feat: make discovery source explicit

Author: DaleSeo

conformance/src/bin/client.rs

@@ -509,8 +509,8 @@ async fn migration_token(
         return Ok(manager.get_access_token().await?);
     }
 
-    let metadata = manager.discover_metadata().await?;
-    manager.set_metadata(metadata);
+    let discovered = manager.discover_metadata().await?;
+    manager.set_metadata(discovered.metadata);
     manager
         .register_client("conformance-client", REDIRECT_URI, &[])
         .await?;
@@ -609,9 +609,9 @@ async fn run_client_credentials_basic(
         .unwrap_or("conformance-test-secret");
 
     let mut manager = AuthorizationManager::new(server_url).await?;
-    let metadata = manager.discover_metadata().await?;
-    let token_endpoint = metadata.token_endpoint.clone();
-    manager.set_metadata(metadata);
+    let discovered = manager.discover_metadata().await?;
+    let token_endpoint = discovered.metadata.token_endpoint.clone();
+    manager.set_metadata(discovered.metadata);
 
     let http = reqwest::Client::new();
     let resp = http

crates/rmcp/src/transport/auth.rs

@@ -591,6 +591,11 @@ pub enum AuthorizationMetadataSource {
     /// URL (`/authorize`, `/token`, `/register`) for compatibility with the
     /// 2025-03-26 MCP spec's default-endpoint fallback. The server gave no
     /// evidence that it supports OAuth.
+    ///
+    /// [Newer MCP revisions] require metadata discovery and do not define an
+    /// endpoint-synthesis fallback.
+    ///
+    /// [Newer MCP revisions]: https://modelcontextprotocol.io/specification/draft/basic/authorization/authorization-server-discovery#protected-resource-metadata-discovery-requirements
     LegacyEndpointFallback,
 }
 
@@ -1268,8 +1273,10 @@ impl AuthorizationManager {
 
     /// Set OAuth2 authorization metadata
     ///
-    /// This should be called after discovering metadata via `discover_metadata()`
-    /// and before creating an `AuthorizationSession`.
+    /// This should be called with
+    /// [`DiscoveredAuthorizationMetadata::metadata`] after
+    /// [`Self::discover_metadata`] and before creating an
+    /// [`AuthorizationSession`].
     pub fn set_metadata(&mut self, metadata: AuthorizationMetadata) {
         self.metadata = Some(metadata);
     }
@@ -1284,8 +1291,8 @@ impl AuthorizationManager {
             && stored.token_response.is_some()
         {
             if self.metadata.is_none() {
-                let metadata = self.discover_metadata().await?;
-                self.metadata = Some(metadata);
+                let discovered = self.discover_metadata().await?;
+                self.metadata = Some(discovered.metadata);
             }
 
             if let (Some(stored_issuer), Some(current_issuer)) =
@@ -1352,27 +1359,32 @@ impl AuthorizationManager {
         Ok(())
     }
 
-    /// discover oauth2 metadata (per SEP-985: Protected Resource Metadata first, then direct OAuth)
+    /// Discover OAuth 2.0 metadata and report how it was obtained.
     ///
-    /// When no metadata can be discovered, this falls back to legacy default
-    /// endpoints derived from the base URL rather than returning an error, so
-    /// a successful result does not prove the server supports OAuth. Callers
-    /// that need to tell verified discovery apart from the synthesized
-    /// fallback should use [`Self::discover_metadata_with_source`] instead.
-    pub async fn discover_metadata(&self) -> Result<AuthorizationMetadata, AuthError> {
-        Ok(self.discover_metadata_with_source().await?.metadata)
-    }
-
-    /// Discover oauth2 metadata along with how it was obtained.
+    /// Discovery follows SEP-985: protected resource metadata first, then
+    /// direct OAuth 2.0 Authorization Server Metadata or OpenID Connect
+    /// Discovery. When discovery finds nothing, the result contains legacy
+    /// default endpoints derived from the base URL and
+    /// [`AuthorizationMetadataSource::LegacyEndpointFallback`].
     ///
-    /// Unlike [`Self::discover_metadata`], the returned
-    /// [`AuthorizationMetadataSource`] lets callers distinguish metadata the
-    /// server actually published from the legacy default-endpoint fallback
-    /// that is synthesized when discovery finds nothing
-    /// ([`AuthorizationMetadataSource::LegacyEndpointFallback`]).
-    pub async fn discover_metadata_with_source(
-        &self,
-    ) -> Result<DiscoveredAuthorizationMetadata, AuthError> {
+    /// # Examples
+    ///
+    /// ```no_run
+    /// use rmcp::transport::auth::{AuthorizationManager, AuthorizationMetadataSource};
+    ///
+    /// # async fn discover() -> Result<(), Box<dyn std::error::Error>> {
+    /// let mut manager = AuthorizationManager::new("https://mcp.example.com").await?;
+    /// let discovered = manager.discover_metadata().await?;
+    ///
+    /// if discovered.source == AuthorizationMetadataSource::LegacyEndpointFallback {
+    ///     println!("the server did not publish OAuth metadata");
+    /// }
+    ///
+    /// manager.set_metadata(discovered.metadata);
+    /// # Ok(())
+    /// # }
+    /// ```
+    pub async fn discover_metadata(&self) -> Result<DiscoveredAuthorizationMetadata, AuthError> {
         if let Some(metadata) = self.discover_oauth_server_via_resource_metadata().await? {
             return Ok(DiscoveredAuthorizationMetadata {
                 metadata,
@@ -3473,8 +3485,8 @@ impl OAuthState {
 
             *manager.current_scopes.write().await = granted_scopes.clone();
 
-            let metadata = manager.discover_metadata().await?;
-            manager.metadata = Some(metadata);
+            let discovered = manager.discover_metadata().await?;
+            manager.metadata = Some(discovered.metadata);
 
             let stored = StoredCredentials {
                 client_id: client_id.to_string(),
@@ -3529,7 +3541,7 @@ impl OAuthState {
         };
         debug!("start discovery");
         let metadata = match manager.discover_metadata().await {
-            Ok(metadata) => metadata,
+            Ok(discovered) => discovered.metadata,
             Err(e) => {
                 *self = OAuthState::Unauthorized(manager);
                 return Err(e);
@@ -3721,8 +3733,8 @@ impl OAuthState {
         };
 
         // Discover metadata
-        let metadata = manager.discover_metadata().await?;
-        manager.metadata = Some(metadata);
+        let discovered = manager.discover_metadata().await?;
+        manager.metadata = Some(discovered.metadata);
 
         // Validate server supports the requested auth method
         manager.validate_client_credentials_metadata(&config)?;
@@ -3857,7 +3869,7 @@ mod tests {
         .await
         .unwrap();
 
-        let metadata = manager.discover_metadata().await.unwrap();
+        let metadata = manager.discover_metadata().await.unwrap().metadata;
 
         assert_eq!(metadata.token_endpoint, "https://auth.example.com/token");
         assert_eq!(
@@ -3920,7 +3932,7 @@ mod tests {
         .await
         .unwrap();
 
-        let metadata = manager.discover_metadata().await.unwrap();
+        let metadata = manager.discover_metadata().await.unwrap().metadata;
 
         assert_eq!(
             (
@@ -4102,7 +4114,7 @@ mod tests {
         .await
         .unwrap();
 
-        let metadata = manager.discover_metadata().await.unwrap();
+        let metadata = manager.discover_metadata().await.unwrap().metadata;
 
         assert_eq!(
             (
@@ -4137,7 +4149,7 @@ mod tests {
     }
 
     #[tokio::test]
-    async fn discover_metadata_falls_back_to_legacy_default_endpoints() {
+    async fn discover_metadata_reports_legacy_fallback_when_nothing_is_discovered() {
         let client = RecordingOAuthHttpClient::with_responses(vec![
             empty_response(404),
             empty_response(404),
@@ -4152,20 +4164,22 @@ mod tests {
         .await
         .unwrap();
 
-        let metadata = manager.discover_metadata().await.unwrap();
+        let discovered = manager.discover_metadata().await.unwrap();
 
         assert_eq!(
             (
-                metadata.authorization_endpoint.as_str(),
-                metadata.token_endpoint.as_str(),
-                metadata.registration_endpoint.as_deref(),
+                discovered.source,
+                discovered.metadata.authorization_endpoint.as_str(),
+                discovered.metadata.token_endpoint.as_str(),
+                discovered.metadata.registration_endpoint.as_deref(),
                 client
                     .requests()
                     .iter()
                     .map(|request| request.uri.as_str())
                     .collect::<Vec<_>>(),
             ),
             (
+                AuthorizationMetadataSource::LegacyEndpointFallback,
                 "https://legacy.example.com/authorize",
                 "https://legacy.example.com/token",
                 Some("https://legacy.example.com/register"),
@@ -4181,37 +4195,7 @@ mod tests {
     }
 
     #[tokio::test]
-    async fn discover_metadata_with_source_reports_legacy_fallback_when_nothing_is_discovered() {
-        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://legacy.example.com/",
-            Arc::new(client),
-        )
-        .await
-        .unwrap();
-
-        let discovered = manager.discover_metadata_with_source().await.unwrap();
-
-        assert_eq!(
-            (
-                discovered.source,
-                discovered.metadata.authorization_endpoint.as_str(),
-            ),
-            (
-                AuthorizationMetadataSource::LegacyEndpointFallback,
-                "https://legacy.example.com/authorize",
-            )
-        );
-    }
-
-    #[tokio::test]
-    async fn discover_metadata_with_source_reports_protected_resource_metadata() {
+    async fn discover_metadata_reports_protected_resource_metadata() {
         let challenge = oauth2::http::Response::builder()
             .status(401)
             .header(
@@ -4245,7 +4229,7 @@ mod tests {
         .await
         .unwrap();
 
-        let discovered = manager.discover_metadata_with_source().await.unwrap();
+        let discovered = manager.discover_metadata().await.unwrap();
 
         assert_eq!(
             (
@@ -4260,7 +4244,7 @@ mod tests {
     }
 
     #[tokio::test]
-    async fn discover_metadata_with_source_reports_authorization_server_metadata() {
+    async fn discover_metadata_reports_authorization_server_metadata() {
         let client = RecordingOAuthHttpClient::with_responses(vec![
             empty_response(404),
             empty_response(404),
@@ -4281,7 +4265,7 @@ mod tests {
         .await
         .unwrap();
 
-        let discovered = manager.discover_metadata_with_source().await.unwrap();
+        let discovered = manager.discover_metadata().await.unwrap();
 
         assert_eq!(
             (
@@ -4398,7 +4382,7 @@ mod tests {
         )
         .await
         .unwrap();
-        manager.metadata = Some(manager.discover_metadata().await.unwrap());
+        manager.metadata = Some(manager.discover_metadata().await.unwrap().metadata);
 
         let request = AuthorizationRequest::new("http://localhost:8080/callback")
             .with_preregistered_client("preregistered-client");
@@ -4853,7 +4837,7 @@ mod tests {
         .await
         .unwrap();
 
-        let metadata = manager.discover_metadata().await.unwrap();
+        let metadata = manager.discover_metadata().await.unwrap().metadata;
         let requests = client.requests();
 
         assert_eq!(
@@ -4910,7 +4894,7 @@ mod tests {
         .await
         .unwrap();
 
-        let metadata = manager.discover_metadata().await.unwrap();
+        let metadata = manager.discover_metadata().await.unwrap().metadata;
 
         assert_eq!(
             (

docs/OAUTH_SUPPORT.md

@@ -108,6 +108,31 @@ Use this path when OAuth traffic must go through a browser fetch API, a remote
 execution environment, a company gateway, a test fake, or any other non-reqwest
 transport.
 
+#### Inspect discovery provenance directly
+
+Most applications can use `OAuthState` without calling metadata discovery
+directly. When using `AuthorizationManager`, `discover_metadata()` returns both
+the metadata and how it was obtained:
+
+```rust ignore
+use rmcp::transport::auth::{AuthorizationManager, AuthorizationMetadataSource};
+
+let mut manager = AuthorizationManager::new(&server_url).await?;
+let discovered = manager.discover_metadata().await?;
+
+if discovered.source == AuthorizationMetadataSource::LegacyEndpointFallback {
+    tracing::warn!(
+        "the server did not publish OAuth metadata; using the 2025-03-26 fallback endpoints"
+    );
+}
+
+manager.set_metadata(discovered.metadata);
+```
+
+`ProtectedResourceMetadata` and `AuthorizationServerMetadata` indicate
+server-published metadata. `LegacyEndpointFallback` indicates endpoints
+synthesized for compatibility with the 2025-03-26 MCP specification.
+
 ### 3. Start authorization with OAuthState
 
 The `OAuthState` state machine manages the full authorization lifecycle.