Auth.js vs Keycloak for Solo Developers

Choosing between a lightweight library and a full identity platform is one of the most consequential auth decisions a solo developer can make. Auth.js (formerly NextAuth.js) and Keycloak sit at opposite ends of the spectrum. One is a library you add to your JavaScript framework. The other is an entire identity server you deploy and manage. Understanding when each makes sense will save you from either over-engineering or under-building your auth layer.

Auth.js Overview

Auth.js is an open-source authentication library for JavaScript frameworks. It started as NextAuth.js for Next.js and has expanded to support SvelteKit, Express, Remix, and other frameworks. It handles session management, OAuth flows, email/magic link sign-in, and credential-based login.

The library runs inside your existing application. There is no separate server to deploy. You configure providers (Google, GitHub, Discord, etc.), set up a database adapter (Prisma, Drizzle, or others), and Auth.js manages sessions using either JWTs or database sessions. The entire setup lives in a single configuration file.

For solo developers, the appeal is control without complexity. You own the code, the sessions, and the user data. There are no usage limits, no monthly active user caps, and no vendor lock-in. Auth.js is free and always will be.

Keycloak Overview

Keycloak is an enterprise-grade identity and access management server from Red Hat. It provides OAuth 2.0, OpenID Connect, SAML 2.0, LDAP integration, fine-grained RBAC, and advanced features like identity brokering and custom authentication flows.

You deploy Keycloak as a standalone Java application, configure realms, clients, and roles through its admin console, and connect your apps via standard OIDC. It is the go-to solution for organizations managing authentication across multiple applications and services.

The power comes with weight. Keycloak requires its own server, its own database, TLS configuration, and ongoing maintenance. The admin console has an enormous number of settings. For teams with dedicated DevOps resources, this is manageable. For solo developers, it is a significant commitment.

Comparison Table

When to Pick Auth.js

Auth.js is the right fit when you want authentication integrated directly into your JavaScript application with no external dependencies:

• You are building with Next.js, SvelteKit, Remix, or another supported framework.
• You want OAuth social logins with minimal configuration.
• Keeping your infrastructure simple matters more than having an enterprise feature set.
• You are comfortable writing authorization logic in your application code.
• Zero additional infrastructure is a hard requirement.

The biggest advantage is simplicity. Your auth lives in your app. There is no separate service to monitor, no Java process to babysit, and no admin console to learn. When you deploy your app, auth deploys with it.

When to Pick Keycloak

Keycloak makes sense when your authentication requirements go beyond what a library can provide:

• You are running multiple applications or microservices that need centralized auth.
• Enterprise SSO via SAML 2.0 is a hard requirement.
• You need complex role hierarchies, group-based permissions, or realm-level isolation.
• Custom authentication flows (conditional MFA, step-up auth) are part of your product.
• You are building something where the identity layer needs to be a first-class, standalone component.

If you are a solo developer building a single SaaS app and none of these apply, Keycloak is almost certainly overkill. The operational cost of running it will eat into your development time.

Verdict

Auth.js wins for solo developers in almost every typical scenario. If you are building a JavaScript web app and need authentication, Auth.js gives you a production-ready solution with zero infrastructure overhead. It supports more OAuth providers out of the box than most developers will ever need, and the community is active and growing.

Pick Keycloak only when you have a genuine enterprise requirement like SAML SSO, multi-service centralized auth, or complex RBAC that would be painful to implement in application code. For a solo developer, the time spent configuring and maintaining Keycloak would almost always be better spent building product features. Start with Auth.js. If you outgrow it, you will know exactly why, and migrating to a dedicated identity server at that point will be a well-informed decision rather than a premature one.