CORS Preflight Failures with Next.js SSR and Genesys Guest API

NICE CXone API & Integrations
1

Why does the WebSocket handshake fails with a CORS error when embedding the Messenger widget in a Next.js app using Server-Side Rendering? The initial HTTP request to /api/v2/guests/conversations returns a 403.

“Ensure your origin is whitelisted in the Genesys Cloud security settings for WebSocket connections.”

I have whitelisted https://example.com. The issue persists only during SSR hydration. Is there a specific header I need to inject via the next.config.js rewrites or is this a known limitation with the Guest API SDK?

2

Make sure you separate your SSR logic from client-side WebSocket initialization. The Genesys Guest API endpoints, specifically /api/v2/guests/conversations, do not support Cross-Origin Resource Sharing (CORS) for direct browser requests during server-side rendering. This is by design to prevent CSRF and unauthorized access. Your Next.js app attempts to fetch this endpoint on the server, but the server environment lacks the browser context required for the subsequent WebSocket handshake, resulting in a 403 Forbidden.

You need to handle the guest conversation creation via a Next.js API route (Serverless Function) or move the initialization strictly to the client side using useEffect. Here is the recommended pattern:

  • Create a Next.js API route /api/guest/init that acts as a backend proxy.
  • Have this route call the Genesys API using server-side authentication (OAuth2 client credentials).
  • Return the conversationId and guestId to the frontend.
  • Initialize the WebSocket connection only on the client side after hydration.

Example API route handler (/pages/api/guest/init.ts):

import { PureCloudPlatformClientV2 } from '@genesyscloud/purecloud-platform-client-v2';

export default async function handler(req, res) {
 const platformClient = new PureCloudPlatformClientV2();
 await platformClient.loginOAuthClientCredentials({
 clientId: process.env.GC_CLIENT_ID,
 clientSecret: process.env.GC_CLIENT_SECRET,
 grantType: 'client_credentials'
 });

 try {
 const response = await platformClient.GuestApi.postGuestsConversations({
 body: {
 type: 'web',
 attributes: {
 name: 'Web Guest'
 }
 }
 });
 
 res.status(200).json({
 conversationId: response.body.id,
 guestId: response.body.guestId
 });
 } catch (error) {
 res.status(500).json({ error: 'Failed to create conversation' });
 }
}

This bypasses the CORS issue entirely by keeping the REST call server-side and only using the WebSocket for real-time events once the client is hydrated.

3

Check your Next.js build configuration and client-side initialization logic. The 403 error you’re seeing isn’t a standard CORS failure; it’s Genesys Cloud’s security layer rejecting the request because the Origin header from your SSR context doesn’t match the whitelisted domain, or more likely, the request is being made server-side where the browser’s cookie/session context is missing.

Does anyone know why the WebSocket handshake fails with a CORS error when embedding the Messenger widget in a Next.js app using Server-Side Rendering?

The core issue is that you’re trying to initialize the Genesys Guest API connection during server-side rendering. The /api/v2/guests/conversations endpoint requires a valid browser context for subsequent WebSocket upgrades. You cannot establish this handshake on the server. You need to ensure the SDK or API calls only execute on the client side after hydration.

Use a dynamic import with ssr: false for any component that interacts with the Genesys SDK. This prevents the server from attempting to fetch the guest identity or start the conversation.

// pages/chat-widget.js
import dynamic from 'next/dynamic';

const GenesysChatComponent = dynamic(() => import('../components/GenesysChat'), {
 ssr: false,
});

export default function ChatPage() {
 return (
 <div>
 <h1>Support</h1>
 <GenesysChatComponent />
 </div>
 );
}

Inside GenesysChatComponent, you can safely use useEffect to initialize the client. Ensure your Authorization header uses a valid OAuth token generated via the client credentials flow if you’re doing headless initiation, but for the widget, rely on the embedded script’s auto-initialization. Also, verify that your whitelisted origin in Genesys Cloud includes the exact protocol and port. If you’re running locally on http://localhost:3000, you must whitelist that specific origin, not just https://example.com. The security settings are strict about protocol mismatches.