[BUG] Prompt-caching-scope-2026-01-05 for the anthropic-beta header.

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?

API Error: 400 {"error":{"message":"{"type":"error","error":{"type":"invalid_request_error","message":"Unexpected value(s) prompt-caching-scope-2026-01-05 for the anthropic-beta header. Please consult our documentation at docs.anthropic.com or try again without the header."},"request_id":"req_vrtx_011CXcAursNGvi98NDiv74kr"}. Received Model Group=claude-sonnet-4-5-20250929\nAvailable Model Group Fallbacks=None","type":"None","param":"None","code":"400"}}

What Should Happen?

API Error: 400 {"error":{"message":"{"type":"error","error":{"type":"invalid_request_error","message":"Unexpected value(s) prompt-caching-scope-2026-01-05 for the anthropic-beta header. Please consult our documentation at docs.anthropic.com or try again without the header."},"request_id":"req_vrtx_011CXcAursNGvi98NDiv74kr"}. Received Model Group=claude-sonnet-4-5-20250929\nAvailable Model Group Fallbacks=None","type":"None","param":"None","code":"400"}}

Error Messages/Logs

Steps to Reproduce

<img width="404" height="484" alt="Image" src="https://github.com/user-attachments/assets/4386bf94-722c-4733-ad01-82844480bd21" />

Claude Model

Sonnet (default)

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

2.0.75 (Claude Code)

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

_No response_