Worked on the hmrc/individual-calculations-api repository to enhance API contract accuracy by updating the OpenAPI Specification for the Trigger Self Assessment Tax Calculation endpoint. Focused on aligning the documented HTTP method with actual runtime behavior, the change involved switching from PUT to POST, clarifying API usage for downstream consumers and reducing integration errors. This update was limited to the OpenAPI spec, ensuring minimal risk and no impact on functional code. Leveraged skills in API design and OpenAPI Specification, using YAML to deliver a traceable, low-risk improvement that improved developer experience and reduced support overhead for the individual-calculations service.