Server ClientAuthenticator requires client_id in token body, rejecting valid client_secret_basic requests (RFC 6749 §2.3.1)

Summary

The server-side ClientAuthenticator reads client_id only from the token request form body and raises invalid_client: "Missing client_id" when it is absent — even when the client authenticated correctly via HTTP Basic (Authorization: Basic base64(client_id:client_secret)). Per RFC 6749 §2.3.1, a client using client_secret_basic sends its client_id and client_secret in the Authorization header and MAY omit them from the body. Servers advertising client_secret_basic in token_endpoint_auth_methods_supported therefore reject spec-compliant clients (e.g. clients that use Basic and do not duplicate client_id into the body).

Location

src/mcp/server/auth/middleware/client_auth.py, authenticate_request:

form_data = await request.form()
client_id = form_data.get("client_id")
if not client_id:
    raise AuthenticationError("Missing client_id")
...
if client.token_endpoint_auth_method == "client_secret_basic":
    ...
    basic_client_id, request_client_secret = decoded.split(":", 1)
    basic_client_id = unquote(basic_client_id)
    if basic_client_id != client_id:   # only cross-checks; never used as fallback
        raise AuthenticationError("Client ID mismatch in Basic auth")

client_id from the Basic header is only used to cross-check a body-supplied value; it is never used as a fallback source. So a request with credentials solely in the Basic header fails before the client is even looked up.

Steps to reproduce

1. Register a client with token_endpoint_auth_method=client_secret_basic.
2. POST /token with grant_type=authorization_code, Authorization: Basic base64(client_id:client_secret), and no client_id/client_secret in the form body.
3. Response: 401 {"error":"invalid_client","error_description":"Missing client_id"}.

Sending the same credentials via client_secret_post (in the body) works.

Expected

When Authorization: Basic is present, the authenticator should derive client_id from the header if it is absent from the body (RFC 6749 §2.3.1), then verify the secret as it does today.

Notes

This is the mirror image of client-side PR #3536 (which stops MCP clients from putting client_id in the body under client_secret_basic). With that client-side change, compliant clients will send client_id only in the Basic header — which this server-side code rejects. The two need to agree.

Observed on mcp 1.27.0.

Start in src/mcp/server/auth/middleware/client_auth.py at authenticate_request and trace how the form fields and HTTP Basic credentials are parsed. Reproduce the token request with credentials only in the Authorization header, then verify that client_secret_basic authentication succeeds while mismatched credentials still produce the documented authentication error.