Kobithasan Vasantharajah focused on improving API contract accuracy for the hmrc/individual-calculations-api repository by updating the OpenAPI Specification for the Trigger Self Assessment Tax Calculation endpoint. He identified a mismatch between the documented HTTP method and the actual API behavior, and addressed this by changing the method from PUT to POST in the YAML-based OpenAPI spec. This targeted, spec-only update enhanced developer experience and reduced integration errors without introducing functional code changes. Kobithasan applied his skills in API design and OpenAPI Specification to ensure the API documentation accurately reflected runtime behavior, minimizing risk and clarifying usage for downstream consumers.