Background subagent can remain running indefinitely after tool activity stops #4850

Describe the bug

A background review subagent remained marked as running long after observable activity stopped. The status stayed at 100 completed tool calls and zero completed turns while elapsed time continued increasing past 15 minutes. The parent session received no result and could not complete the workflow.

Affected version

1.0.83

Steps to reproduce the behavior

1. Start a background general-purpose subagent for a final branch review.
2. Allow it to perform tool calls.
3. Observe the agent reach approximately 100 completed tool calls.
4. Continue checking its status after several minutes.
5. The agent remains running, with the tool-call count unchanged and no completed turn or result.

Expected behavior

The runtime should detect stalled subagents, surface the blocked tool or execution state, and either recover, fail with an actionable error, or enforce a bounded timeout. The parent should not wait indefinitely with no progress signal.

Additional context

• Linux
• Copilot CLI 1.0.83
• Observed elapsed time: over 15 minutes
• Status remained: running, 100 tool calls completed, 0 turns completed
• Implementation and the full 78-test regression suite had already completed; the stalled operation was only a final read-only review.
• No private repository content or session logs are attached.