tRPC

Overview

tRPC enables end-to-end type-safe APIs by sharing TypeScript types between server and client without code generation or schemas. The server defines procedures (queries, mutations, subscriptions) in a router, and the client infers all types from the exported AppRouter type.

When to use: TypeScript full-stack apps needing type-safe API layers, monorepos sharing types, real-time subscriptions, rapid API iteration without OpenAPI/GraphQL overhead.

When NOT to use: Public APIs consumed by non-TypeScript clients (use REST/GraphQL), polyglot backends, projects requiring OpenAPI spec generation as primary output.

Quick Reference

Pattern	API	Key Points
Initialize	`initTRPC.context<Ctx>().create()`	Single entry point, configure once
Router	`t.router({ ... })`	Nest routers with `t.mergeRouters()`
Query	`t.procedure.input(schema).query(fn)`	Read operations, cached by clients
Mutation	`t.procedure.input(schema).mutation(fn)`	Write operations
Subscription	`t.procedure.subscription(fn)`	Real-time via WebSocket or SSE
Middleware	`t.middleware(fn)`	Chain with `.use()`, extend context via `next({ ctx })`
Standalone middleware	`experimental_standaloneMiddleware<{}>()`	Reusable with explicit type constraints
Context	`createContext({ req, res })`	Per-request, passed to all procedures
Server caller	`t.createCallerFactory(router)`	Type-safe server-side procedure calls
Error	`throw new TRPCError({ code, message })`	Mapped to HTTP status codes
Error formatter	`initTRPC.create({ errorFormatter })`	Customize error shape, expose Zod errors
Input validation	`.input(zodSchema)`	Multiple `.input()` calls merge (intersection)
Output validation	`.output(zodSchema)`	Strip extra fields from responses
React hooks	`trpc.useQuery()` / `trpc.useMutation()`	Built on @tanstack/react-query
React utils	`trpc.useUtils()`	Invalidate, prefetch, setData on cache
Suspense query	`trpc.useSuspenseQuery()`	Suspense-compatible data fetching
React subscription	`trpc.useSubscription(input, opts)`	`onData` callback for real-time events
Vanilla client	`createTRPCClient<AppRouter>({ links })`	No React dependency
Batch link	`httpBatchLink({ url })`	Batches requests in single HTTP call
Stream link	`httpBatchStreamLink({ url })`	Streams responses as they resolve
WS link	`wsLink({ client: wsClient })`	WebSocket transport for subscriptions
Split link	`splitLink({ condition, true, false })`	Route operations to different transports
Logger link	`loggerLink()`	Debug request/response in development
Data transformer	`initTRPC.create({ transformer })`	Serialize Dates, Maps, Sets (superjson)
Fetch adapter	`fetchRequestHandler({ endpoint, req })`	Cloudflare Workers, Deno, Bun, Next.js
Procedure chaining	`publicProcedure.use(auth).use(rateLimit)`	Build reusable procedure bases

Common Mistakes

Mistake	Correct Pattern
Exporting the full `t` object	Export only `t.router`, `t.procedure`, `t.middleware`, `t.createCallerFactory`
Creating context inside procedure	Create context in adapter setup, flows through automatically
Using `any` for context type	Define context type with `initTRPC.context<MyContext>().create()`
Catching errors in procedures	Let errors propagate; use `onError` in adapter or error formatter
Importing server code on client	Import only `type AppRouter`, never runtime server code
Using `createCaller` in production request handlers	Use `createCallerFactory` for type-safe reusable callers
Defining input without validation	Always validate with Zod, Valibot, or ArkType schemas
Nesting routers incorrectly	Use dot notation in keys or `t.mergeRouters()`, not deep nesting
Different transformers on client/server	Both sides must use the same transformer (e.g., superjson)
Creating QueryClient inside component render	Create once outside component or use useState initializer
Manual TypeScript generics on hooks	Type the router procedures; let inference propagate to client
Not passing `signal` to fetch in queryFn	tRPC React handles abort signals automatically

Installation

pnpm add @trpc/server @trpc/client @trpc/react-query @tanstack/react-query zod

For subscriptions, add ws (server) and configure wsLink (client). For SSE-based subscriptions, use httpBatchStreamLink instead.

Delegation

Procedure discovery: Use Explore agent
Router architecture review: Use Task agent
Code review: Delegate to code-reviewer agent

If the tanstack-query skill is available, delegate query/mutation caching, invalidation, and optimistic update patterns to it. If the zod-validation skill is available, delegate input schema design and validation patterns to it. If the drizzle-orm skill is available, delegate database query patterns within procedures to it. If the vitest-testing skill is available, delegate test runner configuration and assertion patterns to it. If the hono skill is available, delegate Hono framework routing and middleware patterns to it.