[BUG] New Session ID gets used when resuming without --fork-session

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?

New Session ID gets used when resuming without --fork-session

What Should Happen?

reuse session id and file

Error Messages/Logs

Steps to Reproduce

claude -c

Claude Model

None

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

2.0.1

Platform

Anthropic API

Operating System

Ubuntu/Debian Linux

Terminal/Shell

Other

Additional Information

I have a suspicion that this might be related to what is happening during session startup.
There sometimes is a summary message added async. When I noop'd the summarizer call in cli.js (in 1.0.12*) the session id and file got reused more often.
But a second factor was MCP servers being started.

Maybe anything happening async during session startup can force new uuid?