🛤️ fix: Base URL Fallback for Path-based OAuth Discovery in Token Refresh (#12164)

* fix: add base URL fallback for path-based OAuth discovery in token refresh

The two `refreshOAuthTokens` paths in `MCPOAuthHandler` were missing the
origin-URL fallback that `initiateOAuthFlow` already had. With MCP SDK
1.27.1, `buildDiscoveryUrls` appends the server path to the
`.well-known` URL (e.g. `/.well-known/oauth-authorization-server/mcp`),
which returns 404 for servers like Sentry that only expose the root
discovery endpoint (`/.well-known/oauth-authorization-server`).

Without the fallback, discovery returns null during refresh, the token
endpoint resolves to the wrong URL, and users are prompted to
re-authenticate every time their access token expires instead of the
refresh token being exchanged silently.

Both refresh paths now mirror the `initiateOAuthFlow` pattern: if
discovery fails and the server URL has a non-root path, retry with just
the origin URL.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* refactor: extract discoverWithOriginFallback helper; add tests

Extract the duplicated path-based URL retry logic from both
`refreshOAuthTokens` branches into a single private static helper
`discoverWithOriginFallback`, reducing the risk of the two paths
drifting in the future.

Add three tests covering the new behaviour:
- stored clientInfo path: asserts discovery is called twice (path then
  origin) and that the token endpoint from the origin discovery is used
- auto-discovered path: same assertions for the branchless path
- root URL: asserts discovery is called only once when the server URL
  already has no path component

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* refactor: use discoverWithOriginFallback in discoverMetadata too

Remove the inline duplicate of the origin-fallback logic from
`discoverMetadata` and replace it with a call to the shared
`discoverWithOriginFallback` helper, giving all three discovery
sites a single implementation.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* test: use mock.calls + .href/.toString() for URL assertions

Replace brittle `toHaveBeenNthCalledWith(new URL(...))` comparisons
with `expect.any(URL)` matchers and explicit `.href`/`.toString()`
checks on the captured call args, consistent with the existing
mock.calls pattern used throughout handler.test.ts.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/api/src/mcp/oauth/handler.ts

@@ -161,20 +161,7 @@ export class MCPOAuthHandler {
     logger.debug(
       `[MCPOAuth] Discovering OAuth metadata from ${sanitizeUrlForLogging(authServerUrl)}`,
     );
-    let rawMetadata = await discoverAuthorizationServerMetadata(authServerUrl, {
-      fetchFn,
-    });
-
-    // If discovery failed and we're using a path-based URL, try the base URL
-    if (!rawMetadata && authServerUrl.pathname !== '/') {
-      const baseUrl = new URL(authServerUrl.origin);
-      logger.debug(
-        `[MCPOAuth] Discovery failed with path, trying base URL: ${sanitizeUrlForLogging(baseUrl)}`,
-      );
-      rawMetadata = await discoverAuthorizationServerMetadata(baseUrl, {
-        fetchFn,
-      });
-    }
+    const rawMetadata = await this.discoverWithOriginFallback(authServerUrl, fetchFn);
 
     if (!rawMetadata) {
       /**
@@ -221,6 +208,27 @@ export class MCPOAuthHandler {
     };
   }
 
+  /**
+   * Discovers OAuth authorization server metadata with origin-URL fallback.
+   * If discovery fails for a path-based URL, retries with just the origin.
+   * Mirrors the fallback behavior in `discoverMetadata` and `initiateOAuthFlow`.
+   */
+  private static async discoverWithOriginFallback(
+    serverUrl: URL,
+    fetchFn: FetchLike,
+  ): ReturnType<typeof discoverAuthorizationServerMetadata> {
+    const metadata = await discoverAuthorizationServerMetadata(serverUrl, { fetchFn });
+    // If discovery failed and we're using a path-based URL, try the base URL
+    if (!metadata && serverUrl.pathname !== '/') {
+      const baseUrl = new URL(serverUrl.origin);
+      logger.debug(
+        `[MCPOAuth] Discovery failed with path, trying base URL: ${sanitizeUrlForLogging(baseUrl)}`,
+      );
+      return discoverAuthorizationServerMetadata(baseUrl, { fetchFn });
+    }
+    return metadata;
+  }
+
   /**
    * Registers an OAuth client dynamically
    */
@@ -735,9 +743,10 @@ export class MCPOAuthHandler {
           throw new Error('No token URL available for refresh');
         } else {
           /** Auto-discover OAuth configuration for refresh */
-          const oauthMetadata = await discoverAuthorizationServerMetadata(metadata.serverUrl, {
-            fetchFn: this.createOAuthFetch(oauthHeaders),
-          });
+          const serverUrl = new URL(metadata.serverUrl);
+          const fetchFn = this.createOAuthFetch(oauthHeaders);
+          const oauthMetadata = await this.discoverWithOriginFallback(serverUrl, fetchFn);
+
           if (!oauthMetadata) {
             /**
              * No metadata discovered - use fallback /token endpoint.
@@ -911,9 +920,9 @@ export class MCPOAuthHandler {
       }
 
       /** Auto-discover OAuth configuration for refresh */
-      const oauthMetadata = await discoverAuthorizationServerMetadata(metadata.serverUrl, {
-        fetchFn: this.createOAuthFetch(oauthHeaders),
-      });
+      const serverUrl = new URL(metadata.serverUrl);
+      const fetchFn = this.createOAuthFetch(oauthHeaders);
+      const oauthMetadata = await this.discoverWithOriginFallback(serverUrl, fetchFn);
 
       let tokenUrl: URL;
       if (!oauthMetadata?.token_endpoint) {