Demo→Stub Pattern Library: 6 Convert‑Ready Templates to Turn Playables into OpenAPI Stubs and Acceptance Tests

A playable demo describes user-visible flows but not the API contract contractors will implement. Teams that skip a spec end up with ambiguous behavior, duplicated assumptions, and wasted iterations. Converting the demo into an OpenAPI stub removes ambiguity: it defines endpoints, request/response shapes, and example scenarios you expect during acceptance testing.

OpenAPI-driven mocks (Prism, MockServer, lightweight mock generators) let frontend, QA, and contractors run the same predictable backend. That reduces handoffs: the demo becomes executable via a mock server, acceptance criteria become automated or manual checks against that server, and telemetry maps point to the exact events and fields you need to validate in production.

Workflow distilled into three artifacts: (1) an OpenAPI file with the demo endpoints and concrete examples; (2) a one‑page acceptance test that lists scenarios (happy path + edge cases) with exact requests and expected responses; (3) a minimal telemetry map that lists events, key fields, and where they should fire in the flow. Together these reduce interpretation and speed up implementation.

Practical handoff: include the OpenAPI YAML/JSON and a short README that shows how to run the mock server (one command using Prism or MockServer) and run the one‑page acceptance checklist. Contractors can start frontend integration against the stub immediately and implement the real backend using the same spec.

We provide six patterns you should keep in a repository of small specs and tests. For each pattern the OpenAPI file contains: path(s), operationId, request/response schemas, and at least one concrete example. The acceptance test is a one‑page list of steps (start state, user action, expected API call and response, expected UI change or telemetry event).

Keep the stub responses focused: prefer precise example responses over broad schema-only mocks. Concrete examples make the mock server produce realistic payloads and make acceptance checks deterministic.

Example files should be deliberately small: 150–400 lines of OpenAPI YAML that cover just the demo flow. Add examples under each response object (OpenAPI examples) so mock servers produce the exact payloads your demo expects. Avoid auto-generating huge specs — smaller, focused specs are easier to keep in sync with the demo.

Telemetry map: a simple CSV with columns (event_name, trigger_point, key_fields, success_criteria). For example: order_created, after /checkout/success response, order_id|amount|user_id, appears in production logs within 5s. This gives contractors clarity on which fields to emit and where QA should look when they run the acceptance test.

Tooling options are mature: use Prism for speedy local mock servers from OpenAPI examples, MockServer for enterprise workflows and generated expectations, or one of the light mock generators to share a URL. Whichever you pick, document the exact command and how to swap examples (to test error cases and edge states).

Pitfalls to avoid: (1) Spec drift — keep the spec in the repo and update it with PRs; (2) Over-ambitious mocks — stub only behaviors your demo uses; (3) Missing error cases — include at least one API error response per endpoint so acceptance tests can verify UI error handling.