[BUG] Claude can present the plan to the user before backgrounded subagents finish executing

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?

I ran into an issue where Claude (Opus 5) spun up a backgrounded design subagent while in plan mode, but then ended up presenting the plan to me without waiting for the result of the subagent. I always implement plans from a clear context, which would presumably have discarded the subagent's work. The only reason I caught on to this is because I had a correction to make to Claude's plan myself, and while it was thinking about that it got a notification that the subagent finished.

What Should Happen?

Claude should wait for all subagents to finish before presenting the plan to the user.

Error Messages/Logs

Steps to Reproduce

This is agent behavior which is inherently nondeterministic in its nature, but since the correct layer to fix this is the harness, it should be fairly easy to hook Claude Code up to an API that returns deterministic responses reproducing the undesirable behavior and verify that the harness blocks the action.

Claude Model

Opus

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.1.212

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

iTerm2

Additional Information

_No response_