Thanks so much @balazsorban44!!

Sorry for the delay, I've been working with the Vercel support team and we now have the page loading... I needed trailingSlash: true in next.config.js.

I still see a NextAuth error though. This occurs after clicking sign in and i'm redirected back to.

https://demo-cloud-api.vercel.app/api/auth/signin/github/

This is what i have currently setup.

• NEXTAUTH_URL=https://cockroachlabs.com/demos/demo-cloud-api/api/auth
• NEXT_PUBLIC_ASSET_PREFIX=/demos/demo-cloud-api

and in the GitHub oAuth app settings i have:

Authorization callback URL *
https://cockroachlabs.com/demos/demo-cloud-api/api/auth/callback/github

The Session Provider looks like this

  <SessionProvider session={session} basePath='https://www.cockroachlabs.com/demos/demo-cloud-api/api/auth/'>

Are either of those incorrect?

The two routes should now work:

• https://demo-cloud-api.vercel.app/demos/demo-cloud-api/api/auth/session/
• https://www.cockroachlabs.com/demos/demo-cloud-api/api/auth/session/

The .vercel.app url does show session info, but when i click the "Sign in" button i'm redirected back to the wrong URL.

What do you think, did i do something wrong?

Summary of the setup:

First, the next-auth app:

• Next.js app, using next-auth latest.
• In next.config.js, set trailingSlash: true.
• Set custom basePath for next-auth: /demos/demo-cloud-api/api/auth
• Deploy this app to Vercel at https://demo-cloud-api.vercel.app/demos/demo-cloud-api/, and set NEXTAUTH_URL to https://demo-cloud-api.vercel.app/demos/demo-cloud-api/

Second, the Netlify configuration

• The main app is deployed at https://www.cockroachlabs.com/
• In the proxy configuration, use splat syntax to proxy /demos/demo-cloud-api to the next-auth app like this: /demos/demo-cloud-api* demo-cloud-api.vercel.app/demos/demo-cloud-api/:splat

Problem: next-auth checks for x-forwarded-host in the request to detect the host - but when the request comes from the Netlify proxy, the value is https://demo-cloud-api.vercel.app - so the host detection is wrong and results in sending us to https://next-auth-cockroachdb.vercel.app/api/auth/signin/github/

IMO this is a problem with either Vercel or Netlify - next-auth will use x-forwarded-host as-is and we don't want to change this logic. cc @balazsorban44

x-forwarded-host looks correct to me, as per https://vercel.com/docs/concepts/edge-network/headers#host

Looks like we need to prefer NEXTAUTH_URL on Vercel as done in #4509.

A temporary solution could be: #6526 (comment)