[FEATURE] Warn users when ANTHROPIC_BASE_URL overrides OAuth/subscription credentials

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

When users have both OAuth credentials (Max/Pro subscription) AND custom environment variables like ANTHROPIC_BASE_URL in their settings.json, Claude Code silently uses the custom URL instead of Anthropic's API.

This causes confusing 404 "model not found" errors even though the user has a valid subscription. The .credentials.json correctly shows subscriptionType: "max", but API calls fail because they're being routed to a non-Anthropic endpoint.

I faced this issue after experimenting with a local LLM setup. I had leftover ANTHROPIC_BASE_URL in my settings.json. It took hours to debug because there was no warning that my subscription credentials were being overridden.

Multiple users have reported similar confusion in issues #20976, #16489, #22972, and #16238.

Proposed Solution

Claude Code should detect and warn users when:

1. Valid OAuth/subscription credentials exist in .credentials.json, AND
2. ANTHROPIC_BASE_URL is set to a non-Anthropic endpoint in settings or environment

Suggested warning message:

Warning: ANTHROPIC_BASE_URL is set to a custom endpoint.
Your subscription credentials will not be used.
Run `/logout` and remove ANTHROPIC_BASE_URL from settings to use your Max/Pro subscription.

This warning should appear:

• On startup when the conflict is detected
• When running /doctor command
• In the UI status bar (instead of just showing "API Usage Billing")

Alternative Solutions

Currently there's no warning. Users have to:

• Manually inspect settings.json and .credentials.json
• Search GitHub issues to find similar problems
• Trial and error to identify the conflict

Other tools like VS Code warn when conflicting settings are detected.

Priority

High - Significant impact on productivity

Feature Category

Configuration and settings

Use Case Example

Example scenario:

1. User sets up Claude Code with Max subscription - works fine
2. User experiments with local LLM (Ollama) and adds ANTHROPIC_BASE_URL to settings.json
3. User switches back to using Claude Code normally
4. User forgets to remove the custom URL setting
5. Claude Code shows "API Usage Billing" and all models return 404
6. User spends hours debugging, checking subscription, re-logging in
7. User finally discovers the leftover setting was overriding their subscription

With this feature:

• Step 5 would show a clear warning about the configuration conflict
• User immediately knows to remove the custom URL
• Problem solved in seconds instead of hours

Additional Context

Related GitHub Issues:

• #20976 - Max plan shows as 'API Usage Billing' in CLI
• #16489 - Claude Code uses API billing despite forceLoginMethod setting
• #22972 - CLI shows 'Claude API' despite OAuth account configured
• #16238 - Similar authentication confusion

The fix is straightforward:

• Check if OAuth credentials exist with valid subscriptionType
• Check if ANTHROPIC_BASE_URL is set and doesn't point to api.anthropic.com
• If both conditions true, show warning

This small addition would prevent significant user frustration and reduce support burden.