Fastify Adapter

Example app

The best way to start with the Fastify adapter is to take a look at the example application.

Description	Links
Fastify server with WebSocket Simple tRPC client in node	CodeSandbox Source

How to use tRPC with Fastify

Install dependencies

bash
yarn add @trpc/server fastify zod

bash
yarn add @trpc/server fastify zod

⚠️ Fastify version requirement

The tRPC v11 Fastify adapter requires Fastify v5+. Using Fastify v4 may cause requests to return empty responses without errors.

Zod isn't a required dependency, but it's used in the sample router below.

AI Agents

If you use an AI coding agent, install tRPC skills for better code generation:

bash
npx @tanstack/intent@latest install

bash
npx @tanstack/intent@latest install

Create the router

First of all you need a router to handle your queries, mutations and subscriptions.

A sample router is given below, save it in a file named router.ts.

router.ts

router.ts
ts
import { initTRPC } from '@trpc/server';
import { z } from 'zod';
 
type User = {
  id: string;
  name: string;
  bio?: string;
};
 
const users: Record<string, User> = {};
 
export const t = initTRPC.create();
 
export const appRouter = t.router({
  getUserById: t.procedure.input(z.string()).query((opts) => {
    return users[opts.input]; // input type is string
  }),
  createUser: t.procedure
    .input(
      z.object({
        name: z.string().min(3),
        bio: z.string().max(142).optional(),
      }),
    )
    .mutation((opts) => {
      const id = Date.now().toString();
      const user: User = { id, ...opts.input };
      users[user.id] = user;
      return user;
    }),
});
 
// export type definition of API
export type AppRouter = typeof appRouter;

router.ts
ts
import { initTRPC } from '@trpc/server';
import { z } from 'zod';
 
type User = {
  id: string;
  name: string;
  bio?: string;
};
 
const users: Record<string, User> = {};
 
export const t = initTRPC.create();
 
export const appRouter = t.router({
  getUserById: t.procedure.input(z.string()).query((opts) => {
    return users[opts.input]; // input type is string
  }),
  createUser: t.procedure
    .input(
      z.object({
        name: z.string().min(3),
        bio: z.string().max(142).optional(),
      }),
    )
    .mutation((opts) => {
      const id = Date.now().toString();
      const user: User = { id, ...opts.input };
      users[user.id] = user;
      return user;
    }),
});
 
// export type definition of API
export type AppRouter = typeof appRouter;

If your router file starts getting too big, split your router into several subrouters each implemented in its own file. Then merge them into a single root appRouter.

Create the context

Then you need a context that will be created for each request.

A sample context is given below, save it in a file named context.ts:

context.ts

context.ts
ts
import { CreateFastifyContextOptions } from '@trpc/server/adapters/fastify';
 
export function createContext({ req, res }: CreateFastifyContextOptions) {
  const user = { name: req.headers.username ?? 'anonymous' };
 
  return { req, res, user };
}
 
export type Context = Awaited<ReturnType<typeof createContext>>;

context.ts
ts
import { CreateFastifyContextOptions } from '@trpc/server/adapters/fastify';
 
export function createContext({ req, res }: CreateFastifyContextOptions) {
  const user = { name: req.headers.username ?? 'anonymous' };
 
  return { req, res, user };
}
 
export type Context = Awaited<ReturnType<typeof createContext>>;

Create Fastify server

tRPC includes an adapter for Fastify out of the box. This adapter lets you convert your tRPC router into a Fastify plugin. In order to prevent errors during large batch requests, make sure to set the maxParamLength Fastify option to a suitable value, as shown.

tip

Due to limitations in Fastify's plugin system and type inference, there might be some issues getting for example onError typed correctly. You can add a satisfies FastifyTRPCPluginOptions<AppRouter>['trpcOptions'] to help TypeScript out and get the correct types.

server.ts
ts
import {
  fastifyTRPCPlugin,
  FastifyTRPCPluginOptions,
} from '@trpc/server/adapters/fastify';
import fastify from 'fastify';
import { createContext } from './context';
import { appRouter, type AppRouter } from './router';
 
const server = fastify({
  routerOptions: {
    maxParamLength: 5000,
  },
});
 
server.register(fastifyTRPCPlugin, {
  prefix: '/trpc',
  trpcOptions: {
    router: appRouter,
    createContext,
    onError({ path, error }) {
      // report to error monitoring
      console.error(`Error in tRPC handler on path '${path}':`, error);
    },
  } satisfies FastifyTRPCPluginOptions<AppRouter>['trpcOptions'],
});
 
(async () => {
  try {
    await server.listen({ port: 3000 });
  } catch (err) {
    server.log.error(err);
    process.exit(1);
  }
})();

server.ts
ts
import {
  fastifyTRPCPlugin,
  FastifyTRPCPluginOptions,
} from '@trpc/server/adapters/fastify';
import fastify from 'fastify';
import { createContext } from './context';
import { appRouter, type AppRouter } from './router';
 
const server = fastify({
  routerOptions: {
    maxParamLength: 5000,
  },
});
 
server.register(fastifyTRPCPlugin, {
  prefix: '/trpc',
  trpcOptions: {
    router: appRouter,
    createContext,
    onError({ path, error }) {
      // report to error monitoring
      console.error(`Error in tRPC handler on path '${path}':`, error);
    },
  } satisfies FastifyTRPCPluginOptions<AppRouter>['trpcOptions'],
});
 
(async () => {
  try {
    await server.listen({ port: 3000 });
  } catch (err) {
    server.log.error(err);
    process.exit(1);
  }
})();

Your endpoints are now available via HTTP!

Endpoint	HTTP URI
getUserById	GET http://localhost:3000/trpc/getUserById?input=INPUT where INPUT is a URI-encoded JSON string.
createUser	POST http://localhost:3000/trpc/createUser with req.body of type User

Enable WebSockets

The Fastify adapter supports WebSockets via the @fastify/websocket plugin. All you have to do in addition to the above steps is install the dependency, add some subscriptions to your router, and activate the useWSS option in the plugin. The minimum @fastify/websocket version required is 3.11.0.

Install dependencies

bash
yarn add @fastify/websocket

bash
yarn add @fastify/websocket

Import and register @fastify/websocket

ts
import ws from '@fastify/websocket';
 
server.register(ws);

ts
import ws from '@fastify/websocket';
 
server.register(ws);

Add some subscriptions

Edit the router.ts file created in the previous steps and add the following code:

router.ts
ts
import { initTRPC } from '@trpc/server';
 
const t = initTRPC.create();
 
export const appRouter = t.router({
  randomNumber: t.procedure.subscription(async function* () {
    while (true) {
      yield { randomNumber: Math.random() };
      await new Promise((resolve) => setTimeout(resolve, 1000));
    }
  }),
});

router.ts
ts
import { initTRPC } from '@trpc/server';
 
const t = initTRPC.create();
 
export const appRouter = t.router({
  randomNumber: t.procedure.subscription(async function* () {
    while (true) {
      yield { randomNumber: Math.random() };
      await new Promise((resolve) => setTimeout(resolve, 1000));
    }
  }),
});

Activate the useWSS option

server.ts
ts
import {
  fastifyTRPCPlugin,
  FastifyTRPCPluginOptions,
} from '@trpc/server/adapters/fastify';
import fastify from 'fastify';
import { createContext } from './context';
import { appRouter, type AppRouter } from './router';
 
const server = fastify();
 
server.register(fastifyTRPCPlugin, {
  useWSS: true,
  trpcOptions: {
    router: appRouter,
    createContext,
    // Enable heartbeat messages to keep connection open (disabled by default)
    keepAlive: {
      enabled: true,
      // server ping message interval in milliseconds
      pingMs: 30000,
      // connection is terminated if pong message is not received in this many milliseconds
      pongWaitMs: 5000,
    },
  },
});

server.ts
ts
import {
  fastifyTRPCPlugin,
  FastifyTRPCPluginOptions,
} from '@trpc/server/adapters/fastify';
import fastify from 'fastify';
import { createContext } from './context';
import { appRouter, type AppRouter } from './router';
 
const server = fastify();
 
server.register(fastifyTRPCPlugin, {
  useWSS: true,
  trpcOptions: {
    router: appRouter,
    createContext,
    // Enable heartbeat messages to keep connection open (disabled by default)
    keepAlive: {
      enabled: true,
      // server ping message interval in milliseconds
      pingMs: 30000,
      // connection is terminated if pong message is not received in this many milliseconds
      pongWaitMs: 5000,
    },
  },
});

You can now subscribe to the randomNumber topic and should receive a random number every second 🚀.

Fastify plugin options

name	type	optional	default	description
prefix	string	true	"/trpc"	URL prefix for tRPC routes
useWSS	boolean	true	false	Enable WebSocket support via @fastify/websocket
trpcOptions	FastifyHandlerOptions<AppRouter, Request, Reply>	false	n/a	tRPC handler options including router, createContext, etc.