---
title: Client Contract Generation
description: Derive a client runtime contract from handler-based route declarations.
---

`generateContractFromType()` derives a client definition from your exported
routes. Use it with the Fetch client or TanStack Query helpers without importing
server code into the client.

## Generate a Contract

```ts client-contract.ts
import { generateContractFromType } from "@rest-rpc/core/generate";
import type { routes } from "./server";

export const api = generateContractFromType<typeof routes>({
	filePath: "./server.ts",
	exportName: "routes",
});
```

```ts
import { initClient } from "@rest-rpc/core";
import { api } from "./client-contract";

const client = initClient(api, { baseUrl: "http://localhost:3000" });
```

## Run Generation at Build Time

Generation needs Node's filesystem APIs and the TypeScript 5 or 6 compiler API.
TypeScript 7 does not currently ship a compiler API, but it can still be used for
project-wide checks. The checked project must be free of TypeScript errors, and
route methods, paths, and response statuses must retain literal types.

For a browser bundle or another runtime without filesystem access, inline the
result with a build-time macro. For example, Bun macros can wrap the generation
step:

```ts client-contract.macro.ts
import { generateContractFromType } from "@rest-rpc/core/generate";
import type { routes } from "./server";

export function getApi() {
	return generateContractFromType<typeof routes>({
		filePath: "./server.ts",
		exportName: "routes",
	});
}
```

```ts client.ts
import { initClient } from "@rest-rpc/core";
import { getApi } from "./client-contract.macro" with { type: "macro" };

const client = initClient(getApi(), { baseUrl: "http://localhost:3000" });
```

Other bundlers can use a macro integration such as `unplugin-macros`. Configure
the integration so generation runs during the build and the resulting runtime
contract is included in the client bundle.

## Validation and OpenAPI

Generated contracts carry client types but no runtime validators. Enabling
`validateResponses` therefore cannot add schema validation to this artifact.
Declared schemas still validate requests and outputs on the server.

Generate OpenAPI from the server routes with explicit output schemas. See
[OpenAPI](/docs/openapi).

If the client needs shared runtime schemas, use a
[separate contract](/docs/contract-first-quickstart).