Build end-to-end type-safe APIs with tRPC — routers, procedures, middleware, subscriptions, and Next.js/React integration patterns.

SKILL.md•trpc-fullstack

tRPC Full-Stack

Overview

tRPC lets you build fully type-safe APIs without writing a schema or code-generation step. Your TypeScript types flow from the server router directly to the client — so every API call is autocompleted, validated at compile time, and refactoring-safe. Use this skill when building TypeScript monorepos, Next.js apps, or any project where the server and client share a codebase.

When to Use This Skill

Use when building a TypeScript full-stack app (Next.js, Remix, Express + React) where the client and server share a single repo
Use when you want end-to-end type safety on API calls without REST/GraphQL schema overhead
Use when adding real-time features (subscriptions) to an existing tRPC setup
Use when designing multi-step middleware (auth, rate limiting, tenant scoping) on tRPC procedures
Use when migrating an existing REST/GraphQL API to tRPC incrementally

Core Concepts

Routers and Procedures

A router groups related procedures (think: endpoints). Procedures are typed functions — query for reads, mutation for writes, subscription for real-time streams.

Input Validation with Zod

All procedure inputs are validated with Zod schemas. The validated, typed input is available in the procedure handler — no manual parsing.

Context

context is shared state passed to every procedure — auth session, database client, request headers, etc. It is built once per request in a context factory. Important: Next.js App Router and Pages Router require separate context factories because App Router handlers receive a fetch Request, not a Node.js NextApiRequest.

Middleware

Middleware chains run before a procedure. Use them for authentication, logging, and request enrichment. They can extend the context for downstream procedures.

How It Works

Step 1: Install and Initialize

npm install @trpc/server @trpc/client @trpc/react-query @tanstack/react-query zod

Create the tRPC instance and reusable builders:

// src/server/trpc.ts
import { initTRPC, TRPCError } from '@trpc/server';
import { type Context } from './context';
import { ZodError } from 'zod';

const t = initTRPC.context<Context>().create({
  errorFormatter({ shape, error }) {
    return {
      ...shape,
      data: {
        ...shape.data,
        zodError:
          error.cause instanceof ZodError ? error.cause.flatten() : null,
      },
    };
  },
});

export const router = t.router;
export const publicProcedure = t.procedure;
export const middleware = t.middleware;

Step 2: Define Two Context Factories

Next.js App Router handlers receive a fetch Request (not a Node.js NextApiRequest), so the context must be built differently depending on the call site. Define one factory per surface:

// src/server/context.ts
import { type FetchCreateContextFnOptions } from '@trpc/server/adapters/fetch';
import { auth } from '@/server/auth'; // Next-Auth v5 / your auth helper
import { db } from './db';

/**
 * Context for the HTTP handler (App Router Route Handler).
 * `opts.req` is the fetch Request — auth is resolved server-side via `auth()`.
 */
export async function createTRPCContext(opts: FetchCreateContextFnOptions) {
  const session = await auth(); // server-side auth — no req/res needed
  return { session, db, headers: opts.req.headers };
}

/**
 * Context for direct server-side callers (Server Components, RSC, cron jobs).
 * No HTTP request is involved, so we call auth() directly from the server.
 */
export async function createServerContext() {
  const session = await auth();
  return { session, db };
}

export type Context = Awaited<ReturnType<typeof createTRPCContext>>;

Step 3: Build an Auth Middleware and Protected Procedure

// src/server/trpc.ts (continued)
const enforceAuth = middleware(({ ctx, next }) => {
  if (!ctx.session?.user) {
    throw new TRPCError({ code: 'UNAUTHORIZED' });
  }
  return next({
    ctx: {
      // Narrows type: session is non-null from here
      session: { ...ctx.session, user: ctx.session.user },
    },
  });
});

export const protectedProcedure = t.procedure.use(enforceAuth);

Step 4: Create Routers

// src/server/routers/post.ts
import { z } from 'zod';
import { router, publicProcedure, protectedProcedure } from '../trpc';
import { TRPCError } from '@trpc/server';

export const postRouter = router({
  list: publicProcedure
    .input(
      z.object({
        limit: z.number().min(1).max(100).default(20),
        cursor: z.string().optional(),
      })
    )
    .query(async ({ ctx, input }) => {
      const posts = await ctx.db.post.findMany({
        take: input.limit + 1,
        cursor: input.cursor ? { id: input.cursor } : undefined,
        orderBy: { createdAt: 'desc' },
      });
      const nextCursor =
        posts.length > input.limit ? posts.pop()!.id : undefined;
      return { posts, nextCursor };
    }),

  byId: publicProcedure
    .input(z.object({ id: z.string() }))
    .query(async ({ ctx, input }) => {
      const post = await ctx.db.post.findUnique({ where: { id: input.id } });
      if (!post) throw new TRPCError({ code: 'NOT_FOUND' });
      return post;
    }),

  create: protectedProcedure
    .input(
      z.object({
        title: z.string().min(1).max(200),
        body: z.string().min(1),
      })
    )
    .mutation(async ({ ctx, input }) => {
      return ctx.db.post.create({
        data: { ...input, authorId: ctx.session.user.id },
      });
    }),

  delete: protectedProcedure
    .input(z.object({ id: z.string() }))
    .mutation(async ({ ctx, input }) => {
      const post = await ctx.db.post.findUnique({ where: { id: input.id } });
      if (!post) throw new TRPCError({ code: 'NOT_FOUND' });
      if (post.authorId !== ctx.session.user.id)
        throw new TRPCError({ code: 'FORBIDDEN' });
      return ctx.db.post.delete({ where: { id: input.id } });
    }),
});

Step 5: Compose the Root Router and Export Types

// src/server/root.ts
import { router } from './trpc';
import { postRouter } from './routers/post';
import { userRouter } from './routers/user';

export const appRouter = router({
  post: postRouter,
  user: userRouter,
});

// Export the type for the client — never import the appRouter itself on the client
export type AppRouter = typeof appRouter;

Step 6: Mount the API Handler (Next.js App Router)

The App Router handler must use fetchRequestHandler and the fetch-based context factory. createTRPCContext receives FetchCreateContextFnOptions (with a fetch Request), not a Pages Router req/res pair.

// src/app/api/trpc/[trpc]/route.ts
import { fetchRequestHandler } from '@trpc/server/adapters/fetch';
import { type FetchCreateContextFnOptions } from '@trpc/server/adapters/fetch';
import { appRouter } from '@/server/root';
import { createTRPCContext } from '@/server/context';

const handler = (req: Request) =>
  fetchRequestHandler({
    endpoint: '/api/trpc',
    req,
    router: appRouter,
    // opts is FetchCreateContextFnOptions — req is the fetch Request
    createContext: (opts: FetchCreateContextFnOptions) => createTRPCContext(opts),
  });

export { handler as GET, handler as POST };

Step 7: Set Up the Client (React Query)

// src/utils/trpc.ts
import { createTRPCReact } from '@trpc/react-query';
import type { AppRouter } from '@/server/root';

export const trpc = createTRPCReact<AppRouter>();

// src/app/providers.tsx
'use client';
import { QueryClient, QueryClientProvider } from '@tanstack/react-query';
import { httpBatchLink } from '@trpc/client';
import { useState } from 'react';
import { trpc } from '@/utils/trpc';

export function TRPCProvider({ children }: { children: React.ReactNode }) {
  const [queryClient] = useState(() => new QueryClient());
  const [trpcClient] = useState(() =>
    trpc.createClient({
      links: [
        httpBatchLink({
          url: '/api/trpc',
          headers: () => ({ 'x-trpc-source': 'react' }),
        }),
      ],
    })
  );

  return (
    <trpc.Provider client={trpcClient} queryClient={queryClient}>
      <QueryClientProvider client={queryClient}>{children}</QueryClientProvider>
    </trpc.Provider>
  );
}

Examples

Example 1: Fetching Data in a Component

// components/PostList.tsx
'use client';
import { trpc } from '@/utils/trpc';

export function PostList() {
  const { data, isLoading, error } = trpc.post.list.useQuery({ limit: 10 });

  if (isLoading) return <p>Loading…</p>;
  if (error) return <p>Error: {error.message}</p>;

  return (
    <ul>
      {data?.posts.map((post) => (
        <li key={post.id}>{post.title}</li>
      ))}
    </ul>
  );
}

Example 2: Mutation with Cache Invalidation

'use client';
import { trpc } from '@/utils/trpc';

export function CreatePost() {
  const utils = trpc.useUtils();

  const createPost = trpc.post.create.useMutation({
    onSuccess: () => {
      // Invalidate and refetch the post list
      utils.post.list.invalidate();
    },
  });

  const handleSubmit = (e: React.FormEvent<HTMLFormElement>) => {
    e.preventDefault();
    const form = e.currentTarget;
    const data = new FormData(form);
    createPost.mutate({
      title: data.get('title') as string,
      body: data.get('body') as string,
    });
    form.reset();
  };

  return (
    <form onSubmit={handleSubmit}>
      <input name="title" placeholder="Title" required />
      <textarea name="body" placeholder="Body" required />
      <button type="submit" disabled={createPost.isPending}>
        {createPost.isPending ? 'Creating…' : 'Create Post'}
      </button>
      {createPost.error && <p>{createPost.error.message}</p>}
    </form>
  );
}

Example 3: Server-Side Caller (Server Components / SSR)

Use createServerContext — the dedicated server-side factory — so that auth() is called correctly without needing a synthetic or empty request object:

// app/posts/page.tsx (Next.js Server Component)
import { appRouter } from '@/server/root';
import { createCallerFactory } from '@trpc/server';
import { createServerContext } from '@/server/context';

const createCaller = createCallerFactory(appRouter);

export default async function PostsPage() {
  // Uses createServerContext — calls auth() server-side, no req/res cast needed
  const caller = createCaller(await createServerContext());
  const { posts } = await caller.post.list({ limit: 20 });

  return (
    <ul>
      {posts.map((post) => (
        <li key={post.id}>{post.title}</li>
      ))}
    </ul>
  );
}

Example 4: Real-Time Subscriptions (WebSocket)

// server/routers/notifications.ts
import { observable } from '@trpc/server/observable';
import { EventEmitter } from 'events';

const ee = new EventEmitter();

export const notificationRouter = router({
  onNew: protectedProcedure.subscription(({ ctx }) => {
    return observable<{ message: string; at: Date }>((emit) => {
      const onNotification = (data: { message: string }) => {
        emit.next({ message: data.message, at: new Date() });
      };

      const channel = `user:${ctx.session.user.id}`;
      ee.on(channel, onNotification);
      return () => ee.off(channel, onNotification);
    });
  }),
});

// Client usage — requires wsLink in the client config
trpc.notification.onNew.useSubscription(undefined, {
  onData(data) {
    toast(data.message);
  },
});

Best Practices

✅ Export only AppRouter type from server code — never import appRouter on the client
✅ Use separate context factories — createTRPCContext for the HTTP handler, createServerContext for Server Components and callers
✅ Validate all inputs with Zod — never trust raw input without a schema
✅ Split routers by domain (posts, users, billing) and merge in root.ts
✅ Extend context in middleware rather than querying the DB multiple times per request
✅ Use utils.invalidate() after mutations to keep the cache fresh
❌ Don't cast context with as any to silence type errors — the mismatch will surface as a runtime failure when auth or session lookups return undefined
❌ Don't use createContext({} as any) in Server Components — use createServerContext() which calls auth() directly
❌ Don't put business logic in the route handler — keep it in the procedure or a service layer
❌ Don't share the tRPC client instance globally — create it per-provider to avoid stale closures

Security & Safety Notes

Always enforce authorization in protectedProcedure — never rely on client-side checks alone
Validate all input shapes with Zod, including pagination cursors and IDs, to prevent injection via malformed inputs
Avoid exposing internal error details to clients — use TRPCError with a public-safe message and keep stack traces server-side only
Rate-limit public procedures using middleware to prevent abuse

Common Pitfalls

Problem: Auth session is null in protected procedures even when the user is logged in Solution: Ensure createTRPCContext uses the correct server-side auth call (e.g. auth() from Next-Auth v5) and is not receiving a Pages Router req/res cast via as any in an App Router handler
Problem: Server Component caller fails for auth-dependent queries Solution: Use createServerContext() (the dedicated server-side factory) instead of passing an empty or synthetic object to createContext
Problem: "Type error: AppRouter is not assignable to AnyRouter" Solution: Import AppRouter as a type import (import type { AppRouter }) on the client, not the full module
Problem: Mutations not reflecting in the UI after success Solution: Call utils.<router>.<procedure>.invalidate() in onSuccess to trigger a refetch via React Query
Problem: "Cannot find module '@trpc/server/adapters/next'" with App Router Solution: Use @trpc/server/adapters/fetch and fetchRequestHandler for the App Router; the nextjs adapter is for Pages Router only
Problem: Subscriptions not connecting Solution: Subscriptions require splitLink — route subscriptions to wsLink and queries/mutations to httpBatchLink

Related Skills

@typescript-expert — Deep TypeScript patterns used inside tRPC routers and generic utilities
@react-patterns — React hooks patterns that pair with trpc.*.useQuery and useMutation
@test-driven-development — Write procedure unit tests using createCallerFactory without an HTTP server
@security-auditor — Review tRPC middleware chains for auth bypass and input validation gaps

Additional Resources

tRPC Official Docs
create-t3-app — Production Next.js starter with tRPC wired in
tRPC GitHub
TanStack Query Docs

> trpc-fullstack