Desktop app (Code tab): background subagent stalls silently, never notifies orchestrator, SendMessage status request goes unanswered — no stall detection

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report
• [x] I am using the latest version of Claude Code

What's Wrong?

In the Claude desktop app (Code tab), the orchestrator session sent a multi-step task to a background subagent (Agent tool, custom agent with model: opus, effort: high): fix a checker, lint, run pnpm run test:unit, then run a push command.

1. The subagent finished the code fix and test:unit (it passed and auto-committed). Its last text was "Now lint changed files." Then it stopped making progress.
2. No completion notification ever arrived. The harness kept treating it as running.
3. No process was running: no lint, test, push or git processes.
4. The orchestrator sent a status request with SendMessage. It was "queued for delivery at its next tool round" but never answered, because there was no next tool round.
5. About 8 minutes passed. The orchestrator got no signal at all and only found the stall because I asked. It had to inspect the process list and logs by hand, then call TaskStop and send the remaining step to a new agent.

What Should Happen?

The harness should detect a background subagent that stops making progress (no tool calls, no output, no running child processes) and tell the orchestrator right away, or mark it as failed or idle with its last output. A SendMessage to an idle agent should wake it or return an error, not sit in a queue forever. Users shouldn't need custom rules or watcher scripts to catch this.

Steps to Reproduce

Intermittent. Send a long multi-step background Agent task in the desktop app's Code tab (e.g. edit → lint → test → push). Sometimes the agent stops after an intermediate text block with no completion notification. Sending SendMessage to it then gets no response.

Is this a regression?

I don't know

Claude Code Version

2.1.287

Platform

Anthropic API

Operating System

macOS (Darwin 25.6.0)

Terminal/Shell

Claude desktop app, Code tab (zsh)

Claude Model

Opus 5.5 (orchestrator and subagent)

Additional Information

Related: #83848 (stalls with no final text), #82858 (SendMessage undelivered on idle), #89124 (no subagent timeout), #92264 (background session stops advancing), #61405 (closed for inactivity without a fix: subagent timeout/monitoring/abort). Several of these are open, labelled stale, and have no maintainer response. This needs platform-level stall detection.