simplify(session): fold trace-only usage/error events into load-bearing events

The session event vocabulary carried two standalone trace-only events that
were not load-bearing as separate records. Fold their facts into nearby
load-bearing events and delete the standalone variants.

- Token usage now rides on `assistant/message` as an optional `usage` field —
  the assembled model output and its accounting travel together. The loop folds
  `assembler.usage` onto the append instead of emitting a separate `usage`
  event.
- The max-tokens path is the no-data-loss host: 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 }`. `deriveMessages()` skips empty-content assistant messages, so the
  usage host never injects a spurious content-less assistant turn into the
  provider transcript. A step with neither content nor usage appends nothing.
- An operational error's step number now rides on `turn/end.reason` for
  `kind: 'error'` (`{ kind: 'error', step, message, code? }`) — the durable
  turn outcome ACP and resume already consume. `failTurn` sets the reason
  directly (no separate session `error` event). `agent/error` + logging are
  unchanged for live diagnostics.
- No format-version bump: pre-release, no persisted data, so per the format
  policy there is nothing to migrate or reject (the RFC's "refresh the format
  version" criterion over-reached). `version` stays 1.
- ACP fixtures + goldens re-recorded (keyless replay): dropped standalone
  usage/error lines, usage folded onto assistant/message, error step on
  turn/end.reason.

RFC moved proposed -> implemented with an implementation note recording the two
scope refinements.

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).
 

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`
 

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
 

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' }
   /**

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
 

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.

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.