Explore a New API

Explore an OpenAPI spec before production access, credentials, or a running backend are available. Get a feel for the API before committing to an integration.

Why teams use this

The real API may not be ready, or it may be the wrong place to experiment. Counterfact gives you a safe, editable server so you can start building from the contract today.

How it works

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.

Example

npx counterfact@latest https://api.example.com/openapi.json api

Browse 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.

What you get

• 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.

Keep exploring

• 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