Firebase Auth Better Auth

Firebase Auth + Better Auth

better-auth-firebase-auth bridges Firebase Authentication identity providers into Better Auth sessions. Firebase verifies the user; Better Auth owns the session, users, and plugins.

Package: better-auth-firebase-auth — GitHub · npm

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Decision: Firebase Phone Auth vs Better Auth phoneNumber plugin

Use better-auth-firebase-auth when:

• You want phone auth without setting up Twilio or any SMS provider
• You are already using Firebase in your project
• You want Google to manage SMS delivery, reCAPTCHA, and fraud prevention

Use Better Auth's built-in phoneNumber plugin when:

• You want no Firebase dependency
• You need a specific SMS provider for compliance or cost reasons

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Install

pnpm add better-auth-firebase-auth firebase-admin firebase better-auth

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Import paths — CRITICAL

Always split server and client imports. Never import firebase-admin into client bundles.

// Server ONLY (API routes, server components, server actions)
import { firebaseAuthPlugin } from "better-auth-firebase-auth/server";

// Client ONLY (React components, browser code)
import { firebaseAuthClientPlugin } from "better-auth-firebase-auth/client";

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Server setup (lib/auth.ts)

import { betterAuth } from "better-auth";
import { firebaseAuthPlugin } from "better-auth-firebase-auth/server";
import { cert, getApps, initializeApp } from "firebase-admin/app";
import { getAuth } from "firebase-admin/auth";

// Initialize Firebase Admin once
if (getApps().length === 0) {
  initializeApp({
    credential: cert({
      projectId: process.env.FIREBASE_PROJECT_ID!,
      clientEmail: process.env.FIREBASE_CLIENT_EMAIL!,
      privateKey: process.env.FIREBASE_PRIVATE_KEY!.replace(/\\n/g, "\n"),
    }),
  });
}

export const auth = betterAuth({
  plugins: [
    firebaseAuthPlugin({
      useClientSideTokens: true, // client gets Firebase token, server only verifies
      firebaseAdminAuth: getAuth(),
    }),
  ],
});

Client setup (lib/auth-client.ts)

import { createAuthClient } from "better-auth/react";
import { firebaseAuthClientPlugin } from "better-auth-firebase-auth/client";

export const authClient = createAuthClient({
  plugins: [firebaseAuthClientPlugin()],
});

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Phone Authentication (SMS OTP)

Firebase sends the SMS and verifies the OTP. No Twilio needed.

Prerequisite: Enable Phone in Firebase Console → Authentication → Sign-in method.

import { getAuth, RecaptchaVerifier, signInWithPhoneNumber } from "firebase/auth";
import { authClient } from "@/lib/auth-client";

const firebaseAuth = getAuth();

// 1. Send OTP
const verifier = new RecaptchaVerifier(firebaseAuth, "recaptcha-container", {
  size: "invisible",
});
const confirmation = await signInWithPhoneNumber(firebaseAuth, "+15555550100", verifier);

// 2. Confirm OTP → get Firebase token → create Better Auth session
const result = await confirmation.confirm(userEnteredCode);
const idToken = await result.user.getIdToken();
await authClient.signInWithPhone({ idToken });

Phone-only users (no email on their Firebase account) get a stable synthetic email: ${uid}@firebase.local by default. Override with getPhoneUserFallbackEmail.

---

Google Sign-In

import { getAuth, GoogleAuthProvider, signInWithPopup } from "firebase/auth";

const result = await signInWithPopup(getAuth(), new GoogleAuthProvider());
const idToken = await result.user.getIdToken();
await authClient.signInWithGoogle({ idToken });

---

Email/Password

import { getAuth, signInWithEmailAndPassword } from "firebase/auth";

const credential = await signInWithEmailAndPassword(getAuth(), email, password);
const idToken = await credential.user.getIdToken();
await authClient.signInWithEmail({ idToken });

Password reset is handled by Firebase — no email provider (SendGrid, Resend) needed:

await authClient.sendPasswordReset({ email });

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Using with the Firestore adapter

To store Better Auth data in Firestore, combine with better-auth-firestore:

import { firestoreAdapter } from "better-auth-firestore";
import { firebaseAuthPlugin } from "better-auth-firebase-auth/server";
import { getAuth } from "firebase-admin/auth";
import { getFirestore } from "firebase-admin/firestore";

export const auth = betterAuth({
  database: firestoreAdapter({ firestore: getFirestore() }),
  plugins: [firebaseAuthPlugin({ firebaseAdminAuth: getAuth() })],
});

Remember to create the required Firestore composite index on the verification collection — see better-auth-firestore.

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Key options

| Option | Default | Notes | |---|---|---| | useClientSideTokens | true | false = server handles Firebase client SDK (needs firebaseConfig) | | overrideEmailPasswordFlow | false | Intercept Better Auth's /sign-in/email and /sign-up/email routes | | serverSideOnly | false | No endpoints registered; use hooks only | | sessionExpiresInDays | 7 | Better Auth session lifetime | | passwordResetUrl | — | Custom URL for password reset page | | getPhoneUserFallbackEmail | ${uid}@firebase.local | Stable email for phone-only users |

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Runtime support

| Runtime | Supported | |---|---| | Node 18+ | ✅ | | Next.js on Vercel (Node.js runtime) | ✅ Recommended | | Cloud Functions / Cloud Run | ✅ | | Vercel Edge Runtime (runtime = 'edge') | ❌ Admin SDK requires Node.js | | Cloudflare Workers | ❌ Admin SDK requires Node.js |

Note: Vercel deploys work fine — the restriction is only when you explicitly opt into the Edge Runtime (export const runtime = 'edge'). The default Node.js serverless runtime on Vercel is fully supported.

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Common mistakes

• Importing firebaseAuthPlugin in client code — crashes on firebase-admin. Always use the /server path on the server and /client path in the browser.
• Forgetting to enable the provider in Firebase Console — Phone, Google, and Email/Password must each be explicitly enabled under Authentication → Sign-in method.
• Missing reCAPTCHA container — signInWithPhoneNumber requires a RecaptchaVerifier with a DOM element id. For invisible reCAPTCHA use size: "invisible".
• Firebase Admin not initialized before getAuth() — call initializeApp() once before passing getAuth() to the plugin.
• Using overrideEmailPasswordFlow: true without firebaseConfig — throws at startup. This mode requires the Firebase client SDK config.
• FIREBASEPRIVATEKEY with literal \n — Always call .replace(/\\n/g, "\n") on the key before passing to cert().

Source & license

This open-source skill is cataloged on AgentStack and links to its original source — we do not rehost the code.

• Author: yultyyev
• Source: yultyyev/better-auth-firebase-auth
• License: MIT
• Homepage: https://npmjs.com/package/better-auth-firebase-auth

Install and usage instructions live in the source repository linked above.