ACP: a sub-agent that needs approval waits forever; the request never reaches the client

With fx acp in ask mode, a sub-agent that needs tool approval never finishes. fx moves the child to awaiting_approval, but never sends the ACP client a session/request_permission, or anything else. The parent's subagent tool call stays in_progress, and session/prompt never returns. The only way out is cancelling the turn.

Repro

fx 0.0.14. The code path is unchanged on main at e71a663b.

1. Set "permission_mode": "ask" in ~/.fx/settings.json.
2. Create a workspace containing a package.json and a tsconfig.json.
3. Run the attached probe: node acp-subagent-probe.mjs <path-to-fx> <workspace> out.ndjson. It:
   • sends initialize and session/new;
   • prompts "Spawn 2 subagents, one to read package.json and one to read tsconfig.json";
   • approves any session/request_permission with allow_once;
   • logs every frame.
4. One child finishes in about 6 s: its tool_call_update is completed. The other's tool call stays in_progress. No permission request or other frame follows, and the prompt never returns; the probe gives up after 240 s.
5. The session's subagent/children.json shows that child with "phase":"awaiting_approval" and "permission_mode":"ask".

It happened every time: three of three runs, from this probe, from T3 Code, and from T3 Code's replay recorder. In auto mode, the same prompt completes.

Where it seems to come from

• runSubagentChild runs the child with the parent's tool context.
• When the child needs approval, TurnContext.registerApproval moves it to awaiting_approval and registers the request in the sub-agent approval registry.
• Only the terminal app drains that registry, via pendingApprovalRequest / resolveApproval in input_approval_runtime.zig and app_worker_runtime.zig. Nothing under src/acp calls either function, so over ACP the request is never shown to the user and never resolved.

Expected

The child's approval reaches the client as a session/request_permission on the parent session, ideally saying which sub-agent asks, and the client's answer resolves it. If fx can't route it, the child should fail rather than wait forever.

Attachments

• fx-0.0.14-acp-subagent-supervised.ndjson: every ACP frame from the probe run, with home paths replaced and the host's skill list omitted. The last frames are the stuck call's in_progress and the other call's completed.
• acp-subagent-probe.mjs: the probe client.
• children.json: the session's sub-agent state after the hang.

Found while adding fx to T3 Code (pingdotgg/t3code#18277). T3 Code shows fx sub-agents in their own threads, and for now tells users to switch to Auto when delegating.