An API contract is a crucial element in software development, establishing a formalized agreement on how an API should function and enabling seamless interaction between systems, teams, and machines. Historically, API contracts evolved from WSDLs used for SOAP-based services to the more flexible and widely adopted OpenAPI standard. OpenAPI provides a structured and predictable framework for defining API behavior, including endpoints, request types, and responses, ensuring clear communication and preventing integration issues. Its dual readability by both humans and machines facilitates collaboration among developers and automated systems, allowing for automated testing and validation. This approach ensures that, whether in human-to-human, human-to-machine, or machine-to-machine contexts, APIs perform as expected, supporting reliable and scalable development across various applications.