API Contracts and Schema Design Questions

Defining the interface contract between producers and consumers: request/response payload shapes, data models, field-level validation, nullability, and enum/typing decisions (including safely evolving an enum's allowed values without breaking clients). Covers contract-first design with OpenAPI and JSON Schema (authoring specs, generating SDKs and mock servers, catching breaking changes in CI), mapping internal domain models to external DTOs as both evolve independently, and designing a stable error-response contract (structured error codes, correlation IDs, retryable classification). Also covers the leadership and behavioral practice of establishing and governing contract standards across teams. The contract is the durable artifact clients depend on.