diff --git a/docs/architecture.md b/docs/architecture.md
index c7c84ce3b4..891e851337 100644
--- a/docs/architecture.md
+++ b/docs/architecture.md
@@ -83,7 +83,7 @@ Streaming is a raw chunk protocol (`block-start`, `text-delta`, `reasoning-delta
 A `Session` is an append-only log of typed `SessionEvent`s — the single source of truth. The LLM message history is *derived* from the log (`deriveMessages()`):
 
 - `user/message` → user message
-- `assistant/message` → assistant message (raw `assistant/chunk` events are replay/UI data and are skipped in derivation)
+- `assistant/message` → assistant message (raw `assistant/chunk` events are replay/UI data and are skipped in derivation; an empty-content `assistant/message`, which exists only to host a max-tokens step's `usage`, is skipped too)
 - `tool/result` → user message carrying a `tool-result` block
 - `context/message`, `steering/message` → user-role messages wrapped in a tagged envelope (`<context source="…">…</context>`) at their chronological position — the "system-reminder" pattern; models distinguish them from real user prompts by the envelope. **TODO(review)**: the real adapters now exist (the original precondition); the envelope still wants a deliberate review against live model behavior (`TODO(review)` in dsh-session).
 
@@ -142,7 +142,7 @@ forever:
                                                          step error (turn ends error/aborted,
                                                          not a normal completed message)
       msg = waterfall agent/step-result               ⟵ runs BEFORE the log append, so the
-      session('assistant/message', 'usage')              log records what tool dispatch uses
+      session('assistant/message' {content, usage?})     log records what tool dispatch uses
       each tool-call (sequential, abort-checked between calls):
         session('tool/call'); ctx.tools.execute()     ⟵ waterfall tools/execute
         session('tool/result')
@@ -158,11 +158,11 @@ forever:
   emit agent/status(idle) unless more queued
 ```
 
-Error containment: a throwing `agent/turn-continuation` listener or a broken step ends the **turn** with an `error` event (appended INSIDE the turn, before `turn/end`) — never the driver loop. An adapter that ends its stream with a `finish {kind:'error'}` or `{kind:'aborted'}` chunk (the in-band error path, for adapters that can't throw mid-stream) is likewise translated into a step error, so the turn ends `error`/`aborted` instead of logging a normal `completed` assistant message. A `cancel()` is honored mid-stream **and** between tool calls; disposal mid-turn ends the turn with reason `disposed` and emits `agent/status('disposed')`.
+Error containment: a throwing `agent/turn-continuation` listener or a broken step ends the **turn** with `turn/end { reason: { kind: 'error', step, message, code? } }` — the failure's step number rides on the durable turn reason (there is no separate session `error` event); live diagnostics fire via `agent/error`. Never the driver loop. An adapter that ends its stream with a `finish {kind:'error'}` or `{kind:'aborted'}` chunk (the in-band error path, for adapters that can't throw mid-stream) is likewise translated into a step error, so the turn ends `error`/`aborted` instead of logging a normal `completed` assistant message. A `cancel()` is honored mid-stream **and** between tool calls; disposal mid-turn ends the turn with reason `disposed` and emits `agent/status('disposed')`.
 
 Turn-end reasons: a turn ends with one `TurnEndReason` — `completed`, `aborted`, `error`, `disposed`, or `max-tokens`. `max-tokens` mirrors the model-call `FinishReason` of the same name (DeepSeek's `length`): a step that hit the output-token ceiling makes the turn end `max-tokens` rather than `completed`, by the rule *any `max-tokens` step in the turn surfaces as `max-tokens`* (a continuation plugin may run further steps after one, but the cut-short fact wins; the `disposed`/`aborted`/`error` outcomes still take precedence). This lets a consumer distinguish a clean stop from a truncated one (the ACP bridge maps it to the `max_tokens` stop reason). `TurnEndReason` is merge-extensible; `refusal` and `max_turn_requests` are the next variants to add when an adapter/loop first emits them.
 
-A failure that happens once the turn is already closed has no in-turn position for a session `error` event (appending one after `turn/end` would put it past the persistence commit boundary, where it is dropped as a crash tail — [the turn-enclosure invariant](rfc/implemented/architecture/2026-06-15-turn-enclosure-invariant.md)). So a rejecting `session/flush` (the post-`turn/end` durability checkpoint) and a throwing `agent/turn-end` listener are reported via `agent/error` + the logger only, NOT as a session event; the turn stays balanced and the persistence backend keeps its buffered events for the next flush.
+A failure that happens once the turn is already closed has no in-turn position for a turn-end error reason (the turn already ended). So a rejecting `session/flush` (the post-`turn/end` durability checkpoint) and a throwing `agent/turn-end` listener are reported via `agent/error` + the logger only, NOT as a session event; the turn stays balanced and the persistence backend keeps its buffered events for the next flush.
 
 **Turn-enclosure invariant**: every session event lives inside a turn (between a `turn/start` and its `turn/end`). The loop appends queued `user/message` events *after* `turn/start`, and an idle `agent.inject()` wraps its `context/message` in a one-shot `injection` turn. This makes the turn the single durability/replay boundary: a persistence backend can treat anything after the last `turn/end` as an interrupted-crash tail without risking the loss of legitimately-recorded between-turn context. The `dsh-invariants` plugin enforces it in dev (a message event outside an open turn throws). See [the turn-enclosure invariant](rfc/implemented/architecture/2026-06-15-turn-enclosure-invariant.md).
 
diff --git a/docs/cordis-catalog/events-and-services.md b/docs/cordis-catalog/events-and-services.md
index afa4356880..fbb2ab10ba 100644
--- a/docs/cordis-catalog/events-and-services.md
+++ b/docs/cordis-catalog/events-and-services.md
@@ -390,7 +390,7 @@ get(id: SessionId): Session | undefined
 list(): Session[]
 ```
 
-Source: [`packages/core/session/src/index.ts:222`](../../packages/core/session/src/index.ts)
+Source: [`packages/core/session/src/index.ts:229`](../../packages/core/session/src/index.ts)
 
 ### `ctx.systemPrompt` — `SystemPrompt`
 
diff --git a/docs/core-data-structures/core.md b/docs/core-data-structures/core.md
index ae6086b7b7..ec4805c798 100644
--- a/docs/core-data-structures/core.md
+++ b/docs/core-data-structures/core.md
@@ -189,7 +189,7 @@ type SessionEvent<T extends SessionEventType = SessionEventType> = {
 }[T]
 ```
 
-The thirteen event variants (`turn/start`, `turn/end`, `step/start`, `step/end`, `user/message`, `context/message`, `assistant/chunk`, `assistant/message`, `tool/call`, `tool/result`, `steering/message`, `usage`, `error`), the `deriveMessages()` projection rules, the `TurnTrigger`/`TurnEndReason` reasons, and the turn-enclosure invariant are on **[session.md](session.md)**. How the log is made durable — the `SessionPersistence` seam, JSONL/SQLite backends, the `session/flush` checkpoint, crash recovery, and `SessionHeader` — is on **[persistence.md](persistence.md)**.
+The eleven event variants (`turn/start`, `turn/end`, `step/start`, `step/end`, `user/message`, `context/message`, `assistant/chunk`, `assistant/message`, `tool/call`, `tool/result`, `steering/message`), the `deriveMessages()` projection rules, the `TurnTrigger`/`TurnEndReason` reasons, and the turn-enclosure invariant are on **[session.md](session.md)**. How the log is made durable — the `SessionPersistence` seam, JSONL/SQLite backends, the `session/flush` checkpoint, crash recovery, and `SessionHeader` — is on **[persistence.md](persistence.md)**.
 
 ## The agent handle
 
diff --git a/docs/core-data-structures/session.md b/docs/core-data-structures/session.md
index d60b738b6a..d73ad0cb4e 100644
--- a/docs/core-data-structures/session.md
+++ b/docs/core-data-structures/session.md
@@ -24,14 +24,17 @@ interface SessionEventMap {
   'context/message': { content: ContentBlock[]; source: MessageSource }
   /** Raw stream chunk — token-level replay fidelity. */
   'assistant/chunk': { turn: number; step: number; chunk: StreamChunk }
-  /** Assembled assistant message for one step (derived history uses this). */
-  'assistant/message': { turn: number; step: number; content: ContentBlock[] }
+  /**
+   * Assembled assistant message for one step (derived history uses this).
+   * Carries the step's `usage` when the adapter reported token accounting, so
+   * the model output and its accounting travel together (there is no separate
+   * usage record). `usage` is absent when the adapter reported none.
+   */
+  'assistant/message': { turn: number; step: number; content: ContentBlock[]; usage?: TokenUsage }
   'tool/call': { turn: number; step: number; callId: CallId; name: string; arguments: string }
   'tool/result': { turn: number; step: number; callId: CallId; content: ContentBlock[]; isError: boolean; error?: { name: string; code: string } }
   /** Steering content injected between steps of a running turn. */
   'steering/message': { turn: number; content: ContentBlock[]; source: MessageSource }
-  'usage': { turn: number; step: number; usage: TokenUsage }
-  'error': { turn: number; step: number; message: string; code?: string }
 }
 ```
 
@@ -59,11 +62,11 @@ type SessionEvent<T extends SessionEventType = SessionEventType> = {
 `Session.deriveMessages()` projects the event log into the `Message[]` the model sees. The projection rules:
 
 - `user/message` → a user message.
-- `assistant/message` → an assistant message. Raw `assistant/chunk` events are replay/UI data and are **skipped** in derivation (the assembled message is authoritative).
+- `assistant/message` → an assistant message. Raw `assistant/chunk` events are replay/UI data and are **skipped** in derivation (the assembled message is authoritative). An **empty-content** `assistant/message` is also skipped — a max-tokens step cut off with no content still records an `assistant/message` to host its `usage`, but a content-less assistant turn must not enter the provider transcript.
 - `tool/result` → a user message carrying a `tool-result` block.
 - `context/message`, `steering/message` → user-role messages wrapped in a tagged envelope (`<context source="…">…</context>`) at their chronological position — the "system-reminder" pattern; the model distinguishes them from real prompts by the envelope.
 
-Everything else (`turn/*`, `step/*`, `usage`, `error`) is structural/telemetry and does not project into a message.
+Everything else (`turn/*`, `step/*`) is structural and does not project into a message. Token usage is observed on `assistant/message.usage` (the step that produced it); an operational error's step number is on `turn/end.reason` for `kind: 'error'`.
 
 ## What started a turn: `TurnTriggerMap`
 
@@ -89,7 +92,13 @@ interface TurnTriggerMap {
 interface TurnEndReasonMap {
   completed: { kind: 'completed' }
   aborted: { kind: 'aborted'; reason?: string }
-  error: { kind: 'error'; message: string; code?: string }
+  /**
+   * The turn failed: a step threw or the model reported a failure. `step` is the
+   * step number the failure occurred on (the operational error's location — the
+   * single durable record of an in-turn failure; live diagnostics also fire via
+   * `agent/error`). `code` is the error's code when one was attached.
+   */
+  error: { kind: 'error'; step: number; message: string; code?: string }
   disposed: { kind: 'disposed' }
   'max-tokens': { kind: 'max-tokens' }
   /**
diff --git a/docs/rfc/README.md b/docs/rfc/README.md
index fae21e0351..e37f162164 100644
--- a/docs/rfc/README.md
+++ b/docs/rfc/README.md
@@ -51,7 +51,6 @@ Do NOT write one for a mechanical or local choice (a variable name, a one-file r
 |---|---|
 | [Unify the agent id and the session id](proposed/simplification/2026-06-20-unify-agent-and-session-id.md) | 2026-06-20 |
 | [Stop mirroring durable boundaries as agent events](proposed/simplification/2026-06-20-remove-agent-boundary-mirror-events.md) | 2026-06-20 |
-| [Fold trace-only session facts into load-bearing events](proposed/simplification/2026-06-20-collapse-trace-only-session-events.md) | 2026-06-20 |
 
 ### Architecture
 
@@ -95,6 +94,7 @@ Do NOT write one for a mechanical or local choice (a variable name, a one-file r
 | [Drop the unconsumed `llm/adapter-change` event](implemented/simplification/2026-06-20-drop-unconsumed-llm-adapter-change-event.md) | 2026-06-20 |
 | [Prune dead methods from the persistence and bash seams](implemented/simplification/2026-06-20-prune-dead-seam-methods.md) | 2026-06-20 |
 | [Keep one public stop primitive](implemented/simplification/2026-06-20-public-agent-stop-surface.md) | 2026-06-20 |
+| [Fold trace-only session facts into load-bearing events](implemented/simplification/2026-06-20-collapse-trace-only-session-events.md) | 2026-06-20 |
 
 ### Architecture
 
diff --git a/docs/rfc/proposed/simplification/2026-06-20-collapse-trace-only-session-events.md b/docs/rfc/implemented/simplification/2026-06-20-collapse-trace-only-session-events.md
similarity index 68%
rename from docs/rfc/proposed/simplification/2026-06-20-collapse-trace-only-session-events.md
rename to docs/rfc/implemented/simplification/2026-06-20-collapse-trace-only-session-events.md
index ecba30bcbd..3a2c11bdc6 100644
--- a/docs/rfc/proposed/simplification/2026-06-20-collapse-trace-only-session-events.md
+++ b/docs/rfc/implemented/simplification/2026-06-20-collapse-trace-only-session-events.md
@@ -1,6 +1,6 @@
 # RFC: Fold trace-only session facts into load-bearing events
 
-Status: proposed
+Status: implemented (proposed and accepted 2026-06-20)
 
 ## Problem
 
@@ -31,3 +31,12 @@ If analytics become real, add a projection helper or a dedicated telemetry store
 ## What we give up
 
 A consumer can no longer filter the canonical log for standalone `usage` or step-level `error` rows. It must read those facts from the assistant/failure events that carry them. That is a reasonable simplification only if the implementing PR proves the same facts remain present; otherwise the standalone events should stay.
+
+## Implementation note
+
+Shipped as proposed, with two scope refinements (per AGENTS.md "RFCs are proposals, not golden truth"):
+
+- **No format-version bump.** The acceptance criterion "the session format version and recorded fixtures are refreshed" over-reached: the harness is pre-release with no persisted user data, so per the pre-release format policy there is nothing to migrate or reject. The session `version` stays `1`; only event shapes and recorded fixtures change. `turn/end.reason.error.step` is therefore optional-on-read for any hypothetical pre-existing log but guaranteed for newly-written ones — no migration shim.
+- **Empty-content `assistant/message` hosts usage with no data loss.** The proof the proposal demanded (no persisted usage chunk becomes unrepresented) lands on the max-tokens path: a step cut off with usage but empty content (e.g. only a dropped tool call) previously emitted a standalone `usage`. It now records an empty-content `assistant/message { content: [], usage }`. To keep that from injecting a spurious content-less assistant turn into the provider transcript, `deriveMessages()` skips empty-content `assistant/message` events. A regression test asserts usage stays represented AND derived history is uncorrupted.
+
+Usage is now observed on `assistant/message.usage`; an operational error's step on `turn/end.reason` for `kind: 'error'`. `agent/error` + logging are unchanged for live diagnostics.
diff --git a/docs/rfc/implemented/testing/2026-06-19-acp-snapshot-tests.md b/docs/rfc/implemented/testing/2026-06-19-acp-snapshot-tests.md
index af8c40c55e..a5875993a3 100644
--- a/docs/rfc/implemented/testing/2026-06-19-acp-snapshot-tests.md
+++ b/docs/rfc/implemented/testing/2026-06-19-acp-snapshot-tests.md
@@ -18,7 +18,7 @@ A snapshot test boots the **real** `examples/acp-agent` subprocess, drives it ov
 
 ### The fixture is the persisted session JSONL
 
-The per-scenario fixture is `<scenario>/session.jsonl`: the exact log produced by running the scenario once against the real API (the snapshot harness harvests the file the JSONL persistence backend writes). This log already contains everything needed to reproduce the run deterministically: its `assistant/chunk` events carry every parsed `StreamChunk` (the LLM's behavior), and its `tool/call`/`tool/result`/`turn/*`/`assistant/message`/`usage` events carry the harness's behavior. One artifact captures both, and it is the format the codebase already treats as the authoritative replay record ([packages/core/session/src/types.ts](../../../../packages/core/session/src/types.ts): "raw chunks are the replay record").
+The per-scenario fixture is `<scenario>/session.jsonl`: the exact log produced by running the scenario once against the real API (the snapshot harness harvests the file the JSONL persistence backend writes). This log already contains everything needed to reproduce the run deterministically: its `assistant/chunk` events carry every parsed `StreamChunk` (the LLM's behavior), and its `tool/call`/`tool/result`/`turn/*`/`assistant/message` events carry the harness's behavior (token usage rides on `assistant/message.usage`). One artifact captures both, and it is the format the codebase already treats as the authoritative replay record ([packages/core/session/src/types.ts](../../../../packages/core/session/src/types.ts): "raw chunks are the replay record").
 
 An earlier draft used a hand-authored `llm.json` of model chunks; reusing the real session log instead means the fixture is a genuine product of the system (not a hand-built mock), and it doubles as a behavioral golden (see below). A byte-level HTTP-record library (Polly/nock/MSW) was rejected: adapter-specific, awkward with streaming SSE, and lower-level than the thing under test.
 
@@ -55,7 +55,7 @@ A snapshot run asserts **two** normalized goldens, because the harness's externa
 1. The **stdout transcript** — the framed `session/update` JSON-RPC the editor sees. Catches regressions in the ACP bridge's event→update translation (`streamSessionEventUpdate`).
 2. The **re-derived session JSONL** — the log the replay run itself persists, compared against the recorded fixture. Catches regressions in the loop, tool dispatch, and turn/step structure that never surface on stdout.
 
-The two are genuinely additive: stdout is the bridge's *lossy projection* of the log (it drops `usage`, `step/*`, exact `seq`/`time`, and renders tool I/O differently), so a loop/tool/turn-structure regression can change the JSONL while leaving the stdout projection identical, and a bridge-translation regression can change stdout while the JSONL is untouched. Asserting the JSONL equality also echoes the proposed [universal replay fixture](../../proposed/testing/2026-06-11-deterministic-and-stress-testing.md) idea.
+The two are genuinely additive: stdout is the bridge's *lossy projection* of the log (it drops `assistant/message.usage`, `step/*`, exact `seq`/`time`, and renders tool I/O differently), so a loop/tool/turn-structure regression can change the JSONL while leaving the stdout projection identical, and a bridge-translation regression can change stdout while the JSONL is untouched. Asserting the JSONL equality also echoes the proposed [universal replay fixture](../../proposed/testing/2026-06-11-deterministic-and-stress-testing.md) idea.
 
 Both surfaces contain non-deterministic values that a pure normalization function scrubs **before** the snapshot: `randomUUID()` session ids → `{{sessionId}}`, the temp `mkdtemp` cwd → `{{cwd}}` (it appears in terminal-card `_meta` and the log header), JSON-RPC ids → a stable sequence, and the log's per-event `time` (epoch ms) + header `createdAt` dropped or zeroed (the log's `seq` is left intact — it is deterministic by contract, `seq = log.length`). Real bash runs during replay, so the JSONL normalizer additionally stabilizes tool-output volatility (any embedded paths/pids/timestamps) — scenarios keep bash commands tightly constrained (`echo`, file writes; no `date`/`env`/background/large-output) so this surface is small. The goldens are themselves **JSONL** — one compact, normalized record per line, in the same shape as the surfaces they mirror (NDJSON on the wire, JSONL on disk: `stdout.golden.jsonl`, `session.golden.jsonl`), so they stay `grep`/`jq`-able and faithful to what the agent actually emits. A separate raw-purity assertion keeps the guarantee that every stdout line parses as JSON (no logger leak onto the protocol channel). Vitest's `toMatchFileSnapshot` provides the golden store and the `-u`/`--update` "accept the diff" workflow.
 
diff --git a/examples/acp-agent/tests/snapshots/error-finish/session.golden.jsonl b/examples/acp-agent/tests/snapshots/error-finish/session.golden.jsonl
index fcf2cde49f..9f6ee27674 100644
--- a/examples/acp-agent/tests/snapshots/error-finish/session.golden.jsonl
+++ b/examples/acp-agent/tests/snapshots/error-finish/session.golden.jsonl
@@ -3,5 +3,4 @@
 {"type":"user/message","seq":1,"time":0,"data":{"content":[{"type":"text","text":"This prompt triggers a recorded provider error."}],"source":{"kind":"user"}}}
 {"type":"step/start","seq":2,"time":0,"data":{"turn":1,"step":1}}
 {"type":"step/end","seq":3,"time":0,"data":{"turn":1,"step":1}}
-{"type":"error","seq":4,"time":0,"data":{"turn":1,"step":1,"message":"simulated provider error (HTTP 401)","code":"AUTH"}}
-{"type":"turn/end","seq":5,"time":0,"data":{"turn":1,"reason":{"kind":"error","message":"simulated provider error (HTTP 401)","code":"AUTH"}}}
+{"type":"turn/end","seq":4,"time":0,"data":{"turn":1,"reason":{"kind":"error","step":1,"message":"simulated provider error (HTTP 401)","code":"AUTH"}}}
diff --git a/examples/acp-agent/tests/snapshots/multi-turn/session.golden.jsonl b/examples/acp-agent/tests/snapshots/multi-turn/session.golden.jsonl
index 20fe3a727b..92f0465e66 100644
--- a/examples/acp-agent/tests/snapshots/multi-turn/session.golden.jsonl
+++ b/examples/acp-agent/tests/snapshots/multi-turn/session.golden.jsonl
@@ -27,40 +27,38 @@
 {"type":"assistant/chunk","seq":25,"time":0,"data":{"turn":1,"step":1,"chunk":{"type":"block-end","index":1,"block":{"type":"text","text":"ONE"}}}}
 {"type":"assistant/chunk","seq":26,"time":0,"data":{"turn":1,"step":1,"chunk":{"type":"usage","usage":{"inputTokens":106,"outputTokens":20,"cacheReadTokens":768,"reasoningTokens":18}}}}
 {"type":"assistant/chunk","seq":27,"time":0,"data":{"turn":1,"step":1,"chunk":{"type":"finish","reason":{"kind":"stop"}}}}
-{"type":"assistant/message","seq":28,"time":0,"data":{"turn":1,"step":1,"content":[{"type":"reasoning","text":"The user wants me to reply with exactly the word \"ONE\" and use no tools."},{"type":"text","text":"ONE"}]}}
-{"type":"usage","seq":29,"time":0,"data":{"turn":1,"step":1,"usage":{"inputTokens":106,"outputTokens":20,"cacheReadTokens":768,"reasoningTokens":18}}}
-{"type":"step/end","seq":30,"time":0,"data":{"turn":1,"step":1}}
-{"type":"turn/end","seq":31,"time":0,"data":{"turn":1,"reason":{"kind":"completed"}}}
-{"type":"turn/start","seq":32,"time":0,"data":{"turn":2,"trigger":{"kind":"message","source":{"kind":"user"}}}}
-{"type":"user/message","seq":33,"time":0,"data":{"content":[{"type":"text","text":"Reply with exactly the word: TWO. No tools."}],"source":{"kind":"user"}}}
-{"type":"step/start","seq":34,"time":0,"data":{"turn":2,"step":1}}
-{"type":"assistant/chunk","seq":35,"time":0,"data":{"turn":2,"step":1,"chunk":{"type":"block-start","index":0,"blockType":"reasoning"}}}
-{"type":"assistant/chunk","seq":36,"time":0,"data":{"turn":2,"step":1,"chunk":{"type":"reasoning-delta","index":0,"text":"The"}}}
-{"type":"assistant/chunk","seq":37,"time":0,"data":{"turn":2,"step":1,"chunk":{"type":"reasoning-delta","index":0,"text":" user"}}}
-{"type":"assistant/chunk","seq":38,"time":0,"data":{"turn":2,"step":1,"chunk":{"type":"reasoning-delta","index":0,"text":" wants"}}}
-{"type":"assistant/chunk","seq":39,"time":0,"data":{"turn":2,"step":1,"chunk":{"type":"reasoning-delta","index":0,"text":" me"}}}
-{"type":"assistant/chunk","seq":40,"time":0,"data":{"turn":2,"step":1,"chunk":{"type":"reasoning-delta","index":0,"text":" to"}}}
-{"type":"assistant/chunk","seq":41,"time":0,"data":{"turn":2,"step":1,"chunk":{"type":"reasoning-delta","index":0,"text":" reply"}}}
-{"type":"assistant/chunk","seq":42,"time":0,"data":{"turn":2,"step":1,"chunk":{"type":"reasoning-delta","index":0,"text":" with"}}}
-{"type":"assistant/chunk","seq":43,"time":0,"data":{"turn":2,"step":1,"chunk":{"type":"reasoning-delta","index":0,"text":" exactly"}}}
-{"type":"assistant/chunk","seq":44,"time":0,"data":{"turn":2,"step":1,"chunk":{"type":"reasoning-delta","index":0,"text":" the"}}}
-{"type":"assistant/chunk","seq":45,"time":0,"data":{"turn":2,"step":1,"chunk":{"type":"reasoning-delta","index":0,"text":" word"}}}
-{"type":"assistant/chunk","seq":46,"time":0,"data":{"turn":2,"step":1,"chunk":{"type":"reasoning-delta","index":0,"text":" \""}}}
-{"type":"assistant/chunk","seq":47,"time":0,"data":{"turn":2,"step":1,"chunk":{"type":"reasoning-delta","index":0,"text":"T"}}}
-{"type":"assistant/chunk","seq":48,"time":0,"data":{"turn":2,"step":1,"chunk":{"type":"reasoning-delta","index":0,"text":"WO"}}}
-{"type":"assistant/chunk","seq":49,"time":0,"data":{"turn":2,"step":1,"chunk":{"type":"reasoning-delta","index":0,"text":"\""}}}
-{"type":"assistant/chunk","seq":50,"time":0,"data":{"turn":2,"step":1,"chunk":{"type":"reasoning-delta","index":0,"text":" and"}}}
-{"type":"assistant/chunk","seq":51,"time":0,"data":{"turn":2,"step":1,"chunk":{"type":"reasoning-delta","index":0,"text":" no"}}}
-{"type":"assistant/chunk","seq":52,"time":0,"data":{"turn":2,"step":1,"chunk":{"type":"reasoning-delta","index":0,"text":" tools"}}}
-{"type":"assistant/chunk","seq":53,"time":0,"data":{"turn":2,"step":1,"chunk":{"type":"reasoning-delta","index":0,"text":"."}}}
-{"type":"assistant/chunk","seq":54,"time":0,"data":{"turn":2,"step":1,"chunk":{"type":"block-start","index":1,"blockType":"text"}}}
-{"type":"assistant/chunk","seq":55,"time":0,"data":{"turn":2,"step":1,"chunk":{"type":"text-delta","index":1,"text":"T"}}}
-{"type":"assistant/chunk","seq":56,"time":0,"data":{"turn":2,"step":1,"chunk":{"type":"text-delta","index":1,"text":"WO"}}}
-{"type":"assistant/chunk","seq":57,"time":0,"data":{"turn":2,"step":1,"chunk":{"type":"block-end","index":0,"block":{"type":"reasoning","text":"The user wants me to reply with exactly the word \"TWO\" and no tools."}}}}
-{"type":"assistant/chunk","seq":58,"time":0,"data":{"turn":2,"step":1,"chunk":{"type":"block-end","index":1,"block":{"type":"text","text":"TWO"}}}}
-{"type":"assistant/chunk","seq":59,"time":0,"data":{"turn":2,"step":1,"chunk":{"type":"usage","usage":{"inputTokens":122,"outputTokens":21,"cacheReadTokens":768,"reasoningTokens":18}}}}
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diff --git a/examples/acp-agent/tests/snapshots/text-turn/session.golden.jsonl b/examples/acp-agent/tests/snapshots/text-turn/session.golden.jsonl
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--- a/examples/acp-agent/tests/snapshots/text-turn/session.golden.jsonl
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index 8306f3d4de..ed34f9a727 100644
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--- a/examples/acp-agent/tests/snapshots/tool-call-turn/session.golden.jsonl
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--- a/examples/acp-agent/tests/snapshots/workspace-edit/session.golden.jsonl
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-{"type":"assistant/message","seq":187,"time":1781834687489,"data":{"turn":1,"step":3,"content":[{"type":"reasoning","text":"The file now has two lines. I'll reply with DONE."},{"type":"text","text":"DONE"}]}}
-{"type":"usage","seq":188,"time":1781834687489,"data":{"turn":1,"step":3,"usage":{"inputTokens":156,"outputTokens":17,"cacheReadTokens":1024,"reasoningTokens":14}}}
-{"type":"step/end","seq":189,"time":1781834687489,"data":{"turn":1,"step":3}}
-{"type":"turn/end","seq":190,"time":1781834687489,"data":{"turn":1,"reason":{"kind":"completed"}}}
+{"type":"assistant/message","seq":114,"time":1781834685387,"data":{"turn":1,"step":1,"content":[{"type":"reasoning","text":"The user wants me to:\n1. Append a second line containing \"WORLD\" to greeting.txt\n2. Read the file back with `cat greeting.txt` to confirm\n3. Reply with \"DONE\"\n\nBut they want \"a single bash call per action\" - so I'll do two separate bash calls: one for appending, one for reading."},{"type":"tool-call","id":"call_00_r3tvHl3fD0tmV0GKQt032338","name":"bash","arguments":"{\"command\": \"echo 'WORLD' >> greeting.txt\", \"description\": \"Append WORLD line to greeting.txt\"}"}],"usage":{"inputTokens":161,"outputTokens":146,"cacheReadTokens":768,"reasoningTokens":74}}}
+{"type":"tool/call","seq":115,"time":1781834685387,"data":{"turn":1,"step":1,"callId":"call_00_r3tvHl3fD0tmV0GKQt032338","name":"bash","arguments":"{\"command\": \"echo 'WORLD' >> greeting.txt\", \"description\": \"Append WORLD line to greeting.txt\"}"}}
+{"type":"tool/result","seq":116,"time":1781834685400,"data":{"turn":1,"step":1,"callId":"call_00_r3tvHl3fD0tmV0GKQt032338","content":[{"type":"text","text":"(no output)"}],"isError":false}}
+{"type":"step/end","seq":117,"time":1781834685400,"data":{"turn":1,"step":1}}
+{"type":"step/start","seq":118,"time":1781834685400,"data":{"turn":1,"step":2}}
+{"type":"assistant/chunk","seq":119,"time":1781834686163,"data":{"turn":1,"step":2,"chunk":{"type":"block-start","index":0,"blockType":"reasoning"}}}
+{"type":"assistant/chunk","seq":120,"time":1781834686163,"data":{"turn":1,"step":2,"chunk":{"type":"reasoning-delta","index":0,"text":"App"}}}
+{"type":"assistant/chunk","seq":121,"time":1781834686261,"data":{"turn":1,"step":2,"chunk":{"type":"reasoning-delta","index":0,"text":"ended"}}}
+{"type":"assistant/chunk","seq":122,"time":1781834686290,"data":{"turn":1,"step":2,"chunk":{"type":"reasoning-delta","index":0,"text":" successfully"}}}
+{"type":"assistant/chunk","seq":123,"time":1781834686318,"data":{"turn":1,"step":2,"chunk":{"type":"reasoning-delta","index":0,"text":"."}}}
+{"type":"assistant/chunk","seq":124,"time":1781834686319,"data":{"turn":1,"step":2,"chunk":{"type":"reasoning-delta","index":0,"text":" Now"}}}
+{"type":"assistant/chunk","seq":125,"time":1781834686319,"data":{"turn":1,"step":2,"chunk":{"type":"reasoning-delta","index":0,"text":" read"}}}
+{"type":"assistant/chunk","seq":126,"time":1781834686352,"data":{"turn":1,"step":2,"chunk":{"type":"reasoning-delta","index":0,"text":" the"}}}
+{"type":"assistant/chunk","seq":127,"time":1781834686352,"data":{"turn":1,"step":2,"chunk":{"type":"reasoning-delta","index":0,"text":" file"}}}
+{"type":"assistant/chunk","seq":128,"time":1781834686381,"data":{"turn":1,"step":2,"chunk":{"type":"reasoning-delta","index":0,"text":"."}}}
+{"type":"assistant/chunk","seq":129,"time":1781834686469,"data":{"turn":1,"step":2,"chunk":{"type":"block-start","index":1,"blockType":"tool-call"}}}
+{"type":"assistant/chunk","seq":130,"time":1781834686469,"data":{"turn":1,"step":2,"chunk":{"type":"tool-call-delta","index":1,"id":"call_00_SkCP8dgN8aCbLiZDcYa68316","name":"bash","argumentsDelta":""}}}
+{"type":"assistant/chunk","seq":131,"time":1781834686497,"data":{"turn":1,"step":2,"chunk":{"type":"tool-call-delta","index":1,"id":"call_00_SkCP8dgN8aCbLiZDcYa68316","name":"bash","argumentsDelta":"{"}}}
+{"type":"assistant/chunk","seq":132,"time":1781834686498,"data":{"turn":1,"step":2,"chunk":{"type":"tool-call-delta","index":1,"id":"call_00_SkCP8dgN8aCbLiZDcYa68316","name":"bash","argumentsDelta":"\""}}}
+{"type":"assistant/chunk","seq":133,"time":1781834686498,"data":{"turn":1,"step":2,"chunk":{"type":"tool-call-delta","index":1,"id":"call_00_SkCP8dgN8aCbLiZDcYa68316","name":"bash","argumentsDelta":"command"}}}
+{"type":"assistant/chunk","seq":134,"time":1781834686498,"data":{"turn":1,"step":2,"chunk":{"type":"tool-call-delta","index":1,"id":"call_00_SkCP8dgN8aCbLiZDcYa68316","name":"bash","argumentsDelta":"\""}}}
+{"type":"assistant/chunk","seq":135,"time":1781834686498,"data":{"turn":1,"step":2,"chunk":{"type":"tool-call-delta","index":1,"id":"call_00_SkCP8dgN8aCbLiZDcYa68316","name":"bash","argumentsDelta":": "}}}
+{"type":"assistant/chunk","seq":136,"time":1781834686530,"data":{"turn":1,"step":2,"chunk":{"type":"tool-call-delta","index":1,"id":"call_00_SkCP8dgN8aCbLiZDcYa68316","name":"bash","argumentsDelta":"\""}}}
+{"type":"assistant/chunk","seq":137,"time":1781834686530,"data":{"turn":1,"step":2,"chunk":{"type":"tool-call-delta","index":1,"id":"call_00_SkCP8dgN8aCbLiZDcYa68316","name":"bash","argumentsDelta":"cat"}}}
+{"type":"assistant/chunk","seq":138,"time":1781834686530,"data":{"turn":1,"step":2,"chunk":{"type":"tool-call-delta","index":1,"id":"call_00_SkCP8dgN8aCbLiZDcYa68316","name":"bash","argumentsDelta":" greeting"}}}
+{"type":"assistant/chunk","seq":139,"time":1781834686530,"data":{"turn":1,"step":2,"chunk":{"type":"tool-call-delta","index":1,"id":"call_00_SkCP8dgN8aCbLiZDcYa68316","name":"bash","argumentsDelta":".txt"}}}
+{"type":"assistant/chunk","seq":140,"time":1781834686559,"data":{"turn":1,"step":2,"chunk":{"type":"tool-call-delta","index":1,"id":"call_00_SkCP8dgN8aCbLiZDcYa68316","name":"bash","argumentsDelta":"\""}}}
+{"type":"assistant/chunk","seq":141,"time":1781834686591,"data":{"turn":1,"step":2,"chunk":{"type":"tool-call-delta","index":1,"id":"call_00_SkCP8dgN8aCbLiZDcYa68316","name":"bash","argumentsDelta":", "}}}
+{"type":"assistant/chunk","seq":142,"time":1781834686591,"data":{"turn":1,"step":2,"chunk":{"type":"tool-call-delta","index":1,"id":"call_00_SkCP8dgN8aCbLiZDcYa68316","name":"bash","argumentsDelta":"\""}}}
+{"type":"assistant/chunk","seq":143,"time":1781834686591,"data":{"turn":1,"step":2,"chunk":{"type":"tool-call-delta","index":1,"id":"call_00_SkCP8dgN8aCbLiZDcYa68316","name":"bash","argumentsDelta":"description"}}}
+{"type":"assistant/chunk","seq":144,"time":1781834686591,"data":{"turn":1,"step":2,"chunk":{"type":"tool-call-delta","index":1,"id":"call_00_SkCP8dgN8aCbLiZDcYa68316","name":"bash","argumentsDelta":"\""}}}
+{"type":"assistant/chunk","seq":145,"time":1781834686591,"data":{"turn":1,"step":2,"chunk":{"type":"tool-call-delta","index":1,"id":"call_00_SkCP8dgN8aCbLiZDcYa68316","name":"bash","argumentsDelta":": "}}}
+{"type":"assistant/chunk","seq":146,"time":1781834686623,"data":{"turn":1,"step":2,"chunk":{"type":"tool-call-delta","index":1,"id":"call_00_SkCP8dgN8aCbLiZDcYa68316","name":"bash","argumentsDelta":"\""}}}
+{"type":"assistant/chunk","seq":147,"time":1781834686623,"data":{"turn":1,"step":2,"chunk":{"type":"tool-call-delta","index":1,"id":"call_00_SkCP8dgN8aCbLiZDcYa68316","name":"bash","argumentsDelta":"Read"}}}
+{"type":"assistant/chunk","seq":148,"time":1781834686623,"data":{"turn":1,"step":2,"chunk":{"type":"tool-call-delta","index":1,"id":"call_00_SkCP8dgN8aCbLiZDcYa68316","name":"bash","argumentsDelta":" greeting"}}}
+{"type":"assistant/chunk","seq":149,"time":1781834686655,"data":{"turn":1,"step":2,"chunk":{"type":"tool-call-delta","index":1,"id":"call_00_SkCP8dgN8aCbLiZDcYa68316","name":"bash","argumentsDelta":".txt"}}}
+{"type":"assistant/chunk","seq":150,"time":1781834686655,"data":{"turn":1,"step":2,"chunk":{"type":"tool-call-delta","index":1,"id":"call_00_SkCP8dgN8aCbLiZDcYa68316","name":"bash","argumentsDelta":" to"}}}
+{"type":"assistant/chunk","seq":151,"time":1781834686684,"data":{"turn":1,"step":2,"chunk":{"type":"tool-call-delta","index":1,"id":"call_00_SkCP8dgN8aCbLiZDcYa68316","name":"bash","argumentsDelta":" confirm"}}}
+{"type":"assistant/chunk","seq":152,"time":1781834686684,"data":{"turn":1,"step":2,"chunk":{"type":"tool-call-delta","index":1,"id":"call_00_SkCP8dgN8aCbLiZDcYa68316","name":"bash","argumentsDelta":"\""}}}
+{"type":"assistant/chunk","seq":153,"time":1781834686713,"data":{"turn":1,"step":2,"chunk":{"type":"tool-call-delta","index":1,"id":"call_00_SkCP8dgN8aCbLiZDcYa68316","name":"bash","argumentsDelta":"}"}}}
+{"type":"assistant/chunk","seq":154,"time":1781834686745,"data":{"turn":1,"step":2,"chunk":{"type":"block-end","index":0,"block":{"type":"reasoning","text":"Appended successfully. Now read the file."}}}}
+{"type":"assistant/chunk","seq":155,"time":1781834686745,"data":{"turn":1,"step":2,"chunk":{"type":"block-end","index":1,"block":{"type":"tool-call","id":"call_00_SkCP8dgN8aCbLiZDcYa68316","name":"bash","arguments":"{\"command\": \"cat greeting.txt\", \"description\": \"Read greeting.txt to confirm\"}"}}}}
+{"type":"assistant/chunk","seq":156,"time":1781834686745,"data":{"turn":1,"step":2,"chunk":{"type":"usage","usage":{"inputTokens":193,"outputTokens":74,"cacheReadTokens":896,"reasoningTokens":9}}}}
+{"type":"assistant/chunk","seq":157,"time":1781834686745,"data":{"turn":1,"step":2,"chunk":{"type":"finish","reason":{"kind":"tool-calls"}}}}
+{"type":"assistant/message","seq":158,"time":1781834686745,"data":{"turn":1,"step":2,"content":[{"type":"reasoning","text":"Appended successfully. Now read the file."},{"type":"tool-call","id":"call_00_SkCP8dgN8aCbLiZDcYa68316","name":"bash","arguments":"{\"command\": \"cat greeting.txt\", \"description\": \"Read greeting.txt to confirm\"}"}],"usage":{"inputTokens":193,"outputTokens":74,"cacheReadTokens":896,"reasoningTokens":9}}}
+{"type":"tool/call","seq":159,"time":1781834686745,"data":{"turn":1,"step":2,"callId":"call_00_SkCP8dgN8aCbLiZDcYa68316","name":"bash","arguments":"{\"command\": \"cat greeting.txt\", \"description\": \"Read greeting.txt to confirm\"}"}}
+{"type":"tool/result","seq":160,"time":1781834686758,"data":{"turn":1,"step":2,"callId":"call_00_SkCP8dgN8aCbLiZDcYa68316","content":[{"type":"text","text":"hello\nWORLD\n"}],"isError":false}}
+{"type":"step/end","seq":161,"time":1781834686758,"data":{"turn":1,"step":2}}
+{"type":"step/start","seq":162,"time":1781834686758,"data":{"turn":1,"step":3}}
+{"type":"assistant/chunk","seq":163,"time":1781834687255,"data":{"turn":1,"step":3,"chunk":{"type":"block-start","index":0,"blockType":"reasoning"}}}
+{"type":"assistant/chunk","seq":164,"time":1781834687255,"data":{"turn":1,"step":3,"chunk":{"type":"reasoning-delta","index":0,"text":"The"}}}
+{"type":"assistant/chunk","seq":165,"time":1781834687336,"data":{"turn":1,"step":3,"chunk":{"type":"reasoning-delta","index":0,"text":" file"}}}
+{"type":"assistant/chunk","seq":166,"time":1781834687365,"data":{"turn":1,"step":3,"chunk":{"type":"reasoning-delta","index":0,"text":" now"}}}
+{"type":"assistant/chunk","seq":167,"time":1781834687365,"data":{"turn":1,"step":3,"chunk":{"type":"reasoning-delta","index":0,"text":" has"}}}
+{"type":"assistant/chunk","seq":168,"time":1781834687365,"data":{"turn":1,"step":3,"chunk":{"type":"reasoning-delta","index":0,"text":" two"}}}
+{"type":"assistant/chunk","seq":169,"time":1781834687366,"data":{"turn":1,"step":3,"chunk":{"type":"reasoning-delta","index":0,"text":" lines"}}}
+{"type":"assistant/chunk","seq":170,"time":1781834687396,"data":{"turn":1,"step":3,"chunk":{"type":"reasoning-delta","index":0,"text":"."}}}
+{"type":"assistant/chunk","seq":171,"time":1781834687425,"data":{"turn":1,"step":3,"chunk":{"type":"reasoning-delta","index":0,"text":" I"}}}
+{"type":"assistant/chunk","seq":172,"time":1781834687426,"data":{"turn":1,"step":3,"chunk":{"type":"reasoning-delta","index":0,"text":"'ll"}}}
+{"type":"assistant/chunk","seq":173,"time":1781834687455,"data":{"turn":1,"step":3,"chunk":{"type":"reasoning-delta","index":0,"text":" reply"}}}
+{"type":"assistant/chunk","seq":174,"time":1781834687455,"data":{"turn":1,"step":3,"chunk":{"type":"reasoning-delta","index":0,"text":" with"}}}
+{"type":"assistant/chunk","seq":175,"time":1781834687455,"data":{"turn":1,"step":3,"chunk":{"type":"reasoning-delta","index":0,"text":" D"}}}
+{"type":"assistant/chunk","seq":176,"time":1781834687455,"data":{"turn":1,"step":3,"chunk":{"type":"reasoning-delta","index":0,"text":"ONE"}}}
+{"type":"assistant/chunk","seq":177,"time":1781834687455,"data":{"turn":1,"step":3,"chunk":{"type":"reasoning-delta","index":0,"text":"."}}}
+{"type":"assistant/chunk","seq":178,"time":1781834687484,"data":{"turn":1,"step":3,"chunk":{"type":"block-start","index":1,"blockType":"text"}}}
+{"type":"assistant/chunk","seq":179,"time":1781834687484,"data":{"turn":1,"step":3,"chunk":{"type":"text-delta","index":1,"text":"D"}}}
+{"type":"assistant/chunk","seq":180,"time":1781834687484,"data":{"turn":1,"step":3,"chunk":{"type":"text-delta","index":1,"text":"ONE"}}}
+{"type":"assistant/chunk","seq":181,"time":1781834687488,"data":{"turn":1,"step":3,"chunk":{"type":"block-end","index":0,"block":{"type":"reasoning","text":"The file now has two lines. I'll reply with DONE."}}}}
+{"type":"assistant/chunk","seq":182,"time":1781834687488,"data":{"turn":1,"step":3,"chunk":{"type":"block-end","index":1,"block":{"type":"text","text":"DONE"}}}}
+{"type":"assistant/chunk","seq":183,"time":1781834687488,"data":{"turn":1,"step":3,"chunk":{"type":"usage","usage":{"inputTokens":156,"outputTokens":17,"cacheReadTokens":1024,"reasoningTokens":14}}}}
+{"type":"assistant/chunk","seq":184,"time":1781834687488,"data":{"turn":1,"step":3,"chunk":{"type":"finish","reason":{"kind":"stop"}}}}
+{"type":"assistant/message","seq":185,"time":1781834687489,"data":{"turn":1,"step":3,"content":[{"type":"reasoning","text":"The file now has two lines. I'll reply with DONE."},{"type":"text","text":"DONE"}],"usage":{"inputTokens":156,"outputTokens":17,"cacheReadTokens":1024,"reasoningTokens":14}}}
+{"type":"step/end","seq":186,"time":1781834687489,"data":{"turn":1,"step":3}}
+{"type":"turn/end","seq":187,"time":1781834687489,"data":{"turn":1,"reason":{"kind":"completed"}}}
diff --git a/packages/core/agent-loop/src/loop.ts b/packages/core/agent-loop/src/loop.ts
index b5d7aca146..f0eb6fb88a 100644
--- a/packages/core/agent-loop/src/loop.ts
+++ b/packages/core/agent-loop/src/loop.ts
@@ -154,7 +154,7 @@ export interface LoopHandle {
  *       stream ctx.llm.stream(req)                    ⟵ waterfall llm/stream (raw chunks)
  *         session('assistant/chunk'); emit agent/stream-chunk
  *       msg = waterfall agent/step-result             ⟵ BEFORE the log append, so the
- *       session('assistant/message','usage')             session records what actually ran
+ *       session('assistant/message' {content, usage?})   session records what actually ran
  *       each tool-call in msg (sequential, abort-checked):
  *         session('tool/call'); ctx.tools.execute()   ⟵ waterfall tools/execute
  *         session('tool/result')
@@ -313,41 +313,24 @@ async function runTurn(ctx: Context, agent: ReactLoopAgent, handle: LoopHandle,
     return false
   }
 
-  // Record a step/turn failure exactly once: append the single `error` event
-  // (only while the turn is still open — see below), set the error reason, and
-  // emit agent/error (contained — trap: a throwing agent/error listener must not
-  // re-escape and strand the turn). Disposal and abort set `reason` directly
-  // without calling this (no `error` event for those — they are not failures).
+  // Record a step/turn failure exactly once: set the error reason (carrying the
+  // failing `step` — the durable failure lives entirely on turn/end.reason, there
+  // is no separate session error event) and emit agent/error (contained — trap: a
+  // throwing agent/error listener must not re-escape and strand the turn).
+  // Disposal and abort set `reason` directly without calling this (they are not
+  // failures).
   const failTurn = (err: CodedError): void => {
     if (errorReported) return
     errorReported = true
-    // Only append the session `error` INSIDE the turn (before turn/end). If the
-    // turn has already ended — the only way here is a throwing agent/turn-end
-    // listener after closeTurn(true) already appended turn/end — appending now
-    // would land the error AFTER the last turn/end, where the persistence
-    // backend treats it as a crash tail and drops it on resume (the turn-enclosure RFC). In
-    // that case report via agent/error + the logger only; the turn is balanced.
-    if (!turnEnded) {
-      // Set `reason` BEFORE the append: Session.append pushes the error event
-      // before notifying session/event listeners, so a throwing listener would
-      // otherwise leave `reason` unset (and closeTurn would record the wrong
-      // reason / the outer catch would skip closeTurn). The append is contained
-      // — the error event is already in the log either way; a throwing listener
-      // must not abort finalization.
-      reason = { kind: 'error', ...errorData(err) }
-      try {
-        session.append('error', { turn, step, ...errorData(err) })
-      } catch (appendError: unknown) {
-        ctx.logger.warn(`agent "${agent.id}": session/event listener threw on the error event at turn ${turn}: ${toError(appendError).message}`)
-      }
-    } else {
-      ctx.logger.warn(`agent "${agent.id}": agent/turn-end listener threw after turn ${turn} closed: ${err.message}`)
-    }
+    // Set `reason` here so the durable failure is captured before closeTurn
+    // appends turn/end. The step number rides along so the operational error's
+    // location survives in the durable log.
+    reason = { kind: 'error', step, ...errorData(err) }
     try {
       ctx.emit('agent/error', agent, turn, step, err)
     } catch {
-      // contained: the error is already logged; a throwing agent/error
-      // listener must not prevent the turn from closing.
+      // contained: the error is already captured on `reason`; a throwing
+      // agent/error listener must not prevent the turn from closing.
     }
   }
 
@@ -608,11 +591,14 @@ async function runStep(
   if (assembler.finish.kind === 'max-tokens') {
     let message: Message = withoutToolCalls(assembler.message())
     message = withoutToolCalls(await ctx.waterfall('agent/step-result', agent, turn, step, message, () => Promise.resolve(message)))
-    if (message.content.length > 0) {
-      session.append('assistant/message', { turn, step, content: message.content })
-    }
-    if (assembler.usage) {
-      session.append('usage', { turn, step, usage: assembler.usage })
+    // Fire the assistant/message when there is content OR usage: a max-tokens
+    // step can be cut off with empty content but still carry token accounting,
+    // and assistant/message is the only host for usage (there is no standalone
+    // usage event). An empty-content assistant/message is skipped by
+    // deriveMessages(), so hosting usage on it never injects a spurious assistant
+    // turn into derived history.
+    if (message.content.length > 0 || assembler.usage) {
+      session.append('assistant/message', { turn, step, content: message.content, ...(assembler.usage ? { usage: assembler.usage } : {}) })
     }
     return { hadToolCalls: false, finish: assembler.finish }
   }
@@ -623,10 +609,7 @@ async function runStep(
   let message: Message = assembler.message()
   message = await ctx.waterfall('agent/step-result', agent, turn, step, message, () => Promise.resolve(message))
 
-  session.append('assistant/message', { turn, step, content: message.content })
-  if (assembler.usage) {
-    session.append('usage', { turn, step, usage: assembler.usage })
-  }
+  session.append('assistant/message', { turn, step, content: message.content, ...(assembler.usage ? { usage: assembler.usage } : {}) })
 
   // --- Tool execution (sequential; parallel execution is a TODO) ---
   // ToolRegistry.execute converts tool failures (including aborts) into
diff --git a/packages/core/agent-loop/tests/coverage-edges.spec.ts b/packages/core/agent-loop/tests/coverage-edges.spec.ts
index 12036d7aec..3eefbf6986 100644
--- a/packages/core/agent-loop/tests/coverage-edges.spec.ts
+++ b/packages/core/agent-loop/tests/coverage-edges.spec.ts
@@ -213,9 +213,9 @@ describe('toError normalization', () => {
     expect(errors).toHaveLength(1)
     expect(errors[0]!.message).toBe('naked string error')
     // A non-Error throw is wrapped in a HarnessError with code UNKNOWN, so the
-    // session error event carries a routable code instead of degrading.
-    const errorEvent = agent.session.events.find(e => e.type === 'error')
-    expect(errorEvent?.type === 'error' && errorEvent.data.code).toBe('UNKNOWN')
+    // turn-end error reason carries a routable code instead of degrading.
+    const turnEnd = agent.session.events.find(e => e.type === 'turn/end')
+    expect(turnEnd?.type === 'turn/end' && turnEnd.data.reason.kind === 'error' && turnEnd.data.reason.code).toBe('UNKNOWN')
   })
 
   it('normalizes non-Error throws from agent/request waterfall via inline toError in runStep catch', async () => {
@@ -240,8 +240,8 @@ describe('toError normalization', () => {
     expect(errors).toHaveLength(1)
     // String() of { code: 500 } is '[object Object]'
     expect(errors[0]!.message).toBe('[object Object]')
-    const errorEvent = agent.session.events.find(e => e.type === 'error')
-    expect(errorEvent?.type === 'error' && errorEvent.data.code).toBe('UNKNOWN')
+    const turnEnd = agent.session.events.find(e => e.type === 'turn/end')
+    expect(turnEnd?.type === 'turn/end' && turnEnd.data.reason.kind === 'error' && turnEnd.data.reason.code).toBe('UNKNOWN')
   })
 })
 
@@ -268,11 +268,11 @@ describe('coded error data emission', () => {
     expect(errors).toHaveLength(1)
     expect(errors[0]!.message).toBe('server overloaded')
 
-    // session error event includes the code
-    const errorEvent = agent.session.events.find(e => e.type === 'error')
-    expect(errorEvent).toBeDefined()
-    if (errorEvent!.type === 'error') {
-      expect(errorEvent!.data.code).toBe('RATE_LIMIT')
+    // turn-end error reason includes the code
+    const turnEnd = agent.session.events.find(e => e.type === 'turn/end')
+    expect(turnEnd).toBeDefined()
+    if (turnEnd?.type === 'turn/end' && turnEnd.data.reason.kind === 'error') {
+      expect(turnEnd.data.reason.code).toBe('RATE_LIMIT')
     }
   })
 })
diff --git a/packages/core/agent-loop/tests/loop.spec.ts b/packages/core/agent-loop/tests/loop.spec.ts
index 18dbafeb04..8cb60e99b7 100644
--- a/packages/core/agent-loop/tests/loop.spec.ts
+++ b/packages/core/agent-loop/tests/loop.spec.ts
@@ -58,11 +58,13 @@ describe('agent loop', () => {
 
     const types = agent.session.events.map(e => e.type)
     // turn/start opens the turn, THEN the queued user message is recorded inside
-    // it (every event is turn-enclosed), then assembled message + usage.
+    // it (every event is turn-enclosed), then the assembled message (carrying the
+    // step's usage).
     expect(types[0]).toBe('turn/start')
     expect(types[1]).toBe('user/message')
     expect(types).toContain('assistant/message')
-    expect(types).toContain('usage')
+    const assistantMessage = agent.session.events.find(e => e.type === 'assistant/message')
+    expect(assistantMessage?.type === 'assistant/message' && assistantMessage.data.usage).toEqual({ inputTokens: 10, outputTokens: 'hello there'.length })
     expect(types.at(-1)).toBe('turn/end')
 
     // derived history: user + assistant
@@ -442,6 +444,47 @@ describe('agent loop', () => {
     expect(agent.session.events.some(e => e.type === 'tool/call')).toBe(false)
     expect(agent.session.deriveMessages()).toEqual([{ role: 'user', content: [{ type: 'text', text: 'go' }] }])
     expect(reasons).toEqual([{ kind: 'max-tokens' }])
+    // No-data-loss: a max-tokens step whose only content was a dropped tool call
+    // has EMPTY assistant content, but its usage must still be represented. It
+    // rides on an (empty-content) assistant/message — there is no standalone
+    // usage event — and that empty message is skipped by deriveMessages(), so
+    // the derived history above is NOT corrupted by a spurious assistant turn.
+    const assistantMessage = agent.session.events.find(e => e.type === 'assistant/message')
+    expect(assistantMessage?.type === 'assistant/message' && assistantMessage.data).toEqual({
+      turn: 1, step: 1, content: [], usage: { inputTokens: 10, outputTokens: 5 },
+    })
+  })
+
+  it('appends no assistant/message for a max-tokens step with empty content and no usage', async () => {
+    // A max-tokens step truncated to a dropped tool call AND with no usage chunk
+    // has nothing to record: empty content and no accounting → no assistant/message
+    // (the empty-content host exists only to carry usage). The turn still ends
+    // max-tokens.
+    const callId = CallId('c1')
+    const adapter = new MockAdapter([[
+      { type: 'block-start', index: 0, blockType: 'tool-call' },
+      { type: 'tool-call-delta', index: 0, id: callId, name: 'echo', argumentsDelta: '{"text":"x"}' },
+      { type: 'block-end', index: 0, block: { type: 'tool-call', id: callId, name: 'echo', arguments: '{"text":"x"}' } },
+      { type: 'finish', reason: { kind: 'max-tokens' } },
+    ]])
+    const ctx = await harness(adapter)
+    ctx.tools.register(defineTool({
+      name: 'echo',
+      description: '',
+      parameters: { text: { type: 'string' } },
+      async execute() { return [{ type: 'text', text: 'should not run' }] },
+    }))
+    const agent = ctx.agentLoop.create(AgentId('a1'), { model: 'mock' })
+
+    const reasons: TurnEndReason[] = []
+    ctx.on('agent/turn-end', (_agent, _turn, reason) => void reasons.push(reason))
+
+    send(agent, 'go')
+    await waitForIdle(ctx, agent)
+
+    expect(reasons).toEqual([{ kind: 'max-tokens' }])
+    expect(agent.session.events.some(e => e.type === 'assistant/message')).toBe(false)
+    expect(agent.session.deriveMessages()).toEqual([{ role: 'user', content: [{ type: 'text', text: 'go' }] }])
   })
 
   it('keeps safe max-tokens assistant content while dropping truncated tool calls', async () => {
@@ -563,7 +606,10 @@ describe('agent loop', () => {
     expect(errors).toHaveLength(1)
     expect(errors[0]!.message).toContain('script exhausted')
     expect(reasons[0]).toMatchObject({ kind: 'error' })
-    expect(agent.session.events.some(e => e.type === 'error')).toBe(true)
+    // The durable failure lives entirely on turn/end.reason (with the failing
+    // step), not a standalone error event.
+    const turnEnd = agent.session.events.find(e => e.type === 'turn/end')
+    expect(turnEnd?.type === 'turn/end' && turnEnd.data.reason).toMatchObject({ kind: 'error', step: 1 })
   })
 
   it('disposing the loop fiber mid-turn stops the loop (HMR safety)', async () => {
diff --git a/packages/core/agent-loop/tests/review-fixes.spec.ts b/packages/core/agent-loop/tests/review-fixes.spec.ts
index 0d2441c7db..3695904322 100644
--- a/packages/core/agent-loop/tests/review-fixes.spec.ts
+++ b/packages/core/agent-loop/tests/review-fixes.spec.ts
@@ -492,11 +492,13 @@ describe('HIGH: a finish-error stream chunk ends the turn as error, not complete
     send(agent, 'go')
     await waitForIdle(ctx, agent)
 
-    expect(reasons).toEqual([{ kind: 'error', message: 'provider 401', code: 'AUTH' }])
+    expect(reasons).toEqual([{ kind: 'error', step: 1, message: 'provider 401', code: 'AUTH' }])
 
     const events = [...agent.session.events]
-    expect(events.some(event => event.type === 'error'
-      && event.data.message === 'provider 401' && event.data.code === 'AUTH')).toBe(true)
+    // The durable failure lives on turn/end.reason (with the failing step), not
+    // a standalone error event.
+    const turnEnd = events.find(event => event.type === 'turn/end')
+    expect(turnEnd?.type === 'turn/end' && turnEnd.data.reason).toEqual({ kind: 'error', step: 1, message: 'provider 401', code: 'AUTH' })
     // Crucially: no assistant/message was logged for the failed step.
     expect(events.some(event => event.type === 'assistant/message')).toBe(false)
   })
@@ -515,7 +517,7 @@ describe('HIGH: a finish-error stream chunk ends the turn as error, not complete
     send(agent, 'go')
     await waitForIdle(ctx, agent)
 
-    expect(reasons).toEqual([{ kind: 'error', message: 'model stream aborted', code: 'ABORTED' }])
+    expect(reasons).toEqual([{ kind: 'error', step: 1, message: 'model stream aborted', code: 'ABORTED' }])
     expect([...agent.session.events].some(event => event.type === 'assistant/message')).toBe(false)
   })
 
@@ -533,7 +535,7 @@ describe('HIGH: a finish-error stream chunk ends the turn as error, not complete
     send(agent, 'go')
     await waitForIdle(ctx, agent)
 
-    expect(reasons).toEqual([{ kind: 'error', message: 'codeless failure' }])
+    expect(reasons).toEqual([{ kind: 'error', step: 1, message: 'codeless failure' }])
   })
 })
 
@@ -592,7 +594,7 @@ describe('P1-5: a started turn (and any open step) is always closed on a boundar
       turnEnd: e.filter(x => x.type === 'turn/end').length,
       stepStart: e.filter(x => x.type === 'step/start').length,
       stepEnd: e.filter(x => x.type === 'step/end').length,
-      errors: e.filter(x => x.type === 'error').length,
+      errors: e.filter(x => x.type === 'turn/end' && x.data.reason.kind === 'error').length,
       lastTurnEnd: e.findLast(x => x.type === 'turn/end'),
     }
   }
@@ -611,10 +613,10 @@ describe('P1-5: a started turn (and any open step) is always closed on a boundar
     await waitForIdle(ctx, agent)
 
     const c = boundaryCounts(agent)
-    // turn opened and closed; no step ran; exactly one error logged + emitted.
+    // turn opened and closed; no step ran; exactly one error turn-end + emitted.
     expect(c).toMatchObject({ turnStart: 1, turnEnd: 1, stepStart: 0, stepEnd: 0, errors: 1 })
     expect(errors.map(e => e.message)).toEqual(['boom turn-start'])
-    expect(c.lastTurnEnd?.type === 'turn/end' && c.lastTurnEnd.data.reason).toEqual({ kind: 'error', message: 'boom turn-start' })
+    expect(c.lastTurnEnd?.type === 'turn/end' && c.lastTurnEnd.data.reason).toEqual({ kind: 'error', step: 0, message: 'boom turn-start' })
     // model was never called (we threw before the step's request).
     expect(adapter.requests).toHaveLength(0)
   })
@@ -664,7 +666,7 @@ describe('P1-5: a started turn (and any open step) is always closed on a boundar
     expect(c.turnStart).toBe(1)
     expect(c.turnEnd).toBe(1)
     expect(c.stepStart).toBe(c.stepEnd)
-    expect(c.lastTurnEnd?.type === 'turn/end' && c.lastTurnEnd.data.reason).toMatchObject({ kind: 'error', message: 'provider 500' })
+    expect(c.lastTurnEnd?.type === 'turn/end' && c.lastTurnEnd.data.reason).toMatchObject({ kind: 'error', step: 1, message: 'provider 500' })
 
     // loop survives: a second turn runs to completion (invariants oracle would
     // throw on its turn/start if turn 1 had been left open).
@@ -701,8 +703,8 @@ describe('P1-5: a started turn (and any open step) is always closed on a boundar
     expect(turnStarts).toBe(1)
     expect(turnEnds).toBe(1) // balanced — the turn was closed despite disposal
     expect(reasons).toEqual([{ kind: 'disposed' }])
-    // no error event: disposal is not a failure.
-    expect(e.some(x => x.type === 'error')).toBe(false)
+    // no error reason: disposal is not a failure.
+    expect(e.some(x => x.type === 'turn/end' && x.data.reason.kind === 'error')).toBe(false)
   })
 
   it('preserves reason disposed when the turn-end emit throws during disposal (outer-catch disposed branch)', async () => {
@@ -741,9 +743,10 @@ describe('P1-5: a started turn (and any open step) is always closed on a boundar
     expect(e.filter(x => x.type === 'turn/start')).toHaveLength(1)
     const turnEnd = e.findLast(x => x.type === 'turn/end')
     expect(turnEnd?.type === 'turn/end' && turnEnd.data.reason).toEqual({ kind: 'disposed' })
-    // The throwing turn-end listener is contained: no error event is logged and
-    // no agent/error is emitted (disposal is not a failure; the throw is swallowed).
-    expect(e.some(x => x.type === 'error')).toBe(false)
+    // The throwing turn-end listener is contained: the turn/end carries the
+    // disposed reason (not an error) and no agent/error is emitted (disposal is
+    // not a failure; the throw is swallowed).
+    expect(e.some(x => x.type === 'turn/end' && x.data.reason.kind === 'error')).toBe(false)
     expect(errorEmits).toHaveLength(0)
   })
 
@@ -840,11 +843,11 @@ describe('P1-5: a started turn (and any open step) is always closed on a boundar
     await waitForIdle(ctx, agent)
 
     const c = boundaryCounts(agent)
-    // step opened and closed; exactly one error; turn balanced; turn ends error.
+    // step opened and closed; exactly one error turn-end; turn balanced.
     expect(c).toMatchObject({ turnStart: 1, turnEnd: 1, stepStart: 1, stepEnd: 1, errors: 1 })
     expect(errors.map(e => e.message)).toEqual(['boom step-end'])
     expect(c.lastTurnEnd?.type === 'turn/end' && c.lastTurnEnd.data.reason)
-      .toEqual({ kind: 'error', message: 'boom step-end' })
+      .toEqual({ kind: 'error', step: 1, message: 'boom step-end' })
 
     // step/end precedes turn/end (ordering contract)
     const e = [...agent.session.events]
@@ -882,12 +885,12 @@ describe('P1-5: a started turn (and any open step) is always closed on a boundar
     await waitForIdle(ctx, agent)
 
     const c = boundaryCounts(agent)
-    // exactly one error event + one agent/error emit, despite two failTurn calls.
+    // exactly one error turn-end + one agent/error emit, despite two failTurn calls.
     expect(c.errors).toBe(1)
     expect(errors.map(e => e.message)).toEqual(['provider down'])
     expect(c.turnStart).toBe(1)
     expect(c.turnEnd).toBe(1) // single turn/end, balanced
-    expect(c.lastTurnEnd?.type === 'turn/end' && c.lastTurnEnd.data.reason).toMatchObject({ kind: 'error', message: 'provider down' })
+    expect(c.lastTurnEnd?.type === 'turn/end' && c.lastTurnEnd.data.reason).toMatchObject({ kind: 'error', step: 1, message: 'provider down' })
 
     // loop survives the compound failure.
     send(agent, 'again')
@@ -895,42 +898,6 @@ describe('P1-5: a started turn (and any open step) is always closed on a boundar
     expect(boundaryCounts(agent).turnEnd).toBe(2)
   })
 
-  it('a throwing session/event listener on the error event still closes the turn (finalizer containment)', async () => {
-    // failTurn appends the `error` event; Session.append pushes it BEFORE
-    // notifying session/event listeners, so a throwing listener leaves `error`
-    // in the log but must NOT abort finalization — `reason` is set before the
-    // append and the throw is contained, so closeTurn(false) still runs and
-    // turn/end is appended (the turn is balanced, not left open).
-    // Plain harness (no invariants oracle): the throwing listener is itself a
-    // session/event subscriber. A finish-error drives the boundary-error path.
-    const errorStream: StreamChunk[] = [{ type: 'finish', reason: { kind: 'error', message: 'provider down' } }]
-    const adapter = new MockAdapter([errorStream, textResponse('turn 2 ok')])
-    const ctx = await harness(adapter)
-    const agent = ctx.agentLoop.create(AgentId('a-errthrow'), { model: 'mock' })
-
-    let threw = false
-    ctx.on('session/event', (_s, event) => {
-      if (!threw && event.type === 'error') { threw = true; throw new Error('boom error-event listener') }
-    })
-
-    send(agent, 'go')
-    await waitForIdle(ctx, agent)
-
-    const e = [...agent.session.events]
-    // The error event is in the log (pushed before the listener threw)…
-    expect(e.some(x => x.type === 'error')).toBe(true)
-    // …and the turn was still closed with the error reason (finalization did not
-    // abort): the last event is turn/end carrying the error reason.
-    const last = e.at(-1)
-    expect(last?.type).toBe('turn/end')
-    expect(last?.type === 'turn/end' && last.data.reason).toMatchObject({ kind: 'error', message: 'provider down' })
-
-    // loop survives: a second turn runs normally.
-    send(agent, 'again')
-    await waitForIdle(ctx, agent)
-    expect(adapter.requests).toHaveLength(2)
-  })
-
   it('a throwing session/event listener on step/end during finalization still appends turn/end', async () => {
     // A throwing agent/step-start listener drives the outer catch, which calls
     // closeStep() during finalization. closeStep appends step/end; a
diff --git a/packages/core/session/README.md b/packages/core/session/README.md
index 8c2f62cc1b..74c543fc07 100644
--- a/packages/core/session/README.md
+++ b/packages/core/session/README.md
@@ -45,7 +45,7 @@ Plain class (not a Cordis Service). Create via `ctx.sessions.create()`.
 
 ### Session event vocabulary (`types.ts`)
 
-The append-only log: `turn/start`, `turn/end`, `step/start`, `step/end`, `user/message`, `assistant/message`, `assistant/chunk`, `tool/call`, `tool/result`, `steering/message`, `context/message`, `usage`, `error`.
+The append-only log: `turn/start`, `turn/end`, `step/start`, `step/end`, `user/message`, `assistant/message`, `assistant/chunk`, `tool/call`, `tool/result`, `steering/message`, `context/message`. Token usage rides on `assistant/message.usage`; an operational error's step is on `turn/end.reason` for `kind: 'error'`.
 
 Merge-extensible via `SessionEventMap` — a compaction plugin adds `compaction/marker`, etc.
 
diff --git a/packages/core/session/src/index.ts b/packages/core/session/src/index.ts
index f86916d37f..55eeb523a5 100644
--- a/packages/core/session/src/index.ts
+++ b/packages/core/session/src/index.ts
@@ -160,7 +160,10 @@ export class Session {
    *
    * - `user/message` → user message
    * - `assistant/message` → assistant message (chunks are skipped — they are
-   *   replay/UI data; the assembled message is authoritative for history)
+   *   replay/UI data; the assembled message is authoritative for history). An
+   *   EMPTY-content assistant/message is skipped: a max-tokens step cut off with
+   *   no content still records an assistant/message to host its `usage`, but a
+   *   content-less assistant turn must not enter the provider transcript.
    * - `tool/result` → user message carrying a tool-result block
    * - `context/message` / `steering/message` → tagged synthetic user messages
    *   at their chronological position
@@ -186,6 +189,10 @@ export class Session {
           break
         }
         case 'assistant/message': {
+          // Skip an empty-content assistant/message: it exists only to host a
+          // max-tokens step's usage and must not inject a content-less assistant
+          // turn into the provider transcript.
+          if (event.data.content.length === 0) break
           messages.push({ role: 'assistant', content: structuredClone(event.data.content) })
           break
         }
diff --git a/packages/core/session/src/types.ts b/packages/core/session/src/types.ts
index 4b7334b207..ab9159377d 100644
--- a/packages/core/session/src/types.ts
+++ b/packages/core/session/src/types.ts
@@ -88,7 +88,13 @@ export type TurnTrigger = TurnTriggerMap[keyof TurnTriggerMap]
 export interface TurnEndReasonMap {
   completed: { kind: 'completed' }
   aborted: { kind: 'aborted'; reason?: string }
-  error: { kind: 'error'; message: string; code?: string }
+  /**
+   * The turn failed: a step threw or the model reported a failure. `step` is the
+   * step number the failure occurred on (the operational error's location — the
+   * single durable record of an in-turn failure; live diagnostics also fire via
+   * `agent/error`). `code` is the error's code when one was attached.
+   */
+  error: { kind: 'error'; step: number; message: string; code?: string }
   disposed: { kind: 'disposed' }
   'max-tokens': { kind: 'max-tokens' }
   /**
@@ -140,14 +146,17 @@ export interface SessionEventMap {
   'context/message': { content: ContentBlock[]; source: MessageSource }
   /** Raw stream chunk — token-level replay fidelity. */
   'assistant/chunk': { turn: number; step: number; chunk: StreamChunk }
-  /** Assembled assistant message for one step (derived history uses this). */
-  'assistant/message': { turn: number; step: number; content: ContentBlock[] }
+  /**
+   * Assembled assistant message for one step (derived history uses this).
+   * Carries the step's `usage` when the adapter reported token accounting, so
+   * the model output and its accounting travel together (there is no separate
+   * usage record). `usage` is absent when the adapter reported none.
+   */
+  'assistant/message': { turn: number; step: number; content: ContentBlock[]; usage?: TokenUsage }
   'tool/call': { turn: number; step: number; callId: CallId; name: string; arguments: string }
   'tool/result': { turn: number; step: number; callId: CallId; content: ContentBlock[]; isError: boolean; error?: { name: string; code: string } }
   /** Steering content injected between steps of a running turn. */
   'steering/message': { turn: number; content: ContentBlock[]; source: MessageSource }
-  'usage': { turn: number; step: number; usage: TokenUsage }
-  'error': { turn: number; step: number; message: string; code?: string }
 }
 
 export type SessionEventType = keyof SessionEventMap
diff --git a/packages/core/session/tests/properties.spec.ts b/packages/core/session/tests/properties.spec.ts
index 4d0b1b1e71..42149515f2 100644
--- a/packages/core/session/tests/properties.spec.ts
+++ b/packages/core/session/tests/properties.spec.ts
@@ -24,6 +24,7 @@ const textContentArb = fc.array(
 const messageEventArb: fc.Arbitrary<Appendable> = fc.oneof(
   textContentArb.map((content): Appendable => ({ type: 'user/message', data: { content, source: { kind: 'user' } } })),
   textContentArb.map((content): Appendable => ({ type: 'assistant/message', data: { turn: 1, step: 1, content } })),
+  textContentArb.map((content): Appendable => ({ type: 'assistant/message', data: { turn: 1, step: 1, content, usage: { inputTokens: 1, outputTokens: 1 } } })),
   fc.record({ id: fc.string({ minLength: 1 }), content: textContentArb, isError: fc.boolean() })
     .map((r): Appendable => ({ type: 'tool/result', data: { turn: 1, step: 1, callId: CallId(r.id), content: r.content, isError: r.isError } })),
 )
@@ -35,8 +36,6 @@ const nonMessageEventArb: fc.Arbitrary<Appendable> = fc.oneof(
   fc.constant<Appendable>({ type: 'step/start', data: { turn: 1, step: 1 } }),
   fc.constant<Appendable>({ type: 'step/end', data: { turn: 1, step: 1 } }),
   fc.string().map((text): Appendable => ({ type: 'assistant/chunk', data: { turn: 1, step: 1, chunk: { type: 'text-delta', index: 0, text } } })),
-  fc.constant<Appendable>({ type: 'usage', data: { turn: 1, step: 1, usage: { inputTokens: 1, outputTokens: 1 } } }),
-  fc.constant<Appendable>({ type: 'error', data: { turn: 1, step: 1, message: 'x' } }),
 )
 
 const anyEventArb = fc.oneof(messageEventArb, nonMessageEventArb)
diff --git a/packages/support/invariants/src/index.ts b/packages/support/invariants/src/index.ts
index 9b4b4ddfa0..804093380f 100644
--- a/packages/support/invariants/src/index.ts
+++ b/packages/support/invariants/src/index.ts
@@ -192,8 +192,8 @@ function checkEvent(trace: SessionTrace, event: SessionEvent): void {
     // turn as its commit/replay boundary (the JSONL backend treats anything
     // after the last turn/end as a crash tail), so a bare event between turns is
     // silently dropped on reload. The loop records queued user messages after
-    // turn/start, an idle agent.inject() wraps its context/message in a one-shot
-    // turn, and usage/error are only appended inside an open turn. A `default`
+    // turn/start, and an idle agent.inject() wraps its context/message in a
+    // one-shot turn. A `default`
     // (not an enumerated list) is deliberate: SessionEventMap is
     // merge-extensible, so a PLUGIN-added event type appended while idle must
     // also fail here rather than fall through and be dropped on resume.
diff --git a/packages/support/invariants/tests/invariants.spec.ts b/packages/support/invariants/tests/invariants.spec.ts
index f1bf2a2e74..b9add0479f 100644
--- a/packages/support/invariants/tests/invariants.spec.ts
+++ b/packages/support/invariants/tests/invariants.spec.ts
@@ -95,14 +95,12 @@ describe('session-log invariants', () => {
       .toThrow(/outside any open turn/)
   })
 
-  it('rejects usage/error and plugin-added events appended outside any open turn', async () => {
+  it('rejects steering and plugin-added events appended outside any open turn', async () => {
     const { ctx } = await setup({ freeze: false })
     const session = ctx.sessions.create()
-    // usage and error are turn-scoped: outside a turn they would land past the
+    // steering/message is turn-scoped: outside a turn it would land past the
     // commit boundary and be dropped on resume (the turn-enclosure RFC).
-    expect(() => session.append('usage', { turn: 1, step: 1, usage: { inputTokens: 1, outputTokens: 1 } }))
-      .toThrow(/outside any open turn/)
-    expect(() => session.append('error', { turn: 1, step: 1, message: 'boom' }))
+    expect(() => session.append('steering/message', { turn: 1, content: [{ type: 'text', text: 'go' }], source: { kind: 'user' } }))
       .toThrow(/outside any open turn/)
     // A PLUGIN-added (merge-extensible) event type is caught by the default too.
     expect(() => session.append('compaction/marker' as never, { foo: 'bar' } as never))
@@ -156,7 +154,7 @@ describe('session-log invariants', () => {
       session.append('step/start', { turn: 1, step: 1 })
       session.append('tool/call', { turn: 1, step: 1, callId: CallId('c1'), name: 'echo', arguments: '{}' })
       session.append('step/end', { turn: 1, step: 1 })
-      session.append('turn/end', { turn: 1, reason: { kind: 'error', message: 'boom' } })
+      session.append('turn/end', { turn: 1, reason: { kind: 'error', step: 1, message: 'boom' } })
     }).not.toThrow()
   })
 
diff --git a/packages/support/llm-replay/tests/llm-replay.spec.ts b/packages/support/llm-replay/tests/llm-replay.spec.ts
index f16e988035..556fb5abff 100644
--- a/packages/support/llm-replay/tests/llm-replay.spec.ts
+++ b/packages/support/llm-replay/tests/llm-replay.spec.ts
@@ -130,11 +130,11 @@ describe('deriveReplayScript', () => {
   })
 
   it('throws on a group that lacks a terminal finish chunk (a thrown stream)', () => {
-    // A thrown stream(): prefix chunks logged, then error/turn/end, NO finish.
+    // A thrown stream(): prefix chunks logged, then turn/end (error reason), NO finish.
     const events: SessionEvent[] = [
       chunkEvent(1, 1, 1, { type: 'block-start', index: 0, blockType: 'text' }),
       chunkEvent(2, 1, 1, { type: 'text-delta', index: 0, text: 'par' }),
-      { type: 'turn/end', seq: 3, time: 0, data: { turn: 1, reason: { kind: 'error', message: 'x' } } },
+      { type: 'turn/end', seq: 3, time: 0, data: { turn: 1, reason: { kind: 'error', step: 1, message: 'x' } } },
     ]
     expect(() => deriveReplayScript(events)).toThrow(/without a finish chunk.*replay\.override\.json/s)
   })
diff --git a/packages/ui/acp/src/index.ts b/packages/ui/acp/src/index.ts
index d7662cfebf..ec79e97443 100644
--- a/packages/ui/acp/src/index.ts
+++ b/packages/ui/acp/src/index.ts
@@ -775,7 +775,7 @@ function validateMcpServers(params: { mcpServers?: unknown[] }): void {
  * generic fallback (title = tool name, raw args as input) when no registry is
  * available (e.g. pure translator tests).
  *
- * Other event types (turn/step boundaries, context/message, usage, …) produce
+ * Other event types (turn/step boundaries, context/message, …) produce
  * no client update.
  */
 export function streamSessionEventUpdate(
@@ -873,7 +873,7 @@ export function streamSessionEventUpdate(
       })
       return
     }
-    // turn/step boundaries, context/message, steering, usage, error,
+    // turn/step boundaries, context/message, steering,
     // assistant/message — no direct ACP client update.
     default:
       return
diff --git a/packages/ui/acp/tests/codec.spec.ts b/packages/ui/acp/tests/codec.spec.ts
index 38a7a6cb41..9d82fe7533 100644
--- a/packages/ui/acp/tests/codec.spec.ts
+++ b/packages/ui/acp/tests/codec.spec.ts
@@ -16,7 +16,7 @@ describe('turnEndToStopReason', () => {
     expect(turnEndToStopReason({ kind: 'max-tokens' })).toBe('max_tokens')
     expect(turnEndToStopReason({ kind: 'aborted', reason: 'x' })).toBe('cancelled')
     expect(turnEndToStopReason({ kind: 'disposed' })).toBe('cancelled')
-    expect(turnEndToStopReason({ kind: 'error', message: 'boom' })).toBe('end_turn')
+    expect(turnEndToStopReason({ kind: 'error', step: 1, message: 'boom' })).toBe('end_turn')
   })
 
   it('falls back to end_turn for an unknown (merge-extensible) future kind', () => {
diff --git a/packages/ui/acp/tests/stream-update.spec.ts b/packages/ui/acp/tests/stream-update.spec.ts
index 737ad3804a..30cbd17c40 100644
--- a/packages/ui/acp/tests/stream-update.spec.ts
+++ b/packages/ui/acp/tests/stream-update.spec.ts
@@ -116,7 +116,7 @@ describe('streamSessionEventUpdate', () => {
   it('produces no update for boundary/other event types', () => {
     expect(updatesFor(evt('turn/start', { turn: 1, trigger: { kind: 'message', source: { kind: 'user' } } }))).toEqual([])
     expect(updatesFor(evt('turn/end', { turn: 1, reason: { kind: 'completed' } }))).toEqual([])
-    expect(updatesFor(evt('usage', { turn: 1, step: 1, usage: { inputTokens: 1, outputTokens: 1 } }))).toEqual([])
+    expect(updatesFor(evt('step/start', { turn: 1, step: 1 }))).toEqual([])
   })
 })