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.

One thing changed since I first wrote this comparison, and it matters. The maintainer sunset the Lucia library. The last published release was v3.2.2 in October 2024, and the npm package now carries a deprecation notice that points to a migration guide. Lucia today is positioned as an open-source learning resource for implementing sessions and auth in JavaScript and TypeScript, not a package you take a long-term dependency on. The code still works and the repo still has 10,466 GitHub stars with roughly 203,700 weekly npm downloads, so plenty of projects still run on it. But if you adopt Lucia in 2026 you are effectively adopting a reference implementation you will own and maintain yourself, alongside the maintainer's companion packages Oslo (crypto primitives) and Arctic (OAuth 2.0 clients). Read that as a feature if you wanted to own your auth anyway, and as a warning if you wanted something maintained for you.

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, since it lets users try the app before committing to an account. Lucia doesn't have this concept built in.

By the Numbers (2026)

Here is the real, current state of both tools, checked on 2026-05-28.

Lucia

• Latest published release: v3.2.2, released 20 October 2024. The npm package is flagged deprecated with a pointer to the v3 migration guide.
• GitHub: 10,466 stars, repository active and not archived, primary language TypeScript and JavaScript.
• npm downloads: roughly 203,700 in the week of 21 to 27 May 2026 for the core lucia package, so real usage continues despite the sunset.
• Cost: free and open source, MIT-style permissive licensing, zero per-user fees at any scale.
• Status: maintainer-recommended as a learning resource rather than a maintained library. Companion packages Oslo and Arctic carry the actively maintained primitives.

Firebase Auth (Google Cloud Identity Platform)

• Free tier: 50,000 monthly active users (MAU) for email/password, anonymous, phone, and social sign-in. SAML and OIDC enterprise providers are free only up to 50 MAU.
• Paid MAU tiers above the free 50,000: $0.0055 per MAU for 50,001 to 100,000, $0.0046 per MAU for 100,001 to 1,000,000, $0.0032 per MAU for 1,000,001 to 10,000,000, and $0.0025 per MAU above 10 million.
• Enterprise SSO (SAML and OIDC): $0.015 per MAU after the first 50.
• Phone authentication (SMS): billed per message and region-dependent. United States, Canada, and India are about $0.01 per SMS, United Kingdom about $0.04, Brazil about $0.05, most other countries about $0.06, with premium carrier routes reaching $0.34. Confirm current rates before relying on these.
• Platforms: official SDKs for web, iOS, Android, Flutter, Unity, and C++, plus Admin SDKs for Node.js, Java, Python, and Go. FirebaseUI provides an open-source drop-in UI for web, iOS, and Android.

Real Cost at Solo-Dev Scale

The pricing only matters once you know your numbers, so let me work a concrete example. Lucia is free at every line item, so the entire exercise is about what Firebase Auth actually costs you.

Assumptions. Say your solo project grows to 60,000 monthly active users, all on email/password and Google sign-in, with no phone verification and no enterprise SSO. That is a real business, well past hobby scale.

The math. The first 50,000 MAU are free. The remaining 10,000 MAU fall in the 50,001 to 100,000 band at $0.0055 each.

• 10,000 MAU times $0.0055 = $55.00 per month for authentication.

So at 60,000 MAU you pay about $55 per month, and Lucia would be $0. Push to 100,000 MAU and the billable 50,000 above the free tier still sits in the same band, so 50,000 times $0.0055 equals $275 per month. The first time most solo projects feel Firebase Auth pricing at all is somewhere north of 50,000 MAU, which is a milestone most never reach.

Where the bill jumps unexpectedly. Add phone verification and the model changes, because SMS is billed per message, not per MAU. If 10,000 of those US users verify by SMS once in a month at roughly $0.01 each, that is another $100 on top, and international SMS at $0.06 would make the same volume $600. Enterprise SSO is the other surprise. Turn on SAML or OIDC and you pay $0.015 per MAU after the first 50, which is nearly three times the standard MAU rate. For a solo developer on email and social login the practical takeaway is simple. Firebase Auth is genuinely free until you have a real audience, and even then the bill is modest unless you lean on SMS or enterprise SSO. Lucia removes the bill entirely, but you pay in the hours you spend building and maintaining the flows yourself.

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 is to 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. And with Lucia now sunset to a learning resource, choosing it means choosing to maintain your own implementation, which is a fine choice as long as you make it on purpose.

Sources

All figures checked on 2026-05-28.

• Lucia repository (GitHub stars, status): https://github.com/lucia-auth/lucia
• Lucia project site (now a learning resource): https://lucia-auth.com/
• Lucia v3 deprecation and migration guide: https://lucia-auth.com/lucia-v3/migrate
• Lucia latest version and weekly downloads (npm registry API): https://registry.npmjs.org/lucia and https://api.npmjs.org/downloads/point/last-week/lucia
• Firebase pricing overview (free tier MAU): https://firebase.google.com/pricing
• Google Cloud Identity Platform pricing (per-MAU tiers, SSO): https://cloud.google.com/identity-platform/pricing
• Firebase Auth supported methods and SDKs: https://firebase.google.com/docs/auth
• Firebase Auth cost breakdown reference (per-MAU tiers and SMS rates): https://www.metacto.com/blogs/the-complete-guide-to-firebase-auth-costs-setup-integration-and-maintenance