Merge's approach to creating and testing SDKs for a Unified API platform involves using OpenAPI to generate standardized, scalable templates across various programming languages. By leveraging the OpenAPI Generator tool, Merge customizes Mustache template files to produce SDKs that integrate seamlessly with their API, ensuring clients can interface with HR, ATS, CRM, Accounting, and Project Management platforms. The article explores the challenges and strategies of implementing unit tests within these templates, particularly highlighting the use of OpenAPI's custom vendor extensions for introducing example payloads. Merge's process includes adding these tests to continuous integration and delivery pipelines, ensuring that any API changes are validated against the SDK client code, thereby maintaining the reliability and functionality of their integration services for customers.