Type-Providers

Type Providers

Type Providers are a TypeScript feature that enables Fastify to infer type information from inline JSON Schema. They are an alternative to specifying generic arguments on routes and can reduce the need to keep associated types for each schema in a project.

Providers

Official Type Provider packages follow the @fastify/type-provider-{provider-name} naming convention. Several community providers are also available.

The following inference packages are supported:

• json-schema-to-ts
• typebox
• zod

See also the Type Provider wrapper packages for each of the packages respectively:

• @fastify/type-provider-json-schema-to-ts
• @fastify/type-provider-typebox
• @fastify/type-provider-zod

Json Schema to Ts

The following sets up a json-schema-to-ts Type Provider:

$ npm i @fastify/type-provider-json-schema-to-ts

import Fastify from 'fastify'
import { JsonSchemaToTsProvider } from '@fastify/type-provider-json-schema-to-ts'

const server = Fastify().withTypeProvider<JsonSchemaToTsProvider>()

server.get('/route', {
  schema: {
    querystring: {
      type: 'object',
      properties: {
        foo: { type: 'number' },
        bar: { type: 'string' },
      },
      required: ['foo', 'bar']
    }
  }
}, (request, reply) => {

  // type Query = { foo: number, bar: string }
  const { foo, bar } = request.query // type safe!
})

When using raw JSON Schema objects with JsonSchemaToTsProvider, TypeScript must be able to see the exact literal values in the schema. Inline schemas, like the example above, are inferred from the route call. If the schema is moved into a separate variable, use as const:

const querystringSchema = {
  type: 'object',
  properties: {
    foo: { type: 'number' },
    bar: { type: 'string' },
  },
  required: ['foo', 'bar']
} as const

Without as const, TypeScript may widen schema values such as type: 'object' to type: string, which prevents json-schema-to-ts from inferring the route types. This assertion has no effect on the schema used by Fastify at runtime.

TypeBox

The following sets up a TypeBox Type Provider:

$ npm i typebox @fastify/type-provider-typebox

import Fastify from 'fastify'
import { TypeBoxTypeProvider } from '@fastify/type-provider-typebox'
import { Type } from 'typebox'

const server = Fastify().withTypeProvider<TypeBoxTypeProvider>()

server.get('/route', {
  schema: {
    querystring: Type.Object({
      foo: Type.Number(),
      bar: Type.String()
    })
  }
}, (request, reply) => {

  // type Query = { foo: number, bar: string }
  const { foo, bar } = request.query // type safe!
})

See the TypeBox documentation for setting-up AJV to work with TypeBox.

Zod

The following sets up a Zod Type Provider:

$ npm i zod @fastify/type-provider-zod

import fastify from 'fastify'
import { ZodTypeProvider, serializerCompiler, validatorCompiler } from '@fastify/type-provider-zod'
import { z } from 'zod/v4'

const server = fastify()
server.setValidatorCompiler(validatorCompiler)
server.setSerializerCompiler(serializerCompiler)

server.withTypeProvider<ZodTypeProvider>().get('/route', {
  schema: {
    querystring: z.object({
      foo: z.number(),
      bar: z.string()
    })
  }
}, (request, reply) => {

  // type Query = { foo: number, bar: string }
  const { foo, bar } = request.query // type safe!
})

Scoped Type-Provider

The provider types don't propagate globally. In encapsulated usage, one can remap the context to use one or more providers (for example, typebox and json-schema-to-ts can be used in the same application).

Example:

import Fastify from 'fastify'
import { TypeBoxTypeProvider } from '@fastify/type-provider-typebox'
import { JsonSchemaToTsProvider } from '@fastify/type-provider-json-schema-to-ts'
import { Type } from 'typebox'

const fastify = Fastify()

function pluginWithTypebox(fastify: FastifyInstance, _opts, done): void {
  fastify.withTypeProvider<TypeBoxTypeProvider>()
    .get('/', {
      schema: {
        body: Type.Object({
          x: Type.String(),
          y: Type.Number(),
          z: Type.Boolean()
        })
      }
    }, (req) => {
        const { x, y, z } = req.body // type safe
    });
  done()
}

function pluginWithJsonSchema(fastify: FastifyInstance, _opts, done): void {
  fastify.withTypeProvider<JsonSchemaToTsProvider>()
    .get('/', {
      schema: {
        body: {
          type: 'object',
          properties: {
            x: { type: 'string' },
            y: { type: 'number' },
            z: { type: 'boolean' }
          },
        }
      }
    }, (req) => {
      const { x, y, z } = req.body // type safe
    });
  done()
}

fastify.register(pluginWithJsonSchema)
fastify.register(pluginWithTypebox)

It is important to note that since the types do not propagate globally, it is currently not possible to avoid multiple registrations on routes when dealing with several scopes, as shown below:

import Fastify from 'fastify'
import { TypeBoxTypeProvider } from '@fastify/type-provider-typebox'
import { Type } from 'typebox'

const server = Fastify().withTypeProvider<TypeBoxTypeProvider>()

server.register(plugin1) // wrong
server.register(plugin2) // correct

function plugin1(fastify: FastifyInstance, _opts, done): void {
  fastify.get('/', {
    schema: {
      body: Type.Object({
        x: Type.String(),
        y: Type.Number(),
        z: Type.Boolean()
      })
    }
  }, (req) => {
    // In a new scope, call `withTypeProvider` again to ensure it works
    const { x, y, z } = req.body
  });
  done()
}

function plugin2(fastify: FastifyInstance, _opts, done): void {
  const server = fastify.withTypeProvider<TypeBoxTypeProvider>()

  server.get('/', {
    schema: {
      body: Type.Object({
        x: Type.String(),
        y: Type.Number(),
        z: Type.Boolean()
      })
    }
  }, (req) => {
    // works
    const { x, y, z } = req.body
  });
  done()
}

Type Definition of FastifyInstance + TypeProvider

When working with modules, use FastifyInstance with Type Provider generics. See the example below:

// index.ts
import Fastify from 'fastify'
import { TypeBoxTypeProvider } from '@fastify/type-provider-typebox'
import { registerRoutes } from './routes'

const server = Fastify().withTypeProvider<TypeBoxTypeProvider>()

registerRoutes(server)

server.listen({ port: 3000 })

// routes.ts
import { Type } from 'typebox'
import {
  FastifyInstance,
  FastifyBaseLogger,
  RawReplyDefaultExpression,
  RawRequestDefaultExpression,
  RawServerDefault
} from 'fastify'
import { TypeBoxTypeProvider } from '@fastify/type-provider-typebox'

type FastifyTypebox = FastifyInstance<
  RawServerDefault,
  RawRequestDefaultExpression<RawServerDefault>,
  RawReplyDefaultExpression<RawServerDefault>,
  FastifyBaseLogger,
  TypeBoxTypeProvider
>;

export function registerRoutes(fastify: FastifyTypebox): void {
  fastify.get('/', {
    schema: {
      body: Type.Object({
        x: Type.String(),
        y: Type.Number(),
        z: Type.Boolean()
      })
    }
  }, (req) => {
    // works
    const { x, y, z } = req.body
  });
}