chore(backend): Fix ClerkRequest helper instance check

.changeset/five-numbers-scream.md

@@ -0,0 +1,5 @@
+---
+"@clerk/backend": patch
+---
+
+Fixed an issue where TanStack React Start middleware fails to properly handle requests.

integration/templates/tanstack-react-start/package.json

@@ -13,7 +13,6 @@
     "@tanstack/react-start": "1.132.51",
     "react": "18.3.1",
     "react-dom": "18.3.1",
-    "srvx": "0.8.15",
     "tailwind-merge": "^2.5.4"
   },
   "devDependencies": {

packages/backend/src/tokens/__tests__/clerkRequest.test.ts

@@ -171,4 +171,39 @@ describe('createClerkRequest', () => {
       expect(json.cookies).toBe('{}');
     });
   });
+
+  describe('duck typing detection (instanceof workaround)', () => {
+    it('should create a new ClerkRequest from a regular Request', () => {
+      const regularRequest = new Request('http://localhost:3000');
+      const clerkRequest = createClerkRequest(regularRequest);
+
+      expect(clerkRequest).not.toBe(regularRequest);
+      expect(clerkRequest.clerkUrl).toBeDefined();
+      expect(clerkRequest.cookies).toBeDefined();
+    });
+
+    it('should return an existing ClerkRequest instance unchanged', () => {
+      const firstClerkRequest = createClerkRequest(new Request('http://localhost:3000'));
+      const secondClerkRequest = createClerkRequest(firstClerkRequest);
+
+      expect(secondClerkRequest).toBe(firstClerkRequest);
+    });
+
+    it('should work correctly with bundler-scoped Request classes', () => {
+      // Simulate bundler creating a scoped Request class (like Request$1)
+      class RequestScoped extends Request {
+        constructor(input: RequestInfo | URL, init?: RequestInit) {
+          super(input, init);
+        }
+      }
+
+      const scopedRequest = new RequestScoped('http://localhost:3000');
+      const clerkRequest = createClerkRequest(scopedRequest);
+
+      // Should create a new ClerkRequest even though scopedRequest is a different Request class
+      expect(clerkRequest).not.toBe(scopedRequest);
+      expect(clerkRequest.clerkUrl).toBeDefined();
+      expect(clerkRequest.cookies).toBeDefined();
+    });
+  });
 });

packages/backend/src/tokens/clerkRequest.ts

@@ -77,7 +77,12 @@ class ClerkRequest extends Request {
 }
 
 export const createClerkRequest = (...args: ConstructorParameters<typeof ClerkRequest>): ClerkRequest => {
-  return args[0] instanceof ClerkRequest ? args[0] : new ClerkRequest(...args);
+  // Use duck typing instead of instanceof to avoid issues with polyfilled Request classes
+  // (e.g., in TanStack Start or other environments with multiple Request class instances)
+  // ClerkRequest has unique properties 'clerkUrl' and 'cookies' that distinguish it from Request
+  const isClerkRequest = args[0] && typeof args[0] === 'object' && 'clerkUrl' in args[0] && 'cookies' in args[0];
+
+  return isClerkRequest ? (args[0] as ClerkRequest) : new ClerkRequest(...args);
 };
 
 export type { ClerkRequest };