GitHubProvider returns 400 when claude.ai skips Dynamic Client Registration

[saidsef]

What happened?

When adding a FastMCP server backed by GitHubProvider to claude.ai as a remote MCP server, the OAuth flow fails with a 400 "Client Not Registered" error. The browser popup shows:

The client ID Ov23lifBJkuc3N90PTD4 was not found in the server's client registry.

Clearing auth tokens and reconnecting does not fix it — the error reappears immediately. The server works fine with MCP Inspector and Claude Desktop.

The cause: claude.ai doesn't call /register before /authorize. It sends the GitHub OAuth App's client_id (the one you configured in GitHubProvider) directly in the authorization request. OAuthProxy.get_client() returns None for any id not previously registered via /register, and the MCP SDK's authorize handler returns 400 before the provider code is reached at all.

Example Code

from fastmcp import FastMCP
from fastmcp.server.auth.providers.github import GitHubProvider

auth = GitHubProvider(
    client_id="Ov23lifBJkuc3N90PTD4",   # GitHub OAuth App client_id
    client_secret="...",
    base_url="https://your-server.example.com",
)
mcp = FastMCP("My Server", auth=auth)

Steps:

1. Run the server above (publicly accessible over HTTPS)
2. In claude.ai → Settings → Integrations → Add remote MCP server
3. Set the server URL; when prompted for OAuth credentials, enter the GitHub OAuth App client_id
4. Browser popup opens and immediately shows the 400 error page

Expected: GitHub authorization screen (or FastMCP consent screen)
Actual: "Client Not Registered" error — flow never starts

Version Information

fastmcp 3.2.5
mcp 1.6.0
Python 3.14
Client: claude.ai web