Auth is not just login
Authentication gets treated as a checkbox — "we need login" — but the real complexity is in everything around it: session management, role-based access control, multi-tenancy, organization switching, and compliance requirements like SOC 2 and GDPR. Before evaluating providers, list every auth-adjacent feature your application needs in the next 12 months.
The build vs. buy decision is already made
Unless you're building a security product, you should not be building auth from scratch. Rolling your own authentication is the single most reliably bad decision in web development. The question is which provider to buy, not whether to buy. Even if you use an open-source solution like Keycloak or SuperTokens, you're still "buying" in the sense that you're adopting someone else's auth system.
Pricing models that bite you at scale
Auth pricing diverges wildly at scale. Clerk charges per monthly active user (MAU), which is predictable but expensive for consumer apps with millions of users. Auth0's pricing jumps sharply past 10K MAU. Supabase Auth is included free with their database tier. Firebase Auth is free up to 50K MAU. The right choice depends entirely on your expected user count trajectory.
Developer experience vs. feature depth
Clerk offers the best developer experience in the market — pre-built components, excellent docs, and fast integration. Auth0 offers the deepest feature set — custom flows, machine-to-machine auth, and extensive enterprise features. Supabase Auth wins for simplicity if you're already using Supabase. The tradeoff is always between "fast to start" and "handles every edge case."
The migration reality check
Switching auth providers is harder than switching databases because auth touches every surface of your application — API routes, middleware, frontend components, and user data. If you're likely to outgrow your initial choice, pick a provider with a clear migration path or standard protocol support (OAuth 2.0, OIDC). Avoid proprietary lock-in patterns in your application code.