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CI NPM version Known Vulnerabilities js-standard-style

Fastify plugin to parse the multipart content-type. Supports:

  • Async / Await
  • Async iterator support to handle multiple parts
  • Stream & Disk mode
  • Accumulate whole file in memory
  • Mode to attach all fields to the request body
  • Tested across Linux/Mac/Windows

Under the hood it uses busboy.


npm i fastify-multipart
# Typescript support
npm i @types/busboy


If you are looking for the documentation for the legacy callback-api please see here.

const fastify = require('fastify')()
const fs = require('fs')
const util = require('util')
const path = require('path')
const { pipeline } = require('stream')
const pump = util.promisify(pipeline)

fastify.register(require('fastify-multipart'))'/', async function (req, reply) {
  // process a single file
  // also, consider that if you allow to upload multiple files
  // you must consume all files otherwise the promise will never fulfill
  const data = await req.file()

  data.file // stream
  data.fields // other parsed parts

  // to accumulate the file in memory! Be careful!
  // await data.toBuffer() // Buffer
  // or

  await pump(data.file, fs.createWriteStream(data.filename))

  // be careful of permission issues on disk and not overwrite
  // sensitive files that could cause security risks
  // also, consider that if the file stream is not consumed, the promise will never fulfill


fastify.listen(3000, err => {
  if (err) throw err
  console.log(`server listening on ${fastify.server.address().port}`)

You can also pass optional arguments to busboy when registering with Fastify. This is useful for setting limits on the content that can be uploaded. A full list of available options can be found in the busboy documentation.

Note: if the file stream that is provided by data.file is not consumed, like in the example below with the usage of pump, the promise will not be fulfilled at the end of the multipart processing. This behavior is inherited from busboy.

fastify.register(require('fastify-multipart'), {
  limits: {
    fieldNameSize: 100, // Max field name size in bytes
    fieldSize: 1000000, // Max field value size in bytes
    fields: 10,         // Max number of non-file fields
    fileSize: 100,      // For multipart forms, the max file size
    files: 1,           // Max number of file fields
    headerPairs: 2000   // Max number of header key=>value pairs

If you do set upload limits, be sure to catch the error. An error or exception will occur if a limit is reached. These events are documented in more detail here.

Note: if the file stream that is provided by data.file is not consumed, like in the example below with the usage of pump, the promise will not be fulfilled at the end of the multipart processing. This behavior is inherited from busboy.

Note: if you set a fileSize limit and you want to know if the file limit was reached you can listen to data.file.on('limit') or check at the end of the stream the property data.file.truncated.

try {
  const data = await req.file()
  await pump(data.file, fs.createWriteStream(data.filename))
} catch (error) {
  if (error instanceof fastify.multipartErrors.FilesLimitError) {
    // handle error

Additionally, you can pass per-request options to the req.file, req.files, req.saveRequestFiles or req.multipartIterator function.'/', async function (req, reply) {
  const options = { limits: { fileSize: 1000 } };
  const data = await req.file(options)
  await pump(data.file, fs.createWriteStream(data.filename))

Handle multiple file streams'/', async function (req, reply) {
  const parts = await req.files()
  for await (const part of parts) {
    await pump(part.file, fs.createWriteStream(part.filename))

Handle multiple file streams and fields'/upload/raw/any', async function (req, reply) {
  const parts = await
  for await (const part of parts) {
    if (part.file) {
      await pump(part.file, fs.createWriteStream(part.filename))
    } else {

Accumulate whole file in memory'/upload/raw/any', async function (req, reply) {
  const data = await req.file()
  const buffer = await data.toBuffer()
  // upload to S3

Upload files to disk and work with temporary file paths

This will store all files in the operating system default directory for temporary files. As soon as the response ends all files are removed.'/upload/files', async function (req, reply) {
  // stores files to tmp dir and return files
  const files = await req.saveRequestFiles()
  files[0].fields // other parsed parts


Handle file size limitation

If you set a fileSize limit, it is able to throw a RequestFileTooLargeError error when limit reached.'/upload/files', async function (req, reply) {
  try {
    //const file = await req.file({ limits: { fileSize: 17000 } })
    //const files = await req.files({ limits: { fileSize: 17000 } })
    //const parts = await{ limits: { fileSize: 17000 } })
    const files = await req.saveRequestFiles({ limits: { fileSize: 17000 } })
  } catch (error) {
    // error instanceof fastify.multipartErrors.RequestFileTooLargeError

If you want to fallback to the handling before 4.0.0, you can disable the throwing behavior by passing throwFileSizeLimit. Note: It will not affect the behavior of saveRequestFiles()

// globally disable
fastify.register(fastifyMultipart, { throwFileSizeLimit: false })'/upload/file', async function (req, reply) {
  const file = await req.file({ throwFileSizeLimit: false, limits: { fileSize: 17000 } })
  //const files = await req.files({ throwFileSizeLimit: false, limits: { fileSize: 17000 } })
  //const parts = await{ throwFileSizeLimit: false, limits: { fileSize: 17000 } })
  //const files = await req.saveRequestFiles({ throwFileSizeLimit: false, limits: { fileSize: 17000 } })

Parse all fields and assign them to the body

This allows you to parse all fields automatically and assign them to the request.body. By default files are accumulated in memory (Be careful!) to buffer objects. Uncaught errors are handled by Fastify.

fastify.register(require('fastify-multipart'), { attachFieldsToBody: true })'/upload/files', async function (req, reply) {
  const uploadValue = await req.body.upload.toBuffer()  // access files
  const fooValue = await           // other fields

You can also define an onFile handler to avoid accumulating all files in memory.

async function onFile(part) {
  await pump(part.file, fs.createWriteStream(part.filename))

fastify.register(require('fastify-multipart'), { attachFieldsToBody: true, onFile })'/upload/files', async function (req, reply) {
  const fooValue = await           // other fields

JSON Schema body validation

If you enable attachFieldsToBody and set sharedSchemaId a shared JSON Schema is added, which can be used to validate parsed multipart fields.

const opts = {
  attachFieldsToBody: true,
  sharedSchemaId: '#mySharedSchema'
fastify.register(require('fastify-multipart'), opts)'/upload/files', {
  schema: {
    body: {
      type: 'object',
      required: ['myField'],
      properties: {
        myField: { $ref: '#mySharedSchema'},
        // or
        myFiles: { type: 'array', items: fastify.getSchema('mySharedSchema') },
        // or
        hello: {
          properties: {
            value: { 
              type: 'string',
              enum: ['male']
}, function (req, reply) {
  console.log({ body: req.body })

Access all errors

We export all custom errors via a server decorator fastify.multipartErrors. This is useful if you want to react to specific errors. They are derived from fastify-error and include the correct statusCode property.'/upload/files', async function (req, reply) {
  const { FilesLimitError } = fastify.multipartErrors


This project is kindly sponsored by:


Licensed under MIT.