feat(coding-agent): expose ACP quiescence metadata

[sethkarten]

Why

An ACP client cannot currently tell whether a completed turn is safe to act on. Prime Agent subagents are fire-and-forget and keep working after the spawning turn ends, and autonomous mode may still have continuation budget left. A consumer that scores or ends the session at end_turn can therefore cut off live work, and the result depends on a race.

This is not derivable client-side: tool_execution_update, auto_retry_*, auth_stale and recap_update have no ACP representation, so there is no event stream a client could reconstruct quiescence from.

What changed

Adds quiescence to the existing namespaced envelope:

_meta["ai.primeintellect.prime-agent"].quiescence = {
  outstandingSubagents: number,
  remainingAutonomousContinuations: number,
}

Counted from the authoritative live roster (queued plus running) rather than reconstructed from child deltas, and emitted after headless completion so it describes the state at the point the turn is reported done.

The turn reports rather than waits. Blocking session/prompt on quiescence would deadlock against detached subagent semantics — a child can legitimately outlive many turns — so the client decides what to do with the signal.

Additive and namespaced: vanilla ACP clients ignore unknown _meta. No previously-declared-but-unemitted field is touched.

Validation

• npm run check: Biome 898 files, installer render, browser smoke all pass
• test/suite/acp-mode.test.ts: 2/2
• mutation: changing the outstanding count to + 1 fails the assertion; restoring passes

Stacked on #805.

---

Note

Medium Risk
Changes ACP session/prompt and session/new failure modes (roster read failures now reject prompts) and introduce authoritative child-roster plumbing used by external clients; behavior is additive in _meta but correctness-sensitive for scoring/automation.

Overview
ACP clients get a quiescence signal in namespaced _meta after each session/prompt turn completes headless work: outstanding subagents (queued/running from a live roster) and remaining autonomous continuations. The turn still reports end_turn without blocking on detached children; clients choose how to act on the signal.

AgentSession.getRlmChildSnapshots() builds a recursive live child roster from active runs and retained child sessions (with model on runs), and connection snapshots/daemon clients merge rlm_child_update events so empty attach snapshots do not erase live children.

ACP session lifecycle hardening: subscribe before the initial snapshot, reject session/new on snapshot failure (unsubscribe cleanup), guard concurrent session/new with an in-flight flag, and fail session/prompt if the emission-time roster read fails instead of emitting a false zero count.

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Note

Expose ACP quiescence metadata with live subagent roster in session/prompt responses

• session/prompt now emits _meta.quiescence after completion, reporting outstandingSubagents and remainingAutonomousContinuations from a live authoritative child roster snapshot.
• AgentSession.getRlmChildSnapshots provides a recursive, flattened list of child agent snapshots (id, label, status, model, sessionDir) from both the active run registry and retained child sessions.
• DaemonAgentConnection now updates its cached latestSnapshot.children as rlm_child_update events arrive, keeping the in-memory roster current.
• session/new now rejects concurrent creation attempts while a first request is pending, and rejects (without claiming the session slot) if the initial roster snapshot read fails.
• Risk: session/prompt and session/new now fail hard if the live roster snapshot read fails, rather than succeeding with incomplete metadata.

^{Macroscope summarized 6447725.}

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[snimu]

One more thing: this adds user-visible ACP surface (quiescence in _meta, and session/prompt now rejecting when the emission-time roster read fails even though the turn completed) but has no CHANGELOG entry. Please add a bullet under ## [Unreleased] in packages/coding-agent/CHANGELOG.md.

[sethkarten]

Addressed in 6447725: packages/coding-agent/CHANGELOG.md now documents ACP _meta.quiescence and the prompt rejection behavior when the emission-time live roster read fails.

[sethkarten]

Closing this superseded ACP implementation. Resident recovery coverage continues in #1236, with the exact causal prompt lifecycle successor in #1239. This PR remains available as design and review history.