[BUG] /context command doesn't work if the previous prompt has 'ultrathink' in it.

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?

/context doesn't show the context usage and throws a 400 error.

What Should Happen?

I need to be able the see the memory consumption by different resources with this command.

Error Messages/Logs

> /context 
  ⎿ Error: 400 {"type":"error","error":{"type":"invalid_request_error","message":"messages.3.content.0: When thinking is disabled, an `assistant` message in the final position cannot 
    contain `thinking`. To use thinking blocks, enable `thinking` in your request."},"request_id":"req_vrtx_XXXX"}

Steps to Reproduce

Go to an existing project.

Switch to Sonnet 4.5 and ask for it to provide project analysis and ask it to 'Ultrathink'. Make sure that 'Ultrathink' is in the prompt.

After the analysis is complete and the response is provided by Claude, try /context to see the error.

Claude Model

Sonnet (default)

Is this a regression?

Yes, this worked in a previous version

Last Working Version

1.0.126

Claude Code Version

2.0.0

Platform

Google Vertex AI

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

_No response_