`app.register()`

Loads a plugin, creating an encapsulated child context for its routes and decorators.

Since Fastify 5 Spec ↗

Syntax

app.register(plugin, options?)

Parameters

Name	Type	Required	Description
`plugin`	`FastifyPluginCallback`	No	A plugin function or imported plugin module.
`options`	`object`	No	Plugin options, including `prefix` for route namespacing.

Returns

FastifyInstance — The instance, for chaining.

Examples

import Fastify from 'fastify';
import cors from '@fastify/cors';

const app = Fastify();

await app.register(cors, { origin: true });

await app.register(
  async (instance) => {
    instance.get('/cats', async () => ['kitty']);
  },
  { prefix: '/api/v1' },
);

Notes

Each registered plugin runs in its own encapsulated scope; decorators and hooks added inside do not leak to the parent unless wrapped with fastify-plugin. Use `prefix` to mount route groups under a base path.