`onResponse hook`

Runs after the response has been fully sent to the client.

Since Fastify 5 Spec ↗

Syntax

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

Parameters

Name	Type	Required	Description
`request`	`FastifyRequest`	No	The completed request.
`reply`	`FastifyReply`	No	The reply, with statusCode and elapsedTime set.

Returns

void | Promise<void> — Side-effect only; cannot modify the response.

Examples

import Fastify from 'fastify';

const app = Fastify({ logger: true });

app.addHook('onResponse', async (req, reply) => {
  req.log.info({
    status: reply.statusCode,
    ms: reply.elapsedTime,
    route: req.routeOptions.url,
  }, 'request completed');
});

Notes

The response is already flushed, so this hook is for metrics, audit logging, and cleanup only. reply.elapsedTime gives the handler duration in milliseconds.