An API’s data contract — what it accepts as input, what it returns as output — exists whether you define it formally or not. Leaving it informal means the contract lives only in documentation prose and developer intuition, is inconsistently enforced, and drifts between what the documentation says and what the code actually handles. JSON Schema provides a standard, machine-readable format for expressing data contracts that can drive validation, documentation, and testing from a single source of truth.

Documentation is the first thing a developer encounters and the last thing most teams prioritize. The result is a familiar pattern: the API is built, the launch deadline approaches, documentation gets written in a hurry by someone who did not build the API and does not fully understand it, and integrators spend the next six months filing support tickets that could have been answered by better documentation. OpenAPI exists to break that cycle by making documentation a first-class artifact of the API itself.