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Version: v4.1.x

V4 Migration Guide

This guide is intended to help with migration from Fastify v3 to v4.

Before migrating to v4, please ensure that you have fixed all deprecation warnings from v3. All v3 deprecations have been removed and they will no longer work after upgrading.

Breaking Changes

Deprecation of app.use()

Starting this version of Fastify, we have deprecated the use of app.use(). We have decided not to support the use of middlewares. Both @fastify/middie and @fastify/express will still be there and maintained. Use Fastify's hooks instead.

reply.res moved to reply.raw

If you previously used the reply.res attribute to access the underlying Request object you'll instead need to depend on reply.raw.

Need to return reply to signal a "fork" of the promise chain

In some situations, like when a response is sent asynchronously or when you're just not explicitly returning a response, you'll need to return the reply argument from your router handler.

exposeHeadRoutes true by default

Starting from v4, all the GET routes will create a sibling HEAD route. You can revert this behaviour by setting the server's option exposeHeadRoutes to false.

Non Breaking Changes

Change of schema for multiple types

Since Fastify v4 has upgraded to Ajv v8. The "type" keywords with multiple types (other than with "null") are prohibited. Read more 'here'

You may encounter a console warning such as

strict mode: use allowUnionTypes to allow union type keyword at "#/properties/image" (strictTypes)

So schemas like below will need to be changed from

type: 'object',
properties: {
api_key: { type: 'string' },
image: { type: ['object', 'array'] }


type: 'object',
properties: {
api_key: { type: 'string' },
image: {
anyOf: [
{ type: 'array' },
{ type: 'object' }