The frontend and the backend agreed on the API last week — the endpoints, the shapes, the status codes — and wrote it down in an OpenAPI file. But the backend doesn't exist yet, and won't for a sprint or two. So the frontend is stuck: you can read the contract, but you can't hit it. You can't see a real 404 come back, you can't wire a fetch call against it, you can't hand QA something runnable. The contract is a document when what you need is a server. API Contract Lab exists to close exactly that gap — to turn the contract you already have into mock responses, client code, and a runnable collection you can use today, before a single backend route is written. This is a look inside how it does that, entirely in your browser.
Three Front Doors: The Forms Your Contract Already Takes
Most mocking tools make you describe your API again, by hand, in their UI. API Contract Lab assumes you've already written it down somewhere — so it reads whichever form you have. Three inputs, one internal model:
OpenAPI Spec
The formal version, YAML or JSON. Paths, methods, parameters, and response schemas are read straight off it.
JSON Example
A sample response you already have — from a colleague, a docs page, or a real endpoint. It infers the shape from the object.
Endpoint List
The bare minimum: a list of paths and methods. Enough to scaffold mock responses and start exploring the surface.
Whichever you paste, the parser normalizes it into a single internal representation of your API — so everything downstream works the same way regardless of how you described it. That's the small architectural decision that keeps the rest of the tool simple: three front doors, one room.
One Parser, One Model of Your API
The job of the first stage is to stop caring how you typed your contract. An OpenAPI document, a raw JSON blob, and a plain endpoint list look nothing alike, but they describe the same thing: a set of operations (a method + a path), each with a request shape and one or more response shapes. The parser pulls those out and builds a uniform internal model — endpoints, their parameters, and the schema of what each returns. Once your API is that single normalized structure, generating a mock, a snippet, or an export is the same operation applied three different ways. The messy part is the parsing; everything after it is clean.
Mock Responses, Including the Unhappy Paths
This is the part that earns its keep. Plenty of tools will hand you a 200 OK with some placeholder data. Real clients have to survive much more than the happy path — the validation error, the expired token, the missing record, the server that fell over. So API Contract Lab generates realistic data per endpoint with the full spread of status-code variants, not just the success case:
| Status | The case it lets you build against |
|---|---|
200 / 201 | Success — a record read back, or a resource created |
400 | A malformed or invalid request — the shape your form validation has to handle |
401 | Missing or bad auth — the redirect-to-login path |
404 | The resource doesn't exist — your empty and not-found states |
500 | The server fell over — the error boundary you keep forgetting to test |
Being able to render an endpoint's 401 and 500 before the backend can even produce them is the difference between a UI that handles errors and one that only handles demos. You build the unhappy paths while they're cheap.
Client Snippets in Four Flavors
A mock response is more useful when you don't have to hand-write the call to fetch it. For each endpoint, API Contract Lab emits ready-to-paste client code in four shapes, so whatever your stack is, the call is already written:
fetch— the no-dependency browser baseline, for plain JS/TS frontends.curl— for the terminal, a quick smoke test, or a bug report.- Python
requests— for a script, a backend integration, or a notebook. - React Query — the hook wired up with its key, for a real React data layer.
Each snippet is scoped to the endpoint you're looking at, so it's a copy-paste away from a working call — not a template you still have to fill in.
Three Ways Out: Postman, OpenAPI, a Mock Server
Generated mocks are only useful where your team actually works, so there are three exports, each aimed at a different destination:
Postman Collection
A Postman Collection v2.1 — import it and every endpoint is there to hit and share with the team in seconds.
OpenAPI Bundle
An OpenAPI example bundle — your spec enriched with concrete example responses, for docs or codegen.
Mock Server
Mock handler scripts for Express.js or FastAPI — a runnable local server that answers like the real one.
That last one is the bridge from "a mock in a browser tab" to "a server my app can actually call." Drop the Express or FastAPI handlers into a tiny project, run it, and your frontend has a backend to talk to — one that returns the right shapes and the right status codes — while the real one is still being built.
Where It Fits Next to Mock Data Lab
It's worth being precise about scope, because API Contract Lab has a sibling. Mock Data Lab takes one schema and fills it with realistic, edge-case-laden data — the names, the nulls, the thousand rows. API Contract Lab works a level up: it's about the contract — the endpoints, the request/response cycle, the status codes, the runnable mock server. One answers "what does a row look like?"; the other answers "what does the API do?" Reach for Mock Data Lab when you need volume to fill a table; reach for API Contract Lab when you need a backend to call. Used together, they're the two halves of mocking an API before the backend exists — which is the whole point of working contract-first in the first place.
Why It Runs Entirely in Your Browser
Like the rest of the Developer Labs tools, API Contract Lab does all of this client-side — the parsing and generation run in your browser, nothing is uploaded, and there's no account to create. That's a privacy decision as much as a convenience one: the contract you paste is often an unreleased internal API, and it never leaves your machine. Free, browser-based, no signup — paste a spec, get a mock, move on.
The Whole Point
You already have your API contract — as an OpenAPI spec, a JSON example, or a list of endpoints. API Contract Lab reads it, generates mock responses with real status-code variants, hands you fetch, curl, Python, and React Query snippets, and exports a Postman collection, an OpenAPI bundle, or a runnable Express/FastAPI mock server — in the browser, with nothing uploaded. Stop waiting on the backend to start building against it.
Mock Your API Before the Backend Exists
Paste an OpenAPI spec, a JSON example, or an endpoint list and get mocks, snippets, and an exportable collection — free, in your browser, no account required.
Open API Contract Lab