Can't make basePath to work

[ivan-kleshnin]

I want to run NextJS + Next-Auth under an /auth prefix with a CredentialsProvider (just exploring the lib).
From issues/discussions it follows that basePath is a supported feature but I can't make it work for the simplest setup:

next.config.js

module.exports = {
  basePath: "/auth",
}

pages/_app.js

export default function MyApp({Component, pageProps: {session, ...pageProps}}: AppProps) {
  return <>
    <SessionProvider session={session} basePath="/auth"> -- also tried "/auth/api/auth" and "auth"
      <Component {...pageProps}/>
    </SessionProvider>
  </>
}

Now /auth/api/auth/signin does render a form but for invalid credentials it redirects to /api/auth/callback/credentials which is 404 instead of displaying a flash message above the form.

When I open http://localhost:3000/auth/api/auth/providers it shows incorrect values:

{ 
  credentials: {
    id: "credentials",
    name: "Credentials",
    type: "credentials",
    signinUrl: "http://localhost:3000/api/auth/signin/credentials", -- no basePath!!!
    callbackUrl: "http://localhost:3000/api/auth/callback/credentials" -- no basePath!!!
  }
}

[...nextauth].js

Is basic, copy-pasted from the docs:

export default NextAuth({
  providers: [
    CredentialsProvider({
      name: "Credentials",

      credentials: {
        username: {label: "Username", type: "text"},
        password: {label: "Password", type: "password"},
      },

      async authorize(credentials, req) {
        ...
      }
    })
  ],

❗ If I remove basePath from both next.config and SessionProvider everything behaves correctly.
So the problem is about basePath.

What do I miss?

---

"next": "12.1.6",
"next-auth": "^4.5.0",

Demo repo: https://github.com/ivan-kleshnin/next-auth-issue

[ivan-kleshnin]

Update

The above issue can be fixed by defining NEXTAUTH_URL like:

NEXTAUTH_URL=http://localhost:3000/auth/api/auth

It's still unclear why it's necessary to set basePath twice but whatever 🤷

[gyurisc]

Where do you set this environment variable? In your nextjs .env.local or someplace else?

[ivan-kleshnin]

I think it was working as a normal ENV so .env or .env.local worked. But I'm not sure if it applies today, as Next-Auth has gone through multiple code rewrites since 2022. I don't use this library anymore.