29 lines
2.6 KiB
Markdown
29 lines
2.6 KiB
Markdown
# RFC: Drop durable step boundary events
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Status: proposed
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## Problem
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The session log stores `step/start` and `step/end` events even though every step-scoped event already carries `{ turn, step }`: assistant chunks, assistant messages, tool calls, tool results, usage, and errors. `deriveMessages()` ignores step boundaries, ACP ignores them for UI, and the main consumers are invariants, tests, snapshot goldens, and crash repair.
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The boundary events make the log more ceremonial than informative. The loop tracks open steps solely to close them, repair synthesizes `step/end` when a crash leaves a step open, invariants track a second nesting stack inside the turn, and snapshots carry lines that do not affect replayed message history. A model request that crashes before producing any step-scoped event is the only information represented by a bare `step/start`, and that case has no useful resumable content.
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## Proposal
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Make the turn the only durable boundary. Remove `step/start` and `step/end` from `SessionEventMap`; keep the numeric `step` field on events that need grouping. The loop increments the step counter and records step-scoped events with that number, but it no longer appends open/close boundary events. Consumers infer step groups from contiguous events sharing `(turn, step)`.
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The invariants plugin should enforce that step-scoped events have valid positive step numbers within an open turn, not that separate boundary records surround them. Crash repair should not synthesize `step/end`; if [interrupted turns are truncated](2026-06-20-truncate-interrupted-turns.md), the repair path disappears entirely.
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## Acceptance criteria
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- `SessionEventMap` no longer includes `step/start` or `step/end`.
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- The loop has no `closeStep()` finalization path.
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- ACP snapshots and persistence contract fixtures stop expecting step-boundary lines.
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- `deriveMessages()` and replay derive the same message history from step-scoped events.
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- The [event taxonomy docs](../../architecture.md) describe turns as the durable boundary and steps as a field on step-scoped records.
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- The session format version and recorded fixtures are refreshed; non-current stored logs are rejected per the pre-release format policy.
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## What we give up
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The log no longer records "a model request started but produced no event before the process died" as a durable fact. That is acceptable: there is no assistant content, tool call, usage, or error to replay from that empty request. A live UI can still show an in-progress step from a transient event if it needs one; the durable log should not store an empty bracket.
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