Fastify

The fastify plugin generates type-safe route handler definitions for Fastify servers, ensuring your route handlers match your OpenAPI specification at compile time.

Installation

Install OpenAPI TypeScript:

npm install @hey-api/openapi-ts --save-dev

The Fastify plugin generates types only. You’ll need the fastify package for your runtime server.

Configuration

Add the fastify plugin to your OpenAPI TypeScript configuration:

openapi-ts.config.ts

import { defineConfig } from '@hey-api/openapi-ts';

export default defineConfig({
  input: 'https://api.example.com/openapi.yaml',
  output: {
    path: './src/client',
  },
  plugins: [
    'fastify',
    '@hey-api/typescript',
  ],
});

What Gets Generated

The plugin generates a RouteHandlers type that maps each operation to a Fastify RouteHandler type with proper typing for:

Request body (Body)
Query parameters (Querystring)
Path parameters (Params)
Headers (Headers)
Response data (Reply)

Example Generated Code

Given this OpenAPI operation:

openapi.yaml

paths:
  /pet/{petId}:
    get:
      operationId: showPetById
      parameters:
        - name: petId
          in: path
          required: true
          schema:
            type: string
      responses:
        '200':
          description: Pet details
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/Pet'

The plugin generates:

fastify.gen.ts

import type { RouteHandler } from 'fastify';
import type { ShowPetByIdData, ShowPetByIdResponses } from './types.gen';

export type RouteHandlers = {
  showPetById: RouteHandler<{
    Params: ShowPetByIdData['path'];
    Reply: ShowPetByIdResponses;
  }>;
};

Usage

Implement your route handlers with full type safety:

handlers.ts

import type { RouteHandlers } from './client/fastify.gen';

export const handlers: RouteHandlers = {
  showPetById(request, reply) {
    // request.params is typed as { petId: string }
    const petId = request.params.petId;
    
    // reply.send() expects the correct response type
    reply.send({
      id: petId,
      name: 'Fluffy',
      status: 'available',
    });
  },
  
  createPet(request, reply) {
    // request.body is fully typed
    const pet = request.body;
    
    // TypeScript ensures you return the correct response
    reply.code(201).send(pet);
  },
  
  listPets(request, reply) {
    // request.query is typed with query parameters
    const limit = request.query?.limit ?? 10;
    
    reply.send([]);
  },
};

Integration with Fastify

Use the generated types with popular Fastify plugins like fastify-openapi-glue:

server.ts

import Fastify from 'fastify';
import glue from 'fastify-openapi-glue';
import { readFileSync } from 'fs';
import { handlers } from './handlers';

const fastify = Fastify();

// Load your OpenAPI specification
const specification = JSON.parse(
  readFileSync('./openapi.json', 'utf-8')
);

// Register with type-safe handlers
fastify.register(glue, {
  specification,
  serviceHandlers: handlers,
});

fastify.listen({ port: 3000 });

Type Safety Benefits

Request Parameter Typing

The plugin extracts parameter types from your OpenAPI spec:

// Path parameters
showPetById(request, reply) {
  request.params.petId // string (required)
}

// Query parameters
listPets(request, reply) {
  request.query?.limit    // number | undefined
  request.query?.status   // 'available' | 'pending' | 'sold' | undefined
}

// Request body
createPet(request, reply) {
  request.body.name    // string
  request.body.status  // 'available' | 'pending' | 'sold'
}

Response Type Safety

Return types are validated against your API spec:

showPetById(request, reply) {
  reply.send({
    id: 123,
    name: 'Fluffy',
    status: 'available',
  }); // ✓ Type-safe!
  
  reply.send({ invalid: 'data' }); // ✗ Type error!
}

Error Response Typing

Error responses from your OpenAPI spec are included in the Reply type:

showPetById(request, reply) {
  if (!petExists) {
    // 404 error response is typed
    reply.code(404).send({
      code: 404,
      message: 'Pet not found',
    });
  }
}

Configuration Options

The fastify plugin has minimal configuration:

includeInEntry

boolean

default:false

Whether to export the RouteHandlers type from the main index.ts entry file.

openapi-ts.config.ts

{
  name: 'fastify',
  includeInEntry: true,
}

Complete Example

Define your OpenAPI specification

openapi.yaml

openapi: 3.1.0
info:
  title: Pet Store API
  version: 1.0.0
paths:
  /pets:
    get:
      operationId: listPets
      parameters:
        - name: limit
          in: query
          schema:
            type: integer
            maximum: 100
      responses:
        '200':
          description: List of pets
          content:
            application/json:
              schema:
                type: array
                items:
                  $ref: '#/components/schemas/Pet'
    post:
      operationId: createPet
      requestBody:
        required: true
        content:
          application/json:
            schema:
              $ref: '#/components/schemas/NewPet'
      responses:
        '201':
          description: Pet created
  /pets/{petId}:
    get:
      operationId: showPetById
      parameters:
        - name: petId
          in: path
          required: true
          schema:
            type: string
      responses:
        '200':
          description: Pet details
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/Pet'
components:
  schemas:
    Pet:
      type: object
      required:
        - id
        - name
      properties:
        id:
          type: integer
        name:
          type: string
        status:
          type: string
          enum: [available, pending, sold]
    NewPet:
      type: object
      required:
        - name
      properties:
        name:
          type: string
        status:
          type: string
          enum: [available, pending, sold]

Configure OpenAPI TypeScript

openapi-ts.config.ts

import { defineConfig } from '@hey-api/openapi-ts';

export default defineConfig({
  input: './openapi.yaml',
  output: {
    path: './src/client',
  },
  plugins: ['fastify', '@hey-api/typescript'],
});

Generate types

npx @hey-api/openapi-ts

Implement handlers

src/handlers.ts

import type { RouteHandlers } from './client/fastify.gen';

const pets: Array<{ id: number; name: string; status: string }> = [];
let nextId = 1;

export const serviceHandlers: RouteHandlers = {
  listPets(request, reply) {
    const limit = request.query?.limit ?? 10;
    reply.send(pets.slice(0, limit));
  },
  
  createPet(request, reply) {
    const newPet = {
      id: nextId++,
      ...request.body,
    };
    pets.push(newPet);
    reply.code(201).send(newPet);
  },
  
  showPetById(request, reply) {
    const pet = pets.find(p => p.id === Number(request.params.petId));
    if (!pet) {
      reply.code(404).send({ error: 'Pet not found' });
      return;
    }
    reply.send(pet);
  },
};

Create server

src/server.ts

import Fastify from 'fastify';
import glue from 'fastify-openapi-glue';
import { readFileSync } from 'fs';
import { serviceHandlers } from './handlers';

export async function buildServer() {
  const fastify = Fastify({ logger: true });
  
  const specification = JSON.parse(
    readFileSync('./openapi.json', 'utf-8')
  );
  
  await fastify.register(glue, {
    specification,
    serviceHandlers,
  });
  
  return fastify;
}

buildServer().then(server => {
  server.listen({ port: 3000 }, (err) => {
    if (err) {
      server.log.error(err);
      process.exit(1);
    }
  });
});

Generated Code Structure

The plugin generates a single file:

src/client/
├── fastify.gen.ts    # RouteHandlers type
├── types.gen.ts      # Request/response types
└── index.ts          # Main entry (if includeInEntry: true)

Dependencies

The fastify plugin declares a dependency on @hey-api/typescript for type generation:

plugins: [
  'fastify',        // Generates RouteHandlers
  '@hey-api/typescript',  // Generates request/response types
]

How It Works

For each operation in your OpenAPI spec, the plugin:

Extracts operation metadata - Reads operationId, parameters, request body, and responses
Maps to Fastify types - Converts OpenAPI parameters to Fastify’s type system:
- parameters[in=path] → Params
- parameters[in=query] → Querystring
- parameters[in=header] → Headers
- requestBody → Body
- responses → Reply
Generates handler signature - Creates a RouteHandler<T> type with all request/response types
Combines into RouteHandlers - Exports a single type mapping operation IDs to handler types

Best Practices

Use with fastify-openapi-glue - This plugin works perfectly with fastify-openapi-glue to automatically route requests to your handlers based on the OpenAPI spec.

Keep handlers and spec in sync

Run npx @hey-api/openapi-ts after updating your OpenAPI specification to regenerate types.

Leverage TypeScript strict mode

Enable strict: true in tsconfig.json to catch type mismatches at compile time.

Use operationId consistently

Ensure every operation in your OpenAPI spec has a unique operationId - this becomes your handler function name.

Handle all response types

The Reply type includes all possible responses from your spec. Handle success and error cases appropriately.

Limitations

The plugin generates types only, not runtime validation
Default responses are omitted from the Reply type to avoid overly broad types
Only operations with operationId are included in the RouteHandlers type

Migration from Untyped Handlers

Before:

const handlers = {
  getPet(request, reply) {
    const id = request.params.id; // any
    reply.send({ id, name: 'Fluffy' }); // no type checking
  },
};

After:

import type { RouteHandlers } from './client/fastify.gen';

const handlers: RouteHandlers = {
  getPet(request, reply) {
    const id = request.params.id; // string (typed!)
    reply.send({ id, name: 'Fluffy' }); // type-checked!
  },
};

TypeScript will immediately flag any mismatches between your handlers and your OpenAPI specification.

Get Started

Configuration

Core Plugins

HTTP Clients

Validators

State Management

Web Frameworks

Advanced

Installation

Configuration

What Gets Generated

Example Generated Code

Usage

Integration with Fastify

Type Safety Benefits

Request Parameter Typing

Response Type Safety

Error Response Typing

Configuration Options

Complete Example

Generated Code Structure

Dependencies

How It Works

Best Practices

Limitations

Migration from Untyped Handlers

Build docs developers (and LLMs) love

Get Started

Configuration

Core Plugins

HTTP Clients

Validators

State Management

Web Frameworks

Advanced

​Installation

​Configuration

​What Gets Generated

​Example Generated Code

​Usage

​Integration with Fastify

​Type Safety Benefits

​Request Parameter Typing

​Response Type Safety

​Error Response Typing

​Configuration Options

​Complete Example

​Generated Code Structure

​Dependencies

​How It Works

​Best Practices

​Limitations

​Migration from Untyped Handlers

Build docs developers (and LLMs) love

Installation

Configuration

What Gets Generated

Example Generated Code

Usage

Integration with Fastify

Type Safety Benefits

Request Parameter Typing

Response Type Safety

Error Response Typing

Configuration Options

Complete Example

Generated Code Structure

Dependencies

How It Works

Best Practices

Limitations

Migration from Untyped Handlers