What best describes an API contract?

• A. A list of API endpoints only
• B. Internal implementation details
• C. A user guides
• D. A formal specification of inputs, outputs, and behavior

Answer: (D)

An API contract is the formal agreement that defines exactly how two systems will interact. It specifies what can be sent (inputs), what will be returned (outputs), and how the API behaves in different situations. This means data formats, required versus optional parameters, the meaning of each field, and how errors are represented, including status codes and messages. It also covers how authentication works, how limits or pagination are handled, and how versions are managed. Because this is a precise, machine-readable specification, clients and servers can evolve independently while staying compatible, and you can generate documentation, tests, or client code from it. Endpoints are part of the picture, but without the detailed input/output definitions and behavior, you don’t have the full contract. Internal implementation details and user guides don’t define the agreement between caller and service, so they don’t capture the contract’s formal, interoperable nature.