[BUG] No Effort slider available when using Claude code via vertex AI

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 opus 4.6 is selected on Claude code connected to vertex AI on the latest version, no Effort slider is show.

<img width="882" height="225" alt="Image" src="https://github.com/user-attachments/assets/f74fec29-c682-45b7-8f5c-1c47f2f2d27c" />

What Should Happen?

Claude code show an effort slider when opus 4.6 is selected to adjust how much "thinking" it does.

Error Messages/Logs

Steps to Reproduce

1.Setup Claude code to use Vertex AI [make sure Claude Opus 4.6 is made available in model garden].

1. Update Claude code to latest version.
2. open Claude code and run /model and select opus 4.6

Claude Model

Opus

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

2.1.34

Platform

Google Vertex AI

Operating System

macOS

Terminal/Shell

iTerm2

Additional Information

_No response_