diff --git a/api/server/routes/__tests__/mcp.spec.js b/api/server/routes/__tests__/mcp.spec.js
index 3a50c0f6cf..9e3ed7a351 100644
--- a/api/server/routes/__tests__/mcp.spec.js
+++ b/api/server/routes/__tests__/mcp.spec.js
@@ -315,7 +315,6 @@ describe('MCP Routes', () => {
       it('should fail the flow when OAuth error is received with valid CSRF cookie', async () => {
         const flowId = 'test-user-id:test-server';
         const mockFlowManager = {
-          getFlowState: jest.fn().mockResolvedValue({ status: 'PENDING', createdAt: Date.now() }),
           failFlow: jest.fn().mockResolvedValue(true),
         };
 
@@ -342,10 +341,38 @@ describe('MCP Routes', () => {
         );
       });
 
+      it('should fail the flow when OAuth error is received with valid session cookie', async () => {
+        const flowId = 'test-user-id:test-server';
+        const mockFlowManager = {
+          failFlow: jest.fn().mockResolvedValue(true),
+        };
+
+        getLogStores.mockReturnValueOnce({});
+        require('~/config').getFlowStateManager.mockReturnValueOnce(mockFlowManager);
+        MCPOAuthHandler.resolveStateToFlowId.mockResolvedValueOnce(flowId);
+
+        const sessionToken = generateTestCsrfToken('test-user-id');
+        const response = await request(app)
+          .get('/api/mcp/test-server/oauth/callback')
+          .set('Cookie', [`oauth_session=${sessionToken}`])
+          .query({
+            error: 'invalid_client',
+            state: flowId,
+          });
+        const basePath = getBasePath();
+
+        expect(response.status).toBe(302);
+        expect(response.headers.location).toBe(`${basePath}/oauth/error?error=invalid_client`);
+        expect(mockFlowManager.failFlow).toHaveBeenCalledWith(
+          flowId,
+          'mcp_oauth',
+          'invalid_client',
+        );
+      });
+
       it('should NOT fail the flow when OAuth error is received without cookies (DoS prevention)', async () => {
         const flowId = 'test-user-id:test-server';
         const mockFlowManager = {
-          getFlowState: jest.fn().mockResolvedValue({ status: 'PENDING', createdAt: Date.now() }),
           failFlow: jest.fn(),
         };
 
diff --git a/packages/api/src/mcp/MCPConnectionFactory.ts b/packages/api/src/mcp/MCPConnectionFactory.ts
index d6c3b5290c..c517140b8a 100644
--- a/packages/api/src/mcp/MCPConnectionFactory.ts
+++ b/packages/api/src/mcp/MCPConnectionFactory.ts
@@ -495,7 +495,8 @@ export class MCPConnectionFactory {
   /**
    * Checks whether an error indicates the OAuth client registration was rejected.
    * Includes RFC 6749 §5.2 standard codes (`invalid_client`, `unauthorized_client`)
-   * and known vendor-specific patterns (Okta: `client_id mismatch`, Auth0: `client not found`).
+   * and known vendor-specific patterns (Okta: `client_id mismatch`, Auth0: `client not found`,
+   * generic: `unknown client`).
    */
   static isClientRejection(error: unknown): boolean {
     if (!error || typeof error !== 'object') {
diff --git a/packages/api/src/mcp/__tests__/MCPConnectionFactory.test.ts b/packages/api/src/mcp/__tests__/MCPConnectionFactory.test.ts
index 5983710f65..d90ca5b345 100644
--- a/packages/api/src/mcp/__tests__/MCPConnectionFactory.test.ts
+++ b/packages/api/src/mcp/__tests__/MCPConnectionFactory.test.ts
@@ -354,8 +354,8 @@ describe('MCPConnectionFactory', () => {
 
       await oauthRequiredHandler!({ serverUrl: 'https://api.example.com' });
 
-      // Wait for the background .catch() handler to run
-      await new Promise((r) => setTimeout(r, 50));
+      // Drain microtasks so the background .catch() handler completes
+      await new Promise((r) => setImmediate(r));
 
       // deleteClientRegistration should have been called via clearStaleClientIfRejected
       expect(mockMCPTokenStorage.deleteClientRegistration).toHaveBeenCalledWith(