Fix OAuth metadata validation for compliant servers #778
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The OAuthMetadata validation was too restrictive, rejecting valid OAuth 2.0 server configurations that advertise additional authentication methods beyond the minimum required set. Changes: - Allow 'client_secret_basic' in token_endpoint_auth_methods_supported - Allow 'plain' in code_challenge_methods_supported This fixes compatibility with MCP servers like Asana (mcp.asana.com) that advertise support for multiple OAuth authentication methods as allowed by the OAuth 2.0 specification and MCP specification. The MCP specification does not restrict which authentication methods servers can support, only requiring that PKCE is used. These changes ensure the client can connect to any compliant MCP server regardless of which optional authentication methods they also support.
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Does anyone know why this isn’t being actively discussed? I couldn’t find any documentation mentioning that this isn’t supported I only found out by digging through the source code. cc @dsp-ant |
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Motivation and Context
The OAuth metadata validation was overly restrictive, causing connection failures with
compliant MCP servers like Asana that support multiple OAuth authentication methods.
The validation rejected servers advertising additional authentication methods beyond
the minimum required set, even though these methods are allowed by both the MCP and
OAuth 2.0 specifications.
Specifically, servers returning:
token_endpoint_auth_methods_supported:["client_secret_basic","client_secret_post","none"]code_challenge_methods_supported:["plain","S256"]Were rejected because the client only accepted a limited subset of these methods.
How Has This Been Tested?
https://mcp.asana.com/sseBreaking Changes
No - this change maintains backward compatibility while expanding support for
additional compliant servers.
Types of changes
change)
Checklist
Additional context
Changes made:
client_secret_basicto allowedtoken_endpoint_auth_methods_supportedplainto allowedcode_challenge_methods_supportedCompliance:
ocol.io/specification/2025-03-26/basic/authorization.md) requires PKCE support but
doesn't restrict which authentication methods servers can advertise
relevant RFCs
This fix enables the MCP Python SDK to work with any compliant MCP server regardless
of which optional OAuth authentication methods they advertise.