`preHandler hook`

Runs after validation, just before the route handler executes.

Since Fastify 5 Spec ↗

Syntax

app.addHook('preHandler', async (request, reply) => {})

Parameters

Name	Type	Required	Description
`request`	`FastifyRequest`	No	The validated request.
`reply`	`FastifyReply`	No	The reply object.

Returns

void | Promise<void> — Throw or send to short-circuit.

Examples

import Fastify from 'fastify';

const app = Fastify();

const requireAdmin = async (req, reply) => {
  if (req.user?.role !== 'admin') {
    reply.code(403).send({ error: 'Forbidden' });
  }
};

app.delete('/cats/:id', { preHandler: requireAdmin }, async () => ({
  deleted: true,
}));

Notes

The body is parsed and validated here, making preHandler the right place for authorization and resource loading. Route-level preHandler arrays run in order before the handler.