Which of the following defines the limits and requirements for OpenAPI compliance in external service schemas?

The concept of OpenAPI compliance is fundamentally grounded in following a specific set of guidelines and structures outlined in the OpenAPI Specification. This specification serves as a blueprint for describing the capabilities of an API, including its endpoints, request and response formats, and authentication methods.

By adhering to this specification, developers ensure that their API documentation is consistent, predictable, and user-friendly. It provides a standardized method to generate APIs that can be understood and consumed not only by developers but also by automated tools for tasks such as testing, documentation generation, and client SDK generation.

The other options mentioned do not encapsulate the essence of OpenAPI compliance. For example, while the size of response code output could be relevant in discussing API responses, it does not specifically dictate compliance with the OpenAPI framework. Similarly, the presence of complex object parameters and the use of descriptive parameter names are important aspects of API design but are not comprehensive enough to define overall compliance with OpenAPI standards.