CORS preflight failing on /api/v2/messenger/config despite valid origin in Genesys Cloud

The docs state: “Ensure the Origin header matches a registered domain in the Genesys Cloud web messaging configuration.”

I’m embedding the Genesys Cloud Messenger widget into a Next.js app (App Router). The widget loads, but the initial handshake to fetch the config fails with a CORS error. The browser console shows:

Access to fetch at 'https://api.mypurecloud.com/api/v2/messenger/config' from origin 'http://localhost:3000' has been blocked by CORS policy: Response to preflight request doesn't pass access control check: No 'Access-Control-Allow-Origin' header is present on the requested resource.

I’ve added http://localhost:3000 to the Allowed Origins list in the Genesys Cloud UI under Web Messaging > Configuration. The list looks like this in the JSON response from the config endpoint:

{
 "allowedOrigins": [
 "https://prod.example.com",
 "http://localhost:3000"
 ]
}

The Next.js server is making the request server-side during SSR (or ISR), which might be stripping headers or sending a different User-Agent, but the Origin header in the network tab definitely says http://localhost:3000.

I’m using the standard @genesyscloud/messenger package. The init call is straightforward:

import { Messenger } from '@genesyscloud/messenger';

const messenger = new Messenger({
 orgGuid: 'my-org-guid',
 deploymentId: 'my-deployment-id',
 origin: 'http://localhost:3000'
});

await messenger.init();

The init() method hangs and eventually throws the network error. If I disable SSR and run it purely client-side on a static page, it works. But we need this to hydrate properly.

Is there a specific header the Next.js server needs to pass along with the Origin, or is the Genesys Cloud API rejecting preflights from localhost even when explicitly allowed? I’ve tried adding Access-Control-Request-Headers manually via a custom fetch wrapper, but that just returns a 403. The docs don’t mention any special handling for Next.js server components.

// Genesys Cloud Messenger Init
window.initMessenger = (function () {
var messenger = window.messenger || ;
var methods = [‘fetchConfig’, ‘sendEvent’, ‘sendMessage’];
methods.forEach(function (method) {
messenger[method] = function () {
messenger.push({ method: method, args: arguments });
};
});
return messenger;
})();

// Critical: Set the origin BEFORE loading the script
window.messengerOrigin = ‘http://localhost:3000’;

var s = document.createElement(‘script’);
s.async = true;
s.src = ‘https://webchat.mypurecloud.com/webchat.js’;
document.head.appendChild(s);

The issue isn't just the Origin header in the request. The Genesys Cloud web messenger SDK checks the `window.messengerOrigin` variable before it even fires the fetch request. If that variable is undefined or mismatches the allowed origins in your Genesys Cloud org settings, the SDK might bypass the standard CORS flow or send a malformed preflight.

You're using Next.js App Router, which likely renders server-side. You need to ensure that `window.messengerOrigin` is set in a client-side component that runs after hydration. If you set it in a layout that runs on the server, it gets stripped out.

Put that `window.messengerOrigin` assignment in a `useEffect` hook or a dynamic import that only loads on the client. Then verify that `http://localhost:3000` is actually added to the "Allowed Origins" list in Genesys Cloud under Admin > Web Messaging > Settings. The docs are vague on this, but the SDK enforces it strictly.

Also check if you're using a custom domain. If your app is behind a reverse xy or nginx, the Origin header might be getting rewritten. The API endpoint `/api/v2/messenger/config` is strict about matching the exact string in the config.