[BUG] Effort not supported for Opus when using LiteLLM proxy with VertexAI

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?

CC /model Effort not supported for Opus and slider is greyed out.

<img width="1088" height="256" alt="Image" src="https://github.com/user-attachments/assets/41eb147d-461f-4bd7-b425-e40b3465c840" />

What Should Happen?

There should be an option to set effort for Opus 4.6

Error Messages/Logs

N/A

Steps to Reproduce

1. Setup CC with LiteLLM, select vertex ai opus 4.6 as default opus model using ANTHROPIC_DEFAULT_OPUS_MODEL env var.
2. Run CC and open /model.

Claude Model

Opus

Is this a regression?

I don't know

Last Working Version

N/A

Claude Code Version

2.1.37

Platform

Other

Operating System

Windows

Terminal/Shell

Windows Terminal

Additional Information

_No response_