Counterfact

Local APIs from OpenAPI

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

Counterfact turns an OpenAPI document into a useful local API in one command. Point your frontend at localhost and start building against generated responses before the real backend is ready.

Try it nowRead the docs

Start with your OpenAPI document

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

Requires Node.js 22+. For a repeatable project or CI workflow,install a pinned versionand commit the lockfile.

Quick start

A useful API before you write a handler

  1. Run the command

    Counterfact reads the document, writes editable routes and generated types, then starts a server.

  2. Point the frontend at localhost

    The base URL is http://localhost:3100. A browser app can make an ordinary request:

    const response = await fetch("http://localhost:3100/pet/1");
const pet = await response.json();

  3. Get a generated response

    Supported operations return schema-derived sample data immediately. For example:

    {
  "id": 1,
  "name": "string",
  "status": "available"
}

The defaults are a useful starting point for frontend work and exploration—not a claim that your backend behavior is complete.

When your frontend needs more

Replace a generated response with a custom handler when a screen needs a specific case. Add only the state, failures, latency, proxying, or tests that the workflow needs.

Custom responsesStateFailures and latencyProxyingTestsREPL