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

Start in src/mcp/server/auth/middleware/client_auth.py at the linked ClientAuthenticator logic. Reproduce the case where token_endpoint_auth_method is none while a client secret is stored, then verify that request credentials are not required for this method and that other methods retain their checks. Add or update the relevant authentication test to cover the behavior.