Auth.js (NextAuth.js)

Overview

Auth.js, formerly NextAuth.js, is a widely used open-source authentication library for Next.js and the broader JavaScript ecosystem, originating around 2020. It is a library you wire into your own application code, not a hosted service or standalone identity provider. It targets startups and product teams who want to add authentication directly in their app without running or paying for an external IdP. It is fully open source under a permissive license with no managed tier.

What it is good at

Breadth of providers and ease of adoption are the strengths. Auth.js ships dozens of built-in OAuth and OIDC providers, plus email and credentials flows, and integrates tightly with Next.js sessions and middleware. It is the de facto default for many Next.js apps, so there is a large body of examples, adapters for common databases, and community knowledge. Because it runs entirely in your codebase and infrastructure, there is no per-user cost and full control over the auth flow.

Where it falls short

It is a library, so you own the surrounding work: session storage, security hardening, and keeping integrations current. Enterprise SSO with SAML, SCIM provisioning, governance, and audit are not its focus, and there is no admin console or vendor support. As apps grow into multi-tenant B2B needs, teams often outgrow it toward a full platform like Zitadel or Keycloak, or a more featureful library such as Better Auth.

Pricing

Free and open source. There is no license cost; your cost is the engineering time to integrate and maintain it. Compare against running or buying an IdP with the TCO calculator.

Best for, and who should look elsewhere

A strong fit for Next.js and JavaScript apps that want to add authentication in-code with minimal fuss and many provider options. Look elsewhere if you need a managed IdP, enterprise SSO and SCIM, or built-in governance and support.

Bottom line

The default authentication library for many Next.js apps, excellent for in-app auth but not a managed enterprise identity platform.