Summary

ACP mode: "run" sessions spawned via sessions_spawn without thread binding have no completion delivery mechanism back to the parent/requester session. Subagent mode: "run" sessions auto-announce completion to the parent, but equivalent ACP sessions silently complete with no notification.

Steps to Reproduce

1. From a main or agent session (e.g. Telegram direct chat, no thread/topic), spawn an ACP run:

{
  "runtime": "acp",
  "agentId": "claude",
  "mode": "run",
  "task": "Run date and report the time",
  "runTimeoutSeconds": 60
}

2. The task executes successfully — the session JSONL shows correct assistant output.
3. The parent session never receives a completion event or auto-announce.

Observed Behavior

• Subagent mode: "run": Completion auto-announces to parent session ✅
• ACP mode: "run" without thread binding: Task completes, result written to session JSONL, but parent session receives nothing ❌
• ACP mode: "run" with streamTo: "parent": Only a start system event is delivered; no completion or end event arrives ❌

Timeline from gateway logs (streamTo: "parent" case)

13:57:25 — stream log: kind=system_event, text="Started claude session..."
13:57:45 — session JSONL: assistant responds with correct output, stopReason=stop
           (no further stream log entries, no completion event)

Expected Behavior

ACP mode: "run" should deliver completion results back to the requester session, consistent with subagent mode: "run" behavior. At minimum when streamTo: "parent" is set, a completion/end system event should be emitted.

Context

• ACP delivery appears designed primarily for thread-bound scenarios (Discord threads / Telegram topics). Non-thread-bound spawns from direct sessions have no delivery target.
• Startup identity reconcile also fails consistently (checked=3 resolved=0 failed=3) because it runs ~80ms after acpx backend registered but ~1s before backend ready — possibly related but not the direct cause of this delivery gap.
• Gateway version: 2026.3.11 (updated today)
• acpx version: 0.1.16
• ACP backend: claude (Claude Code CLI)

Workarounds

• Poll the ACP session JSONL manually after spawning
• Use thread-bound ACP sessions (requires forum-mode Telegram or Discord threads)
• Use native subagent runtime instead of ACP when auto-announce is needed

Suggestion

1. For mode: "run" without thread binding, deliver completion to the requester session the same way subagents do (auto-announce).
2. For streamTo: "parent", ensure a completion/end event is written to the stream log when the ACP run finishes.
3. Consider adding a startup reconcile retry or gate on backend ready (separate issue if preferred).