[BUG] Model discovery incompatible with hybrid claude.ai/gateway authentication

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 using Claude Code with a model gateway, where Claude Code is configured to authenticate to both the model gateway and claude.ai (such as LiteLLM following this guide), model discovery does not function even if CLAUDE_CODE_ENABLE_GATEWAY_MODEL_DISCOVERY=1 is set.

In this configuration, the model proxy authentication token is supplied using ANTHROPIC_CUSTOMER_HEADER: "x-litellm-api-key: Bearer ....", and Claude Code handles the Anthropic Claude.ai credential. Although the docs state that the value of custom headers set in ANTHROPIC_CUSTOMER_HEADER will be included in the request, it appears as if Claude Code does not attempt model discovery unless ANTHROPIC_AUTH_TOKEN or ANTHROPIC_API_KEY is also set. This is undesirable, as we want Claude Code to be using OAuth to Claude.ai, not a static token, but we still want model discovery to occur.

What Should Happen?

So long as CLAUDE_CODE_ENABLE_GATEWAY_MODEL_DISCOVERY=1 is set, model discovery should occur. The presence or absence of ANTHROPIC_AUTH_TOKEN or ANTHROPIC_API_KEY should be irrelevant.

Error Messages/Logs

None

Steps to Reproduce

1. Configure a model proxy following this guide
2. Configure Claude Code to connect through this model proxy, and to use model discovery
3. Observe that model discovery was not used (e.g. by looking at /model, or by looking at model proxy logs)
4. Configure Claude Code to use ANTHROPIC_AUTH_TOKEN
5. Observe that model discovery now works

Claude Model

None

Is this a regression?

No, this never worked

Last Working Version

_No response_

Claude Code Version

2.1.179

Platform

Anthropic API

Operating System

Ubuntu/Debian Linux

Terminal/Shell

Other

Additional Information

_No response_