"])
+    expect(userMessages(engine.view().messages)).toHaveLength(1)
+    engine.stop()
+  })
+
+  test("2. echo determinism: folding the echo does not change rendered messages", async () => {
+    const server = new FakeSessionServer("session-echo")
+    const engine = await Engine.createSessionEngine(server.sessionID, server, { now: () => server.time })
+    await until(() => server.admitted.length === 0 && engine.view().seq === 0)
+
+    engine.submit({ id: "msg_1", text: "instant" })
+    const before = engine.view().messages
+    await engine.settled()
+
+    expect(engine.view().messages).toEqual(before)
+    engine.stop()
+  })
+
+  test("3. sync opacity: the fold cannot see intents or the engine", () => {
+    const source = readFileSync(new URL("../src/solid/engine/fold.ts", import.meta.url), "utf8")
+    const code = source
+      .split("\n")
+      .filter((line) => !line.trim().startsWith("//") && !line.trim().startsWith("*"))
+      .join("\n")
+
+    expect(code).not.toContain("outbox")
+    expect(code).not.toContain("./engine")
+    expect(code).not.toContain("intent")
+  })
+
+  test("4. ordering: a burst admits in submission order", async () => {
+    const server = new FakeSessionServer("session-ordering")
+    const engine = await Engine.createSessionEngine(server.sessionID, server)
+
+    for (const value of [1, 2, 3, 4, 5]) engine.submit({ id: `msg_${value}`, text: `m${value}` })
+    await engine.settled()
+
+    expect(server.admitted).toEqual(["msg_1", "msg_2", "msg_3", "msg_4", "msg_5"])
+    engine.stop()
+  })
+
+  test("5. convergence: drained clients equal the server fold", async () => {
+    const server = new FakeSessionServer("session-convergence")
+    const a = await Engine.createSessionEngine(server.sessionID, server, { makeID: () => "msg_a" })
+    const b = await Engine.createSessionEngine(server.sessionID, server, { makeID: () => "msg_b" })
+
+    a.submit({ text: "from a" })
+    b.submit({ text: "from b" })
+    await Promise.all([a.settled(), b.settled()])
+    await until(() => a.view().seq === server.events.length && b.view().seq === server.events.length)
+
+    expect(a.view()).toEqual(server.truth())
+    expect(b.view()).toEqual(server.truth())
+    a.stop()
+    b.stop()
+  })
+
+  test("6. failure atomicity: typed rejection removes and surfaces the intent", async () => {
+    const server = new FakeSessionServer("session-failure")
+    server.faults.reject = 1
+    const engine = await Engine.createSessionEngine(server.sessionID, server)
+    const failures: Array<Engine.IntentFailure> = []
+    engine.subscribeFailures((failure) => failures.push(failure))
+    const before = engine.view()
+
+    const intent = engine.submit({ id: "msg_1", text: "doomed" })
+    expect(userMessages(engine.view().messages)).toHaveLength(1)
+    await until(() => failures.length === 1)
+
+    expect(engine.view()).toEqual(before)
+    expect(failures).toEqual([{ intent, reason: "rejected" }])
+    engine.stop()
+  })
+
+  test("7. lossy history: reconnect without retained events recovers via snapshot", async () => {
+    const server = new FakeSessionServer("session-lossy")
+    let release: (() => void) | undefined
+    const engine = await Engine.createSessionEngine(server.sessionID, server, {
+      now: () => server.time,
+      reconnect: () => new Promise<void>((resolve) => (release = resolve)),
+    })
+    engine.submit({ id: "msg_1", text: "first" })
+    await engine.settled()
+
+    server.cutConnections()
+    await until(() => release !== undefined)
+    // While disconnected the session advances, then history is dropped: the
+    // reconnect cursor cannot be replayed and must recover via snapshot.
+    await server.submit({ id: "msg_2", sessionID: server.sessionID, request: { text: "second" } })
+    server.prune()
+    release!()
+
+    await until(() => engine.view().seq === 2)
+    expect(engine.view()).toEqual(server.truth())
+    engine.stop()
+  })
+
+  test("8. attach gaps: a synced marker past the fold forces snapshot recovery", async () => {
+    const server = new FakeSessionServer("session-marker-gap")
+    await server.submit({ id: "msg_1", sessionID: server.sessionID, request: { text: "hello" } })
+    const stale = { ...server.snapshotValue(), messages: [], inbox: [], seq: 0 }
+    let attempts = 0
+    const engine = await Engine.createSessionEngine(
+      server.sessionID,
+      {
+        snapshot: (sessionID) => (attempts === 0 ? Promise.resolve(stale) : server.snapshot(sessionID)),
+        async *stream(sessionID, after, signal) {
+          attempts++
+          if (attempts === 1) {
+            // Dishonest attach: the marker admits the cursor but skips the replay range.
+            yield { type: "log.synced" as const, aggregateID: sessionID, seq: server.snapshotValue().seq }
+            return
+          }
+          yield* server.stream(sessionID, after, signal)
+        },
+        submit: (input) => server.submit(input),
+      },
+      { reconnect: async () => {} },
+    )
+    await engine.ready()
+
+    expect(attempts).toBe(2)
+    expect(engine.view()).toEqual(server.truth())
+    engine.stop()
+  })
+
+  test("snapshot refresh cannot move the fold behind the live log", async () => {
+    const server = new FakeSessionServer("session-refresh-race")
+    const stale = server.snapshotValue()
+    let refresh = false
+    const transport: Engine.SessionTransport = {
+      snapshot: (sessionID) => (refresh ? Promise.resolve(stale) : server.snapshot(sessionID)),
+      stream: (sessionID, after) => server.stream(sessionID, after),
+      submit: (input) => server.submit(input),
+    }
+    const engine = await Engine.createSessionEngine(server.sessionID, transport)
+    await engine.ready()
+
+    engine.submit({ id: "msg_1", text: "newer than snapshot" })
+    await until(() => engine.view().seq === 1)
+    refresh = true
+    await engine.refresh()
+
+    expect(engine.view().seq).toBe(1)
+    expect(engine.view().pending.map((item) => item.id)).toEqual(["msg_1"])
+    engine.stop()
+  })
+})
@@ -0,0 +1,113 @@
+import { describe, expect, test } from "bun:test"
+import { Engine } from "../src/solid/engine/engine"
+import { FakeSessionServer, until, userMessages } from "./fixture/sync-engine"
+
+type Client = {
+  readonly name: string
+  readonly engine: Engine.SessionEngine
+  readonly submitted: Array<string>
+  readonly rejected: Set<string>
+  readonly views: Array<Engine.SessionView>
+}
+
+describe("session sync engine simulation", () => {
+  for (const seed of [1, 2, 3, 42, 1337, 90210]) {
+    test(`seed ${seed}: two clients converge through chaotic transport faults`, async () => {
+      const random = mulberry32(seed)
+      const server = new FakeSessionServer(`session-sim-${seed}`)
+      const clients = await Promise.all([makeClient("a", server), makeClient("b", server)])
+
+      for (let step = 0; step < 80; step++) {
+        const roll = random()
+        if (roll < 0.45) {
+          const client = pick(clients, random)
+          const intent = client.engine.submit({ text: `step-${step}` })
+          client.submitted.push(intent.id)
+        } else if (roll < 0.55) {
+          server.cutConnections()
+        } else if (roll < 0.65) {
+          server.faults.loseResponses++
+        } else if (roll < 0.73) {
+          server.faults.loseRequests += 1 + Math.floor(random() * 2)
+        } else if (roll < 0.8) {
+          server.faults.reject++
+        } else {
+          server.faults.latency = Math.floor(random() * 6)
+        }
+        await advance(2 + Math.floor(random() * 8))
+      }
+
+      server.faults.latency = 0
+      server.faults.loseRequests = 0
+      server.faults.loseResponses = 0
+      server.faults.reject = 0
+      for (let attempt = 0; attempt < 100; attempt++) {
+        server.cutConnections()
+        await advance(4)
+        const accounted = clients.every(
+          (client) =>
+            client.submitted.filter((id) => server.admitted.includes(id) || client.rejected.has(id)).length ===
+            client.submitted.length,
+        )
+        if (accounted) break
+      }
+      await until(
+        () => clients.every((client) => client.engine.view().seq === server.events.length),
+        `seed ${seed} did not converge`,
+      )
+
+      expect(new Set(server.admitted).size).toBe(server.admitted.length)
+      for (const client of clients) {
+        const expected = client.submitted.filter((id) => !client.rejected.has(id))
+        const observed = server.admitted.filter((id) => client.submitted.includes(id))
+        expect(observed).toEqual(expected)
+        expect(client.engine.view()).toEqual(server.truth())
+        assertNoFlicker(client.views, server.admitted, `${seed}/${client.name}`)
+        client.engine.stop()
+      }
+    })
+  }
+})
+
+async function makeClient(name: string, server: FakeSessionServer): Promise<Client> {
+  let counter = 0
+  const engine = await Engine.createSessionEngine(server.sessionID, server, {
+    makeID: () => `msg_${name}${String(++counter).padStart(4, "0")}`,
+    now: () => server.time,
+    reconnect: async () => {},
+  })
+  const client: Client = { name, engine, submitted: [], rejected: new Set(), views: [engine.view()] }
+  engine.subscribe((view) => client.views.push(view))
+  engine.subscribeFailures((failure) => client.rejected.add(failure.intent.id))
+  return client
+}
+
+function assertNoFlicker(views: ReadonlyArray<Engine.SessionView>, admitted: ReadonlyArray<string>, label: string) {
+  for (const id of admitted) {
+    const first = views.findIndex((view) => userMessages(view.messages).some((message) => message.id === id))
+    expect(first, `${label}: ${id} never rendered`).toBeGreaterThanOrEqual(0)
+    for (const view of views.slice(first)) {
+      const rows = userMessages(view.messages).filter((message) => message.id === id)
+      expect(rows, `${label}: ${id} disappeared or duplicated`).toHaveLength(1)
+    }
+  }
+}
+
+async function advance(steps: number) {
+  for (let step = 0; step < steps; step++) await Promise.resolve()
+  await Bun.sleep(0)
+}
+
+function pick<Value>(values: ReadonlyArray<Value>, random: () => number) {
+  return values[Math.floor(random() * values.length)]!
+}
+
+function mulberry32(seed: number) {
+  return () => {
+    seed |= 0
+    seed = (seed + 0x6d2b79f5) | 0
+    const first = Math.imul(seed ^ (seed >>> 15), 1 | seed)
+    const second = (first + Math.imul(first ^ (first >>> 7), 61 | first)) ^ first
+    return ((second ^ (second >>> 14)) >>> 0) / 4294967296
+  }
+}
@@ -44,6 +44,21 @@ export const reserveSequence = Effect.fn("Bus.reserveSequence")(function* (
    .pipe(Effect.orDie)
 })

+export const retainedCount = Effect.fn("Bus.retainedCount")(function* (
+  db: Database.Interface["db"],
+  aggregateID: string,
+  after: number,
+  through: number,
+) {
+  const row = yield* db
+    .select({ count: sql<number>`count(*)` })
+    .from(EventTable)
+    .where(and(eq(EventTable.aggregate_id, aggregateID), gt(EventTable.seq, after), lte(EventTable.seq, through)))
+    .get()
+    .pipe(Effect.orDie)
+  return row?.count ?? 0
+})
+
 export type SerializedEvent = {
  readonly id: Event.ID
  readonly type: string
@@ -150,6 +165,7 @@ export interface Interface {
    readonly aggregateID: string
    readonly after?: number
    readonly follow?: boolean
+    readonly includeLive?: (event: Event.Payload) => boolean
  }) => Stream.Stream<LogItem>
  /** @deprecated Use `subscribe()` and consume the returned stream. */
  readonly listen: (listener: Subscriber) => Effect.Effect<Unsubscribe>
@@ -768,6 +784,7 @@ export function configured(options?: Options) {
          readonly aggregateID: string
          readonly after?: number
          readonly follow?: boolean
+          readonly includeLive?: (event: Event.Payload) => boolean
        }): Stream.Stream<LogItem> =>
          Stream.unwrap(
            Effect.gen(function* () {
@@ -791,7 +808,8 @@ export function configured(options?: Options) {
                )
              // Subscribing before the historical read means events committed during
              // replay either appear in the read or arrive through a post-marker wake.
-              const wakes = input.follow ? yield* subscribeDurable(input.aggregateID) : undefined
+              const subscription = input.follow && input.includeLive ? yield* PubSub.subscribe(pubsub.live) : undefined
+              const wakes = input.follow && !subscription ? yield* subscribeDurable(input.aggregateID) : undefined
              const target = yield* latestSequence(db, input.aggregateID)
              const marker: EventLog.Synced = {
                type: "log.synced",
@@ -802,6 +820,14 @@ export function configured(options?: Options) {
                Stream.map((event): LogItem => event),
                Stream.concat(Stream.make(marker)),
              )
+              if (subscription && input.includeLive) {
+                const follow: Stream.Stream<LogItem> = Stream.fromSubscription(subscription).pipe(
+                  Stream.filter(input.includeLive),
+                  Stream.filter((event) => !event.durable || event.durable.seq > target),
+                  Stream.map((event): LogItem => event),
+                )
+                return Stream.concat(replay, follow)
+              }
              if (!wakes) return replay
              const live: Stream.Stream<LogItem> = Stream.fromSubscription(wakes).pipe(
                Stream.mapEffect(() => latestSequence(db, input.aggregateID)),
@@ -141,6 +141,11 @@ export class InboxConflictError extends Schema.TaggedError<InboxConflictError>()
  sessionID: SessionSchema.ID,
  inboxID: SessionMessage.ID,
 }) {}
+export class SeqUnavailableError extends Schema.TaggedError<SeqUnavailableError>()("Session.SeqUnavailableError", {
+  sessionID: SessionSchema.ID,
+  after: Event.Seq,
+  head: Schema.optional(Event.Seq),
+}) {}
 type InboxItemRef = { readonly sessionID: SessionSchema.ID; readonly inboxID: SessionMessage.ID }
 export class SkillNotFoundError extends Schema.TaggedError<SkillNotFoundError>()("Session.SkillNotFoundError", {
  skill: Skill.ID,
@@ -194,13 +199,33 @@ export interface Interface {
   * unhandled compaction barriers.
   */
  readonly inbox: (sessionID: SessionSchema.ID) => Effect.Effect<SessionInbox.Info[], NotFoundError>
+  readonly snapshot: (input: {
+    sessionID: SessionSchema.ID
+    recent?: number
+  }) => Effect.Effect<
+    {
+      readonly session: SessionSchema.Info
+      readonly children: SessionSchema.Info[]
+      readonly inbox: SessionInbox.Info[]
+      readonly messages: SessionMessage.Info[]
+      readonly seq: Event.Seq
+    },
+    NotFoundError | MessageDecodeError
+  >
  readonly cancelInbox: (input: InboxItemRef) => Effect.Effect<void, NotFoundError | InboxConflictError>
  readonly steerInbox: (input: InboxItemRef) => Effect.Effect<void, NotFoundError | InboxConflictError>
  readonly queueInbox: (input: InboxItemRef) => Effect.Effect<void, NotFoundError | InboxConflictError>
+  readonly openLog: (input: {
+    sessionID: SessionSchema.ID
+    after?: number
+    follow?: boolean
+    ephemeral?: boolean
+  }) => Effect.Effect<Stream.Stream<SessionEvent.Event | EventLog.Synced>, NotFoundError | SeqUnavailableError>
  /**
-   * Durable, ordered session log read. Replays durable session bus after
-   * the exclusive `after` cursor, emits a `Synced` marker at the captured
-   * replay watermark, then continues live when `follow` is set.
+   * Ordered session log read. Replays durable session events after the
+   * exclusive `after` cursor, emits a `Synced` marker at the captured replay
+   * watermark, then continues live when `follow` is set. Ephemeral events are
+   * included only in the live phase when explicitly requested.
   * The marker's seq may exceed the last emitted event because other durable
   * bus share the aggregate's sequence space.
   */
@@ -208,7 +233,8 @@ export interface Interface {
    sessionID: SessionSchema.ID
    after?: number
    follow?: boolean
-  }) => Stream.Stream<SessionEvent.DurableEvent | EventLog.Synced, NotFoundError>
+    ephemeral?: boolean
+  }) => Stream.Stream<SessionEvent.Event | EventLog.Synced, NotFoundError | SeqUnavailableError>
  readonly switchAgent: (input: { sessionID: SessionSchema.ID; agent: Agent.ID }) => Effect.Effect<void, NotFoundError>
  readonly switchModel: (input: { sessionID: SessionSchema.ID; model: Model.Ref }) => Effect.Effect<void, NotFoundError>
  readonly rename: (input: { sessionID: SessionSchema.ID; title: string }) => Effect.Effect<void, NotFoundError>
@@ -326,6 +352,7 @@ const layer = Layer.effect(
    })
    const decodeMessage = Schema.decodeUnknownEffect(SessionMessage.Info)
    const isDurableSessionEvent = Schema.is(SessionEvent.Durable)
+    const isSessionEvent = Schema.is(SessionEvent.All)
    const persistProject = (project: Project.Resolved) => upsertProject(db, project).pipe(Effect.orDie)
    const decode = (row: typeof SessionMessageTable.$inferSelect) =>
      decodeMessage({ ...row.data, id: row.id, type: row.type }).pipe(
@@ -557,20 +584,90 @@ const layer = Layer.effect(
        yield* result.get(sessionID)
        return yield* SessionInbox.list(db, sessionID)
      }),
+      snapshot: Effect.fn("Session.snapshot")(function* (input) {
+        return yield* db
+          .transaction(() =>
+            Effect.gen(function* () {
+              const row = yield* db
+                .select()
+                .from(SessionTable)
+                .where(eq(SessionTable.id, input.sessionID))
+                .get()
+                .pipe(Effect.orDie)
+              if (!row) return yield* new NotFoundError({ sessionID: input.sessionID })
+              const children = yield* db
+                .select()
+                .from(SessionTable)
+                .where(eq(SessionTable.parent_id, input.sessionID))
+                .orderBy(desc(SessionTable.time_updated), desc(SessionTable.id))
+                .all()
+                .pipe(Effect.orDie)
+              const inbox = yield* SessionInbox.list(db, input.sessionID)
+              const messages = yield* db
+                .select()
+                .from(SessionMessageTable)
+                .where(eq(SessionMessageTable.session_id, input.sessionID))
+                .orderBy(desc(SessionMessageTable.seq))
+                .limit(input.recent ?? 200)
+                .all()
+                .pipe(Effect.orDie)
+              const seq = yield* Bus.latestSequence(db, input.sessionID)
+              if (seq < 0) return yield* Effect.die(new Error(`Session ${input.sessionID} has no event sequence`))
+              return {
+                session: fromRow(row),
+                children: children.map(fromRow),
+                inbox,
+                messages: yield* Effect.forEach(messages.toReversed(), decode),
+                seq: Event.Seq.make(seq),
+              }
+            }),
+          )
+          .pipe(Effect.catchTag("SqlError", Effect.die))
+      }),
      cancelInbox: Effect.fn("Session.cancelInbox")((input) => mutatePending(input, SessionInbox.cancel)),
      steerInbox: Effect.fn("Session.steerInbox")((input) => mutatePending(input, SessionInbox.steer, true)),
      queueInbox: Effect.fn("Session.queueInbox")((input) => mutatePending(input, SessionInbox.queue)),
-      log: (input) =>
-        Stream.unwrap(
-          result
-            .get(input.sessionID)
-            .pipe(Effect.as(bus.log({ aggregateID: input.sessionID, after: input.after, follow: input.follow }))),
-        ).pipe(
-          Stream.filter(
-            (item): item is SessionEvent.DurableEvent | EventLog.Synced =>
-              Bus.isSynced(item) || isDurableSessionEvent(item),
-          ),
-        ),
+      openLog: Effect.fn("Session.openLog")(function* (input) {
+        yield* result.get(input.sessionID)
+        if (input.after !== undefined) {
+          const head = yield* Bus.latestSequence(db, input.sessionID)
+          if (input.after > head)
+            return yield* new SeqUnavailableError({
+              sessionID: input.sessionID,
+              after: Event.Seq.make(input.after),
+              head: head >= 0 ? Event.Seq.make(head) : undefined,
+            })
+          // A cursor claims the caller already holds everything through `after`, so
+          // replay of (after, head] must be provably complete. Without retained rows
+          // covering the range (events.persist off, or pruned history) replaying
+          // nothing would silently desync the caller; fail so it re-snapshots instead.
+          if (input.after < head) {
+            const retained = yield* Bus.retainedCount(db, input.sessionID, input.after, head)
+            if (retained < head - input.after)
+              return yield* new SeqUnavailableError({
+                sessionID: input.sessionID,
+                after: Event.Seq.make(input.after),
+                head: Event.Seq.make(head),
+              })
+          }
+        }
+        return bus
+          .log({
+            aggregateID: input.sessionID,
+            after: input.after,
+            follow: input.follow,
+            includeLive: input.ephemeral
+              ? (event) => isSessionEvent(event) && event.data.sessionID === input.sessionID
+              : undefined,
+          })
+          .pipe(
+            Stream.filter(
+              (item): item is SessionEvent.Event | EventLog.Synced =>
+                Bus.isSynced(item) || (input.ephemeral ? isSessionEvent(item) : isDurableSessionEvent(item)),
+            ),
+          )
+      }),
+      log: (input) => Stream.unwrap(result.openLog(input)),
      prompt: Effect.fn("Session.prompt")((input) =>
        Effect.uninterruptible(
          Effect.gen(function* () {
@@ -4,8 +4,10 @@ import { Database } from "@opencode-ai/core/database/database"
 import { Agent } from "@opencode-ai/core/agent"
 import { AppNodeBuilder } from "@opencode-ai/core/effect/app-node-builder"
 import { LayerNode } from "@opencode-ai/util/effect/layer-node"
+import { and, eq } from "drizzle-orm"
 import { Bus } from "@opencode-ai/core/bus"
 import { Event } from "@opencode-ai/schema/event"
+import { EventTable } from "@opencode-ai/core/event/sql"
 import { Location } from "@opencode-ai/core/location"
 import { Project } from "@opencode-ai/core/project"
 import { ProjectTable } from "@opencode-ai/core/project/sql"
@@ -15,6 +17,8 @@ import { SessionProjector } from "@opencode-ai/core/session/projector"
 import { SessionExecution } from "@opencode-ai/core/session/execution"
 import { SessionStore } from "@opencode-ai/core/session/store"
 import { SessionTable } from "@opencode-ai/core/session/sql"
+import { SessionEvent } from "@opencode-ai/core/session/event"
+import { SessionMessage } from "@opencode-ai/core/session/message"
 import { testEffect } from "./lib/effect"
 import { globalProjectLayer } from "./lib/project"

@@ -28,6 +32,16 @@ const it = testEffect(
    ],
  ),
 )
+// Default bus: durable payloads are not retained (`events.persist` off).
+const itVolatile = testEffect(
+  AppNodeBuilder.build(
+    LayerNode.group([Database.node, Bus.node, SessionProjector.node, SessionStore.node, Session.node]),
+    [
+      [Project.node, globalProjectLayer],
+      [SessionExecution.node, SessionExecution.noopLayer],
+    ],
+  ),
+)
 const location = Location.Ref.make({ directory: AbsolutePath.make("/project") })

 describe("Session.log", () => {
@@ -60,6 +74,43 @@ describe("Session.log", () => {
    }),
  )

+  it.effect("accepts a cursor exactly at the aggregate head", () =>
+    Effect.gen(function* () {
+      const session = yield* Session.Service
+      const created = yield* session.create({ location })
+
+      const items = Array.from(
+        yield* Stream.runCollect(session.log({ sessionID: created.id, after: Event.Seq.make(0) })),
+      )
+
+      expect(items).toEqual([{ type: "log.synced", aggregateID: created.id, seq: Event.Seq.make(0) }])
+    }),
+  )
+
+  it.effect("fails with SeqUnavailable when the cursor is beyond the aggregate head", () =>
+    Effect.gen(function* () {
+      const session = yield* Session.Service
+      const created = yield* session.create({ location })
+
+      const errors = yield* Effect.forEach([1, 10], (after) =>
+        Effect.flip(Stream.runCollect(session.log({ sessionID: created.id, after: Event.Seq.make(after) }))),
+      )
+
+      expect(errors.map((error) => error._tag)).toEqual([
+        "Session.SeqUnavailableError",
+        "Session.SeqUnavailableError",
+      ])
+      expect(errors.map((error) => (error._tag === "Session.SeqUnavailableError" ? error.after : undefined))).toEqual([
+        Event.Seq.make(1),
+        Event.Seq.make(10),
+      ])
+      expect(errors.map((error) => (error._tag === "Session.SeqUnavailableError" ? error.head : undefined))).toEqual([
+        Event.Seq.make(0),
+        Event.Seq.make(0),
+      ])
+    }),
+  )
+
  it.effect("fails with NotFound for an unknown session", () =>
    Effect.gen(function* () {
      const session = yield* Session.Service
@@ -68,6 +119,109 @@ describe("Session.log", () => {
    }),
  )

+  it.effect("orders live ephemeral deltas after their durable start", () =>
+    Effect.gen(function* () {
+      const session = yield* Session.Service
+      const bus = yield* Bus.Service
+      const created = yield* session.create({ location })
+      const assistantMessageID = SessionMessage.ID.create()
+      const fiber = yield* session
+        .log({ sessionID: created.id, after: Event.Seq.make(0), follow: true, ephemeral: true })
+        .pipe(Stream.take(3), Stream.runCollect, Effect.forkScoped)
+      yield* Effect.yieldNow
+
+      yield* bus.publish(SessionEvent.Text.Started, {
+        sessionID: created.id,
+        assistantMessageID,
+        ordinal: 0,
+      })
+      yield* bus.publish(SessionEvent.Text.Delta, {
+        sessionID: created.id,
+        assistantMessageID,
+        ordinal: 0,
+        delta: "hello",
+      })
+
+      expect(Array.from(yield* Fiber.join(fiber)).map((item) => item.type)).toEqual([
+        "log.synced",
+        "session.text.started",
+        "session.text.delta",
+      ])
+    }),
+  )
+
+  it.effect("never includes ephemeral events in replay", () =>
+    Effect.gen(function* () {
+      const session = yield* Session.Service
+      const bus = yield* Bus.Service
+      const created = yield* session.create({ location })
+      const assistantMessageID = SessionMessage.ID.create()
+      yield* bus.publish(SessionEvent.Text.Started, {
+        sessionID: created.id,
+        assistantMessageID,
+        ordinal: 0,
+      })
+      yield* bus.publish(SessionEvent.Text.Delta, {
+        sessionID: created.id,
+        assistantMessageID,
+        ordinal: 0,
+        delta: "not retained",
+      })
+      yield* bus.publish(SessionEvent.Text.Ended, {
+        sessionID: created.id,
+        assistantMessageID,
+        ordinal: 0,
+        text: "complete",
+      })
+
+      const items = Array.from(yield* Stream.runCollect(session.log({ sessionID: created.id, ephemeral: true })))
+
+      expect(items.map((item) => item.type)).toEqual([
+        "session.created",
+        "session.text.started",
+        "session.text.ended",
+        "log.synced",
+      ])
+    }),
+  )
+
+  it.effect("keeps the default follow stream durable-only", () =>
+    Effect.gen(function* () {
+      const session = yield* Session.Service
+      const bus = yield* Bus.Service
+      const created = yield* session.create({ location })
+      const assistantMessageID = SessionMessage.ID.create()
+      const fiber = yield* session
+        .log({ sessionID: created.id, after: Event.Seq.make(0), follow: true })
+        .pipe(Stream.take(3), Stream.runCollect, Effect.forkScoped)
+      yield* Effect.yieldNow
+
+      yield* bus.publish(SessionEvent.Text.Started, {
+        sessionID: created.id,
+        assistantMessageID,
+        ordinal: 0,
+      })
+      yield* bus.publish(SessionEvent.Text.Delta, {
+        sessionID: created.id,
+        assistantMessageID,
+        ordinal: 0,
+        delta: "filtered",
+      })
+      yield* bus.publish(SessionEvent.Text.Ended, {
+        sessionID: created.id,
+        assistantMessageID,
+        ordinal: 0,
+        text: "complete",
+      })
+
+      expect(Array.from(yield* Fiber.join(fiber)).map((item) => item.type)).toEqual([
+        "log.synced",
+        "session.text.started",
+        "session.text.ended",
+      ])
+    }),
+  )
+
  it.effect("reads across undecodable gaps in aggregate order and marks the true log position", () =>
    Effect.gen(function* () {
      const GapEvent = Bus.durable({
@@ -87,12 +241,33 @@ describe("Session.log", () => {
      const items = Array.from(yield* Stream.runCollect(session.log({ sessionID: created.id, after: 1 })))

      expect(
-        items.map((item): number | string | undefined => (Bus.isSynced(item) ? item.type : item.durable?.seq)),
+        items.map((item): number | string | undefined =>
+          Bus.isSynced(item) ? item.type : "durable" in item ? item.durable.seq : undefined,
+        ),
      ).toEqual([3, 4, "log.synced"])
      expect(items.at(-1)).toEqual({ type: "log.synced", aggregateID: created.id, seq: Event.Seq.make(4) })
    }),
  )

+  it.effect("fails with SeqUnavailable when the replay range is only partially retained", () =>
+    Effect.gen(function* () {
+      const db = (yield* Database.Service).db
+      const session = yield* Session.Service
+      const created = yield* session.create({ location })
+      yield* session.rename({ sessionID: created.id, title: "pruned" })
+      yield* db
+        .delete(EventTable)
+        .where(and(eq(EventTable.aggregate_id, created.id), eq(EventTable.seq, 1)))
+        .run()
+
+      const error = yield* Effect.flip(
+        Stream.runCollect(session.log({ sessionID: created.id, after: Event.Seq.make(0) })),
+      )
+
+      expect(error._tag).toBe("Session.SeqUnavailableError")
+    }),
+  )
+
  it.effect("completes with a bare synced marker for a migrated Session with no event sequence", () =>
    Effect.gen(function* () {
      const db = (yield* Database.Service).db
@@ -121,3 +296,46 @@ describe("Session.log", () => {
    }),
  )
 })
+
+describe("Session.log without retained events", () => {
+  itVolatile.effect("accepts a cursor exactly at the head", () =>
+    Effect.gen(function* () {
+      const session = yield* Session.Service
+      const created = yield* session.create({ location })
+      yield* session.rename({ sessionID: created.id, title: "at head" })
+
+      const items = Array.from(
+        yield* Stream.runCollect(session.log({ sessionID: created.id, after: Event.Seq.make(1) })),
+      )
+
+      expect(items).toEqual([{ type: "log.synced", aggregateID: created.id, seq: Event.Seq.make(1) }])
+    }),
+  )
+
+  itVolatile.effect("fails with SeqUnavailable for a cursor behind the head", () =>
+    Effect.gen(function* () {
+      const session = yield* Session.Service
+      const created = yield* session.create({ location })
+      yield* session.rename({ sessionID: created.id, title: "behind head" })
+
+      const error = yield* Effect.flip(
+        Stream.runCollect(session.log({ sessionID: created.id, after: Event.Seq.make(0) })),
+      )
+
+      expect(error._tag).toBe("Session.SeqUnavailableError")
+      expect(error._tag === "Session.SeqUnavailableError" ? error.head : undefined).toEqual(Event.Seq.make(1))
+    }),
+  )
+
+  itVolatile.effect("replays nothing but stays live for a cursorless read", () =>
+    Effect.gen(function* () {
+      const session = yield* Session.Service
+      const created = yield* session.create({ location })
+      yield* session.rename({ sessionID: created.id, title: "cursorless" })
+
+      const items = Array.from(yield* Stream.runCollect(session.log({ sessionID: created.id })))
+
+      expect(items).toEqual([{ type: "log.synced", aggregateID: created.id, seq: Event.Seq.make(1) }])
+    }),
+  )
+})
@@ -0,0 +1,87 @@
+import { describe, expect } from "bun:test"
+import { Effect } from "effect"
+import { Database } from "@opencode-ai/core/database/database"
+import { AppNodeBuilder } from "@opencode-ai/core/effect/app-node-builder"
+import { LayerNode } from "@opencode-ai/util/effect/layer-node"
+import { Bus } from "@opencode-ai/core/bus"
+import { Location } from "@opencode-ai/core/location"
+import { Project } from "@opencode-ai/core/project"
+import { AbsolutePath } from "@opencode-ai/core/schema"
+import { Session } from "@opencode-ai/core/session"
+import { SessionProjector } from "@opencode-ai/core/session/projector"
+import { SessionExecution } from "@opencode-ai/core/session/execution"
+import { SessionStore } from "@opencode-ai/core/session/store"
+import { SessionEvent } from "@opencode-ai/core/session/event"
+import { Event } from "@opencode-ai/schema/event"
+import { testEffect } from "./lib/effect"
+import { globalProjectLayer } from "./lib/project"
+
+const it = testEffect(
+  AppNodeBuilder.build(
+    LayerNode.group([Database.node, Bus.node, SessionProjector.node, SessionStore.node, Session.node]),
+    [
+      [Bus.node, Bus.configured({ persist: true })],
+      [Project.node, globalProjectLayer],
+      [SessionExecution.node, SessionExecution.noopLayer],
+    ],
+  ),
+)
+const location = Location.Ref.make({ directory: AbsolutePath.make("/project") })
+
+describe("Session.snapshot", () => {
+  it.effect("returns an empty projected session at its aggregate watermark", () =>
+    Effect.gen(function* () {
+      const sessions = yield* Session.Service
+      const created = yield* sessions.create({ location })
+
+      expect(yield* sessions.snapshot({ sessionID: created.id })).toEqual({
+        session: created,
+        children: [],
+        inbox: [],
+        messages: [],
+        seq: Event.Seq.make(0),
+      })
+    }),
+  )
+
+  it.effect("returns the most recent messages in aggregate order", () =>
+    Effect.gen(function* () {
+      const sessions = yield* Session.Service
+      const bus = yield* Bus.Service
+      const created = yield* sessions.create({ location })
+      yield* Effect.forEach(["first", "second", "third"], (text) =>
+        bus.publish(SessionEvent.Synthetic, { sessionID: created.id, text }),
+      )
+
+      const snapshot = yield* sessions.snapshot({ sessionID: created.id, recent: 2 })
+
+      expect(snapshot.messages.map((message) => (message.type === "synthetic" ? message.text : message.type))).toEqual([
+        "second",
+        "third",
+      ])
+      expect(snapshot.seq).toBe(Event.Seq.make(3))
+    }),
+  )
+
+  it.effect("keeps rows and watermark consistent during concurrent publication", () =>
+    Effect.gen(function* () {
+      const sessions = yield* Session.Service
+      const bus = yield* Bus.Service
+      const created = yield* sessions.create({ location })
+      const publish = Effect.forEach(
+        Array.from({ length: 40 }, (_, index) => index + 1),
+        (index) => bus.publish(SessionEvent.Synthetic, { sessionID: created.id, text: String(index) }),
+      )
+      const read = Effect.forEach(Array.from({ length: 40 }), () => sessions.snapshot({ sessionID: created.id }))
+
+      const [, snapshots] = yield* Effect.all([publish, read], { concurrency: "unbounded" })
+
+      snapshots.forEach((snapshot) => {
+        expect(snapshot.messages).toHaveLength(snapshot.seq)
+        expect(
+          snapshot.messages.map((message) => (message.type === "synthetic" ? Number(message.text) : -1)),
+        ).toEqual(Array.from({ length: snapshot.seq }, (_, index) => index + 1))
+      })
+    }),
+  )
+})
@@ -1,5 +1,6 @@
 import { Schema } from "effect"
 import { Skill } from "@opencode-ai/schema/skill"
+import { Event } from "@opencode-ai/schema/event"

 export class InvalidRequestError extends Schema.TaggedError<InvalidRequestError>()(
  "InvalidRequestError",
@@ -35,6 +36,17 @@ export class SessionBusyError extends Schema.TaggedError<SessionBusyError>()(
  { httpApiStatus: 409 },
 ) {}

+export class SeqUnavailableError extends Schema.TaggedError<SeqUnavailableError>()(
+  "SeqUnavailableError",
+  {
+    sessionID: Schema.String,
+    after: Event.Seq,
+    head: Schema.optional(Event.Seq),
+    message: Schema.String,
+  },
+  { httpApiStatus: 409 },
+) {}
+
 export class ServiceUnavailableError extends Schema.TaggedError<ServiceUnavailableError>()(
  "ServiceUnavailableError",
  {
@@ -18,6 +18,7 @@ import {
  InvalidRequestError,
  MessageNotFoundError,
  ServiceUnavailableError,
+  SeqUnavailableError,
  SessionBusyError,
  SessionNotFoundError,
  SkillNotFoundError,
@@ -219,6 +220,30 @@ export const makeSessionGroup = <I extends HttpApiMiddleware.AnyId, S>(sessionLo
        }),
      ),
    )
+    .add(
+      HttpApiEndpoint.get("session.snapshot", "/api/session/:sessionID/snapshot", {
+        params: { sessionID: Session.ID },
+        query: {
+          recent: Schema.NumberFromString.pipe(Schema.decodeTo(PositiveInt), Schema.optional),
+        },
+        success: Schema.Struct({
+          data: Schema.Struct({
+            session: Session.Info,
+            children: Schema.Array(Session.Info),
+            inbox: Schema.Array(SessionInbox.Info),
+            messages: Schema.Array(SessionMessage.Info),
+            seq: Event.Seq,
+          }),
+        }).annotate({ identifier: "SessionSnapshotResponse" }),
+        error: [SessionNotFoundError, UnknownError],
+      }).annotateMerge(
+        OpenApi.annotations({
+          identifier: "v2.session.snapshot",
+          summary: "Snapshot session state",
+          description: "Retrieve projected session state and its aggregate sequence from one consistent read.",
+        }),
+      ),
+    )
    .add(
      HttpApiEndpoint.delete("session.remove", "/api/session/:sessionID", {
        params: { sessionID: Session.ID },
@@ -633,17 +658,18 @@ export const makeSessionGroup = <I extends HttpApiMiddleware.AnyId, S>(sessionLo
        query: {
          after: Schema.NumberFromString.pipe(Schema.decodeTo(Event.Seq), Schema.optional),
          follow: BooleanFromString.pipe(Schema.optional),
+          ephemeral: BooleanFromString.pipe(Schema.optional),
        },
        success: HttpApiSchema.StreamSse({
-          data: Schema.Union([SessionEvent.Durable, EventLog.Synced]).annotate({ identifier: "SessionLogItem" }),
+          data: Schema.Union([SessionEvent.All, EventLog.Synced]).annotate({ identifier: "SessionLogItem" }),
        }),
-        error: SessionNotFoundError,
+        error: [SessionNotFoundError, SeqUnavailableError],
      }).annotateMerge(
        OpenApi.annotations({
          identifier: "v2.session.log",
          summary: "Read the session log",
          description:
-            "Experimental durable session event log. Reads events after an exclusive aggregate sequence and continues with live events when follow=true.",
+            "Experimental session event log. Replay is durable-only; follow mode can opt into live ephemeral events.",
        }),
      ),
    )
@@ -1,7 +1,7 @@
 import { Session } from "@opencode-ai/core/session"
 import { SessionTransfer } from "@opencode-ai/core/session/transfer"
 import { InstructionEntry } from "@opencode-ai/core/session/instruction-entry"
-import { DateTime, Effect, Stream } from "effect"
+import { DateTime, Effect } from "effect"
 import { HttpApiBuilder, HttpApiSchema } from "effect/unstable/httpapi"
 import { Api } from "../api"
 import { SessionsCursor } from "@opencode-ai/protocol/groups/session"
@@ -13,6 +13,7 @@ import {
  InvalidCursorError,
  MessageNotFoundError,
  ServiceUnavailableError,
+  SeqUnavailableError,
  SessionBusyError,
  SessionNotFoundError,
  SkillNotFoundError,
@@ -26,16 +27,23 @@ export const SessionHandler = HttpApiBuilder.group(Api, "server.session", (handl
  Effect.gen(function* () {
    const session = yield* Session.Service
    const transfer = yield* SessionTransfer.Service
+    const sessionNotFound = (error: Session.NotFoundError) =>
+      new SessionNotFoundError({
+        sessionID: error.sessionID,
+        message: `Session not found: ${error.sessionID}`,
+      })
+    const messageDecodeFailed = (error: Session.MessageDecodeError) => {
+      const ref = `err_${crypto.randomUUID().slice(0, 8)}`
+      return Effect.logError("failed to decode session message").pipe(
+        Effect.annotateLogs({ ref, sessionID: error.sessionID, messageID: error.messageID }),
+        Effect.andThen(
+          Effect.fail(new UnknownError({ message: "Unexpected server error. Check server logs for details.", ref })),
+        ),
+      )
+    }
    const pendingMutation = (effect: ReturnType<typeof session.cancelInbox>, conflict: string) =>
      effect.pipe(
-        Effect.catchTag(
-          "Session.NotFoundError",
-          (error) =>
-            new SessionNotFoundError({
-              sessionID: error.sessionID,
-              message: `Session not found: ${error.sessionID}`,
-            }),
-        ),
+        Effect.catchTag("Session.NotFoundError", sessionNotFound),
        Effect.catchTag(
          "Session.InboxConflictError",
          (error) => new ConflictError({ resource: error.inboxID, message: `${conflict}: ${error.inboxID}` }),
@@ -131,25 +139,8 @@ export const SessionHandler = HttpApiBuilder.group(Api, "server.session", (handl
        Effect.fn(function* (ctx) {
          return {
            data: yield* transfer.export({ sessionID: ctx.params.sessionID, sanitize: ctx.query.sanitize }).pipe(
-              Effect.catchTag(
-                "Session.NotFoundError",
-                (error) =>
-                  new SessionNotFoundError({
-                    sessionID: error.sessionID,
-                    message: `Session not found: ${error.sessionID}`,
-                  }),
-              ),
-              Effect.catchTag("Session.MessageDecodeError", (error) => {
-                const ref = `err_${crypto.randomUUID().slice(0, 8)}`
-                return Effect.logError("failed to decode session message").pipe(
-                  Effect.annotateLogs({ ref, sessionID: error.sessionID, messageID: error.messageID }),
-                  Effect.andThen(
-                    Effect.fail(
-                      new UnknownError({ message: "Unexpected server error. Check server logs for details.", ref }),
-                    ),
-                  ),
-                )
-              }),
+              Effect.catchTag("Session.NotFoundError", sessionNotFound),
+              Effect.catchTag("Session.MessageDecodeError", messageDecodeFailed),
            ),
          }
        }),
@@ -180,6 +171,19 @@ export const SessionHandler = HttpApiBuilder.group(Api, "server.session", (handl
          }
        }),
      )
+      .handle(
+        "session.snapshot",
+        Effect.fn(function* (ctx) {
+          return {
+            data: yield* session
+              .snapshot({ sessionID: ctx.params.sessionID, recent: ctx.query.recent })
+              .pipe(
+                Effect.catchTag("Session.NotFoundError", sessionNotFound),
+                Effect.catchTag("Session.MessageDecodeError", messageDecodeFailed),
+              ),
+          }
+        }),
+      )
      .handle(
        "session.remove",
        Effect.fn(function* (ctx) {
@@ -664,25 +668,8 @@ export const SessionHandler = HttpApiBuilder.group(Api, "server.session", (handl
        Effect.fn(function* (ctx) {
          return {
            data: yield* session.context(ctx.params.sessionID).pipe(
-              Effect.catchTag("Session.NotFoundError", (error) =>
-                Effect.fail(
-                  new SessionNotFoundError({
-                    sessionID: error.sessionID,
-                    message: `Session not found: ${error.sessionID}`,
-                  }),
-                ),
-              ),
-              Effect.catchTag("Session.MessageDecodeError", (error) => {
-                const ref = `err_${crypto.randomUUID().slice(0, 8)}`
-                return Effect.logError("failed to decode session message").pipe(
-                  Effect.annotateLogs({ ref, sessionID: error.sessionID, messageID: error.messageID }),
-                  Effect.andThen(
-                    Effect.fail(
-                      new UnknownError({ message: "Unexpected server error. Check server logs for details.", ref }),
-                    ),
-                  ),
-                )
-              }),
+              Effect.catchTag("Session.NotFoundError", sessionNotFound),
+              Effect.catchTag("Session.MessageDecodeError", messageDecodeFailed),
            ),
          }
        }),
@@ -773,19 +760,25 @@ export const SessionHandler = HttpApiBuilder.group(Api, "server.session", (handl
      .handle(
        "session.log",
        Effect.fn(function* (ctx) {
-          yield* session.get(ctx.params.sessionID).pipe(
-            Effect.catchTag(
-              "Session.NotFoundError",
-              (error) =>
-                new SessionNotFoundError({
-                  sessionID: error.sessionID,
-                  message: `Session not found: ${error.sessionID}`,
-                }),
-            ),
-          )
-          return session
-            .log({ sessionID: ctx.params.sessionID, after: ctx.query.after, follow: ctx.query.follow })
-            .pipe(Stream.orDie)
+          return yield* session
+            .openLog({
+              sessionID: ctx.params.sessionID,
+              after: ctx.query.after,
+              follow: ctx.query.follow,
+              ephemeral: ctx.query.ephemeral,
+            })
+            .pipe(
+              Effect.mapError((error) =>
+                error._tag === "Session.NotFoundError"
+                  ? sessionNotFound(error)
+                  : new SeqUnavailableError({
+                      sessionID: error.sessionID,
+                      after: error.after,
+                      head: error.head,
+                      message: `Session log is unavailable after sequence ${error.after}`,
+                    }),
+              ),
+            )
        }),
      )
      .handle(
@@ -208,6 +208,11 @@ export function Prompt(props: PromptProps) {
  const config = useConfig().data
  const dialog = useDialog()
  const toast = useToast()
+  onCleanup(
+    data.session.failures.listen((failure) => {
+      toast.show({ title: "Prompt rejected", message: failure.reason, variant: "error" })
+    }),
+  )
  const status = createMemo(() => data.session.status(props.sessionID ?? ""))
  const history = usePromptHistory()
  const stash = usePromptStash()
@@ -1304,7 +1309,7 @@ export function Prompt(props: PromptProps) {
          return false
        }
      }
-      const error = await client.api.session
+      const error = await data.session
        .prompt({
          sessionID,
          text: inputText,
@@ -1,4 +1,4 @@
-import { createData } from "@opencode-ai/client/solid"
+import { createEngineData } from "@opencode-ai/client/solid"
 import type { Plugin } from "@opencode-ai/plugin/tui"
 import { createSimpleContext } from "./helper"
 import { useClient } from "./client"
@@ -10,7 +10,7 @@ export const { use: useData, provider: DataProvider } = createSimpleContext({
  name: "Data",
  init: () => {
    const client = useClient()
-    const data = createData({
+    const data = createEngineData({
      api: () => client.api,
      event: client.event,
      connection: client.connection,
@@ -213,13 +213,25 @@ export function createSessionRows(sessionID: Accessor<string>, onSynced?: (sessi
  const input = (event: SessionInboxEnqueued) => {
    if (
      event.data.sessionID === sessionID() &&
+      event.data.item.delivery !== "queue" &&
      (event.data.item.type === "user" ||
        (event.data.item.type === "synthetic" && event.data.item.payload.description?.trim()))
    )
      appendMessage(event.data.inboxID)
  }
+  const delivery = (event: { data: { sessionID: string; inboxID: string; delivery: "steer" | "queue" } }) => {
+    if (event.data.sessionID !== sessionID()) return
+    if (event.data.delivery === "steer") return appendMessage(event.data.inboxID)
+    setRows(
+      produce((draft) => {
+        const index = draft.findIndex((row) => row.type === "message" && row.messageID === event.data.inboxID)
+        if (index !== -1) draft.splice(index, 1)
+      }),
+    )
+  }
  const subscriptions = [
    data.on("session.inbox.enqueued", input),
+    data.on("session.inbox.delivery.changed", delivery),
    data.on("session.compaction.started", (event) => {
      if (event.data.sessionID === sessionID()) appendMessage(event.data.inputID ?? event.id.replace(/^evt_/, "msg_"))
    }),
@@ -2592,18 +2592,7 @@ test("reconciles active session forms when the event stream reconnects", async (

 test("settles pending tools when a live failure arrives", async () => {
  const events = createEventStream()
-  const calls = createFetch((url) => {
-    if (url.pathname === "/api/session/session-1/message/msg_model_1")
-      return json({
-        data: {
-          id: "msg_model_1",
-          type: "model-switched",
-          previous: { id: "model-1", providerID: "provider-1", variant: "medium" },
-          model: { id: "model-1", providerID: "provider-1", variant: "high" },
-          time: { created: 0 },
-        },
-      })
-  }, events)
+  const calls = createFetch(undefined, events)
  let sync!: ReturnType<typeof useData>
  let ready!: () => void
  const mounted = new Promise<void>((resolve) => {
@@ -2630,6 +2619,7 @@ test("settles pending tools when a live failure arrives", async () => {

  try {
    await mounted
+    await sync.session.message.sync("session-1")
    emitEvent(events, {
      id: "evt_agent_1",
      created: 0,
@@ -2645,6 +2635,7 @@ test("settles pending tools when a live failure arrives", async () => {
      data: {
        sessionID: "session-1",
        model: { id: "model-1", providerID: "provider-1", variant: "high" },
+        previous: { id: "model-1", providerID: "provider-1", variant: "medium" },
      },
    })
    emitEvent(events, {
@@ -27,7 +27,10 @@ test("scopes sessions to the active session location", async () => {
  const active = "/tmp/opencode/project-b"
  const events = createEventStream()
  const requestedProjects: string[] = []
-  const calls = createFetch((url) => {
+  const calls = createFetch((url, request) => {
+    // The fixture's snapshot synthesizer reads through this handler; only app
+    // traffic belongs in the scoping assertions.
+    if (request.headers.get("x-fixture-synthetic")) return undefined
    if (url.pathname === "/api/location") {
      const directory = url.searchParams.get("location[directory]") ?? process.cwd()
      const project = directory === active ? "proj_b" : "proj_a"
@@ -16,6 +16,9 @@ export function createEventStream() {
  const encoder = new TextEncoder()
  const v2 = new Set<ReadableStreamDefaultController<Uint8Array>>()
  const pending: Uint8Array[] = []
+  const logs = new Map<string, Set<ReadableStreamDefaultController<Uint8Array>>>()
+  const logSeq = new Map<string, number>()
+  const logHistory = new Map<string, Array<{ readonly seq: number; readonly event: unknown }>>()
  const response = (
    controllers: Set<ReadableStreamDefaultController<Uint8Array>>,
    queued: Uint8Array[],
@@ -27,7 +30,8 @@ export function createEventStream() {
        start(controller) {
          current = controller
          controllers.add(controller)
-          if (initial) controller.enqueue(encoder.encode(`data: ${JSON.stringify(initial)}\n\n`))
+          const values = Array.isArray(initial) ? initial : initial ? [initial] : []
+          for (const value of values) controller.enqueue(encoder.encode(`data: ${JSON.stringify(value)}\n\n`))
          for (const chunk of queued.splice(0)) controller.enqueue(chunk)
        },
        cancel() {
@@ -53,13 +57,47 @@ export function createEventStream() {
  return {
    emit(event: OpenCodeEvent) {
      send(v2, pending, event)
+      const sessionID =
+        "durable" in event
+          ? event.durable.aggregateID
+          : "sessionID" in event.data && typeof event.data.sessionID === "string"
+            ? event.data.sessionID
+            : undefined
+      if (!sessionID) return
+      const seq = (logSeq.get(sessionID) ?? 0) + 1
+      const item = "durable" in event ? { ...event, durable: { ...event.durable, seq } } : event
+      if ("durable" in event) {
+        logSeq.set(sessionID, seq)
+        logHistory.set(sessionID, [...(logHistory.get(sessionID) ?? []), { seq, event: item }])
+      }
+      const controllers = logs.get(sessionID)
+      if (controllers) send(controllers, [], item)
    },
    v2() {
      return response(v2, pending, { id: "evt_connected", type: "server.connected", data: {} })
    },
+    log(sessionID: string, after: number) {
+      const controllers = logs.get(sessionID) ?? new Set<ReadableStreamDefaultController<Uint8Array>>()
+      logs.set(sessionID, controllers)
+      return response(
+        controllers,
+        [],
+        [
+          ...(logHistory.get(sessionID) ?? []).filter((entry) => entry.seq > after).map((entry) => entry.event),
+          { type: "log.synced", aggregateID: sessionID, seq: logSeq.get(sessionID) ?? 0 },
+        ],
+      )
+    },
+    seq(sessionID: string) {
+      return logSeq.get(sessionID) ?? 0
+    },
    disconnect() {
      for (const controller of v2) controller.close()
      v2.clear()
+      for (const controllers of logs.values()) {
+        for (const controller of controllers) controller.close()
+        controllers.clear()
+      }
    },
  }
 }
@@ -68,6 +106,8 @@ export type FetchHandler = (url: URL, request: Request) => Response | undefined

 export function createFetch(override?: FetchHandler, events?: ReturnType<typeof createEventStream>) {
  const session = [] as URL[]
+  const sessionEvents = events ?? createEventStream()
+  const snapshots = new Map<string, number>()
  async function fetch(input: RequestInfo | URL, init?: RequestInit) {
    const request = input instanceof Request ? input : new Request(input, init)
    const url = new URL(request.url)
@@ -75,6 +115,47 @@ export function createFetch(override?: FetchHandler, events?: ReturnType<typeof
    const overridden = await override?.(url, request)
    if (overridden) return overridden
    if (url.pathname === "/api/event" && events) return events.v2()
+    const snapshot = url.pathname.match(/^\/api\/session\/([^/]+)\/snapshot$/)
+    if (snapshot) {
+      const sessionID = decodeURIComponent(snapshot[1])
+      const count = snapshots.get(sessionID) ?? 0
+      snapshots.set(sessionID, count + 1)
+      // Synthetic sub-reads reuse the test override as the data source but are
+      // not app traffic; the marker header lets request-observing tests skip them.
+      const read = async (path: string, fallback: unknown) => {
+        const target = new URL(path, url)
+        const response = await override?.(target, new Request(target, { headers: { "x-fixture-synthetic": "snapshot" } }))
+        if (!response) return fallback
+        const body = await response.json()
+        if (typeof body !== "object" || body === null || !("data" in body)) return fallback
+        return body.data
+      }
+      const children = await read(`/api/session?parentID=${encodeURIComponent(sessionID)}`, [])
+      const messages = await read(`/api/session/${encodeURIComponent(sessionID)}/message`, [])
+      return json({
+        data: {
+          session: await read(`/api/session/${encodeURIComponent(sessionID)}`, {
+            id: sessionID,
+            projectID: "proj_test",
+            location: { directory },
+            cost: 0,
+            tokens: { input: 0, output: 0, reasoning: 0, cache: { read: 0, write: 0 } },
+            time: { created: 0, updated: 0 },
+          }),
+          children: Array.isArray(children)
+            ? children.filter(
+                (child) =>
+                  typeof child === "object" && child !== null && "parentID" in child && child.parentID === sessionID,
+              )
+            : [],
+          inbox: await read(`/api/session/${encodeURIComponent(sessionID)}/inbox`, []),
+          messages: Array.isArray(messages) ? messages.toReversed() : [],
+          seq: count === 0 ? 0 : sessionEvents.seq(sessionID),
+        },
+      })
+    }
+    const log = url.pathname.match(/^\/api\/experimental\/session\/([^/]+)\/log$/)
+    if (log) return sessionEvents.log(decodeURIComponent(log[1]), Number(url.searchParams.get("after") ?? 0))

    if (
      [
Author	SHA1	Message	Date
Kit Langton	6cbf607849	refactor(client): clone only changed subtrees on engine publish	2026-08-18 21:29:12 -04:00
Kit Langton	378d46c076	chore(client): add session sync streaming benchmark	2026-08-18 21:29:11 -04:00
Kit Langton	e0d4a0374c	chore(client): regenerate on rebased protocol	2026-08-18 20:56:08 -04:00
Kit Langton	f6f8606155	docs: record cursor honesty contract	2026-08-18 20:44:57 -04:00
Kit Langton	ef31823a6f	fix(client): recover from lossy log attach via snapshot	2026-08-18 20:44:57 -04:00
Kit Langton	4a42f4a724	fix(core): refuse log cursors without retained replay	2026-08-18 20:44:57 -04:00
Kit Langton	ce27f07f15	fix(client): preserve minted prompt IDs	2026-08-18 20:44:57 -04:00
Kit Langton	80587d936f	docs: wiring complete, regression dispositions	2026-08-18 20:44:57 -04:00
Kit Langton	09772d89d5	fix(tui): restore data-layer guarantees under engine wiring	2026-08-18 20:44:57 -04:00
Kit Langton	4f07025067	fix(client): restore engine refresh guarantees	2026-08-18 20:44:57 -04:00
Kit Langton	f253bdaeef	feat(tui): wire engine data layer	2026-08-18 20:44:57 -04:00
Kit Langton	93c24cafa7	feat(client): engine-backed data layer	2026-08-18 20:44:57 -04:00
Kit Langton	0963826120	docs: record lane status	2026-08-18 20:44:56 -04:00
Kit Langton	c5df2e5c28	refactor(core): simplify snapshot and log changes	2026-08-18 20:44:56 -04:00
Kit Langton	13f0cdd0ea	feat(core): session.log follow includes ephemeral events	2026-08-18 20:44:32 -04:00
Kit Langton	5887911c26	feat(core): typed seq-unavailable on session.log	2026-08-18 20:44:01 -04:00
Kit Langton	59bc653465	feat(protocol): session.snapshot endpoint	2026-08-18 20:44:01 -04:00
Kit Langton	8a4c14aed5	refactor(client): simplify sync engine	2026-08-18 20:43:47 -04:00
Kit Langton	15aef5f0f2	test(client): seeded chaos sim	2026-08-18 20:43:47 -04:00
Kit Langton	9b87120db6	test(client): six laws	2026-08-18 20:43:47 -04:00
Kit Langton	af6407a1b1	feat(client): engine core	2026-08-18 20:43:47 -04:00
Kit Langton	e4ff762483	feat(client): session fold	2026-08-18 20:43:47 -04:00
Kit Langton	a00822b06e	feat(core): Session.snapshot	2026-08-18 20:43:47 -04:00
Kit Langton	58c33f6870	docs: deletion inventory at adoption	2026-08-18 20:43:21 -04:00
Kit Langton	82d13c6715	docs: outbox ack contract, feature-grouped commits, opt-in ephemeral stream	2026-08-18 20:43:21 -04:00
Kit Langton	6c1d066a65	docs: drop adopt-strictness (S2), renumber lanes	2026-08-18 20:43:21 -04:00
Kit Langton	9b8801a3c6	docs: commit-by-commit implementation lanes, snapshot recent param	2026-08-18 20:43:21 -04:00
Kit Langton	8dad59cf99	docs: delivery is a parallel build for local comparison, not a staged migration	2026-08-18 20:43:21 -04:00
Kit Langton	a023ece71f	docs: snapshot-only entry contract, compaction/revert/fork, ambient bus filter (S4)	2026-08-18 20:43:21 -04:00
Kit Langton	2b5cc0090a	docs: single session stream, overlay semantics, mid-stream reconnect	2026-08-18 20:43:21 -04:00
Kit Langton	fc6b9e0dde	docs: fold prior-art findings into snapshot decision	2026-08-18 20:43:20 -04:00
Kit Langton	ab535c3875	docs: decide snapshot endpoint shape (tentative)	2026-08-18 20:43:20 -04:00
Kit Langton	4ca2f9dd83	docs: sync engine plan outline	2026-08-18 20:43:20 -04:00