I've only tried the sticky bucketing solutions that use cookies, BTW. I think the sticky bucketing could work with the redis solution, but that is more challenging to set up.

The Express SB service should work in theory, provided you have access to cookies and a stable user id. It might get a bit more complicated if you’re doing hybrid testing with SSR/CSR or using RSCs. Could you tell us a bit more about your setup?

This is an app router page. We do have the next js middleware, but the issue is that NextJS does not allow us to return cookies unless it's a server action or an api call that will return the cookies.

ah, yeah I see the problem. We don't have a Next-friendly component-level sticky bucket service partly due to this issue. One thing you could do however is create a component that injects a <script> onto the DOM which sets the cookie from the client.