Lucia vs Firebase Auth for Solo Developers

Quick Comparison

Lucia Overview

Lucia is an open-source auth library that gives you the building blocks for authentication without imposing opinions about how your auth should work. You install it, configure a database adapter, and use its functions for session management, cookie handling, and token generation. The login forms, registration flows, and OAuth integrations are yours to build.

That's the core philosophy: Lucia handles the hard cryptographic parts (session IDs, secure cookies, token validation) and leaves the application logic to you. Your database schema, your forms, your middleware, your rules. There's no vendor dashboard, no external dependency, and no recurring cost.

I used Lucia in a project where I needed sessions to carry custom metadata that no managed service would let me attach. Because Lucia is a library, I extended the session object and stored exactly what I needed. The flexibility was worth the extra setup time.

Firebase Auth Overview

Firebase Auth is Google's managed authentication service. It handles email/password, phone verification, anonymous auth, and social OAuth (Google, Apple, Facebook, GitHub, Twitter) through simple SDK calls. You call signInWithPopup() or createUserWithEmailAndPassword() and Firebase manages the rest: tokens, refresh, session persistence, and user storage.

The 50,000 MAU free tier is extremely generous. For a solo developer, your auth is effectively free until you have a real business. Firebase Auth also integrates deeply with Firestore and Cloud Functions, where security rules can reference the authenticated user directly.

I use Firebase Auth in a mobile project. The Flutter SDK made Google and Apple sign-in trivially easy. Users authenticate, Firestore rules enforce access, and I never wrote a line of token verification code. For that project, the speed was unbeatable.

Key Differences

Setup time. Firebase Auth takes 15-20 minutes. Install the SDK, initialize with your config, call the auth methods. Social login providers need OAuth app setup on each platform, but Firebase handles the token exchange. Lucia takes several hours. You build the registration form, login handler, session middleware, and password hashing flow yourself. The time difference is real.

Control vs convenience. Lucia gives you total control. Custom session schemas, custom token logic, custom flows that nobody anticipated. Firebase Auth gives you convenience: pre-built flows that work out of the box but offer limited customization. If your auth needs are standard (login, signup, social, logout), Firebase handles them perfectly. If they're unusual, Lucia doesn't fight you.

Cost model. Both are free for most solo developers. Lucia is free forever with no limits. Firebase Auth is free up to 50,000 MAU, then pay-as-you-go pricing kicks in. Phone auth (SMS verification) costs money even on the free tier. For practical purposes, the cost difference only matters at significant scale.

Platform support. Firebase Auth has SDKs for web, iOS, Android, Flutter, Unity, and C++. Lucia is a JavaScript/TypeScript library that works in Node.js environments. If you're building a mobile app, Firebase Auth has native SDKs ready. With Lucia, you'd build an API that the mobile app calls.

Vendor dependency. Lucia runs in your codebase with no external calls during authentication. Firebase Auth depends on Google's servers. If Firebase has an outage (it happens), your users can't log in. With Lucia, auth availability matches your server availability. If you value independence, Lucia provides it.

Anonymous auth. Firebase Auth supports anonymous authentication, where users get a temporary account that can later be linked to a permanent identity. This is powerful for onboarding: let users try the app before committing to an account. Lucia doesn't have this concept built in.

When to Choose Lucia

• You want complete ownership of your authentication logic
• You have non-standard session or token requirements
• Zero vendor dependency is important to you
• You're comfortable building auth flows from scratch
• You want to learn how authentication really works under the hood

When to Choose Firebase Auth

• You're building a mobile app (Firebase has excellent native SDKs)
• Speed of implementation matters more than customization depth
• You want 50,000 MAU free without managing any auth infrastructure
• You're using Firebase or Google Cloud for your backend
• Anonymous auth for progressive user onboarding is useful

The Verdict

For solo developers who want to ship fast, Firebase Auth is the clear winner. The SDK is simple, the free tier is massive, and the integration with Firebase's ecosystem eliminates boilerplate. You'll have working auth in under an hour, and it'll scale to tens of thousands of users for free.

Lucia is the pick for developers who value control and ownership. If you want to understand every line of your auth code, if you have custom requirements that managed services can't accommodate, or if you're philosophically opposed to depending on Google's infrastructure, Lucia gives you that freedom.

My recommendation: use Firebase Auth unless you have a specific reason to own your auth stack. The time savings matter when you're a solo developer with limited hours. If you hit the limits of Firebase Auth later, migrating to a custom solution is always possible. Building custom auth from day one when you don't need to is over-engineering.