Authentication not working when using a basePath

[jezza1245]

Hello folks! I am currently using next-auth@5.0.0-beta.19 on next 14.1.3 and am having an issue with the auth callback action no longer working after deploying my site on a basePath (All was working fine without a base path and I have added the basePath property to my config). I am wondering if anyone has experienced similar or has any tips for things I could try; as I've followed the documentation as far as I could but it doesn't seem to be recognising the callback action now?

Current walkthrough of the issue:

1. Click login button calls the signIn method exported from my NextAuth initialisation where the basePath property is set to include the location of my [...nextauth] handlers including the subpath community/api/auth
2. The authorization endoint for my B2C application /oauth2/v2.0/authorize is successful and returns to the configured redirect URI at http://localhost:3000/community/api/auth/callback/azureb2c
3. I then see the issue
   [auth][error] UnknownAction: Cannot parse action at /api/auth/callback/azureb2c.

The application seems to not be handling this correctly but I'm unsure where the real issue lies; as the redirect uri provided in my b2c application is correct 'http://localhost:3000/community/api/auth/callback/azureb2c' (includes the basepath), but the error I am seeing seems to suggest it's receiving and trying to invoke an action at /api/auth/callback/azureb2c which does not contain the subpath but is 'internally' correct for the application.

I have some middleware where I have console logged the incoming request and the result is as follows:

export async function middleware(request: NextRequest) {
  console.log('->', request)
  return NextResponse.next()
}

returns:

{
    nextUrl: {
        href: 'http://localhost:3000/community/api/auth/callback/azureb2c?code=<code>,
        ...
        pathname: '/api/auth/callback/azureb2c',
    },
    ...,
    url: 'http://localhost:3000/community/api/auth/callback/azureb2c?code=<code>',
    ...
}

Project setup:
My auth api is setup at the following directory app/api/auth/[...nextauth]
Both my NextJS config and NextAuth config have both had the relevant basePath properties added like so:

next.config.mjs

const nextConfig = {
  output: 'standalone',
  basePath:'/community',
  ...
}

src/auth/auth.ts

export const {
  handlers: { GET, POST },
  auth,
  signIn, // The used signIn method
  signOut,
} = NextAuth({
  providers: [MyAzureB2CProvider],
  ...,
  trustHost: true,
  basePath: '/community/api/auth',
  debug: true,
})

I would really appreciate any help if there is something I'm missing or could try; else I will raise an issue for this.

Thanks :)
Jez

[nktnet1]

There's an open pull request summarising the issue here:

• fix(next-auth): Add support for Next.js basePath configuration #10797

More discussions here:

• Custom Base Path not working #9274