[BUG] Parent resumes as if subagent completed while subagent is waiting in Monitor for a background process

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

A foreground Opus 4.8 subagent starts a background process and waits for it using the \Monitor\ tool. While \Monitor\ is still waiting and the subagent has not returned a final response, the parent Fable 5 agent resumes as though the subagent completed.

Setup:

• Parent: Fable 5
• Subagent: Opus 4.8
• Normal Claude Code authentication
• Claude Max 5x subscription
• No proxy or custom API endpoint

What Should Happen?

The parent should remain blocked until the foreground subagent returns a final response, fails, or is explicitly stopped. A pending \Monitor\ call must not be treated as subagent completion.

Error Messages/Logs

No error is shown. The parent is incorrectly resumed while the subagent is still waiting.

Steps to Reproduce

1. Use Fable 5 as the parent agent.
2. Launch an Opus 4.8 subagent in the foreground.
3. Have the subagent start a long-running background process.
4. Have it wait for that process using \Monitor\.
5. Observe that the parent resumes before the process finishes and before the subagent returns a final response.

Claude Model

Not sure / Multiple models

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.1.212 (Claude Code)

Platform

Anthropic API

Operating System

Ubuntu/Debian Linux

Terminal/Shell

Other

Additional Information

_No response_