`onRegister hook`

Runs when a new encapsulated plugin context is created.

Since Fastify 5 Spec ↗

Syntax

app.addHook('onRegister', (instance, opts) => {})

Parameters

Name	Type	Required	Description
`instance`	`FastifyInstance`	No	The newly created child instance.
`opts`	`object`	No	The options passed to register().

Returns

void — Synchronous; inspect or decorate the child.

Examples

import Fastify from 'fastify';

const app = Fastify();

app.addHook('onRegister', (instance, opts) => {
  instance.log.info({ prefix: opts.prefix }, 'plugin registered');
});

Notes

Triggered only for plugins that create their own encapsulation context (not those wrapped with fastify-plugin). Useful for per-plugin setup, diagnostics, or scoped decoration.