Content Type Parser

Natively, Fastify only supports 'application/json' and 'text/plain' content types. The default charset is utf-8. If you need to support different content types, you can use the addContentTypeParser API. The default JSON and/or plain text parser can be changed.

As with the other APIs, addContentTypeParser is encapsulated in the scope in which it is declared. This means that if you declare it in the root scope it will be available everywhere, while if you declare it inside a register it will be available only in that scope and its children.

Fastify adds automatically the parsed request payload to the Fastify request object, you can reach it with request.body.


fastify.addContentTypeParser('application/jsoff', function (req, done) {
  jsoffParser(req, function (err, body) {
    done(err, body)
// handle multiple content types as the same
fastify.addContentTypeParser(['text/xml', 'application/xml'], function (req, done) {
  xmlParser(req, function (err, body) {
    done(err, body)
// async also supported in Node versions >= 8.0.0
fastify.addContentTypeParser('application/jsoff', async function (req) {
  var res = await new Promise((resolve, reject) => resolve(req))
  return res

You can also use the hasContentTypeParser API to find if a specific content type parser already exists.

if (!fastify.hasContentTypeParser('application/jsoff')){
  fastify.addContentTypeParser('application/jsoff', function (req, done) {
    //code to parse request body /payload for given content type

Body Parser

You can parse the body of the request in two ways. The first one is shown above: you add a custom content type parser and handle the request stream. In the second one you should pass a parseAs option to the addContentTypeParser API, where you declare how you want to get the body, it could be 'string' or 'buffer'. If you use the parseAs option Fastify will internally handle the stream and perform some checks, such as the maximum size of the body and the content length. If the limit is exceeded the custom parser will not be invoked.

fastify.addContentTypeParser('application/json', { parseAs: 'string' }, function (req, body, done) {
  try {
    var json = JSON.parse(body)
    done(null, json)
  } catch (err) {
    err.statusCode = 400
    done(err, undefined)

As you can see, now the function signature is (req, body, done) instead of (req, done).

See example/parser.js for an example.

Custom Parser Options
  • parseAs (string): Either 'string' or 'buffer' to designate how the incoming data should be collected. Default: 'buffer'.
  • bodyLimit (number): The maximum payload size, in bytes, that the custom parser will accept. Defaults to the global body limit passed to the Fastify factory function.

Catch All

There are some cases where you need to catch all requests regardless of their content type. With Fastify, you just need to add the '*' content type.

fastify.addContentTypeParser('*', function (req, done) {
  var data = ''
  req.on('data', chunk => { data += chunk })
  req.on('end', () => {
    done(null, data)

In this way, all of the requests that do not have a corresponding content type parser will be handled by the specified function.

This is also useful for piping the request stream. You can define a content parser like:

fastify.addContentTypeParser('*', function (req, done) {

and then access the core HTTP request directly for piping it where you want:'/hello', (request, reply) => {

Here is a complete example that logs incoming json line objects:

const split2 = require('split2')
const pump = require('pump')

fastify.addContentTypeParser('*', (req, done) => {
  done(null, pump(req, split2(JSON.parse)))

  method: 'POST',
  url: '/api/log/jsons',
  handler: (req, res) => {
    req.body.on('data', d => console.log(d)) // log every incoming object

For piping file uploads you may want to checkout this plugin.