[Bug] Anthropic API Error: Third-party app routing for Claude Max OAuth sessions

Bug Description
Claude Code version: 2.1.29
Platform: macOS, zsh
Login method: Claude Max Account (OAuth) — confirmed via /status

Getting the 400 error immediately on any prompt, despite primary quota being available and limit having reset. Clean reinstall + cache wipe did not help. Error only disappears when using /extra-usage flag, which draws from the $100 credit instead of the Max subscription.

API Error: 400 {"type":"error","error":{"type":"invalid_request_error",
"message":"Third-party apps now draw from your extra usage, not your plan limits.
We've added a $100 credit to get you started. Claim it at claude.ai/settings/usage
and keep going."},"request_id":"req_011CZqv9gVfycaS3p2jbG5aC"}
Session ID: 4d5a1b2f-9225-4d65-8b74-7f722de7ba94

Note: Claude.ai chat works fine with the same account — issue appears specific to Claude Code API routing treating a Max OAuth session as a third-party app.

Environment Info

• Platform: darwin
• Terminal: vscode
• Version: 2.1.29
• Feedback ID: b3f7494f-1756-4c9f-b762-57f59fc00864