Server Actions vs API Routes in Next.js: When to Use Which

Server Actions showed up promising to delete an entire layer of boilerplate — no more hand-rolled fetch calls, no more matching route handlers to form submissions. They deliver on that promise for a specific shape of problem, and create quiet confusion when used outside it.

Actions are for mutations tied to a form or component

When a user submits a form, toggles a setting, or triggers a mutation from inside a server-rendered tree, a Server Action collapses client and server into one function call with progressive enhancement built in. That's the sweet spot: co-located, type-safe, no separate endpoint to maintain.

Routes are still right for anything external

Webhooks, third-party integrations, mobile clients, and anything a system outside your Next.js app needs to call still belong behind a proper API route. Server Actions are not a stable public contract — they're an internal wiring mechanism, and treating them as a public API invites versioning pain later.

• Form submission or in-app mutation → Server Action.
• Webhook receiver or third-party callback → API route.
• Mobile app or external client needs it → API route with real versioning.
• Needs custom caching headers or streaming control → API route.

Server Actions are RPC for your own UI. API routes are still the contract you owe everyone else.