fix(tanstack-react-start): Handle serialization issue when doing handshake

.changeset/fifty-buses-fix.md

@@ -0,0 +1,5 @@
+---
+'@clerk/tanstack-react-start': patch
+---
+
+Fix serialization errors during handshake

packages/tanstack-react-start/src/server/authenticateRequest.ts

@@ -4,6 +4,7 @@ import { AuthStatus, constants } from '@clerk/backend/internal';
 import { handleNetlifyCacheInDevInstance } from '@clerk/shared/netlifyCacheHandler';
 
 import { errorThrower } from '../utils';
+import { ClerkHandshakeRedirect } from './errors';
 import { patchRequest } from './utils';
 
 export async function authenticateRequest(
@@ -41,9 +42,9 @@ export async function authenticateRequest(
       requestStateHeaders: requestState.headers,
       publishableKey: requestState.publishableKey,
     });
+
     // triggering a handshake redirect
-    // eslint-disable-next-line @typescript-eslint/only-throw-error
-    throw new Response(null, { status: 307, headers: requestState.headers });
+    throw new ClerkHandshakeRedirect(307, requestState.headers);
   }
 
   if (requestState.status === AuthStatus.Handshake) {

packages/tanstack-react-start/src/server/errors.ts

@@ -0,0 +1,11 @@
+export class ClerkHandshakeRedirect extends Error {
+  constructor(
+    public status: number,
+    public headers: Headers,
+  ) {
+    super('Clerk handshake redirect required');
+    this.name = 'ClerkHandshakeRedirect';
+    this.status = status;
+    this.headers = headers;
+  }
+}

packages/tanstack-react-start/src/server/middlewareHandler.ts

@@ -2,6 +2,7 @@ import type { AnyRouter } from '@tanstack/react-router';
 import type { CustomizeStartHandler, HandlerCallback, RequestHandler } from '@tanstack/react-start/server';
 
 import { authenticateRequest } from './authenticateRequest';
+import { ClerkHandshakeRedirect } from './errors';
 import { loadOptions } from './loadOptions';
 import type { LoaderOptions } from './types';
 import { getResponseClerkState } from './utils';
@@ -33,9 +34,12 @@ export function createClerkHandler<TRouter extends AnyRouter>(
 
         await router.load();
       } catch (error) {
-        if (error instanceof Response) {
+        if (error instanceof ClerkHandshakeRedirect) {
           // returning the response
-          return error;
+          return new Response(null, {
+            status: error.status,
+            headers: error.headers,
+          });
         }
 
         // rethrowing the error if it is not a Response