[BUG] 2.1.77 (Claude Code) - ExitPlanMode presents stale plan file content when re-entering plan mode

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?

When re-entering plan mode in a conversation where a previous plan file already exists, ExitPlanMode displays the contents of the old plan file to the user — even if the current planning session has nothing to do with the previous one.

What Should Happen?

ExitPlanMode should either:

• Require the plan file to have been written/edited during the current plan mode session before allowing exit, OR
• Warn Claude (or the user) that the plan file content appears unchanged from a previous session, OR
• Clear the plan file when re-entering plan mode for a different task

Error Messages/Logs

Steps to Reproduce

1. Start a conversation and enter plan mode for Task A
2. Write a detailed plan to the plan file, exit plan mode, and implement it
3. Later in the same conversation, enter plan mode again for Task B (completely unrelated)
4. The system prompt correctly warns: "A plan file exists... evaluate the user's current request against that plan"
5. If Claude calls ExitPlanMode without fully overwriting the plan file, the user is presented with the old Task A plan as if it were the proposal for Task B

Claude Model

Opus

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.1.77 (Claude Code)

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

Windows Terminal

Additional Information

I think is just when... after finishing a 'big' plan and implementing it (but not much tokens in context, just 87k), I ask him to do a very very little thing without plan mode, and I ask him to do another little big plan in plan mode again. Happened several times in several sessions, and I think it was the same in all.