API evolution often stalls due to OpenAPI Generator limitations, specifically the loss of generic type information and duplicated response envelopes. This technical deep-dive analyzes a Spring Boot experiment designed to solve these issues by establishing a single canonical API contract shared between server and client. The core solution involves extracting the generic ServiceResponse wrapper into a standalone, framework-agnostic module, preventing the explosion of near-identical wrapper classes.

By enriching the OpenAPI specification with custom vendor extensions and utilizing Mustache templates, the architecture enables the generation of thin, type-safe client wrappers that extend the shared contract rather than duplicating it. This approach significantly reduces maintenance overhead, ensures deterministic evolution, and strictly separates success paths from RFC 9457 Problem Details error handling. Authored by Baris Sayli, this methodology offers a reference architecture for teams seeking to maintain hexagonal boundaries and achieve production-ready client generation.