create-farcaster-mini-app/src/auth.ts

import { AuthOptions, getServerSession } from "next-auth"
import CredentialsProvider from "next-auth/providers/credentials";
import { createAppClient, viemConnector } from "@farcaster/auth-client";

declare module "next-auth" {
  interface Session {
    user: {
      fid: number;
    };
  }
}

function getDomainFromUrl(urlString: string | undefined): string {
  if (!urlString) {
    console.warn('NEXTAUTH_URL is not set, using localhost:3000 as fallback');
    return 'localhost:3000';
  }
  try {
    const url = new URL(urlString);
    return url.hostname;
  } catch (error) {
    console.error('Invalid NEXTAUTH_URL:', urlString, error);
    console.warn('Using localhost:3000 as fallback');
    return 'localhost:3000';
  }
}

export const authOptions: AuthOptions = {
    // Configure one or more authentication providers
  providers: [
    CredentialsProvider({
      name: "Sign in with Farcaster",
      credentials: {
        message: {
          label: "Message",
          type: "text",
          placeholder: "0x0",
        },
        signature: {
          label: "Signature",
          type: "text",
          placeholder: "0x0",
        },
        // In a production app with a server, these should be fetched from
        // your Farcaster data indexer rather than have them accepted as part
        // of credentials.
        // question: should these natively use the Neynar API?
        name: {
          label: "Name",
          type: "text",
          placeholder: "0x0",
        },
        pfp: {
          label: "Pfp",
          type: "text",
          placeholder: "0x0",
        },
      },
      async authorize(credentials, req) {
        const csrfToken = req?.body?.csrfToken;
        const appClient = createAppClient({
          ethereum: viemConnector(),
        });

        const domain = getDomainFromUrl(process.env.NEXTAUTH_URL);

        const verifyResponse = await appClient.verifySignInMessage({
          message: credentials?.message as string,
          signature: credentials?.signature as `0x${string}`,
          domain,
          nonce: csrfToken,
        });
        const { success, fid } = verifyResponse;

        if (!success) {
          return null;
        }

        return {
          id: fid.toString(),
        };
      },
    }),
  ],
  callbacks: {
    session: async ({ session, token }) => {
      if (session?.user) {
        session.user.fid = parseInt(token.sub ?? '');
      }
      return session;
    },
  }
}

export const getSession = () => getServerSession(authOptions)