What must be necessarily described in the API contract?

The API contract must necessarily describe the following elements:

• Endpoints (URLs) and supported HTTP methods (GET, POST, PUT, DELETE, etc.).
• Request and response formats, including data structures, field types, mandatory and optional parameters.
• HTTP status codes and their meanings for different scenarios (success, errors).
• Authentication and authorization requirements — how the client should identify itself.
• Restrictions and limits (rate limiting, maximum request size).
• Error descriptions with examples and codes.

A well-structured contract allows clients and servers to interact correctly without misunderstandings.