The oldest argument in frontend work is "the API isn't ready yet." It is usually true, and it usually means the UI team sits idle while the backend team builds the contract everyone agreed on in a meeting and then changed twice. The fix is not another standup. The fix is a mock you can build before a single real route exists.

MockDeck (apimock) takes a one-line description of an endpoint and turns it into a mock API spec. You write something like "GET /orders returns a list of orders with id, total, status" and it produces a spec in OpenAPI 3, Postman, or raw routes — whichever your toolchain eats. The point is your frontend team gets something to build against the same afternoon, not three weeks later.

What makes it useful rather than toy is the sample responses. It generates realistic example payloads, not just empty schemas, so your components render against believable data and your error states have something to show. It also includes an auth style in the spec, so the mock reflects whether an endpoint expects a bearer token or an API key instead of silently assuming one.

Where AI does belong in this picture

Generating a spec from a one-line description is exactly the open-ended case where a model earns its place. The input space is unbounded — natural language about an endpoint — and there is no single "correct" output, only a reasonable one. That is different from validation, where correctness is a written spec and a model only adds failure modes.

What it deliberately does not do

  • It is a mock, not a backend. There is no business logic and no persistence. Calling the mock twice will not give you two different records; it returns the sample response, every time.
  • Sample responses are fabricated. They look plausible, but they will not match your real data shape unless you edit them. Treat them as scaffolding, not as a data contract.
  • It does not test your real API. OpenAPI 3, Postman, and raw are output formats. It will not hit your live endpoints to check that the mock matches reality — that comparison is still your job.
  • It will not design your API for you. Garbage in, garbage spec out: a vague one-liner produces a vague mock.

If your frontend is blocked waiting on a backend, MockDeck is $19/mo and gets a usable mock spec in front of your team today. The real API can catch up later.