GitHubProvider: a transient GitHub 5xx invalidates every active session #4854

What happened?

GitHubTokenVerifier.verify_token() returns None for any non-200 response from
https://api.github.com/user, so a GitHub 5xx produces the same result as an
invalid token. Clients receive invalid_token/401, discard the session, and
re-run the OAuth flow — which also calls GitHub, and also fails.

During the GitHub incident on 2026-08-17 ("Partial System Outage"), four
self-hosted servers using GitHubProvider were unusable for roughly five hours,
reconnect-looping. Tokens that had verified successfully seconds earlier were
rejected.

Expected: a transient upstream failure would be distinguishable from a rejected
credential, and would not invalidate sessions that verified moments before.

Related: #3619 (structured auth failure reasons), #4850 (same 401 symptom,
different cause — that report's token is valid offline; here the rejection
follows an upstream 503).

Example Code

"""A 503 from GitHub is indistinguishable from a 401: both yield None."""
import asyncio
import httpx
from fastmcp.server.auth.providers.github import GitHubTokenVerifier


def client_returning(status: int) -> httpx.AsyncClient:
    async def handler(request: httpx.Request) -> httpx.Response:
        return httpx.Response(status, json={"message": "simulated"})
    return httpx.AsyncClient(transport=httpx.MockTransport(handler))


async def main():
    for status in (401, 503):
        verifier = GitHubTokenVerifier(http_client=client_returning(status))
        result = await verifier.verify_token("any-token")
        print(f"GitHub {status} -> verify_token returned {result!r}")

asyncio.run(main())

# GitHub 401 -> verify_token returned None
# GitHub 503 -> verify_token returned None

Version Information

FastMCP version:                                        3.4.6
MCP version:                                           1.29.0
Python version:                                       3.12.14
Platform:          Linux-6.18.38-Unraid-x86_64-with-glibc2.41
FastMCP root path:    /usr/local/lib/python3.12/site-packages

Also reproduced identically on 3.4.7 (same MRE, same output).