`preParsing hook`

Runs before the request body is parsed, allowing the raw stream to be transformed.

Since Fastify 5 Spec ↗

Syntax

app.addHook('preParsing', async (request, reply, payload) => stream)

Parameters

Name	Type	Required	Description
`request`	`FastifyRequest`	No	The incoming request.
`reply`	`FastifyReply`	No	The reply object.
`payload`	`Readable`	No	The raw request body stream.

Returns

Readable | undefined — Return a replacement stream or nothing.

Examples

import Fastify from 'fastify';
import { Transform } from 'node:stream';

const app = Fastify();

app.addHook('preParsing', async (_req, _reply, payload) => {
  // pass through; could decompress or count bytes here
  return payload.pipe(new Transform({
    transform(chunk, _enc, cb) { cb(null, chunk); },
  }));
});

Notes

Runs after onRequest but before content-type parsing. Return a new Readable to replace the incoming stream (e.g. decryption, decompression). Returning undefined leaves the stream unchanged.