[BUG] Non-thinking models from gateway providers fail

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 ANTHROPIC_BASE_URL is pointed to an external gateway and we select a non-thinking model using /model then requests fail because claude-code currently has adaptive thinking enabled.

What Should Happen?

Models should continue without thinking

Error Messages/Logs

Steps to Reproduce

Set ANTHROPIC_BASE_URL to an external provider, select a non-thinking model and send a message in claude code

Claude Model

Other

Is this a regression?

No, this never worked

Last Working Version

_No response_

Claude Code Version

2.1.126

Platform

Other

Operating System

Windows

Terminal/Shell

PowerShell

Additional Information

_No response_