Authentication in High Level MCPServer #3283

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1. saurabhlalsaxena

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   questionFurther information is requestedFurther information is requested

   on Aug 11, 2026
2. maxisbey commented on Aug 11, 2026

   

   maxisbey

   on Aug 11, 2026

   Last edited by maxisbey

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   Yeah, I agree this is awkward. token_verifier= shouldn't make you invent an authorization server, and the docs only ever show the "there's an AS somewhere" shape.

   To answer the direct question: OAuth is a constructor-shape prerequisite today, not a runtime one. Nothing ever contacts issuer_url; in resource-server-only mode it's just echoed into the /.well-known/oauth-protected-resource document. And resource_server_url is typed AnyHttpUrl | None, so if you pass None explicitly that document isn't served at all and an unauthenticated request gets a plain 401 with WWW-Authenticate: Bearer error="invalid_token" and no discovery pointer, which is exactly the flow you drew.

   A few things that I think explain what you hit:

   • The verifier itself
     • AccessToken requires client_id and scopes as well as token.
     • AccessToken(token=token) raises a validation error inside verify_token, so the correct token gets a 500 and everything else a 401. The static token could never succeed as written.
   • The OAuth flow
     • The server config doesn't push a client into OAuth; a 401 does.
     • That "(or origin)" message is from the TypeScript SDK's client. Clients built on it (Inspector etc.) react to a 401 by fetching the metadata document and chasing authorization_servers, which for a placeholder issuer is a dead end.
     • If the client sends Authorization: Bearer … from the first request, none of that runs. In Inspector that's the custom headers section (leave the OAuth settings empty); most hosts take a headers map in the server config.
   • localhost vs 127.0.0.1
     • That's the client correctly (per RFC 9728) rejecting a metadata document whose resource doesn't match the URL it dialed.
     • If you keep resource_server_url, set it to exactly the URL clients connect to.
     • With resource_server_url=None there's no document to mismatch.

   Here's a working example end to end:

   server.py

   import secrets
   
   from pydantic import AnyHttpUrl
   
   from mcp.server import MCPServer
   from mcp.server.auth.middleware.auth_context import get_access_token
   from mcp.server.auth.provider import AccessToken, TokenVerifier
   from mcp.server.auth.settings import AuthSettings
   
   TOKEN = "alice-token"  # read from the environment in real life, and serve over TLS if it's remote
   
   
   class StaticTokenVerifier(TokenVerifier):
       async def verify_token(self, token: str) -> AccessToken | None:
           if secrets.compare_digest(token, TOKEN):
               return AccessToken(token=token, client_id="alice", scopes=[])
           return None
   
   
   mcp = MCPServer(
       "notes",
       token_verifier=StaticTokenVerifier(),
       auth=AuthSettings(
           issuer_url=AnyHttpUrl("https://unused.invalid"),  # placeholder, never contacted
           resource_server_url=None,  # no metadata document, plain 401
       ),
   )
   
   
   @mcp.tool()
   def whoami() -> str:
       token = get_access_token()
       return token.client_id if token else "anonymous"
   
   
   if __name__ == "__main__":
       mcp.run("streamable-http", port=10000)

   client.py

   import anyio
   import httpx2
   
   from mcp import Client
   from mcp.client.streamable_http import streamable_http_client
   
   
   async def main() -> None:
       async with httpx2.AsyncClient(
           headers={"Authorization": "Bearer alice-token"},
           timeout=httpx2.Timeout(30.0, read=300.0),
           follow_redirects=True,
       ) as http:
           transport = streamable_http_client("http://127.0.0.1:10000/mcp", http_client=http)
           async with Client(transport) as client:
               result = await client.call_tool("whoami", {})
               print(result.content[0].text)  # alice
   
   
   anyio.run(main)

   There's a runnable version of this in examples/stories/bearer_auth/ (it keeps resource_server_url set so you can see the metadata route too), and the client-side header pattern is under "Bring your own httpx2.AsyncClient" in docs/client/transports.md.

   Hopefully that gets you going, but let me know if it doesn't :)

   _{AI Disclaimer}

   👍React with 👍1Reacted by saurabhlalsaxena
3. saurabhlalsaxena commented on Aug 11, 2026

   

   saurabhlalsaxena

   on Aug 11, 2026

   Last edited by saurabhlalsaxena

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   Thanks. I tried running your code and using the mcp inspector to test the MCP, I still get the same error:

   Failed to connect to "mcp-server"
   Protected resource http://localhost:10000/mcp does not match expected http://127.0.0.1:10000/mcp (or origin)
   

   Here are the server logs:

   INFO:     Application startup complete.
   INFO:     Uvicorn running on http://127.0.0.1:10000 (Press CTRL+C to quit)
   INFO:     127.0.0.1:51525 - "GET / HTTP/1.1" 404 Not Found
   INFO:     127.0.0.1:51525 - "GET /json/version HTTP/1.1" 404 Not Found
   INFO:     127.0.0.1:51527 - "GET / HTTP/1.1" 404 Not Found
   INFO:     127.0.0.1:51542 - "POST /mcp HTTP/1.1" 401 Unauthorized
   INFO:     127.0.0.1:51545 - "GET /.well-known/oauth-authorization-server HTTP/1.1" 404 Not Found
   INFO:     127.0.0.1:51545 - "GET /.well-known/openid-configuration HTTP/1.1" 404 Not Found
   INFO:     127.0.0.1:51542 - "POST /mcp HTTP/1.1" 401 Unauthorized
   INFO:     127.0.0.1:51545 - "GET /.well-known/oauth-authorization-server HTTP/1.1" 404 Not Found
   INFO:     127.0.0.1:51545 - "GET /.well-known/openid-configuration HTTP/1.1" 404 Not Found
   INFO:     127.0.0.1:51568 - "POST /mcp HTTP/1.1" 401 Unauthorized
   INFO:     127.0.0.1:51569 - "GET /.well-known/oauth-authorization-server HTTP/1.1" 404 Not Found
   INFO:     127.0.0.1:51569 - "GET /.well-known/openid-configuration HTTP/1.1" 404 Not Found
   INFO:     127.0.0.1:51568 - "POST /mcp HTTP/1.1" 401 Unauthorized
   INFO:     127.0.0.1:51569 - "GET /.well-known/oauth-authorization-server HTTP/1.1" 404 Not Found
   INFO:     127.0.0.1:51569 - "GET /.well-known/openid-configuration HTTP/1.1" 404 Not Found
4. saurabhlalsaxena commented on Aug 11, 2026

   

   saurabhlalsaxena

   on Aug 11, 2026

   Last edited by saurabhlalsaxena

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   Ok. So got it to work. Had missed your point on using TypeScript SDK client. Here is what I had to change:

   In MCP Inspector, added this under Custom Headers

   Authorization: Bearer alice-token
   

   Name | Value
   Authorization | Bearer alice-token

   And left the OAuth configuration empty.

   My feedback would still be that the Auth flow is fragmented. Maybe Auth needs to be a separate library (mcp-auth) instead of trying to incorporate it in the MCP SDK, just as you have 'flask-login' for flask.

   Also the issues with the TypeScript SDK client seems like a bug.
5. maxisbey

   added

   P3Nice to haves, rare edge casesNice to haves, rare edge cases

   v1Affects the v1.x maintenance lineAffects the v1.x maintenance line

   v2Affects the v2 line (2.x on main)Affects the v2 line (2.x on main)

   on Aug 12, 2026