feat(auth): add token_endpoint_auth_method to OAuthClientConfig #648
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Some OAuth providers (e.g. HubSpot) require client credentials to be sent as POST body parameters (client_secret_post) instead of via HTTP Basic Auth header. The oauth2 crate defaults to BasicAuth, and rmcp had no way to override this, causing TokenExchangeFailed errors. Add an optional `token_endpoint_auth_method` field to OAuthClientConfig that accepts "client_secret_post" (RequestBody) and "client_secret_basic" (BasicAuth). Unknown values are silently ignored, preserving the default. Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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Move auth method selection from per-client config to server's AuthorizationMetadata, which is the correct OAuth 2.0 approach. Servers like HubSpot advertise token_endpoint_auth_methods_supported in their metadata; reading it from there avoids manual configuration and prevents TokenExchangeFailed errors with non-BasicAuth providers. Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
…ional_fields Move token_endpoint_auth_methods_supported out of AuthorizationMetadata as an explicit field and read it from the serde(flatten) additional_fields HashMap instead. This avoids serializing `null` when the field is absent, which broke Zod validation in downstream consumers like MCP Inspector. Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
… test assertions When token_endpoint_auth_methods_supported contains both client_secret_post and client_secret_basic, default to basic auth per RFC 6749 §2.3.1. Update configure_client tests to assert actual AuthType instead of is_some(). Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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Some OAuth providers (e.g. HubSpot) require client credentials to be sent as POST body parameters (client_secret_post) instead of via HTTP Basic Auth header. The oauth2 crate defaults to BasicAuth, and rmcp had no way to honor the server's advertised auth method, causing TokenExchangeFailed errors.
Derive token_endpoint_auth_method from the server's authorization metadata (token_endpoint_auth_methods_supported) during configure_client. When the server advertises client_secret_post, the client is set to AuthType::RequestBody; otherwise it defaults to
BasicAuth. No new config fields are added — the behavior is driven entirely by server metadata.
Motivation and Context
OAuth servers advertise supported token endpoint auth methods via token_endpoint_auth_methods_supported in their authorization server metadata. rmcp was ignoring this field, always using Basic Auth. Servers like HubSpot that require client_secret_post would reject token requests.
How Has This Been Tested?
Tested with a simple MCP client connecting to HubSpot's remote MCP server (
https://mcp.hubspot.com/mcp), which requiresclient_secret_postauthentication.Breaking Changes
No breaking changes.
Types of changes
Checklist
Additional context
The TypeScript MCP SDK already implements the same
token_endpoint_auth_methods_supportedpattern:packages/core/src/shared/auth.ts— definestoken_endpoint_auth_methods_supportedinOAuthMetadataSchemapackages/client/src/client/auth.ts—selectClientAuthMethod()readstoken_endpoint_auth_methods_supportedfrom server metadata and picks betweenclient_secret_basic,client_secret_post, ornone;applyClientAuthentication()then sends credentials accordingly (Basic header vs POST body)This PR aligns the Rust SDK with the TypeScript SDK's existing behavior.