Bracket agent-initiated ACP turns so unprompted work renders

[SawyerHood]

What was wrong

When an ACP agent streams work with no session/prompt in flight (the concrete case is OMP's async-job auto-delivery), handleAgentNotification in the provider-acp bridge forwarded every session/update to the translator without opening a turn. ACP has no turn bracket for agent-initiated work, the translator is context-free, and the runtime's delta assembler refuses to let item/stream deltas open a turn, so each unprompted chunk was demoted to a thread-scoped provider/unhandled row. Those rows are hidden unless showUnhandledProviderEvents is on, so the thread simply never answered. Root cause and live repro: https://get-bb.github.io/reports/issues/2122.html.

#2123 (thanks @bradhallett) correctly identified the layer and the shape (a bridge-emitted turn.open, per provider-bridge-protocol turn lifecycle rule 3). It was reviewed REQUEST CHANGES because it tracked the vouched turn in a parallel spontaneousTurnOpen boolean, so the existing readers of activePromptKind kept their old semantics: an agent exit left the turn open forever (thread hangs in Working), permission requests inside the turn were auto-cancelled even in full mode, and the 120 s quiet window buffered the last message and reported "Worked for 2m".

What changed

All in plugins/provider-acp (bridge-local; no wire shape changes, so no HOST_DAEMON_PROTOCOL_VERSION bump).

• src/bridge/bridge.ts: agent-initiated work is folded into the bridge's existing open-turn mirror, activePromptKind: "turn" | "compaction" | "agent" | null, instead of a second flag, so every reader had to decide what an agent turn means:
  • handleAgentNotification opens the turn (ACP_TURN_STARTED_METHOD) when activePromptKind === null and the update is a work kind, and re-arms a 5 s quiet timer per work update. The timer does not close the turn while a permission is pending.
  • Settled on every exit path: turn/start settles a still-open agent turn (end_turn) before the check for an active turn, so the next user prompt or compaction never sees "A turn is already active"; thread/stop (interrupt) settles it as cancelled; agent exit emits the settling error through emitSessionError while the turn is still marked open, then clears the mirror, so the assembler closes the turn as failed; a release stop detaches without fabricating a terminal state (Release agent runtimes on thread stop #1584), same as a prompted turn.
  • handlePermissionRequest treats "agent" like "turn": full mode auto-allows, other modes forward to the user.
  • turn/steer during an agent turn still returns NO_ACTIVE_TURN; the runtime already maps that to a fresh turn/start, which settles the agent turn first.
• src/visibility.ts: the work-kind set is derived from the visibility metadata (AGENT_WORK_ACP_UPDATE_KINDS = normalized kinds minus usage_update, exported as isAgentWorkAcpUpdateKind) rather than a second hand-maintained map.
• src/bridge/fake-acp-agent.mjs: new agent-initiated prompt behavior (OMP wire shape: echoed user_message_chunk, agent chunks, a tool call that completes) with :permission, :exit, and :noise variants.

Differences from #2123: one mirror instead of two; settled on agent exit and on release; permission requests inside the turn are handled; 5 s quiet window instead of 120 s; the quiet window never closes a turn with an unanswered permission; no test-only timeout hook (the tests run against the real window).

How you verified

• New tests in src/bridge/bridge.test.ts under "agent-initiated turns (provider-acp silently drops agent-initiated turns (unprompted session updates, e.g. OMP async-job delivery) #2122)". With the source change stashed, 6 of 7 fail (the unprompted text never becomes an agent message; no second turn); with it, all 7 pass:
  • brackets unprompted agent work as a turn (2 turn/started, 0 provider/unhandled, tool item rendered, exactly one input.accepted) and closes it completed after the quiet window
  • does not open a turn for unprompted non-work updates
  • settles the agent turn before the next user turn opens (3 turns, all completed)
  • interrupts the agent turn on thread/stop
  • fails the agent turn when the agent process exits mid-turn (terminal state instead of a hang)
  • auto-allows a permission request inside an agent turn in full mode
  • forwards a permission request inside an agent turn in ask mode and keeps the exchange in one turn
• The report's issue-2122.repro.test.ts and both hostile probes in issue-2122.pr-edges.test.ts (agent exit, permission during vouched turn), which fail on Vouch agent-initiated ACP turns so async job delivery renders #2123, pass on this branch.
• pnpm exec turbo run test typecheck lint --filter=bb-plugin-provider-acp --force: 7/7 tasks green, 15 test files / 203 tests passed.
• Live on my own dev instance (scripts/bb-dev-app current), registering the fake agent via customAcpAgents: thread spawn --provider acp-fakeacp --prompt agent-initiated produced turn t2 with the agent message, a fileRead item, and the closing message, turn/completed completed 4.98 s after the last chunk, no provider/unhandled rows; the timeline API shows the turn and the final assistant message. --prompt agent-initiated:exit produced provider/error + turn/completed failed on turn t4 within 70 ms of the exit and the thread status is error, not a hung Working state.

Fixes #2122

AGENT GENERATED: by Claude Opus 5