[BUG] GitHub MCP OAuth flow fails on claude.ai/code — localhost callback unreachable

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?

The mcp__github__authenticate tool generates an OAuth URL with
redirect_uri=http://localhost:{port}/callback

This only works when Claude Code runs as a local CLI (which spins up
a temporary HTTP server). On claude.ai/code (web) and mobile, the
localhost callback is unreachable:

ERR_CONNECTION_REFUSED on localhost

Steps to reproduce:

1. Use Claude Code on claude.ai/code (web interface)
2. Call mcp__github__authenticate
3. Open the generated OAuth URL
4. Complete Google/GitHub authorization
5. Callback redirects to localhost → connection refused

Expected: OAuth should work on all Claude Code platforms
Suggested fix: Use a hosted callback endpoint (e.g., api.anthropic.com/callback)
that can relay the auth token back to the session, or provide a manual
token paste flow as fallback.

What Should Happen?

Expected: OAuth should work on all Claude Code platforms
Suggested fix: Use a hosted callback endpoint (e.g., api.anthropic.com/callback)
that can relay the auth token back to the session, or provide a manual
token paste flow as fallback.

<img width="1166" height="684" alt="Image" src="https://github.com/user-attachments/assets/b916c50c-bb2b-40b4-bf3f-e58dee2f586b" />

<img width="947" height="668" alt="Image" src="https://github.com/user-attachments/assets/cf774d88-a759-4f44-9caa-9bff99049449" />

Error Messages/Logs

ERR_CONNECTION_REFUSED on localhost

Steps to Reproduce

Steps to reproduce:

1. Use Claude Code on claude.ai/code (web interface)
2. Call mcp__github__authenticate
3. Open the generated OAuth URL
4. Complete Google/GitHub authorization
5. Callback redirects to localhost → connection refused

Claude Model

Opus

Is this a regression?

I don't know

Last Working Version

_No response_

Claude Code Version

Claude Code 2.1.96

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

Other

Additional Information

_No response_