You have an OpenAPI spec but not yet production access, credentials, or a running backend. You want to understand the API's shape and behavior before committing to an integration.
You cannot call the real API — it isn't running, you lack credentials, or calling it has side effects you want to avoid. Yet you need to understand the API well enough to start building against it.
Point Counterfact at the OpenAPI spec. It generates typed TypeScript handlers for every endpoint and starts a live server immediately, with no code to write. Every route returns random, schema-valid responses by default. Use the running server, the generated types, Swagger UI, and the REPL to explore the API's surface area safely and cheaply.
npx counterfact@latest https://api.example.com/openapi.json apiBrowse the API in Swagger UI at http://localhost:3100/counterfact/swagger/, or fire requests directly from the REPL:
⬣> client.get("/users/42")
{ status: 200, body: { id: 42, name: 'Alex', email: 'alex@example.com' } }
Read the generated types in api/types/ to understand exact request and response shapes before writing any integration code. Every field, parameter, and status code defined in the spec is represented there.
- The server is entirely local, so there are no rate limits, costs, or risk of polluting production data.
- Responses are randomly generated from the spec schema — they conform structurally but carry no meaningful data until you customize handlers.
- If the spec is incomplete or inaccurate, the mock reflects those gaps.
- Mock APIs with Dummy Data — once you've explored the API, customize handlers to return realistic data
- AI-Assisted Implementation — use an AI coding agent to replace
random()with a working implementation - Simulate Failures and Edge Cases — explore error paths without triggering them in the real system
- Hybrid Proxy — explore a real backend while mocking the paths that aren't ready