Background subagent can remain running indefinitely after tool activity stops

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.

在 Linux 上复现后台 final-branch review，并在工具活动达到约 100 次调用后对其进行监控。首先跟踪 subagent 状态和 parent 的等待行为；完整的 78 项回归测试套件可用于验证。检测到停滞的工作、显示出可采取行动的结果，并且 parent 不再无限期等待，即表示完成。