[NEXTjs] app router route handler / middleware wrapper auth((req) => {}) not working properly when initializing NextAuth() using a function.

[jwwhangbo]

Environment

System:
OS: macOS 15.2
CPU: (8) arm64 Apple M2
Memory: 167.84 MB / 16.00 GB
Shell: 5.9 - /bin/zsh
Binaries:
Node: 23.5.0 - /usr/local/bin/node
npm: 11.0.0 - /opt/homebrew/bin/npm
pnpm: 9.15.2 - ~/Documents/GitHub/chatgpt-cooking/node_modules/.bin/pnpm
Watchman: 2024.09.02.00 - /opt/homebrew/bin/watchman
Browsers:
Chrome: 131.0.6778.205
Safari: 18.2
npmPackages:
@auth/core: ^0.37.4 => 0.37.4
@auth/pg-adapter: ^1.7.4 => 1.7.4
@auth/prisma-adapter: ^2.7.4 => 2.7.4
next: 15.1.2 => 15.1.2
next-auth: 5.0.0-beta.25 => 5.0.0-beta.25
react: 19.0.0 => 19.0.0

Reproduction URL

https://github.com/jwwhangbo/NibbleAI/blob/PROTOTYPE/app/api/collections/route.ts

Describe the issue

When initializing NextAuth using a function that doesn't take any parameters,

// @/auth.ts
export const { handlers, auth, signIn, signOut } = NextAuth(() => {
  const pool = new Pool({ connectionString: process.env.DATABASE_URL });
  return {
    adapter: PostgresAdapter(pool),
    callbacks: {
      session({ session, user }) {
        // ...
        return session;
      },
      authorized: async ({ request: { nextUrl }, auth }) => {
        // ...
    },
    providers,
    pages: {
      // ...
    },
  };
});

it returns an auth function that doesn't work when used in middleware.ts or route.ts.

// app/api/collections/route.ts
export const POST = auth(function (req) {
  if (req.auth) return NextResponse.json(req.auth)
  return NextResponse.json({ message: "Not authenticated" }, { status: 401 })
}) 

trying to fetch() from the route immediately returns a 500 internal error.

There were no issues when initializing NextAuth with an object instead of a function. Instead of creating a new PostgreSQL Pool instance within NextAuth (as shown in the database integration tutorial), an existing Pool object was imported and used.

// from Auth.js documentation
import NextAuth from "next-auth"
import PostgresAdapter from "@auth/pg-adapter"
import { Pool } from "@neondatabase/serverless"
 
// *DO NOT* create a `Pool` here, outside the request handler.
// Neon's Postgres cannot keep a pool alive between requests.
 
export const { handlers, auth, signIn, signOut } = NextAuth(() => {
  // Create a `Pool` inside the request handler.
  const pool = new Pool({ connectionString: process.env.DATABASE_URL })
  return {
    adapter: PostgresAdapter(pool),
    providers: [],
  }
})

How to reproduce

Instantiate NextAuth using a function that doesn’t take any parameters.

Expected behavior

auth() wrapper for route handlers and middleware should work normally.