# A contract from OpenAPI

> Generate a typed contract module from any OpenAPI 3.0 or 3.1 document.

`generateContractFromOpenApi(document, { prefix?, exportName?, patterns? })` turns an OpenAPI 3.0 or 3.1 document into the source of a `@trpc12/trpc/contract` module. Schemas are Effect Schemas, translated by Effect's `SchemaRepresentation`; each operation becomes a procedure with its `route` meta (under `.concat(openapi())`), so the contract works with `openApiLink` and `validationLink` against any REST API:

```ts
import { generateContractFromOpenApi } from '@trpc12/trpc/openapi';

const document = await (
  await fetch('https://petstore.example/openapi.json')
).json();
await writeFile(
  'src/petstore.ts',
  generateContractFromOpenApi(document, { exportName: 'petstore' }),
);
```

```ts
// src/petstore.ts (generated)
export const Pet = Schema.Struct({
  id: Schema.Number.check(Schema.isInt()),
  name: Schema.String,
});
const c = trpcContract.create().concat(openapi());
export const petstore = {
  pets: {
    list: c
      .input(PetsListInput)
      .output(PetsListOutput)
      .meta({ route: { method: 'GET', path: '/pets' } })
      .query(),
  },
};
```

- Procedure paths come from `operationId`s, with dots nesting (`pets.list`); without one, from the method and path (`deletePetsPetId`).
- `GET` is a query, a `text/event-stream` response a subscription, anything else a mutation. The first 2xx status becomes `successStatus`.
- Path and query parameters merge with the JSON body into one input, like `inputStructure: 'compact'`.
- Objects that don't mention `additionalProperties` become closed structs (extra keys are still ignored when decoding), except where `allOf` merges them.
- Error responses in @trpc12/trpc's own shape (`code` and `data`) become `.errors()`, so a document from `generateOpenApi` round-trips with typed errors. Other APIs' errors stay unexpected errors.
- `pattern`s are ignored unless `patterns: 'apply'`; only use that with documents you trust.