That's my nextauth file:

import { NextApiHandler } from "next";
import NextAuth from "next-auth";
import { PrismaAdapter } from "@next-auth/prisma-adapter";
import GoogleProvider from "next-auth/providers/google";
import prisma from "../../../../lib/prisma";

const authHandler: NextApiHandler = (req, res) => NextAuth(req, res, options);
export default authHandler;

const options = {
  adapter: PrismaAdapter(prisma),
  providers: [
    GoogleProvider({
      clientId: process.env.GOOGLE_ID,
      clientSecret: process.env.GOOGLE_SECRET,
      authorization: {
        params: {
          prompt: "consent",
          access_type: "offline",
          response_type: "code",
        },
      },
    }),
  ],
  callbacks: {
    async session({ session, token }: any) {
      session.user.username = session.user.name
        .split(" ")
        .join("")
        .toLocaleLowerCase();

      session.user.uid = token.sub;
      return session;
    },
    async redirect() {
      return "/";
    },
  },
  secret: process.env.JWT_SECRET,
};

And that's my config in GCP:

Did you set NEXTAUTH_URL on Vercel? If you did, remove it. NextAuth automatically finds out correct NEXTAUTH_URL

I don't have this variable in Vercel since I don't need it there, but thanks 🙂

But I might need it on my .env, regarding this docs, which is weird

https://next-auth.js.org/deployment
1. Make sure to expose the Vercel System Environment Variables in your project settings.
2. Create a NEXTAUTH_SECRET environment variable for all environments.
- You can use openssl rand -base64 32 or https://generate-secret.vercel.app/32 to generate a random value.
- You do not need the NEXTAUTH_URL environment variable in Vercel.
3. Add your provider's client ID and client secret to environment variables. (Skip this step if not using an OAuth Provider)
4. Deploy!

Thanks @maxswjeon ! My env variables in Vercel have this naming convention: