diff --git a/.changeset/full-frogs-play.md b/.changeset/full-frogs-play.md
new file mode 100644
index 00000000000..a57599fba0b
--- /dev/null
+++ b/.changeset/full-frogs-play.md
@@ -0,0 +1,5 @@
+---
+'@clerk/backend': patch
+---
+
+The `instanceof` check in `createClerkRequest` is allowing `Request` instances through, which means the required `cookies` object is never created/set on the instance. This causes an error in the TanStack Start middleware when `AuthenticateContext.getCookie` tries to access `cookies`. Instead of checking `instanceof`, we check for the presence of the `cookies` and `clerkUrl` keys to determine whether or not we're dealing with a `Request` or `ClerkRequest`
diff --git a/packages/backend/src/tokens/clerkRequest.ts b/packages/backend/src/tokens/clerkRequest.ts
index f35b079d779..d17576af50e 100644
--- a/packages/backend/src/tokens/clerkRequest.ts
+++ b/packages/backend/src/tokens/clerkRequest.ts
@@ -82,7 +82,12 @@ class ClerkRequest extends Request {
 }
 
 export const createClerkRequest = (...args: ConstructorParameters<typeof ClerkRequest>): ClerkRequest => {
-  return args[0] instanceof ClerkRequest ? args[0] : new ClerkRequest(...args);
+  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 };