Claude Code injects `effort-2025-11-24` beta header, breaking Vertex AI routing

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?

Version: 2.1.76
Platform: Mac
Setup: Routing through Vertex AI for billing

Error:
API Error: 400 {"error":{"message":"vertex-ai error: Unexpected value(s)
effort-2025-11-24 for the anthropic-beta header."}}

Steps to reproduce:
Run any Claude Code command while configured to use Vertex AI.

Finding:
grep -r "effort-2025" ~/.local/share/claude/versions/
→ matches 2.1.71, 2.1.72, 2.1.76

The beta header appears to be injected automatically by the Claude Code binary
and is not supported by Vertex AI.

What Should Happen?

It should run without the error.

Error Messages/Logs

API Error: 400 {"error":{"message":"vertex-ai error: Unexpected value(s) `adaptive-thinking-2026-01-28`, 
     `effort-2025-11-24` for the `anthropic-beta` header. Please consult our documentation at docs.anthropic.com or  
     try again without the header.","type":"invalid_request_error","param":null,"code":null},"provider":"vertex-ai"}

Steps to Reproduce

Open claude code and run any code

Claude Model

Sonnet (default)

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

2.1.76

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

_No response_