refactor(events): remove the turn boundary mirror events
Complete the boundary-mirror removal begun with the step mirrors: drop `agent/turn-start` and `agent/turn-end` from the agent event taxonomy. Turn and step boundaries are now read exclusively off the durable `session/event` feed (`turn/start`/`turn/end`/`step/start`/`step/end`) — there is no `agent/*` mirror for any boundary. - loop.ts: delete both turn emits; `closeTurn` loses its `emit` parameter and its now-unreachable idempotency guard (it is called exactly once per turn, on mutually exclusive normal/catch paths); `failTurn` loses the dead post-close branch that only a throwing turn-end LISTENER could reach. - ui-stdio: render turn boundaries from `session/event`, recovering the short agent label from an `agent/created`→id map (the `turn/start` event carries only the turn number, and the session id is not reliably the agent id). ui-stdio is a disposable test REPL, so this migration retires the sole justification the event-domain-semantics RFC gave for KEEPING the turn mirrors. - Tests: reason/turn-number collectors and the boundary-ordering test now read `session/event`; the throwing-turn-boundary-LISTENER tests are deleted (that code path no longer exists). A new test covers the outer-catch disposed branch via a pre-step listener that disposes-then-throws (the surviving real path). - Docs: promote the "remove agent boundary mirror events" RFC to implemented (amended/narrowed — `agent/steering` is RETAINED, not a boundary mirror); update the event-domain-semantics + turn-enclosure RFCs, architecture.md, the cookbook, the ACP/agent/ui-stdio prose, and regenerate the cordis catalog. `agent/steering` and `agent/stream-chunk` are explicitly out of scope (not durable-boundary mirrors). ACP is unaffected — it already settles from the log's `turn/end` + `agent/status`; snapshot goldens are byte-unchanged.
This commit is contained in:
Tianyi Cui
@@ -50,7 +50,6 @@ Do NOT write one for a mechanical or local choice (a variable name, a one-file r
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| Title | First proposed |
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| [Unify the agent id and the session id](proposed/simplification/2026-06-20-unify-agent-and-session-id.md) | 2026-06-20 |
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| [Stop mirroring durable boundaries as agent events](proposed/simplification/2026-06-20-remove-agent-boundary-mirror-events.md) | 2026-06-20 |
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### Architecture
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@@ -97,6 +96,7 @@ Do NOT write one for a mechanical or local choice (a variable name, a one-file r
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| [Prune dead methods from the persistence seam](implemented/simplification/2026-06-20-prune-dead-seam-methods.md) | 2026-06-20 |
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| [Keep one public stop primitive](implemented/simplification/2026-06-20-public-agent-stop-surface.md) | 2026-06-20 |
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| [Fold trace-only session facts into load-bearing events](implemented/simplification/2026-06-20-collapse-trace-only-session-events.md) | 2026-06-20 |
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| [Stop mirroring durable boundaries as agent events](implemented/simplification/2026-06-20-remove-agent-boundary-mirror-events.md) | 2026-06-20 |
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### Architecture
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@@ -37,4 +37,4 @@ Costs: `agent.inject()` while idle now writes three log lines instead of one, an
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The rule is intentionally producer-enforced and dev-checked rather than reader-tolerated: a future backend (SQLite/WAL) inherits the same clean boundary for free, and a plugin that records an event outside a turn fails loudly in dev instead of silently losing data on the next reload.
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The invariant also constrains where the loop may record an `error` event. A failure detected while a turn is open is appended INSIDE the turn (before `turn/end`); but a failure that surfaces once the turn is already closed — a rejecting `session/flush` (which runs as the post-`turn/end` durability checkpoint) or a throwing `agent/turn-end` listener (after `closeTurn` already appended `turn/end`) — has no in-turn position left. Appending an `error` there would land it past the last `turn/end`, exactly the crash-tail position a backend discards. So those post-turn failures are reported via the `agent/error` event and the logger only, never as a `SessionEvent`; the turn stays balanced and persistence keeps its buffered events for the next checkpoint. If durable operational diagnostics are ever needed, they belong on a separate telemetry channel, not the replayable session log.
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The invariant also constrains where the loop may record an `error` event. A failure detected while a turn is open is appended INSIDE the turn (before `turn/end`); but a failure that surfaces once the turn is already closed — a rejecting `session/flush`, which runs as the post-`turn/end` durability checkpoint — has no in-turn position left. Appending an `error` there would land it past the last `turn/end`, exactly the crash-tail position a backend discards. So that post-turn failure is reported via the `agent/error` event and the logger only, never as a `SessionEvent`; the turn stays balanced and persistence keeps its buffered events for the next checkpoint. If durable operational diagnostics are ever needed, they belong on a separate telemetry channel, not the replayable session log.
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@@ -22,18 +22,14 @@ This is the foundational change in a stack that adds a Hooks subsystem; it estab
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- **`agent/*` — the LIVE runtime surface.** Always carries the live `Agent`. Two shapes: INTERCEPTION waterfalls (`agent/request`, `agent/step-result`, `agent/turn-continuation`) that mutate or veto, and TRANSIENT emits (`agent/status`, `agent/stream-chunk`, `agent/error`, `agent/created`/`agent/disposed`, `agent/queued`, `agent/steering`, and the turn boundaries) that notify with the `Agent` in hand.
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- **`tools/*` — the tool registry + execution seam.**
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**The boundary rule:** a durable, replayable fact is a `SessionEvent`; a live interception or a transient/live-object signal is an `agent`/`tools` Cordis event. A datum that is BOTH — a turn or step boundary — lives in the session log, and is mirrored as an `agent/*` emit ONLY where a live consumer provably needs the `Agent` handle at that instant.
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**The boundary rule:** a durable, replayable fact is a `SessionEvent`; a live interception or a transient/live-object signal is an `agent`/`tools` Cordis event. A turn or step boundary is a durable fact, so it lives in the session log and is read off the `session/event` feed — it is NOT mirrored as an `agent/*` emit.
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**Applying the rule to the boundary twins (prune case-by-case):**
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- `agent/turn-start` — **KEPT.** The stdio UI (`dsh-ui-stdio`) labels turn output by `agent.id`, which the `turn/start` session event does not carry. A genuine live-object need.
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- `agent/turn-end` — **KEPT.** The stdio UI listens to print the next-prompt glyph. (Note: the ACP bridge does NOT settle on this event — it settles from `session/event` `turn/end` plus `agent/status`; the surviving justification is the stdio UI alone.)
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- `agent/step-start`, `agent/step-end` — **REMOVED.** No production consumer needs the live `Agent` at a step boundary; a consumer that wants per-step boundaries reads the durable `step/start`/`step/end` session events. Removing the two emits also simplifies the loop's `closeStep` (one append, no paired emit).
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**Applying the rule to the boundary twins:** all four boundary mirrors — `agent/turn-start`, `agent/turn-end`, `agent/step-start`, `agent/step-end` — are **REMOVED**. No production consumer needs the live `Agent` at a boundary: the ACP bridge settles from `session/event` `turn/end` plus `agent/status`, and the only turn-mirror consumer (`dsh-ui-stdio`, a disposable test REPL) was migrated to render boundaries from `session/event`, recovering the short agent label from an `agent/created`→id map. The step mirrors were removed first (they had no consumer at all); the turn mirrors followed once ui-stdio was migrated — see [the remove-boundary-mirror-events RFC](../simplification/2026-06-20-remove-agent-boundary-mirror-events.md), which owns that decision. Removing the emits also simplifies the loop's `closeStep`/`closeTurn` (one append each, no paired emit).
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## Consequences
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- The loop no longer emits `agent/step-start`/`agent/step-end`; `closeStep` appends `step/end` only, and a throwing `step/end` session-event listener is the surviving step-boundary-listener failure path (contained by `closeStep` → `failTurn`, the turn closes balanced).
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- Tests that observed step boundaries via the removed emits now observe the durable `step/start`/`step/end` session events — the behavior they pin (boundary ordering, step counting, a throwing boundary listener failing the turn balanced) is unchanged; only the feed they read moved to the canonical one. Per [AGENTS.md "tests document behavior, not golden truth"](../../../../AGENTS.md), the behavior and its test moved together.
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- The loop no longer emits any boundary mirror; `closeStep` appends `step/end` only and `closeTurn` appends `turn/end` only. A throwing `step/end`/`turn/end` session-event listener is the surviving boundary-listener failure path (contained inside `closeStep`/`closeTurn` — `Session.append` pushes the event before notifying listeners, so the boundary is durable and the turn closes balanced regardless).
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- Tests that observed boundaries via the removed emits now observe the durable `turn/start`/`turn/end`/`step/start`/`step/end` session events — the behavior they pin (boundary ordering, step counting) is unchanged; only the feed they read moved to the canonical one. The tests that exercised a *throwing turn-boundary emit listener* were deleted, because that code path no longer exists (there is no emit to throw from). Per [AGENTS.md "tests document behavior, not golden truth"](../../../../AGENTS.md), the behavior and its test moved (or died) together.
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- The loop marks the step open (`stepOpen = true`) BEFORE appending `step/start`, because `Session.append` pushes the event to the log before notifying `session/event` listeners (validation throws happen earlier, before the push — see [the session append contract](../../../core-data-structures/session.md)). So a throwing `step/start` session-event listener runs with the step already open and the event already in the log: the loop's outer catch then calls `closeStep()`, which appends the balancing `step/end`, and the turn closes balanced with an error (`turn/start → step/start → step/end → turn/end` — verified by the invariants oracle in the regression test). Closing the open step is owed precisely because the marker is set first.
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- This is a partial, conservative realization of the broader [proposed simplification "Stop mirroring durable boundaries as agent events"](../../proposed/simplification/2026-06-20-remove-agent-boundary-mirror-events.md): that RFC proposes removing ALL boundary mirrors (including the turn boundaries and `agent/steering`) and migrating the stdio UI's turn rendering onto `session/event`. This RFC removes only the two step mirrors that have no live consumer; the turn mirrors stay until the stdio UI is migrated. The proposed RFC remains the home for finishing that migration.
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- The cordis catalog (`docs/cordis-catalog/events-and-services.md`) is regenerated to drop the two events.
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- The full realization of this is [the simplification RFC "Stop mirroring durable boundaries as agent events"](../simplification/2026-06-20-remove-agent-boundary-mirror-events.md): all four boundary mirrors are removed and every consumer reads boundaries off `session/event`. `agent/steering` (a live control signal, not a boundary mirror) is retained; see that RFC's scope section.
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- The cordis catalog (`docs/cordis-catalog/events-and-services.md`) is regenerated to drop the mirror events.
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@@ -0,0 +1,37 @@
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# RFC: Stop mirroring durable boundaries as agent events
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Status: implemented (accepted 2026-07-01)
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<!-- Shipped in AMENDED, narrowed form: the four turn/step BOUNDARY mirrors are
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removed; `agent/steering` and `agent/stream-chunk` are RETAINED (they are
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not durable-boundary mirrors — see "Scope: what is and isn't removed"). The
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original proposal bundled `agent/steering` into the removal; validating
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against the code showed it is a distinct live-only signal, so it stayed. -->
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## Problem
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The loop records the canonical transcript in `SessionEvent` and also emitted a parallel set of live `agent/*` boundary mirror events: `agent/turn-start`, `agent/turn-end`, `agent/step-start`, and `agent/step-end`. The mirrors made consumers choose between two sources of truth for the SAME durable fact. ACP already chose the session log for the editor-facing transcript because a throwing peer listener can prevent later `agent/*` listeners from observing a boundary, while the session event was already appended. The stdio UI was the only production consumer that still rendered turn boundaries from the mirror events; it already rendered tool calls and results from `session/event`.
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This duplication is not free. Every lifecycle change had to update the session event, the mirror event, docs, invariants, tests, and snapshot expectations. The duplicate boundary events also made failure ordering subtle: a turn can be durably closed before a live `agent/turn-end` listener runs, so a post-boundary listener failure has no valid in-log position left and must be reported out of band.
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## Decision
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Make `session/event` the single live boundary/transcript stream. Consumers that render turns, tool calls, tool results, assistant messages, and durable boundaries subscribe to `session/event` and derive their UI from the same event vocabulary persistence uses.
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The four durable-boundary mirrors — `agent/turn-start`, `agent/turn-end`, `agent/step-start`, `agent/step-end` — are removed from the agent event taxonomy. A UI that wants the agent handle (or its short id) at a boundary keeps a small map from session id to agent id built from `agent/created`/`agent/disposed`; `dsh-ui-stdio` does exactly this to label its `[<agent> turn N]` header, since the `turn/start` session event carries only the turn number. The canonical record remains the event-sourced session log.
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The step mirrors (which had no consumer at all) were removed first, in [the event-domain-semantics RFC](../architecture/2026-06-30-event-domain-semantics.md); that RFC KEPT the turn mirrors on the stated justification that the stdio UI needed the `Agent` handle at the turn boundary. This RFC finishes the job: `dsh-ui-stdio` is a disposable test REPL whose rendering can change freely, so "ui-stdio needs it" is not a reason to keep a mirror — it was migrated to `session/event` + the id map, and the turn mirrors were removed too.
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## Scope: what is and isn't removed
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Removed (durable-boundary mirrors — the session log is authoritative for each): `agent/turn-start`, `agent/turn-end`, `agent/step-start`, `agent/step-end`.
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RETAINED — NOT durable-boundary mirrors, so out of scope for this decision:
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- `agent/steering` — a live control signal, not a boundary. (The original proposal bundled it into the removal; validating against the code, it is not a duplicate of a durable boundary, so removing it here would have been scope creep. Its fate is a separate future decision.)
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- `agent/stream-chunk` — the live token stream. `assistant/chunk` persistence remains load-bearing, so the chunk stream could later be evaluated as a mirror, but that is a separate decision.
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- `agent/created`, `agent/disposed`, `agent/status`, `agent/error`, `agent/queued` — lifecycle/control events that are not transcript data. `agent/queued` in particular is an inbox acknowledgement that fires before any durable event exists (cancelled queued work may never enter the log), so it is deliberately live-only.
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## What we give up
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A plugin can no longer observe turn/step boundaries from a convenient `Agent`-first event. It must either subscribe to `session/event` or maintain a session-to-agent association. That is an acceptable trade: boundary consumers should not depend on a second event feed that can drift from the durable log.
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@@ -1,31 +0,0 @@
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# RFC: Stop mirroring durable boundaries as agent events
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Status: proposed
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## Problem
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The loop records the canonical transcript in `SessionEvent` and also emits a parallel set of live `agent/*` mirror events: `agent/turn-start`, `agent/turn-end`, `agent/step-start`, `agent/step-end`, `agent/stream-chunk`, and `agent/steering`. The mirrors make consumers choose between two sources of truth. ACP already chose the session log for the editor-facing transcript because a throwing peer listener can prevent later `agent/*` listeners from observing a boundary, while the session event was already appended. The stdio UI is the only production consumer that still renders turn boundaries and the token stream from the mirror events; it already renders tool calls and results from `session/event`.
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This duplication is not free. Every lifecycle change has to update the session event, the mirror event, docs, invariants, tests, and snapshot expectations. The duplicate boundary events also make failure ordering subtle: a turn can be durably closed before a live `agent/turn-end` listener runs, so a post-boundary listener failure has no valid in-log position left and must be reported out of band.
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## Proposal
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Make `session/event` the live transcript stream. Consumers that render turns, tool calls, tool results, assistant messages, and durable boundaries subscribe to `session/event` and derive their UI from the same event vocabulary persistence uses. Keep agent lifecycle/control events that are not transcript data: `agent/created`, `agent/disposed`, `agent/status`, `agent/error`, and `agent/queued`. `agent/queued` is an inbox acknowledgement rather than a transcript mirror: it fires before any durable event exists, and cancelled queued work may never enter the log.
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Remove the duplicate durable-boundary mirrors from the agent event taxonomy. If a UI wants an agent handle from a session event, it can keep a small map from session id to agent built from `agent/created`/`agent/disposed`, or the registry can offer an explicit lookup. The canonical record remains the event-sourced session log.
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## Acceptance criteria
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- ACP and stdio render transcript content from `session/event`.
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- `agent/turn-start`, `agent/turn-end`, `agent/step-start`, `agent/step-end`, and `agent/steering` are removed or reduced to private implementation details.
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- `agent/queued` is either retained and documented as live-only inbox/control state, or deleted in a separate proposal that names the queue-acknowledgement capability loss.
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- Tests assert the persisted event stream, not a second mirror stream, for turn and step ordering.
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- Documentation presents `SessionEvent` as both the durable source and the live transcript feed.
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## What we give up
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A plugin can no longer observe turn/step boundaries from a convenient `Agent`-first event. It must either subscribe to `session/event` or maintain a session-to-agent association. That is an acceptable trade: transcript consumers should not depend on a second event feed that can drift from the durable log.
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## Related
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Because high-fidelity `assistant/chunk` persistence remains load-bearing, `agent/stream-chunk` can be evaluated as another mirror of durable session data rather than as the only token stream. If a future proposal moves chunks out of the canonical log, `agent/stream-chunk` would need a fresh decision as a deliberately live-only UI signal.
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