[BUG] [VSCode] Crash when auto-compact triggers during plan mode — ExitPlanMode tool filtered out after compact boundary

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 auto-compact triggers during an active plan mode session in the VS Code extension, the session crashes with an error prompting the user to stop the prompt.
ExitPlanMode is a deferred tool only available while plan mode is active. When the compact agent rebuilds the session, the tool reference carried from pre-compact history is present but the tool itself isn't loaded into the new session context, causing it to be silently filtered out and leaving the session in a broken state.

What Should Happen?

Auto-compact completes and the session resumes normally, still in plan mode.

Error Messages/Logs

Session crashes after compaction. The extension logs show:                  
[DEBUG] autocompact: tokens=172076 threshold=167000 effectiveWindow=180000
[DEBUG] Forked agent [compact] finished: 1 messages ...
[WARN] Filtering out tool_reference for unavailable tool: ExitPlanMode

Steps to Reproduce

1. Open a plan mode session in the VS Code extension
2. Continue working until the session approaches the auto-compact threshold (~167k tokens)
3. Auto-compact triggers
4. Session crashes with an error prompting to stop the prompt

Claude Model

Not sure / Multiple models

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.1.85.82d

Platform

Other

Operating System

macOS

Terminal/Shell

VS Code integrated terminal

Additional Information

_No response_