[BUG] Plan contamination after forked chat.

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 fork a chat several times and create a different plan for each concurrently. If I start execution of one plan A and the other plan B finishes in the middle. The plan that finishes later B sometimes contaminates the other chat A and replaces it, completely trashing the original plan in that session A.

What Should Happen?

A new plan completed should not affect another running chat session

Error Messages/Logs

Steps to Reproduce

Fork conversations several times. Start different plans and start one before the others session complete the plan.

Claude Model

Opus

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.1.39

Platform

Anthropic API

Operating System

Windows 11

Terminal/Shell

VS Code extension

Additional Information

<img width="974" height="610" alt="Image" src="https://github.com/user-attachments/assets/6970d46d-1bf2-4711-b8db-019348332784" />