The session event vocabulary includes first-class events that are not part of replayable conversation history and have little or no production consumption. `usage` is already present as a model stream chunk before the loop also appends a separate `usage` event. `error` duplicates the `turn/end { kind: 'error', message, code }` reason for loop failures; ACP settlement reads the turn-end reason, ACP rendering ignores the `error` event, and `deriveMessages()` skips it.
These events make the canonical transcript look more useful as telemetry than it currently is. They add event variants, invariants, tests, snapshots, and persistence cases, but they are not load-bearing as separate records. The facts they carry can still be useful: token usage should remain available for accounting, and an error's step number should not silently disappear. The simplification is to fold those facts into nearby events consumers already must understand, not to record less information.
- Fold successful-step usage into the matching `assistant/message`, e.g. `assistant/message { turn, step, content, usage? }`, so the assembled model output and its accounting travel together.
- For a failed or aborted step that has usage but no `assistant/message`, carry the usage on the terminal turn reason or another load-bearing failure record in the same turn. The implementing design must prove no usage chunk that is currently persisted becomes unrepresented.
- Fold the step number from the standalone `error` event into `turn/end.reason` for `kind: 'error'`, e.g. `{ kind: 'error', step, message, code? }`. `turn/end` is the durable turn outcome ACP and resume already consume.
- Keep `agent/error` and logging for live diagnostics; do not add a second session-log error record after `turn/end`.
If analytics become real, add a projection helper or a dedicated telemetry store with its own retention policy. The user conversation log should contain what is needed to render, resume, audit, and account for the interaction without requiring consumers to reconcile duplicate trace rows.
- The loop records durable failures through `turn/end { kind: 'error', step, message, code? }` or an equivalent no-information-loss shape and reports live diagnostics through `agent/error`.
- Recorded fixtures are refreshed for the new event shape; the session format version stays pinned at `0` (unstable/pre-release) and backends reject any non-`0` stored log per the pre-release format policy.
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.
- **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.
**Format version.** The persisted `SessionEventMap` shape changed (usage folded onto `assistant/message`, standalone `usage`/`error` removed, `step` on `turn/end.reason.error`), so per the AGENTS.md "bump the version and reject — don't migrate" policy a backend must reject any non-current log. The version literal is centralized in an exported `SESSION_FORMAT_VERSION` constant (read by both write sites and the coordinator's load-time check). While the harness is unreleased the on-disk format is pre-release/unstable, so the constant stays **`0`**: a breaking format change is absorbed at v0 (no monotonic bump until the first tagged release, when a specific format boundary becomes worth distinguishing) and old logs at any other version are rejected on load — there is no v0→vN migration (no persisted user data exists). `turn/end.reason.error.step` is required for newly-written logs.
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.
