Testing REST APIs plays a critical role in guaranteeing user-friendly functionality, reliability, and performance. This importance is magnified when adopting the API design-first methodology, where the API design is defined before any implementation starts. In such a scenario, testing becomes even more fundamental to the entire process. Testing in the API design-first approach is not just about validating code; it is about validating ideas, assumptions, and requirements. By integrating testing into the design phase, teams can build APIs that are not only functional and reliable but also closely aligned with stakeholder needs and expectations from the outset.