feat: expose OAuth metadata discovery provenance

crates/rmcp/src/transport/auth.rs

@@ -578,6 +578,28 @@ pub struct AuthorizationMetadata {
     pub additional_fields: HashMap<String, serde_json::Value>,
 }
 
+/// Describes how authorization metadata was obtained during OAuth discovery.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+#[non_exhaustive]
+pub enum AuthorizationMetadataSource {
+    /// Protected resource metadata identified the authorization server.
+    ProtectedResourceMetadata,
+    /// Authorization server metadata was discovered directly from the base URL.
+    AuthorizationServerMetadata,
+    /// No metadata was discovered; endpoints were synthesized for legacy OAuth.
+    LegacyEndpointFallback,
+}
+
+/// Authorization metadata together with the discovery mechanism that produced it.
+#[derive(Debug, Clone)]
+#[non_exhaustive]
+pub struct DiscoveredAuthorizationMetadata {
+    /// The discovered authorization metadata or synthesized legacy endpoints.
+    pub metadata: AuthorizationMetadata,
+    /// Whether the metadata was discovered or synthesized as a legacy fallback.
+    pub source: AuthorizationMetadataSource,
+}
+
 #[derive(Debug, Clone, Deserialize)]
 struct ResourceServerMetadata {
     resource: Option<String>,
@@ -1320,18 +1342,44 @@ impl AuthorizationManager {
         Ok(())
     }
 
-    /// discover oauth2 metadata (per SEP-985: Protected Resource Metadata first, then direct OAuth)
+    /// Discover OAuth metadata, preserving legacy endpoint fallback compatibility.
+    ///
+    /// Use [`Self::discover_metadata_with_source`] when callers need to
+    /// distinguish discovered metadata from synthesized legacy endpoints.
     pub async fn discover_metadata(&self) -> Result<AuthorizationMetadata, AuthError> {
+        self.discover_metadata_with_source()
+            .await
+            .map(|discovered| discovered.metadata)
+    }
+
+    /// Discover OAuth metadata and identify the mechanism that produced it.
+    ///
+    /// Protected resource metadata is tried first, followed by direct
+    /// authorization server metadata discovery. If neither succeeds, legacy
+    /// endpoints are synthesized for compatibility and explicitly identified
+    /// as [`AuthorizationMetadataSource::LegacyEndpointFallback`].
+    pub async fn discover_metadata_with_source(
+        &self,
+    ) -> Result<DiscoveredAuthorizationMetadata, AuthError> {
         if let Some(metadata) = self.discover_oauth_server_via_resource_metadata().await? {
-            return Ok(metadata);
+            return Ok(DiscoveredAuthorizationMetadata {
+                metadata,
+                source: AuthorizationMetadataSource::ProtectedResourceMetadata,
+            });
         }
 
         if let Some(metadata) = self.try_discover_oauth_server(&self.base_url).await? {
-            return Ok(metadata);
+            return Ok(DiscoveredAuthorizationMetadata {
+                metadata,
+                source: AuthorizationMetadataSource::AuthorizationServerMetadata,
+            });
         }
 
         debug!("falling back to legacy OAuth endpoints derived from the base URL");
-        Ok(Self::legacy_authorization_metadata(&self.base_url))
+        Ok(DiscoveredAuthorizationMetadata {
+            metadata: Self::legacy_authorization_metadata(&self.base_url),
+            source: AuthorizationMetadataSource::LegacyEndpointFallback,
+        })
     }
 
     fn legacy_authorization_metadata(base_url: &Url) -> AuthorizationMetadata {
@@ -3695,10 +3743,10 @@ mod tests {
 
     use super::{
         AuthError, AuthorizationCallback, AuthorizationManager, AuthorizationMetadata,
-        AuthorizationRequest, AuthorizationSession, CredentialStore, InMemoryCredentialStore,
-        InMemoryStateStore, OAuthClientConfig, OAuthHttpClient, OAuthHttpClientError,
-        OAuthHttpClientFuture, OAuthHttpRedirectPolicy, OAuthHttpRequest, ScopeUpgradeConfig,
-        StateStore, StoredAuthorizationState, is_https_url,
+        AuthorizationMetadataSource, AuthorizationRequest, AuthorizationSession, CredentialStore,
+        InMemoryCredentialStore, InMemoryStateStore, OAuthClientConfig, OAuthHttpClient,
+        OAuthHttpClientError, OAuthHttpClientFuture, OAuthHttpRedirectPolicy, OAuthHttpRequest,
+        ScopeUpgradeConfig, StateStore, StoredAuthorizationState, is_https_url,
     };
     use crate::transport::auth::VendorExtraTokenFields;
 
@@ -3830,6 +3878,97 @@ mod tests {
         );
     }
 
+    #[tokio::test]
+    async fn discover_metadata_with_source_identifies_protected_resource_metadata() {
+        let challenge = oauth2::http::Response::builder()
+            .status(401)
+            .header(
+                "www-authenticate",
+                r#"Bearer resource_metadata="https://mcp.example.com/.well-known/oauth-protected-resource""#,
+            )
+            .body(Vec::new())
+            .unwrap();
+        let client = RecordingOAuthHttpClient::with_responses(vec![
+            challenge,
+            http_response(
+                200,
+                serde_json::json!({
+                    "resource": "https://mcp.example.com/mcp",
+                    "authorization_servers": ["https://auth.example.com"]
+                }),
+            ),
+            http_response(
+                200,
+                serde_json::json!({
+                    "issuer": "https://auth.example.com",
+                    "authorization_endpoint": "https://auth.example.com/authorize",
+                    "token_endpoint": "https://auth.example.com/token"
+                }),
+            ),
+        ]);
+        let manager = AuthorizationManager::new_with_oauth_http_client(
+            "https://mcp.example.com/mcp",
+            Arc::new(client),
+        )
+        .await
+        .unwrap();
+
+        let discovered = manager.discover_metadata_with_source().await.unwrap();
+
+        assert_eq!(
+            (
+                discovered.source,
+                discovered.metadata.issuer.as_deref(),
+                discovered.metadata.token_endpoint.as_str(),
+            ),
+            (
+                AuthorizationMetadataSource::ProtectedResourceMetadata,
+                Some("https://auth.example.com"),
+                "https://auth.example.com/token",
+            )
+        );
+    }
+
+    #[tokio::test]
+    async fn discover_metadata_with_source_identifies_authorization_server_metadata() {
+        let client = RecordingOAuthHttpClient::with_responses(vec![
+            empty_response(404),
+            empty_response(404),
+            empty_response(404),
+            http_response(
+                200,
+                serde_json::json!({
+                    "issuer": "https://mcp.example.com",
+                    "authorization_endpoint": "https://mcp.example.com/authorize",
+                    "token_endpoint": "https://mcp.example.com/token"
+                }),
+            ),
+        ]);
+        let manager = AuthorizationManager::new_with_oauth_http_client(
+            "https://mcp.example.com/",
+            Arc::new(client.clone()),
+        )
+        .await
+        .unwrap();
+
+        let discovered = manager.discover_metadata_with_source().await.unwrap();
+
+        assert_eq!(
+            (
+                discovered.source,
+                discovered.metadata.issuer.as_deref(),
+                discovered.metadata.token_endpoint.as_str(),
+                client.requests().len(),
+            ),
+            (
+                AuthorizationMetadataSource::AuthorizationServerMetadata,
+                Some("https://mcp.example.com"),
+                "https://mcp.example.com/token",
+                4,
+            )
+        );
+    }
+
     #[tokio::test]
     async fn protected_resource_metadata_supports_authorization_server_path_insertion() {
         let client = RecordingOAuthHttpClient::with_responses(vec![
@@ -4124,6 +4263,44 @@ mod tests {
         );
     }
 
+    #[tokio::test]
+    async fn discover_metadata_with_source_identifies_legacy_endpoint_fallback() {
+        let client = RecordingOAuthHttpClient::with_responses(vec![
+            empty_response(404),
+            empty_response(404),
+            empty_response(404),
+            empty_response(404),
+            empty_response(404),
+        ]);
+        let manager = AuthorizationManager::new_with_oauth_http_client(
+            "https://public.example.com/",
+            Arc::new(client.clone()),
+        )
+        .await
+        .unwrap();
+
+        let discovered = manager.discover_metadata_with_source().await.unwrap();
+
+        assert_eq!(
+            (
+                discovered.source,
+                discovered.metadata.authorization_endpoint.as_str(),
+                discovered.metadata.token_endpoint.as_str(),
+                discovered.metadata.registration_endpoint.as_deref(),
+                discovered.metadata.issuer.as_deref(),
+                client.requests().len(),
+            ),
+            (
+                AuthorizationMetadataSource::LegacyEndpointFallback,
+                "https://public.example.com/authorize",
+                "https://public.example.com/token",
+                Some("https://public.example.com/register"),
+                None,
+                5,
+            )
+        );
+    }
+
     fn preregistered_as_metadata_response() -> HttpResponse {
         http_response(
             200,