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Tianyi Cui b0422f2a50 fix review findings: bump session format version + restore late turn-end warn

Codex review of the trace-event fold found two merge-blockers.

Blocker #1 — format version. Folding usage onto assistant/message and removing
the standalone usage/error events changed the persisted SessionEventMap shape,
which per the AGENTS.md "bump the version and reject — don't migrate" policy
requires a backend to reject any non-current log. Centralize the version in an
exported SESSION_FORMAT_VERSION constant (dsh-session), read by both write sites
(Session constructor default, SessionStore.prepare header) and the coordinator's
load-time assertVersion check. The constant is pinned at 0: while unreleased the
on-disk format is unstable/pre-release, so breaking shape churn is absorbed at v0
(no monotonic bump until the first tagged release) and any non-0 log is rejected
on load — no migration. Update every test/fixture/doc that stamps a
currently-written header to the constant, bump the ACP snapshot fixture + golden
headers to v0, and keep the version-rejection test meaningful by switching its
bad value to a clearly non-current 99. AGENTS.md documents both the monotonic
(SQLite SCHEMA_VERSION) and pinned-0 (session log) pre-release stances.

Blocker #2 — restore the late turn-end warn. failTurn now sets the error reason
only while the turn is still open; once turn/end is appended (a throwing
agent/turn-end listener after closeTurn) the reason can no longer reach the
durable log, so the late throw is logged via ctx.logger.warn instead of
vanishing into a futile post-close assignment. A regression test asserts the
warn fires.

Also guard the normal-step assistant/message append with the same
content-or-usage condition as the max-tokens branch (a content-less, usage-less
step records no trace-only row), with a covering test.

2026-06-21 11:08:10 +08:00

5.2 KiB

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RFC: Fold trace-only session facts into load-bearing events

Status: implemented (proposed and accepted 2026-06-20)

Problem

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.

Proposal

Remove standalone trace-only events only where their information can be preserved without a parallel record:

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.

Acceptance criteria

SessionEventMap drops standalone usage and error only after their fields are represented on load-bearing session events.
The loop no longer appends a separate usage event for a usage chunk.
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.
ACP snapshots and persistence tests stop asserting trace-only lines.
Documentation explains exactly where token usage and operational errors are observed.
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.

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 one scope refinement (per AGENTS.md "RFCs are proposals, not golden truth"):

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.

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