 [](https://coveralls.io/github/pmcelhaney/counterfact)   
Mock servers work—until you need state, failures, or control mid-run.
Mock servers make it easy to get started, but hard to keep going.
Counterfact is an API simulator without those limits.
Point it at an [OpenAPI](https://www.openapis.org) document and get a live, stateful API in seconds.
Supports Swagger 2.0 and OpenAPI 3.0, 3.1, and 3.2.
- Type-safe TypeScript handlers for every endpoint
- Hot reloading as you edit
- Shared state across routes
- **A built-in REPL to control behavior at runtime**
- Optional proxying to real backends
Flexbile for humans. Stable for [AI agents](https://github.com/counterfact/api-simulator/blob/main/docs/patterns/ai-assisted-implementation.md).
You’re in control—without restarting.
For a *frontend developer* waiting on a backend,
a *test engineer* who needs clean, reproducible state,
or an *AI agent* that needs a stable API
Real enough to be useful. Fake enough to be usable.
## Try it now
```sh
npx counterfact@latest https://petstore3.swagger.io/api/v3/openapi.json api
```
> Starts a local server with a live REPL to inspect and control API behavior
> Requires Node ≥ 22.0.0
## Go deeper
- [Getting started](./docs/getting-started.md) – Detailed walkthrough with state, REPL, and proxy
- [Patterns](./docs/patterns/index.md) – How Counterfact transforms your workflow
- [Example repo](https://github.com/counterfact/example-petstore) – Using Counterfact to implement the Swagger Petstore
- [How it compares](./docs/comparison.md) – json-server, WireMock, Prism, Microcks, MSW
- [Usage](./docs/usage.md) – Explore features and how to use them
- [Reference](./docs/reference.md) – `$` API, CLI flags, architecture
- [FAQ](./docs/faq.md) – State, types, regeneration
[Changelog](./CHANGELOG.md) · [Contributing](./CONTRIBUTING.md)