Auth0 vs AuthJS for Solo Developers

Quick Comparison

Auth0 Overview

Auth0 is Okta's enterprise identity platform. It's a fully managed auth service covering social logins, passwordless, MFA, SAML SSO, machine-to-machine tokens, and role-based access. The Universal Login redirects users to Auth0's domain for authentication, so credentials never touch your server.

Configuration is done through their dashboard: creating applications, wiring up social connections, and writing Actions (serverless hooks) to customize login flows. The SDK ecosystem covers React, Vue, Angular, Django, Rails, Express, and mobile frameworks.

I set up Auth0 for a multi-tenant B2B app. The SAML SSO integration with a customer's Okta instance took half a day, but after that, enterprise login just worked. For that specific use case, Auth0 earned every dollar.

AuthJS Overview

AuthJS (formerly NextAuth.js) is an open-source authentication library for JavaScript frameworks. It started as a Next.js-only solution but now supports SvelteKit, Express, and more. You configure providers, define callbacks, and AuthJS handles OAuth flows, session management, and CSRF protection.

The big appeal is that AuthJS is free and runs in your application. No external service, no monthly bill, no MAU limits. You add providers (Google, GitHub, Discord, email), configure a database adapter if you want persistent sessions, and auth works. The library handles the OAuth dance, token exchange, and session cookies.

I've used AuthJS in multiple Next.js projects. Adding Google and GitHub sign-in takes about 30 minutes, including the OAuth app creation on each provider's side. The session hook gives you user data on every page. For straightforward social login, AuthJS removes nearly all the friction.

Key Differences

Cost. AuthJS is completely free. No limits, no tiers, no per-user charges, shipped under the permissive ISC license. Auth0's free plan now covers up to 25,000 monthly active users (a notable bump from the old 7,500 cap), but the moment you need paid features the Essentials plan starts at $35/month for just 500 MAU and the Professional plan jumps to $240/month. The free tier is generous on headcount yet thin on features, so the real cost question is which capabilities force you off it. AuthJS costs nothing regardless of how many users you have. For a bootstrapping solo developer, this matters.

Managed vs library. Auth0 handles everything: hosting, security patches, uptime, and compliance. AuthJS is a library in your codebase. You're responsible for keeping it updated, handling edge cases, and ensuring your session storage is reliable. Auth0 removes operational burden. AuthJS removes vendor dependency.

Social login setup. Both make social logins straightforward, but the experience differs. Auth0 lets you toggle providers in a dashboard and enter your OAuth credentials. AuthJS requires you to configure each provider in code and handle the callback routes. Auth0 is slightly faster for adding new providers. AuthJS gives you more control over the flow.

Enterprise features. Auth0 has SAML SSO, SCIM provisioning, organizations, and compliance certifications. AuthJS has none of this built in. You could build SAML support on top of AuthJS using additional libraries, but it's a significant effort. If enterprise customers need SSO, Auth0 is the realistic choice.

Framework support. AuthJS supports Next.js, SvelteKit, and Express with official adapters. Auth0 has SDKs for nearly every framework and language. If you're working in Python, Ruby, Go, or mobile, Auth0 has you covered. AuthJS is JavaScript-only.

Customization. AuthJS gives you callbacks at every step of the auth flow: sign-in, JWT creation, session generation. You can modify tokens, add custom claims, redirect users, and control session behavior in code. Auth0 offers similar customization through Actions, but you're writing serverless functions in their platform, not in your codebase.

By the Numbers (2026)

The marketing copy on both sides smooths over the parts that actually decide a solo build. Here are the figures I could verify, checked on 2026-05-28.

AuthJS (open source).

• Latest stable: next-auth 4.24.14, published 14 April 2026. The framework-agnostic core, @auth/core, sits at 0.34.3 and the SvelteKit adapter @auth/sveltekit at 1.11.2.
• GitHub: 28,259 stars, 4,043 forks, written in TypeScript, last pushed 14 April 2026. Active, not abandoned.
• npm pull: roughly 4.24 million weekly downloads for next-auth and about 3.09 million for @auth/core in the week ending 27 May 2026. This is one of the most installed auth libraries in the JavaScript ecosystem.
• License: ISC, which is as permissive as MIT, so you can read, fork, and modify it freely.
• Cost: zero. There is no MAU meter to watch.

Auth0 (managed).

• Free plan: up to 25,000 monthly active users at $0, including unlimited social connections, passwordless authentication, basic MFA, 1 custom domain, and 5 organizations.
• Essentials: from $35/month, billed against 500 MAU at the entry point, adding pro MFA, role-based access control per organization, and audit log streaming.
• Professional: from $240/month at 500 MAU, adding enhanced attack protection, enterprise MFA, custom database logins, and a security center.
• Enterprise: custom pricing with a 99.99 percent SLA and dedicated support.
• An MAU is any user who authenticates at least once in a 30-day billing window, so usage, not signups, drives the bill.

The headline that catches people off guard: Auth0's free tier is generous on raw user count but the features a real product needs (RBAC per org, pro MFA, more than one organization) live behind the $35 Essentials line, and that line only includes 500 MAU before it scales up.

Real Cost at Solo-Dev Scale

Take a realistic indie product. Say you ship a B2C SaaS, grow to 4,000 monthly active users, and use only social and email login plus standard MFA. What does auth actually cost per month?

AuthJS: $0 for the library. You still pay for the infrastructure it runs on (your existing app server and a session database you already operate), but there is no per-user line item. At 4,000 MAU it is $0. At 400,000 MAU it is still $0.

Auth0: 4,000 MAU sits comfortably inside the free tier's 25,000 MAU ceiling, so the raw login count is free. The catch is features. If standard free-tier MFA and a single organization cover you, your bill stays at $0. The moment you need per-organization RBAC, pro MFA, or more than one organization, you move to Essentials at $35/month, and that entry price only bundles 500 MAU. Pushing Essentials up to its 7,500 MAU band raises the monthly figure well above the $35 floor (Auth0 quotes the exact band price in-dashboard at checkout, so confirm current band pricing there).

The honest read: at a few thousand users with plain social login, both can be free. AuthJS is free because the code is yours. Auth0 is free because you are still under its caps. The divergence shows up when you cross into paid Auth0 territory, where cost scales with both users and feature tier, while AuthJS holds flat at zero. For a bootstrapper optimizing runway, that flat zero is the whole argument. Source figures and bands below; verify Auth0's exact MAU-band pricing in your own dashboard before budgeting.

When to Choose Auth0

• You need enterprise SSO (SAML, SCIM) for B2B customers
• Compliance certifications are a hard requirement
• You're working in a non-JavaScript framework
• You want a managed service so you don't maintain auth infrastructure
• You need features like passwordless or phone-based authentication

When to Choose AuthJS

• You're building with Next.js or SvelteKit and want free auth
• Your budget is tight and per-MAU pricing is a concern
• You want auth logic in your codebase, not on a third-party platform
• Social login (Google, GitHub, Discord) covers your needs
• You prefer open-source solutions you can audit and modify

The Verdict

For solo developers building with Next.js or SvelteKit, AuthJS is the practical choice. It's free, it's in your codebase, and it handles social logins without any recurring costs. The community is active, the documentation is solid, and most authentication needs for indie products are covered.

Auth0 makes sense when you're building for enterprise. SAML SSO, compliance, and organization management are features AuthJS simply doesn't have. If your go-to-market involves selling to companies that require SSO, Auth0 solves a real problem.

My recommendation: start with AuthJS for your MVP. It costs nothing and handles the auth patterns most solo developers need. If enterprise customers start asking for SSO, you can add Auth0 for those specific flows later. Don't pay for enterprise auth before you have enterprise customers.

Sources

All figures verified on 2026-05-28.

• Auth0 pricing (Free up to 25,000 MAU, Essentials from $35/mo, Professional from $240/mo): auth0.com/pricing
• AuthJS GitHub repository (28,259 stars, 4,043 forks, TypeScript, last pushed 14 April 2026): github.com/nextauthjs/next-auth
• next-auth latest version 4.24.14 (published 14 April 2026): registry.npmjs.org/next-auth/latest
• next-auth weekly npm downloads (~4.24M, week ending 27 May 2026): api.npmjs.org/downloads/point/last-week/next-auth
• @auth/core weekly npm downloads (~3.09M, week ending 27 May 2026): api.npmjs.org/downloads/point/last-week/@auth/core
• AuthJS official documentation: authjs.dev