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Fastify Server


pnpm add @ts-rest/fastify


import Fastify from 'fastify';
import { initServer } from '@ts-rest/fastify';

const app = Fastify();

const s = initServer();

const router = s.router(contract, {
getPost: async ({ params: { id } }) => {
const post = await{ where: { id } });

return {
status: 200,
body: post,
createPost: async ({ body }) => {
const post = await{
data: body,

return {
status: 201,
body: post,


const start = async () => {
try {
await app.listen({ port: 3000 });
} catch (err) {


s.registerRouter is a function that takes a contract, a corresponding router with implementations for each route and a fastify app, and it will create the corresponding fastify routes for each endpoint with the correct method, paths and middleware and attach them to your fastify app.


You can pass an optional options object as the last argument for s.registerRouter.

type Options = {
logInitialization?: boolean; // print route initialization logs to console
jsonQuery?: boolean;
responseValidation?: boolean;
| 'combined'
| ((
err: TsRestRequestValidationError,
request: fastify.FastifyRequest,
reply: fastify.FastifyReply
) => void);

Response Validation

To enable response parsing and validation, you can use the validateResponses option. If there is a corresponding response Zod schema defined in the contract for the returned status code, the response will be parsed and validated. If validation fails a ResponseValidationError will be thrown causing a 500 response to be returned.

s.registerRouter(contract, router, app, {
validateResponses: true,

Request Validation Error Handling

The default functionality of handling request validation errors is combined which returns a 400 response with all validation errors in the body in this form.

pathParameterErrors: z.ZodError | null;
headerErrors: z.ZodError | null;
queryParameterErrors: z.ZodError | null;
bodyErrors: z.ZodError | null;

You can also pass a custom error handler function to the requestValidationErrorHandler option.

s.registerRouter(contract, router, app, {
requestValidationErrorHandler: (err, req, res, next) => {
// err is typed as ^ RequestValidationError
return res.status(400).json({
message: 'Validation failed',