> ## Documentation Index
> Fetch the complete documentation index at: https://eo.utopy.dev/llms.txt
> Use this file to discover all available pages before exploring further.

# Contract-first APIs

> Implement oRPC contracts with Effect-native handlers.

Use `eoc` and `implementEffect(...)` when you already have an oRPC contract or want contract-first enforcement.

## Define the contract

```ts title="contract.ts" theme={null}
import { eoc, ORPCTaggedError } from "effect-orpc";
import * as z from "zod";

class UserNotFoundError extends ORPCTaggedError("UserNotFoundError", {
  code: "NOT_FOUND",
  status: 404,
  schema: z.object({ id: z.string() }),
}) {}

export const contract = {
  users: {
    get: eoc
      .errors({ UserNotFoundError })
      .input(z.object({ id: z.string() }))
      .output(z.object({ id: z.string(), name: z.string() })),
  },
};
```

## Implement with Effect

```ts title="router.ts" theme={null}
import { implementEffect } from "effect-orpc";

const oe = implementEffect(contract, UsersRepoLive);

export const router = oe.router({
  users: {
    get: oe.users.get.effect(function* ({ input }) {
      const usersRepo = yield* UsersRepo;
      const user = yield* usersRepo.findById(input.id);
      if (!user)
        return yield* new UserNotFoundError({ data: { id: input.id } });
      return user;
    }),
  },
});
```

## What contract leaves expose

Contract leaves preserve the contract-defined input, output, and error surface. They add `.effect(...)` alongside implementer methods such as `.handler(...)` and `.use(...)`.

They do not expose contract-changing methods such as `.input(...)` or `.output(...)` because those belong in the contract definition.

## When to use this path

Use contract-first when:

* multiple teams depend on a stable API contract
* you generate clients from contracts
* you want API shape changes isolated from implementation code
* you want tagged Effect errors declared at contract definition time

Use direct `eos` builders when you want the shortest path to implementing procedures.

## Next step

Read the [`implementEffect` reference](/effect-v4/reference/implement-effect).