In what form was the API contract described? What sections did the specification consist of?

API contracts are usually described in formats such as OpenAPI (formerly Swagger), RAML, or API Blueprint. The specification includes the following main sections:

• Info — general information about the API (name, version, description).
• Paths — description of endpoints, including methods (GET, POST, etc.), request parameters, request body, and responses.
• Components — data schemas (models) used in requests and responses.
• Security — description of authentication and authorization mechanisms.
• Tags — grouping endpoints by categories.

Such a contract clearly defines how the client and server interact and serves as documentation for developers.