The design phase of the API lifecycle is where we start organizing all the business and technical details of an API. The goal is to use JSON Schema, OpenAPI, JSON, and YAML to define as much about an API as possible, that we can then publish documentation and mock servers that teams producing and potential consumers can understand what it will do–all before we write any code.
Like the define stage, the more work and iterations that happens during the design stage will save time and money in future stages of the API lifecycle. A modern API lifecycle gives us the artifacts and tooling necessary to design an API and emulate what it will do, so that we can rapidly iterate, pivot, and change the design before we ever release an API. Within the design stage there will many iterations that occur before an API is ready to move to development.