feat(auth): add token_endpoint_auth_method to OAuthClientConfig

crates/rmcp/src/transport/auth.rs

@@ -2,7 +2,7 @@ use std::{collections::HashMap, sync::Arc, time::Duration};
 
 use async_trait::async_trait;
 use oauth2::{
-    AuthUrl, AuthorizationCode, ClientId, ClientSecret, CsrfToken, EmptyExtraTokenFields,
+    AuthType, AuthUrl, AuthorizationCode, ClientId, ClientSecret, CsrfToken, EmptyExtraTokenFields,
     PkceCodeChallenge, PkceCodeVerifier, RedirectUrl, RefreshToken, RequestTokenError, Scope,
     StandardTokenResponse, TokenResponse, TokenUrl,
     basic::{BasicClient, BasicTokenType},
@@ -482,6 +482,21 @@ impl AuthorizationManager {
             client_builder = client_builder.set_client_secret(ClientSecret::new(secret));
         }
 
+        let uses_secret_post = metadata
+            .additional_fields
+            .get("token_endpoint_auth_methods_supported")
+            .and_then(|v| v.as_array())
+            .map(|arr| {
+                let has_basic = arr.iter().any(|m| m.as_str() == Some("client_secret_basic"));
+                let has_post = arr.iter().any(|m| m.as_str() == Some("client_secret_post"));
+                has_post && !has_basic
+            })
+            .unwrap_or(false);
+
+        if uses_secret_post {
+            client_builder = client_builder.set_auth_type(AuthType::RequestBody);
+        }
+
         self.oauth_client = Some(client_builder);
         Ok(())
     }
@@ -1451,14 +1466,15 @@ impl OAuthState {
 
 #[cfg(test)]
 mod tests {
+    use std::collections::HashMap;
     use std::sync::Arc;
 
-    use oauth2::{CsrfToken, PkceCodeVerifier};
+    use oauth2::{AuthType, CsrfToken, PkceCodeVerifier};
     use url::Url;
 
     use super::{
-        AuthError, AuthorizationManager, InMemoryStateStore, StateStore, StoredAuthorizationState,
-        is_https_url,
+        AuthError, AuthorizationManager, AuthorizationMetadata, InMemoryStateStore,
+        OAuthClientConfig, StateStore, StoredAuthorizationState, is_https_url,
     };
 
     // SEP-991: URL-based Client IDs
@@ -1876,4 +1892,107 @@ mod tests {
         let mut manager = AuthorizationManager::new("http://localhost").await.unwrap();
         manager.set_state_store(TrackingStateStore::default());
     }
+
+    /// Helper: create an AuthorizationManager with minimal metadata so
+    /// `configure_client` can be exercised without a live server.
+    async fn manager_with_metadata(
+        metadata_override: Option<AuthorizationMetadata>,
+    ) -> AuthorizationManager {
+        let mut mgr = AuthorizationManager::new("http://localhost").await.unwrap();
+        mgr.set_metadata(metadata_override.unwrap_or(AuthorizationMetadata {
+            authorization_endpoint: "http://localhost/authorize".to_string(),
+            token_endpoint: "http://localhost/token".to_string(),
+            ..Default::default()
+        }));
+        mgr
+    }
+
+    fn test_client_config() -> OAuthClientConfig {
+        OAuthClientConfig {
+            client_id: "my-client".to_string(),
+            client_secret: Some("my-secret".to_string()),
+            scopes: vec![],
+            redirect_uri: "http://localhost/callback".to_string(),
+        }
+    }
+
+    #[tokio::test]
+    async fn test_configure_client_uses_client_secret_post_from_metadata() {
+        let mut additional_fields = HashMap::new();
+        additional_fields.insert(
+            "token_endpoint_auth_methods_supported".to_string(),
+            serde_json::json!(["client_secret_post"]),
+        );
+        let meta = AuthorizationMetadata {
+            authorization_endpoint: "http://localhost/authorize".to_string(),
+            token_endpoint: "http://localhost/token".to_string(),
+            additional_fields,
+            ..Default::default()
+        };
+        let mut mgr = manager_with_metadata(Some(meta)).await;
+        mgr.configure_client(test_client_config()).unwrap();
+        assert!(matches!(mgr.oauth_client.as_ref().unwrap().auth_type(), AuthType::RequestBody));
+    }
+
+    #[tokio::test]
+    async fn test_configure_client_defaults_to_basic_auth() {
+        let mut mgr = manager_with_metadata(None).await;
+        mgr.configure_client(test_client_config()).unwrap();
+        assert!(matches!(mgr.oauth_client.as_ref().unwrap().auth_type(), AuthType::BasicAuth));
+    }
+
+    #[tokio::test]
+    async fn test_configure_client_with_explicit_basic_in_metadata() {
+        let mut additional_fields = HashMap::new();
+        additional_fields.insert(
+            "token_endpoint_auth_methods_supported".to_string(),
+            serde_json::json!(["client_secret_basic"]),
+        );
+        let meta = AuthorizationMetadata {
+            authorization_endpoint: "http://localhost/authorize".to_string(),
+            token_endpoint: "http://localhost/token".to_string(),
+            additional_fields,
+            ..Default::default()
+        };
+        let mut mgr = manager_with_metadata(Some(meta)).await;
+        mgr.configure_client(test_client_config()).unwrap();
+        assert!(matches!(mgr.oauth_client.as_ref().unwrap().auth_type(), AuthType::BasicAuth));
+    }
+
+    #[tokio::test]
+    async fn test_configure_client_ignores_unsupported_auth_methods_in_metadata() {
+        let mut additional_fields = HashMap::new();
+        additional_fields.insert(
+            "token_endpoint_auth_methods_supported".to_string(),
+            serde_json::json!(["private_key_jwt"]),
+        );
+        let meta = AuthorizationMetadata {
+            authorization_endpoint: "http://localhost/authorize".to_string(),
+            token_endpoint: "http://localhost/token".to_string(),
+            additional_fields,
+            ..Default::default()
+        };
+        let mut mgr = manager_with_metadata(Some(meta)).await;
+        // Unsupported method should fall through to default (basic auth)
+        mgr.configure_client(test_client_config()).unwrap();
+        assert!(matches!(mgr.oauth_client.as_ref().unwrap().auth_type(), AuthType::BasicAuth));
+    }
+
+    #[tokio::test]
+    async fn test_configure_client_prefers_basic_when_both_methods_supported() {
+        let mut additional_fields = HashMap::new();
+        additional_fields.insert(
+            "token_endpoint_auth_methods_supported".to_string(),
+            serde_json::json!(["client_secret_post", "client_secret_basic"]),
+        );
+        let meta = AuthorizationMetadata {
+            authorization_endpoint: "http://localhost/authorize".to_string(),
+            token_endpoint: "http://localhost/token".to_string(),
+            additional_fields,
+            ..Default::default()
+        };
+        let mut mgr = manager_with_metadata(Some(meta)).await;
+        mgr.configure_client(test_client_config()).unwrap();
+        assert!(matches!(mgr.oauth_client.as_ref().unwrap().auth_type(), AuthType::BasicAuth));
+    }
 }