🔐 fix: Respect Server's Token Endpoint Auth Methods for MCP OAuth Refresh (#9717)

* fix: respect server's token endpoint auth methods for MCP OAuth refresh

Previously, LibreChat always used Basic Auth when refreshing OAuth tokens if a
client_secret was present. This caused issues with servers (like FastMCP) that
only support client_secret_post. Now properly checks and respects the server's
advertised token_endpoint_auth_methods_supported.

Fixes token refresh failures with error: "refresh_token.client_id: Field required"

* chore: remove MCP OAuth URL Logging

api/server/services/Tools/mcp.js

@@ -44,7 +44,7 @@ async function reinitMCPServer({
     const oauthStart =
       _oauthStart ??
       (async (authURL) => {
-        logger.info(`[MCP Reinitialize] OAuth URL received: ${authURL}`);
+        logger.info(`[MCP Reinitialize] OAuth URL received for ${serverName}`);
         oauthUrl = authURL;
         oauthRequired = true;
       });

packages/api/src/mcp/__tests__/handler.test.ts

@@ -1,4 +1,5 @@
 import type { MCPOptions } from 'librechat-data-provider';
+import type { AuthorizationServerMetadata } from '@modelcontextprotocol/sdk/shared/auth.js';
 import { MCPOAuthHandler } from '~/mcp/oauth';
 
 jest.mock('@librechat/data-schemas', () => ({
@@ -12,11 +13,19 @@ jest.mock('@librechat/data-schemas', () => ({
 
 jest.mock('@modelcontextprotocol/sdk/client/auth.js', () => ({
   startAuthorization: jest.fn(),
+  discoverAuthorizationServerMetadata: jest.fn(),
 }));
 
-import { startAuthorization } from '@modelcontextprotocol/sdk/client/auth.js';
+import {
+  startAuthorization,
+  discoverAuthorizationServerMetadata,
+} from '@modelcontextprotocol/sdk/client/auth.js';
 
 const mockStartAuthorization = startAuthorization as jest.MockedFunction<typeof startAuthorization>;
+const mockDiscoverAuthorizationServerMetadata =
+  discoverAuthorizationServerMetadata as jest.MockedFunction<
+    typeof discoverAuthorizationServerMetadata
+  >;
 
 describe('MCPOAuthHandler - Configurable OAuth Metadata', () => {
   const mockServerName = 'test-server';
@@ -188,6 +197,432 @@ describe('MCPOAuthHandler - Configurable OAuth Metadata', () => {
     });
   });
 
+  describe('refreshOAuthTokens', () => {
+    const mockRefreshToken = 'refresh-token-12345';
+    const originalFetch = global.fetch;
+    const mockFetch = jest.fn() as unknown as jest.MockedFunction<typeof fetch>;
+
+    beforeEach(() => {
+      jest.clearAllMocks();
+      global.fetch = mockFetch;
+    });
+
+    afterEach(() => {
+      mockFetch.mockClear();
+    });
+
+    afterAll(() => {
+      global.fetch = originalFetch;
+    });
+
+    describe('with stored metadata', () => {
+      it('should use client_secret_post when server only supports that method', async () => {
+        const metadata = {
+          serverName: 'test-server',
+          userId: 'user-123',
+          serverUrl: 'https://auth.example.com',
+          state: 'state-123',
+          clientInfo: {
+            client_id: 'test-client-id',
+            client_secret: 'test-client-secret',
+            grant_types: ['authorization_code', 'refresh_token'],
+            scope: 'read write',
+          },
+        };
+
+        // Mock OAuth metadata discovery
+        mockDiscoverAuthorizationServerMetadata.mockResolvedValueOnce({
+          issuer: 'https://auth.example.com',
+          authorization_endpoint: 'https://auth.example.com/oauth/authorize',
+          token_endpoint: 'https://auth.example.com/oauth/token',
+          token_endpoint_auth_methods_supported: ['client_secret_post'],
+          response_types_supported: ['code'],
+          jwks_uri: 'https://auth.example.com/.well-known/jwks.json',
+          subject_types_supported: ['public'],
+          id_token_signing_alg_values_supported: ['RS256'],
+        } as AuthorizationServerMetadata);
+
+        mockFetch.mockResolvedValueOnce({
+          ok: true,
+          json: async () => ({
+            access_token: 'new-access-token',
+            refresh_token: 'new-refresh-token',
+            expires_in: 3600,
+          }),
+        } as Response);
+
+        const result = await MCPOAuthHandler.refreshOAuthTokens(mockRefreshToken, metadata);
+
+        // Verify the call was made without Authorization header
+        expect(mockFetch).toHaveBeenCalledWith(
+          'https://auth.example.com/oauth/token',
+          expect.objectContaining({
+            method: 'POST',
+            headers: expect.not.objectContaining({
+              Authorization: expect.any(String),
+            }),
+          }),
+        );
+
+        // Verify the body contains client_id and client_secret
+        const callArgs = mockFetch.mock.calls[0];
+        const body = callArgs[1]?.body as URLSearchParams;
+        expect(body.toString()).toContain('client_id=test-client-id');
+        expect(body.toString()).toContain('client_secret=test-client-secret');
+
+        expect(result).toEqual({
+          access_token: 'new-access-token',
+          refresh_token: 'new-refresh-token',
+          expires_in: 3600,
+          obtained_at: expect.any(Number),
+          expires_at: expect.any(Number),
+        });
+      });
+
+      it('should use client_secret_basic when server only supports that method', async () => {
+        const metadata = {
+          serverName: 'test-server',
+          userId: 'user-123',
+          serverUrl: 'https://auth.example.com',
+          state: 'state-123',
+          clientInfo: {
+            client_id: 'test-client-id',
+            client_secret: 'test-client-secret',
+            grant_types: ['authorization_code', 'refresh_token'],
+            scope: 'read write',
+          },
+        };
+
+        // Mock OAuth metadata discovery
+        mockDiscoverAuthorizationServerMetadata.mockResolvedValueOnce({
+          issuer: 'https://auth.example.com',
+          authorization_endpoint: 'https://auth.example.com/oauth/authorize',
+          token_endpoint: 'https://auth.example.com/oauth/token',
+          token_endpoint_auth_methods_supported: ['client_secret_basic'],
+          response_types_supported: ['code'],
+          jwks_uri: 'https://auth.example.com/.well-known/jwks.json',
+          subject_types_supported: ['public'],
+          id_token_signing_alg_values_supported: ['RS256'],
+        } as AuthorizationServerMetadata);
+
+        mockFetch.mockResolvedValueOnce({
+          ok: true,
+          json: async () => ({
+            access_token: 'new-access-token',
+            refresh_token: 'new-refresh-token',
+            expires_in: 3600,
+          }),
+        } as Response);
+
+        await MCPOAuthHandler.refreshOAuthTokens(mockRefreshToken, metadata);
+
+        const expectedAuth = `Basic ${Buffer.from('test-client-id:test-client-secret').toString('base64')}`;
+        expect(mockFetch).toHaveBeenCalledWith(
+          'https://auth.example.com/oauth/token',
+          expect.objectContaining({
+            method: 'POST',
+            headers: expect.objectContaining({
+              Authorization: expectedAuth,
+            }),
+            body: expect.not.stringContaining('client_id='),
+          }),
+        );
+      });
+
+      it('should prefer client_secret_basic when both methods are supported', async () => {
+        const metadata = {
+          serverName: 'test-server',
+          userId: 'user-123',
+          serverUrl: 'https://auth.example.com',
+          state: 'state-123',
+          clientInfo: {
+            client_id: 'test-client-id',
+            client_secret: 'test-client-secret',
+            grant_types: ['authorization_code', 'refresh_token'],
+          },
+        };
+
+        // Mock OAuth metadata discovery
+        mockDiscoverAuthorizationServerMetadata.mockResolvedValueOnce({
+          issuer: 'https://auth.example.com',
+          authorization_endpoint: 'https://auth.example.com/oauth/authorize',
+          token_endpoint: 'https://auth.example.com/oauth/token',
+          token_endpoint_auth_methods_supported: ['client_secret_post', 'client_secret_basic'],
+          response_types_supported: ['code'],
+          jwks_uri: 'https://auth.example.com/.well-known/jwks.json',
+          subject_types_supported: ['public'],
+          id_token_signing_alg_values_supported: ['RS256'],
+        } as AuthorizationServerMetadata);
+
+        mockFetch.mockResolvedValueOnce({
+          ok: true,
+          json: async () => ({
+            access_token: 'new-access-token',
+            refresh_token: 'new-refresh-token',
+            expires_in: 3600,
+          }),
+        } as Response);
+
+        await MCPOAuthHandler.refreshOAuthTokens(mockRefreshToken, metadata);
+
+        const expectedAuth = `Basic ${Buffer.from('test-client-id:test-client-secret').toString('base64')}`;
+        expect(mockFetch).toHaveBeenCalledWith(
+          'https://auth.example.com/oauth/token',
+          expect.objectContaining({
+            headers: expect.objectContaining({
+              Authorization: expectedAuth,
+            }),
+          }),
+        );
+      });
+
+      it('should default to client_secret_basic when no methods are advertised', async () => {
+        const metadata = {
+          serverName: 'test-server',
+          userId: 'user-123',
+          serverUrl: 'https://auth.example.com',
+          state: 'state-123',
+          clientInfo: {
+            client_id: 'test-client-id',
+            client_secret: 'test-client-secret',
+            grant_types: ['authorization_code', 'refresh_token'],
+          },
+        };
+
+        // Mock OAuth metadata discovery with no auth methods specified
+        mockDiscoverAuthorizationServerMetadata.mockResolvedValueOnce({
+          issuer: 'https://auth.example.com',
+          authorization_endpoint: 'https://auth.example.com/oauth/authorize',
+          token_endpoint: 'https://auth.example.com/oauth/token',
+          // No token_endpoint_auth_methods_supported field
+          response_types_supported: ['code'],
+          jwks_uri: 'https://auth.example.com/.well-known/jwks.json',
+          subject_types_supported: ['public'],
+          id_token_signing_alg_values_supported: ['RS256'],
+        } as AuthorizationServerMetadata);
+
+        mockFetch.mockResolvedValueOnce({
+          ok: true,
+          json: async () => ({
+            access_token: 'new-access-token',
+            refresh_token: 'new-refresh-token',
+            expires_in: 3600,
+          }),
+        } as Response);
+
+        await MCPOAuthHandler.refreshOAuthTokens(mockRefreshToken, metadata);
+
+        const expectedAuth = `Basic ${Buffer.from('test-client-id:test-client-secret').toString('base64')}`;
+        expect(mockFetch).toHaveBeenCalledWith(
+          'https://auth.example.com/oauth/token',
+          expect.objectContaining({
+            headers: expect.objectContaining({
+              Authorization: expectedAuth,
+            }),
+          }),
+        );
+      });
+
+      it('should include client_id in body for public clients (no secret)', async () => {
+        const metadata = {
+          serverName: 'test-server',
+          userId: 'user-123',
+          serverUrl: 'https://auth.example.com',
+          state: 'state-123',
+          clientInfo: {
+            client_id: 'test-client-id',
+            // No client_secret - public client
+            grant_types: ['authorization_code', 'refresh_token'],
+          },
+        };
+
+        // Mock OAuth metadata discovery
+        mockDiscoverAuthorizationServerMetadata.mockResolvedValueOnce({
+          issuer: 'https://auth.example.com',
+          authorization_endpoint: 'https://auth.example.com/oauth/authorize',
+          token_endpoint: 'https://auth.example.com/oauth/token',
+          token_endpoint_auth_methods_supported: ['none'],
+          response_types_supported: ['code'],
+          jwks_uri: 'https://auth.example.com/.well-known/jwks.json',
+          subject_types_supported: ['public'],
+          id_token_signing_alg_values_supported: ['RS256'],
+        } as AuthorizationServerMetadata);
+
+        mockFetch.mockResolvedValueOnce({
+          ok: true,
+          json: async () => ({
+            access_token: 'new-access-token',
+            refresh_token: 'new-refresh-token',
+            expires_in: 3600,
+          }),
+        } as Response);
+
+        await MCPOAuthHandler.refreshOAuthTokens(mockRefreshToken, metadata);
+
+        // Verify the call was made without Authorization header
+        expect(mockFetch).toHaveBeenCalledWith(
+          'https://auth.example.com/oauth/token',
+          expect.objectContaining({
+            method: 'POST',
+            headers: expect.not.objectContaining({
+              Authorization: expect.any(String),
+            }),
+          }),
+        );
+
+        // Verify the body contains client_id (public client)
+        const callArgs = mockFetch.mock.calls[0];
+        const body = callArgs[1]?.body as URLSearchParams;
+        expect(body.toString()).toContain('client_id=test-client-id');
+      });
+    });
+
+    describe('with pre-configured OAuth settings', () => {
+      it('should use client_secret_post when configured to only support that method', async () => {
+        const config = {
+          token_url: 'https://auth.example.com/oauth/token',
+          client_id: 'test-client-id',
+          client_secret: 'test-client-secret',
+          token_endpoint_auth_methods_supported: ['client_secret_post'],
+        };
+
+        mockFetch.mockResolvedValueOnce({
+          ok: true,
+          json: async () => ({
+            access_token: 'new-access-token',
+            refresh_token: 'new-refresh-token',
+            expires_in: 3600,
+          }),
+        } as Response);
+
+        await MCPOAuthHandler.refreshOAuthTokens(
+          mockRefreshToken,
+          { serverName: 'test-server' },
+          config,
+        );
+
+        // Verify the call was made without Authorization header
+        expect(mockFetch).toHaveBeenCalledWith(
+          new URL('https://auth.example.com/oauth/token'),
+          expect.objectContaining({
+            method: 'POST',
+            headers: expect.not.objectContaining({
+              Authorization: expect.any(String),
+            }),
+          }),
+        );
+
+        // Verify the body contains client_id and client_secret
+        const callArgs = mockFetch.mock.calls[0];
+        const body = callArgs[1]?.body as URLSearchParams;
+        expect(body.toString()).toContain('client_id=test-client-id');
+        expect(body.toString()).toContain('client_secret=test-client-secret');
+      });
+
+      it('should use client_secret_basic when configured to support that method', async () => {
+        const config = {
+          token_url: 'https://auth.example.com/oauth/token',
+          client_id: 'test-client-id',
+          client_secret: 'test-client-secret',
+          token_endpoint_auth_methods_supported: ['client_secret_basic'],
+        };
+
+        mockFetch.mockResolvedValueOnce({
+          ok: true,
+          json: async () => ({
+            access_token: 'new-access-token',
+            refresh_token: 'new-refresh-token',
+            expires_in: 3600,
+          }),
+        } as Response);
+
+        await MCPOAuthHandler.refreshOAuthTokens(
+          mockRefreshToken,
+          { serverName: 'test-server' },
+          config,
+        );
+
+        const expectedAuth = `Basic ${Buffer.from('test-client-id:test-client-secret').toString('base64')}`;
+        expect(mockFetch).toHaveBeenCalledWith(
+          new URL('https://auth.example.com/oauth/token'),
+          expect.objectContaining({
+            method: 'POST',
+            headers: expect.objectContaining({
+              Authorization: expectedAuth,
+            }),
+            body: expect.not.stringContaining('client_id='),
+          }),
+        );
+      });
+
+      it('should default to client_secret_basic when no auth methods configured', async () => {
+        const config = {
+          token_url: 'https://auth.example.com/oauth/token',
+          client_id: 'test-client-id',
+          client_secret: 'test-client-secret',
+          // No token_endpoint_auth_methods_supported field
+        };
+
+        mockFetch.mockResolvedValueOnce({
+          ok: true,
+          json: async () => ({
+            access_token: 'new-access-token',
+            refresh_token: 'new-refresh-token',
+            expires_in: 3600,
+          }),
+        } as Response);
+
+        await MCPOAuthHandler.refreshOAuthTokens(
+          mockRefreshToken,
+          { serverName: 'test-server' },
+          config,
+        );
+
+        const expectedAuth = `Basic ${Buffer.from('test-client-id:test-client-secret').toString('base64')}`;
+        expect(mockFetch).toHaveBeenCalledWith(
+          new URL('https://auth.example.com/oauth/token'),
+          expect.objectContaining({
+            headers: expect.objectContaining({
+              Authorization: expectedAuth,
+            }),
+          }),
+        );
+      });
+    });
+
+    it('should throw error when refresh fails', async () => {
+      const metadata = {
+        serverName: 'test-server',
+        userId: 'user-123',
+        serverUrl: 'https://auth.example.com',
+        state: 'state-123',
+        clientInfo: {
+          client_id: 'test-client-id',
+          client_secret: 'test-client-secret',
+          grant_types: ['authorization_code', 'refresh_token'],
+        },
+      };
+
+      // Mock OAuth metadata discovery
+      mockDiscoverAuthorizationServerMetadata.mockResolvedValueOnce({
+        token_endpoint: 'https://auth.example.com/oauth/token',
+        token_endpoint_auth_methods_supported: ['client_secret_post'],
+      } as AuthorizationServerMetadata);
+
+      mockFetch.mockResolvedValueOnce({
+        ok: false,
+        status: 400,
+        statusText: 'Bad Request',
+        text: async () =>
+          '{"error":"invalid_request","error_description":"refresh_token.client_id: Field required"}',
+      } as Response);
+
+      await expect(MCPOAuthHandler.refreshOAuthTokens(mockRefreshToken, metadata)).rejects.toThrow(
+        'Token refresh failed: 400 Bad Request - {"error":"invalid_request","error_description":"refresh_token.client_id: Field required"}',
+      );
+    });
+  });
+
   describe('revokeOAuthToken', () => {
     const mockServerName = 'test-server';
     const mockToken = 'test-token-12345';

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

@@ -501,6 +501,7 @@ export class MCPOAuthHandler {
 
         /** Use the stored client information and metadata to determine the token URL */
         let tokenUrl: string;
+        let authMethods: string[] | undefined;
         if (config?.token_url) {
           tokenUrl = config.token_url;
         } else if (!metadata.serverUrl) {
@@ -515,6 +516,7 @@ export class MCPOAuthHandler {
             throw new Error('No token endpoint found in OAuth metadata');
           }
           tokenUrl = oauthMetadata.token_endpoint;
+          authMethods = oauthMetadata.token_endpoint_auth_methods_supported;
         }
 
         const body = new URLSearchParams({
@@ -532,14 +534,36 @@ export class MCPOAuthHandler {
           Accept: 'application/json',
         };
 
-        /** Use client_secret for authentication if available */
+        /** Handle authentication based on server's advertised methods */
         if (metadata.clientInfo.client_secret) {
-          const clientAuth = Buffer.from(
-            `${metadata.clientInfo.client_id}:${metadata.clientInfo.client_secret}`,
-          ).toString('base64');
-          headers['Authorization'] = `Basic ${clientAuth}`;
+          /** Default to client_secret_basic if no methods specified (per RFC 8414) */
+          const tokenAuthMethods = authMethods ?? ['client_secret_basic'];
+          const usesBasicAuth = tokenAuthMethods.includes('client_secret_basic');
+          const usesClientSecretPost = tokenAuthMethods.includes('client_secret_post');
+
+          if (usesBasicAuth) {
+            /** Use Basic auth */
+            logger.debug('[MCPOAuth] Using client_secret_basic authentication method');
+            const clientAuth = Buffer.from(
+              `${metadata.clientInfo.client_id}:${metadata.clientInfo.client_secret}`,
+            ).toString('base64');
+            headers['Authorization'] = `Basic ${clientAuth}`;
+          } else if (usesClientSecretPost) {
+            /** Use client_secret_post */
+            logger.debug('[MCPOAuth] Using client_secret_post authentication method');
+            body.append('client_id', metadata.clientInfo.client_id);
+            body.append('client_secret', metadata.clientInfo.client_secret);
+          } else {
+            /** No recognized method, default to Basic auth per RFC */
+            logger.debug('[MCPOAuth] No recognized auth method, defaulting to client_secret_basic');
+            const clientAuth = Buffer.from(
+              `${metadata.clientInfo.client_id}:${metadata.clientInfo.client_secret}`,
+            ).toString('base64');
+            headers['Authorization'] = `Basic ${clientAuth}`;
+          }
         } else {
           /** For public clients, client_id must be in the body */
+          logger.debug('[MCPOAuth] Using public client authentication (no secret)');
           body.append('client_id', metadata.clientInfo.client_id);
         }
 
@@ -575,9 +599,6 @@ export class MCPOAuthHandler {
         logger.debug(`[MCPOAuth] Using pre-configured OAuth settings for token refresh`);
 
         const tokenUrl = new URL(config.token_url);
-        const clientAuth = config.client_secret
-          ? Buffer.from(`${config.client_id}:${config.client_secret}`).toString('base64')
-          : null;
 
         const body = new URLSearchParams({
           grant_type: 'refresh_token',
@@ -593,10 +614,44 @@ export class MCPOAuthHandler {
           Accept: 'application/json',
         };
 
-        if (clientAuth) {
-          headers['Authorization'] = `Basic ${clientAuth}`;
+        /** Handle authentication based on configured methods */
+        if (config.client_secret) {
+          /** Default to client_secret_basic if no methods specified (per RFC 8414) */
+          const tokenAuthMethods = config.token_endpoint_auth_methods_supported ?? [
+            'client_secret_basic',
+          ];
+          const usesBasicAuth = tokenAuthMethods.includes('client_secret_basic');
+          const usesClientSecretPost = tokenAuthMethods.includes('client_secret_post');
+
+          if (usesBasicAuth) {
+            /** Use Basic auth */
+            logger.debug(
+              '[MCPOAuth] Using client_secret_basic authentication method (pre-configured)',
+            );
+            const clientAuth = Buffer.from(`${config.client_id}:${config.client_secret}`).toString(
+              'base64',
+            );
+            headers['Authorization'] = `Basic ${clientAuth}`;
+          } else if (usesClientSecretPost) {
+            /** Use client_secret_post */
+            logger.debug(
+              '[MCPOAuth] Using client_secret_post authentication method (pre-configured)',
+            );
+            body.append('client_id', config.client_id);
+            body.append('client_secret', config.client_secret);
+          } else {
+            /** No recognized method, default to Basic auth per RFC */
+            logger.debug(
+              '[MCPOAuth] No recognized auth method, defaulting to client_secret_basic (pre-configured)',
+            );
+            const clientAuth = Buffer.from(`${config.client_id}:${config.client_secret}`).toString(
+              'base64',
+            );
+            headers['Authorization'] = `Basic ${clientAuth}`;
+          }
         } else {
-          // Use client_id in body for public clients
+          /** For public clients, client_id must be in the body */
+          logger.debug('[MCPOAuth] Using public client authentication (no secret, pre-configured)');
           body.append('client_id', config.client_id);
         }