refactor(events): remove the agent/steering mirror emit

agent/steering was the last transient mirror of a durable session event:
drainSteering appended the durable steering/message {turn, content, source}
and emitted the identical fact one line later. Zero production listeners
existed — every steering consumer (hook bridges, goldens, deriveMessages)
reads the durable event — and the one regression test subscriber asserted
a fact the log already records.

Remove the declaration (dsh-agent types + JSDoc list + README row), the
emit in drainSteering (its ctx parameter goes too), and the emit line in
the loop-pseudocode blocks (loop.ts module doc, architecture.md); the
cordis catalog is regenerated. The regression test now pins source
preservation on the durable steering/message event. Live-notification
needs keep their surviving homes: agent/queued at enqueue time,
session/event at drain time.

RFC: docs/rfc/implemented/simplification/2026-07-04-remove-agent-steering-mirror.md
(moved from proposed/, amended to shipped reality). The three implemented
RFCs that stated the retention — the boundary-mirror removal, the
stream-chunk removal, and event-domain-semantics — are amended to point
at that RFC as the record of the removal, per implemented/AGENTS.md.
The rejected retire-mid-turn-steering RFC keeps its frozen text (it
records the declined proposal); the steering capability itself —
steer(), the durable event, continuation forcing — is untouched.
This commit is contained in:
Tianyi Cui
2026-07-04 15:36:40 +08:00
parent 226a8b5e4c
commit c6d2eeea6b
12 changed files with 53 additions and 68 deletions

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@@ -176,7 +176,7 @@ forever:
session('tool/result')
append buffered post-execute additionalContext → session('context/message')(s)
⟵ after ALL tool/results (adjacency)
drain steering → session('steering/message'); emit agent/steering
drain steering → session('steering/message')
session('step/end') ⟵ durable step boundary (no agent/* mirror)
cont = waterfall agent/turn-continuation(default = {action: hadToolCalls||steered
? 'continue' : 'stop'}) → ContinuationDecision

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@@ -49,7 +49,7 @@ A step or turn errored. The loop reports a failure here (plus the logger) even w
Types: [Agent](../core-data-structures/core.md)
Source: [`packages/core/agent/src/types.ts:353`](../../packages/core/agent/src/types.ts)
Source: [`packages/core/agent/src/types.ts:348`](../../packages/core/agent/src/types.ts)
#### `agent/pre-step` — serial
@@ -125,18 +125,6 @@ Types: [Agent](../core-data-structures/core.md)
Source: [`packages/core/agent/src/types.ts:246`](../../packages/core/agent/src/types.ts)
#### `agent/steering` — emit
Steering content was injected into a running turn.
```ts cordis-catalog
'agent/steering'(agent: Agent, turn: number, content: ContentBlock[], source: MessageSource): void
```
Types: [Agent](../core-data-structures/core.md) · [ContentBlock](../core-data-structures/core.md) · [MessageSource](../core-data-structures/core.md)
Source: [`packages/core/agent/src/types.ts:347`](../../packages/core/agent/src/types.ts)
#### `agent/step-result` — waterfall
Waterfall: post-process the assembled assistant Message before tool dispatch (validation, content rewriting, …).

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@@ -59,7 +59,6 @@ Do NOT write one for a mechanical or local choice (a variable name, a one-file r
| [Prune dead core-spine surface — `SurfaceManager.invalidate()`, the loop-internal exports, `ToolExecutionResult.callId`](proposed/simplification/2026-07-04-prune-dead-core-spine-surface.md) | 2026-07-04 |
| [Prune producer-less vocabulary variants (block cache hints, the `agent` message source, the `continuation` turn trigger)](proposed/simplification/2026-07-04-prune-producerless-vocabulary-variants.md) | 2026-07-04 |
| [Prune write-only fields and a dead routing knob from the fs seam](proposed/simplification/2026-07-04-prune-write-only-fs-surface.md) | 2026-07-04 |
| [Remove the `agent/steering` mirror emit](proposed/simplification/2026-07-04-remove-agent-steering-mirror.md) | 2026-07-04 |
| [Share the app bins' boot glue instead of maintaining twin copies](proposed/simplification/2026-07-04-share-app-bin-boot-glue.md) | 2026-07-04 |
| [Tighten the hook-protocol contract — dialect, discarded fields, double defaults, and lib-owned `hook/result` semantics](proposed/simplification/2026-07-04-tighten-hook-protocol-contract.md) | 2026-07-04 |
| [Trim unreachable ACP bridge surface — the branding knobs and the kind-sniffing fallback](proposed/simplification/2026-07-04-trim-acp-bridge-unreachable-surface.md) | 2026-07-04 |
@@ -120,6 +119,7 @@ Do NOT write one for a mechanical or local choice (a variable name, a one-file r
| [Stop mirroring durable boundaries as agent events](implemented/simplification/2026-06-20-remove-agent-boundary-mirror-events.md) | 2026-06-20 |
| [Split the filesystem seam — provider text mutations plus the `dsh-fs-policy` plugin](implemented/simplification/2026-06-26-fsspec-style-fs-seam.md) | 2026-06-26 |
| [Stop mirroring the token stream as an agent event](implemented/simplification/2026-07-02-remove-stream-chunk-mirror.md) | 2026-07-02 |
| [Remove the `agent/steering` mirror emit](implemented/simplification/2026-07-04-remove-agent-steering-mirror.md) | 2026-07-04 |
### Architecture

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@@ -19,7 +19,7 @@ This is the foundational change in a stack that adds a Hooks subsystem; it estab
**Three domains, one job each, with a single boundary rule.**
- **`session/*` — the durable, replayable FACT log.** Owns `SessionEventMap`; every entry is JSON-only (no live objects). One `session/event` emit per append, plus the `session/flush` parallel durability checkpoint. It is also the live transcript feed: a consumer that wants to render or react to what happened subscribes here, so live rendering and `session/load` replay share one path.
- **`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/error`, `agent/created`/`agent/disposed`, `agent/queued`, `agent/steering`) that notify with the `Agent` in hand. Turn and step BOUNDARIES are NOT here — they are durable session events read off `session/event`, and so is the token stream (`assistant/chunk`).
- **`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/error`, `agent/created`/`agent/disposed`, `agent/queued`) that notify with the `Agent` in hand. Turn and step BOUNDARIES are NOT here — they are durable session events read off `session/event`, and so are the token stream (`assistant/chunk`) and mid-turn steering (`steering/message`).
- **`tools/*` — the tool registry + execution seam.**
**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.
@@ -31,5 +31,5 @@ This is the foundational change in a stack that adds a Hooks subsystem; it estab
- 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).
- 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.
- 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.
- 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.
- 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` (not a boundary mirror) stayed outside that RFC's scope and was removed by its own follow-up, [Remove the `agent/steering` mirror emit](../simplification/2026-07-04-remove-agent-steering-mirror.md) — it mirrored the durable `steering/message`.
- The cordis catalog (`docs/cordis-catalog/events-and-services.md`) is regenerated to drop the mirror events.

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@@ -5,10 +5,11 @@ Status: implemented (accepted 2026-07-01)
<!-- Shipped in AMENDED, narrowed form: the four turn/step BOUNDARY mirrors are
removed; `agent/steering` and `agent/stream-chunk` were RETAINED here (they
are not durable-boundary mirrors — see "Scope: what is and isn't removed").
The original proposal bundled `agent/steering` into the removal; validating
against the code showed it is a distinct live-only signal, so it stayed.
`agent/stream-chunk` was later removed by its own decision — see
[Stop mirroring the token stream as an agent event](2026-07-02-remove-stream-chunk-mirror.md). -->
The original proposal bundled `agent/steering` into the removal; keeping it
out kept this RFC's scope to boundaries. Each retained event was later
removed by its own decision — see
[Stop mirroring the token stream as an agent event](2026-07-02-remove-stream-chunk-mirror.md)
and [Remove the `agent/steering` mirror emit](2026-07-04-remove-agent-steering-mirror.md). -->
## Problem
@@ -30,7 +31,7 @@ Removed (durable-boundary mirrors — the session log is authoritative for each)
RETAINED — NOT durable-boundary mirrors, so out of scope for this decision:
- `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.)
- `agent/steering`not a boundary, so out of scope for THIS decision (the original proposal bundled it into the removal; that would have been scope creep here). It mirrors the durable `steering/message` control record rather than a boundary, and was removed by its own follow-up: [Remove the `agent/steering` mirror emit](2026-07-04-remove-agent-steering-mirror.md).
- `agent/stream-chunk` — the live token stream. Out of scope for THIS decision (a mirror of the durable `assistant/chunk`, not a boundary), it was removed by its own follow-up: [Stop mirroring the token stream as an agent event](2026-07-02-remove-stream-chunk-mirror.md).
- `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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@@ -33,7 +33,7 @@ Removed: `agent/stream-chunk`.
Not touched:
- `assistant/chunk` (the durable session event) — the authoritative token stream, kept exactly as-is. This RFC removes the LIVE MIRROR, not the persistence (the persistence-removal proposal was separately rejected — see above).
- `agent/steering` — a live control signal with no durable twin, retained (its fate remains a separate future decision, per the boundary RFC).
- `agent/steering` — not touched by THIS decision (a control signal, not the token stream). Its durable twin is `steering/message`, and the mirror emit was removed by its own follow-up: [Remove the `agent/steering` mirror emit](2026-07-04-remove-agent-steering-mirror.md).
- `agent/status`, `agent/error`, `agent/created`/`agent/disposed`, `agent/queued`, `agent/session-start` — lifecycle/control events that are not transcript data and have no durable duplicate.
## What we give up

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@@ -0,0 +1,30 @@
# RFC: Remove the `agent/steering` mirror emit
Status: implemented (accepted 2026-07-04)
## Problem
`agent/steering` was the last remaining transient mirror of a durable session event. The loop's steering drain appends the durable `steering/message { turn, content, source }` and, on the very next line, emitted `agent/steering(agent, turn, content, source)` — the identical fact as a fire-and-forget event (`packages/core/agent-loop/src/loop.ts`, `drainSteering`). It had zero production listeners: the only subscriber anywhere was a loop regression test asserting the emit carried `source` — the same fact the durable event already records one line above.
Both mirror-removal RFCs retained it while explicitly deferring the decision this RFC makes. The [boundary-mirror removal](2026-06-20-remove-agent-boundary-mirror-events.md) kept it as a live control signal rather than a boundary; the [stream-chunk removal](2026-07-02-remove-stream-chunk-mirror.md) retained it on the reading that it had no durable twin. The second rationale did not survive the code: the durable twin is `steering/message`, appended immediately before the emit with the same payload. The mirrored-vs-live-only line the taxonomy actually draws puts it on the mirror side: `agent/queued` is genuinely live-only (it fires at enqueue time, before any durable event exists, and already carries a `steering: boolean` flag — cancelled queued work never enters the log), while `agent/steering` fired at the exact moment its durable twin landed, carrying nothing the log does not.
Steering carries real production traffic — the hook bridges' turn-continuation decisions inject their reasons through `inbox.steer()`, landing as durable `steering/message` events that the hook-matrix goldens pin — and every one of those consumers observes the durable event. Nothing observed the mirror.
## Decision
`agent/steering` is removed from the agent event taxonomy: the declaration in `packages/core/agent/src/types.ts` (and its mention in the live-events JSDoc list there), the emit in `drainSteering` (whose then-unused `ctx` parameter went with it), the row in `packages/core/agent/README.md`, and the emit line in the loop-pseudocode blocks (the `packages/core/agent-loop/src/loop.ts` module doc and [architecture.md](../../../architecture.md)); the cordis catalog is regenerated without it. The one regression test pins source preservation on the durable `steering/message` event — the fact it pins lives on the log.
Three implemented RFCs stated the retention, and each is amended per [implemented/AGENTS.md](../AGENTS.md) to point here as the record of the removal: the [boundary RFC](2026-06-20-remove-agent-boundary-mirror-events.md)'s retained-list entry, the [stream-chunk RFC](2026-07-02-remove-stream-chunk-mirror.md)'s scope clause, and the [event-domain-semantics RFC](../architecture/2026-06-30-event-domain-semantics.md)'s transient-emit enumeration.
## Why not keep it?
"It is a control signal, not a boundary" — but the taxonomy's operative distinction is mirrored-vs-live-only, not control-vs-boundary, and this event mirrored. A consumer that wants enqueue-time notification has `agent/queued` (with its steering flag); a consumer that wants drain-time notification is by definition asking for the moment `steering/message` is appended, which `session/event` delivers with the same payload plus durability. The rejected [retire-mid-turn-steering RFC](../../rejected/simplification/2026-06-20-retire-mid-turn-steering.md) defended the steering *capability*`steer()`, the durable event, continuation forcing — all of which this removal keeps untouched.
## Acceptance criteria
- The `agent/steering` spelling survives only in RFC prose (this RFC, the three amended RFCs above, and the frozen [rejected steering-capability RFC](../../rejected/simplification/2026-06-20-retire-mid-turn-steering.md), whose text records the proposal it declined); the catalog is regenerated and fresh.
- The retargeted test pins source preservation on `steering/message`; the suite is green.
## Risks
None known: zero production listeners existed to migrate, and both live-notification needs (enqueue, drain) have surviving homes (`agent/queued`, `session/event`).

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@@ -1,28 +0,0 @@
# RFC: Remove the `agent/steering` mirror emit
Status: proposed
## Problem
`agent/steering` is the last remaining transient mirror of a durable session event. The loop's steering drain appends the durable `steering/message { turn, content, source }` and, on the very next line, emits `agent/steering(agent, turn, content, source)` — the identical fact as a fire-and-forget event (`packages/core/agent-loop/src/loop.ts`, `drainSteering`). It has zero production listeners: the only subscriber anywhere is a loop regression test asserting the emit carries `source` — the same fact the durable event already records one line above.
Both mirror-removal RFCs retained it while explicitly deferring the decision this RFC now makes. The [boundary-mirror removal](../../implemented/simplification/2026-06-20-remove-agent-boundary-mirror-events.md) kept it as "a live control signal, not a boundary"; the [stream-chunk removal](../../implemented/simplification/2026-07-02-remove-stream-chunk-mirror.md) kept it as "a live control signal with no durable twin, retained (its fate is a separate future decision)". The second rationale does not survive the code: the durable twin is `steering/message`, appended immediately before the emit with the same payload. The mirrored-vs-live-only line the taxonomy actually draws puts it on the mirror side: `agent/queued` is genuinely live-only (it fires at enqueue time, before any durable event exists, and already carries a `steering: boolean` flag — cancelled queued work never enters the log), while `agent/steering` fires at the exact moment its durable twin lands, carrying nothing the log does not.
Steering carries real production traffic — the hook bridges' turn-continuation decisions inject their reasons through `inbox.steer()`, landing as durable `steering/message` events that the hook-matrix goldens pin — and every one of those consumers observes the durable event. Nothing observes the mirror.
## Proposal
Remove the `agent/steering` declaration from `packages/core/agent/src/types.ts` (and its mention in the live-events JSDoc list there), the emit in `drainSteering` (whose `ctx` parameter becomes unused and goes too), the row in `packages/core/agent/README.md`, and the emit line in the loop-pseudocode blocks (`packages/core/agent-loop/src/loop.ts` module doc and [architecture.md](../../../architecture.md)); run `pnpm run gen-cordis-catalog`. Retarget the one regression test at the durable `steering/message` event — the source-preservation fact it pins lives on the log. The implementing PR amends the two retaining RFCs' scope lines per [implemented/AGENTS.md](../../implemented/AGENTS.md): the boundary RFC's retained-list entry and the stream-chunk RFC's "no durable twin" clause.
## Why not keep it?
"It is a control signal, not a boundary" — but the taxonomy's operative distinction is mirrored-vs-live-only, not control-vs-boundary, and this event mirrors. A consumer that wants enqueue-time notification has `agent/queued` (with its steering flag); a consumer that wants drain-time notification is by definition asking for the moment `steering/message` is appended, which `session/event` delivers with the same payload plus durability. The rejected [retire-mid-turn-steering RFC](../../rejected/simplification/2026-06-20-retire-mid-turn-steering.md) defended the steering *capability*`steer()`, the durable event, continuation forcing — all of which this removal keeps untouched.
## Acceptance criteria
- No `agent/steering` spelling outside this RFC and the two amended RFCs; the catalog is regenerated and fresh.
- The retargeted test pins source preservation on `steering/message`; the suite is green.
## Risks
None known: zero production listeners exist to migrate, and both live-notification needs (enqueue, drain) have surviving homes (`agent/queued`, `session/event`).

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@@ -166,7 +166,7 @@ export interface LoopHandle {
* → dispatch → tools/post-execute
* session('tool/result')
* append buffered post-execute additionalContext → session('context/message')(s)
* drain steering → session('steering/message'); emit agent/steering
* drain steering → session('steering/message')
* session('step/end') ⟵ durable step boundary (no agent/* mirror)
* cont = waterfall agent/turn-continuation ⟵ ContinuationDecision; default
* {action: hadToolCalls||steered ? 'continue':'stop'}; a continue.reason is
@@ -420,7 +420,7 @@ async function runTurn(ctx: Context, agent: ReactLoopAgent, handle: LoopHandle,
// Steering from the previous round's continuation listeners joins before
// the request.
drainSteering(ctx, agent, turn)
drainSteering(agent, turn)
// The step's AbortController exists BEFORE any async pre-step work so a
// dispose() or cancel() — in a synchronous turn-start listener or an
@@ -529,7 +529,7 @@ async function runTurn(ctx: Context, agent: ReactLoopAgent, handle: LoopHandle,
if (stepReason) reason = stepReason
// Steering that arrived during streaming/tool execution.
const steered = drainSteering(ctx, agent, turn)
const steered = drainSteering(agent, turn)
if (closeStep()) break
@@ -635,11 +635,10 @@ async function runTurn(ctx: Context, agent: ReactLoopAgent, handle: LoopHandle,
}
/** Drain the steering queue into the session. Returns whether any arrived. */
function drainSteering(ctx: Context, agent: ReactLoopAgent, turn: number): boolean {
function drainSteering(agent: ReactLoopAgent, turn: number): boolean {
const messages = agent.inbox.drainSteering()
for (const message of messages) {
agent.session.append('steering/message', { turn, content: message.content, source: message.source }, { surfaceOp: 'append' })
ctx.emit('agent/steering', agent, turn, message.content, message.source)
}
return messages.length > 0
}

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@@ -410,7 +410,7 @@ describe('MEDIUM: misc registry and config fixes', () => {
expect(agent.session.deriveMessages().at(-1)?.content).toEqual([{ type: 'text', text: 'routed' }])
})
it('agent/queued carries the resolved source; agent/steering carries its source', async () => {
it('agent/queued carries the resolved source; steering/message records its source', async () => {
const adapter = new MockAdapter([toolCallResponse('c1', 'noop', {}), textResponse('done')])
const ctx = await harness(adapter)
const agent = ctx.agentLoop.create(AgentId('a1'), { model: 'mock' })
@@ -425,15 +425,16 @@ describe('MEDIUM: misc registry and config fixes', () => {
}))
const queuedSources: { source: MessageSource; steering: boolean }[] = []
const steeringSources: MessageSource[] = []
ctx.on('agent/queued', (_agent, _content, info) => void queuedSources.push(info))
ctx.on('agent/steering', (_agent, _turn, _content, source) => void steeringSources.push(source))
send(agent, 'go') // no explicit source → default {kind:'user'} must be visible
await waitForIdle(ctx, agent)
expect(queuedSources[0]).toEqual({ source: { kind: 'user' }, steering: false })
expect(queuedSources[1]).toEqual({ source: { kind: 'plugin', plugin: 'goal' }, steering: true })
// The drain appends the durable steering/message with the caller's source
// intact — the log, not a transient emit, is where consumers read it.
const steeringSources = agent.session.events.flatMap(e => e.type === 'steering/message' ? [e.data.source] : [])
expect(steeringSources).toEqual([{ kind: 'plugin', plugin: 'goal' }])
})
})

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@@ -50,9 +50,8 @@ Turn and step boundaries are NOT mirrored as `agent/*` emits: a consumer that ne
Tool interception is the `tools/pre-execute` / `tools/post-execute` pair in [`dsh-tools`](../tools/README.md) (`PreToolDecision` allow/deny/ask, `PostToolDecision` accept/block) — same typed-Decision idiom, owned there because it is the tool registry's seam.
#### Live control notifications (emit)
#### Error notifications (emit)
- `agent/steering` — steering content injected mid-turn
- `agent/error` — step/turn error
The model's token stream is NOT an `agent/*` event: read it off the durable `session/event` feed as `assistant/chunk` (the same feed persistence and the ACP bridge use).

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@@ -20,7 +20,7 @@
* `agent/request`/`agent/step-result`/`agent/turn-continuation` waterfalls and
* the serial `agent/pre-step`) that mutate/veto, and TRANSIENT emits
* (`agent/status`, `agent/error`, `agent/created`/
* `agent/disposed`, `agent/queued`, `agent/steering`, `agent/session-start`)
* `agent/disposed`, `agent/queued`, `agent/session-start`)
* that notify with the `Agent` in hand. Turn/step boundaries are NOT here —
* they are durable `session/event` records. Answers "right now, with the agent
* object — intercept or observe."
@@ -339,12 +339,7 @@ declare module 'cordis' {
*/
'agent/turn-continuation'(agent: Agent, turn: number, defaultDecision: ContinuationDecision, next: () => Promise<ContinuationDecision>): Promise<ContinuationDecision>
// ---- streaming + tool notifications (emit) ----
/**
* Steering content was injected into a running turn.
* @mode emit
*/
'agent/steering'(agent: Agent, turn: number, content: ContentBlock[], source: MessageSource): void
// ---- error notifications (emit) ----
/**
* A step or turn errored. The loop reports a failure here (plus the logger)
* even when the error has no in-turn position for a session `error` event.