`onSend hook`

Runs after serialization, with access to the final payload before it is written.

Since Fastify 5 Spec ↗

Syntax

app.addHook('onSend', async (request, reply, payload) => any)

Parameters

Name	Type	Required	Description
`request`	`FastifyRequest`	No	The request object.
`reply`	`FastifyReply`	No	The reply object.
`payload`	`string \| Buffer \| stream \| null`	No	The serialized response body.

Returns

any — The (possibly modified) payload to send.

Examples

import Fastify from 'fastify';

const app = Fastify();

app.addHook('onSend', async (_req, reply, payload) => {
  reply.header('x-content-type-options', 'nosniff');
  return payload;
});

Notes

Receives the already-serialized payload (string/Buffer/stream). Return a replacement to modify the body, or undefined to leave it unchanged. Last chance to set headers.