`app.setNotFoundHandler()`

Defines a custom handler for requests that match no route.

Since Fastify 5 Spec ↗

Syntax

app.setNotFoundHandler(options?, (request, reply) => any)

Parameters

Name	Type	Required	Description
`options`	`object`	No	Optional preValidation and preHandler hooks.
`handler`	`Function`	No	Receives the request and reply for unmatched routes.

Returns

FastifyInstance — The instance, for chaining.

Examples

import Fastify from 'fastify';

const app = Fastify();

app.setNotFoundHandler((req, reply) => {
  reply.code(404).send({
    error: 'Not Found',
    path: req.url,
  });
});

Notes

Only one not-found handler exists per encapsulated context. Define it inside a plugin to customize 404 responses for a specific route prefix.