ClientAuthenticator ignores token_endpoint_auth_method="none" when client_secret is stored

Initial Checks

• I confirm that I'm using the latest version of MCP Python SDK
• I confirm that I searched for my issue in https://github.com/modelcontextprotocol/python-sdk/issues before opening this issue

Description

ClientAuthenticator behavior is only partially honoring token_endpoint_auth_method="none":

https://github.com/modelcontextprotocol/python-sdk/blob/6b69f6354ad01e08abc8e1b81390c3a45132f594/src/mcp/server/auth/middleware/client_auth.py#L102-L104

Essentially:

• token_endpoint_auth_method="none" => skip extracting credentials from the request (correct)
• client.client_secret exists => raise error if no credentials were extracted from the request (incorrect)

If token_endpoint_auth_method="none" is set, it should never be checking for a client_secret value on the request, regardless of whether a secret has ever been generated for the client.

Suggested fix is to condition the client.client_secret check on token_endpoint_auth_method not being none:

        if token_endpoint_auth_method != "none" and client.client_secret:  # pragma: no branch
            if not request_client_secret:
                raise AuthenticationError("Client secret is required")  # pragma: no cover

Example Code

Python & MCP Python SDK

1.25.0

Commencez dans src/mcp/server/auth/middleware/client_auth.py, au niveau de la logique ClientAuthenticator liée. Reproduisez le cas où token_endpoint_auth_method vaut none alors qu’un secret client est stocké, puis vérifiez que les identifiants de la requête ne sont pas requis pour cette méthode et que les autres méthodes conservent leurs vérifications. Ajoutez ou mettez à jour le test d’authentification concerné pour couvrir ce comportement.