# Counterfact

> Build the frontend. Don’t wait for the backend.

Counterfact turns an OpenAPI document into a useful local API in one command.
Start with an OpenAPI document, point a frontend at `http://localhost:3100`,
and use schema-derived responses before writing custom handlers. Requires Node
22+. Supports Swagger 2.0 and OpenAPI 3.0, 3.1, and 3.2.

```sh
npx counterfact@latest https://petstore3.swagger.io/api/v3/openapi.json api
```

## Start here

- [Getting Started](https://counterfact.dev/docs/getting-started) — prerequisite, one command, localhost browser request, generated files, and one optional custom response.
- [First 10 minutes](https://counterfact.dev/docs/first-10-minutes) — add a small stateful create/read workflow and one frontend-relevant failure.
- [Usage](https://counterfact.dev/docs/usage) — guides grouped by frontend work, team/test work, advanced control, and troubleshooting.

## Add only what the workflow needs

- [Custom responses](https://counterfact.dev/docs/features/routes)
- [State](https://counterfact.dev/docs/features/state)
- [Failures](https://counterfact.dev/docs/patterns/simulate-failures) and [latency](https://counterfact.dev/docs/patterns/simulate-latency)
- [Proxying](https://counterfact.dev/docs/features/proxy)
- [Tests](https://counterfact.dev/docs/patterns/automated-integration-tests) and [REPL](https://counterfact.dev/docs/features/repl)

## Reference

- [Reference](https://counterfact.dev/docs/reference)
- [FAQ](https://counterfact.dev/docs/faq)
- [Without OpenAPI](https://counterfact.dev/docs/features/without-openapi) — use this alternative only when no OpenAPI document exists.
- [GitHub repository](https://github.com/counterfact/api-simulator)