Your backend isn't ready. Your frontend can't wait.

Counterfact turns your OpenAPI spec into a live, stateful API you can program in TypeScript.

This is a five-minute walkthrough. By the end, you’ll have a stateful, type-safe, hot-reloading API simulator running locally—and you’ll understand why it’s different from traditional mock servers.

Built for frontend developers, test engineers, and AI agents that need a predictable API to work against.

Minute 1 — Start the server

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

Requires Node ≥ 22.0.0

That’s it.

Counterfact reads your spec, generates a TypeScript handler for every endpoint, and starts a server at http://localhost:3100.

Open http://localhost:3100/counterfact/swagger/.

Every endpoint is already live, returning random, schema-valid responses. No code written yet.

Minute 2 — Make a route return real data

Open the generated file for GET /pet/{petId}:

import type { HTTP_GET } from "../../types/paths/pet/{petId}.types.js";

export const GET: HTTP_GET = ($) => $.response[200].random();

Replace .random() with your own logic:

export const GET: HTTP_GET = ($) => {
  if ($.path.petId === 99) {
    return $.response[404].text("Pet not found");
  }
  return $.response[200].json({
    id: $.path.petId,
    name: "Fluffy",
    status: "available",
    photoUrls: []
  });
};

Save the file. The server reloads instantly—no restart, no lost state.

TypeScript enforces the contract. If your response doesn’t match the spec, you’ll know before you make the request.

Minute 3 — Add state that survives across requests

Real APIs have memory. Yours should too.

Create api/routes/_.context.ts:

import type { Pet } from "../types/components/pet.types.js";

export class Context {
  private pets = new Map<number, Pet>();
  private nextId = 1;

  add(data: Omit<Pet, "id">): Pet {
    const pet = { ...data, id: this.nextId++ };
    this.pets.set(pet.id, pet);
    return pet;
  }

  get(id: number): Pet | undefined { return this.pets.get(id); }
  list(): Pet[] { return [...this.pets.values()]; }
  remove(id: number): void { this.pets.delete(id); }
}

Use it in your routes:

export const GET: HTTP_GET = ($) => $.response[200].json($.context.list());
export const POST: HTTP_POST = ($) => $.response[200].json($.context.add($.body));

Now your API behaves like a real system:

• POST creates data
• GET returns it
• DELETE removes it

State survives hot reloads. Restarting resets everything—perfect for clean test runs.

Minute 4 — Control the system at runtime (REPL)

This is where Counterfact becomes more than a mock.

The built-in REPL lets you inspect and control the system while it’s running.

Seed data:

⬣> context.add({ name: "Fluffy", status: "available", photoUrls: [] })
⬣> context.add({ name: "Rex", status: "pending", photoUrls: [] })

Make requests:

⬣> client.get("/pet/1")

Simulate failures instantly:

⬣> context.rateLimitExceeded = true
⬣> client.get("/pet/1")
{ status: 429, body: "Too Many Requests" }

No HTTP scripts. No restarts. Just direct control.

Minute 5 — Proxy to the real backend

When parts of your backend are ready, forward them through.

Everything else stays simulated.

npx counterfact@latest openapi.yaml api --proxy-url https://api.example.com

Toggle paths live:

⬣> .proxy on /payments
⬣> .proxy on /auth
⬣> .proxy off

What you just built

In five minutes, you turned a static spec into a working system:

• Schema-valid responses from the moment it starts
• Type-safe handlers generated from your spec
• Shared state across all routes
• Hot reloading without losing that state
• A live control surface (REPL) for runtime behavior
• Selective proxying to real services

Go deeper

Getting started	Detailed walkthrough with state, REPL, and proxy
Usage	Feature index: routes, context, REPL, proxy, middleware, and more
Patterns	Failures, latency, AI sandboxes, integration tests
Reference	$ API, CLI flags, architecture
How it compares	json-server, WireMock, Prism, Microcks, MSW
FAQ	State, types, regeneration
Petstore example	Full working example

Changelog · Contributing