tRPC Patterns

Quick Guide: tRPC carries types from server to client through one exported type rather than through a generated schema, so export type AppRouter = typeof appRouter is the whole bridge and everything downstream fails without it. Procedures take a validated input and return a value; TRPCError codes are what become HTTP statuses; middleware narrows the context type so an authenticated procedure's ctx.user is non-nullable. In v11 the transformer moved inside the link, subscriptions are async generators, and @trpc/tanstack-react-query is the current React integration.

Detailed Resources:

• examples/core.md — initialization, context, a CRUD router, the provider, inferred types, queryOptions
• examples/middleware.md — logging, rate limiting, resource-scoped access
• examples/infinite-queries.md — cursor pagination end to end
• examples/optimistic-updates.md — the full snapshot-and-rollback cycle
• examples/subscriptions.md — async generator subscriptions with resumable event ids
• examples/file-uploads.md — File in an input schema (v11+)
• reference.md — error code to HTTP status table, batching and invalidation notes, v10 → v11 migration

---

Which path applies

• @trpc/tanstack-react-query — the current integration. createTRPCContext yields a useTRPC hook, and each procedure exposes queryOptions(), mutationOptions(), infiniteQueryOptions() and queryKey() that go straight into the standard query hooks. Pattern 5.
• @trpc/react-query — the classic integration, still supported in v11. Procedures carry their own trpc.x.useQuery() hooks instead, and a cache key comes from getQueryKey(trpc.x) rather than from a queryKey() on the procedure. Migrate when convenient; the two can coexist.

---

<critical_requirements>

Before writing tRPC code

Export the router's type: export type AppRouter = typeof appRouter. That single line is the whole client-side contract — without it the client falls back to unknown and every guarantee tRPC offers is gone, with no error at the point the export was forgotten.

Give every procedure that accepts input a validator on .input(). It is both the runtime check and the source of the handler's parameter type, so a procedure without one receives unknown and tempts a cast.

Throw TRPCError with a code rather than a bare Error. The code is what maps to an HTTP status and what the client switches on; a bare Error arrives as an opaque 500.

Place the transformer inside httpBatchLink(), not on createTRPCClient(). v11 moved it, and the old position raises an error at client construction.

</critical_requirements>

---

Auto-detection: initTRPC, createTRPCClient, createTRPCContext, createTRPCOptionsProxy, @trpc/server, @trpc/client, @trpc/react-query, @trpc/tanstack-react-query, TRPCError, publicProcedure, protectedProcedure, httpBatchLink, httpSubscriptionLink, loggerLink, inferRouterInputs, inferRouterOutputs, useTRPC, tracked, AppRouter

Applies to:

• Router and procedure definition, and composing routers into one
• Input validation and the types inferred from it
• Per-request context, and middleware that narrows it
• Error codes and the shape the client receives
• Turning a procedure into typed query and mutation options
• Subscriptions, file inputs and cursor pagination

Handled elsewhere:

• APIs published for third-party or non-TypeScript consumers, which want a language-neutral contract document rather than a shared type
• Caching policy, retries and invalidation semantics — this skill settles how a procedure becomes the options a query client consumes, and the client decides what to do with them
• The schema library used on .input(); any validator the version supports works, and the examples show one
• Session and token issuance; context consumes a session rather than establishing one

---

<philosophy>

Philosophy

There is no API description anywhere — no schema file, no generated client, no build step between the two halves. The router's inferred type is the contract, and it is shared by importing a type across the codebase.

That buys immediate accuracy: a procedure's return type changes and every call site reddens in the same typecheck, with no regeneration step to forget. It costs polyglot support, a published contract, and HTTP caching — calls go out as POST by default, so a CDN in front of the endpoint has nothing to cache. Which is why tRPC suits an internal API in one TypeScript codebase and suits a public one badly.

</philosophy>

---

<patterns>

Core patterns

Pattern 1: Initialization

Initialize once per application and export the factories the routers compose from.

const t = initTRPC.context<Context>().create({
  transformer: superjson, // Date, Map and Set survive the wire
  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;

The error formatter is what turns a validation failure into something a form can render per field, instead of one message.

Full code: examples/core.md — including the per-request context factory

---

Pattern 2: Procedures and input validation

const createUserSchema = z.object({
  email: z.string().email(),
  name: z.string().min(1).max(100),
});

export const userRouter = router({
  create: protectedProcedure
    .input(createUserSchema)
    .mutation(async ({ input, ctx }) => {
      // input: { email: string; name: string } — validated, and typed from the same schema
      return ctx.db.user.create({ data: input });
    }),
});

Full code: examples/core.md

---

Pattern 3: Middleware that narrows the context

const isAuthenticated = middleware(async ({ ctx, next }) => {
  if (!ctx.session || !ctx.user) throw new TRPCError({ code: "UNAUTHORIZED" });
  return next({ ctx: { ...ctx, session: ctx.session, user: ctx.user } });
});

export const protectedProcedure = publicProcedure.use(isAuthenticated);

The next({ ctx }) return type is what makes ctx.user non-nullable in every procedure built on protectedProcedure — so forgetting the check becomes a compile error rather than a review comment.

Full code: examples/middleware.md

---

Pattern 4: The type bridge

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

type RouterInputs = inferRouterInputs<AppRouter>;
type RouterOutputs = inferRouterOutputs<AppRouter>;

type User = RouterOutputs["user"]["getById"];

Component props take RouterOutputs[...] rather than a hand-written interface, so a field removed from a procedure reddens every component that read it.

Full code: examples/core.md

---

Pattern 5: Typed query and mutation options

export const { TRPCProvider, useTRPC } = createTRPCContext<AppRouter>();

const trpc = useTRPC();
const { data } = useQuery(trpc.user.getById.queryOptions({ id: userId }));

Each procedure carries queryOptions(), mutationOptions(), infiniteQueryOptions() and queryKey(). The options object goes into the ordinary hook, so anything the query client can do to an options object works here too.

Full code: examples/core.md — provider, links and a component

---

Pattern 6: Errors

// Server
throw new TRPCError({ code: "NOT_FOUND", message: "User not found" });
throw new TRPCError({
  code: "INTERNAL_SERVER_ERROR",
  message: "Failed to delete",
  cause: error,
});

// Client
onError: (error) => {
  switch (error.data?.code) {
    case "NOT_FOUND":
      return toast.error("Not found");
    case "FORBIDDEN":
      return toast.error("Not allowed");
  }
};

cause keeps the original stack for the server logs while the client still receives only the code and message. The code-to-status table is in reference.md.

---

Pattern 7: Optimistic updates

Cancel, snapshot, write, roll back on failure, invalidate when settled — all five, since the rollback is what makes the optimistic write safe.

const toggleTodo = useMutation({
  ...trpc.todo.toggle.mutationOptions(),
  onMutate: async ({ id }) => {
    await queryClient.cancelQueries({ queryKey: trpc.todo.list.queryKey() });
    const previousTodos = queryClient.getQueryData(trpc.todo.list.queryKey());
    queryClient.setQueryData(trpc.todo.list.queryKey(), (old) =>
      toggle(old, id),
    );
    return { previousTodos };
  },
  onError: (err, vars, context) =>
    queryClient.setQueryData(trpc.todo.list.queryKey(), context?.previousTodos),
  onSettled: () =>
    queryClient.invalidateQueries({ queryKey: trpc.todo.list.queryKey() }),
});

The cancelQueries is not optional: an in-flight refetch that resolves after the optimistic write overwrites it with the pre-mutation server state.

Full code: examples/optimistic-updates.md

</patterns>

---

<red_flags>

Red flags

Breaks at runtime:

• No export type AppRouter — the client infers unknown and every procedure call is untyped, with nothing failing at the file that omitted it.
• A transformer on createTRPCClient() in v11 — client construction throws, naming the link it should have gone in.
• A transformer configured on one side only — the two ends disagree about the wire format, and Date, Map and Set arrive as something else.
• observable() in a subscription — that is the v10 shape; v11 subscriptions are async generators.
• rawInput in middleware — v11 replaced it with await getRawInput().
• An optimistic write with no snapshot and no onError — a failed mutation leaves the UI showing a change that never happened.

Surprising behaviour:

• httpBatchLink merges concurrent calls into one request, so every procedure in a batch shares one HTTP status — a 401 from one is the status the others see too.
• Context is built per request, so anything mutable stored on it is discarded and never shared.
• Middleware runs in declaration order, which decides what each layer can see: auth before rate limiting means the limit is keyed on a known user.
• A retried mutation repeats a write — set retry: false for mutations.
• Resumable subscriptions need lastEventId in the input schema for tracked() to have somewhere to resume from.
• An auth check written in a procedure body leaves ctx.user nullable to the type system, so the next procedure that forgets it compiles.

</red_flags>