Worked on the CAUSW_backend repository to deliver a more robust and maintainable API layer, focusing on backend development using Java, Spring Framework, and JPA. Developed and standardized new user interaction APIs, including post like, comment, and child-comment v2 endpoints, while implementing consistent API responses and comprehensive error handling. Migrated mapper logic from MapStruct to static mappers to reduce dependencies and improve performance, and expanded service-level unit tests using Mockito to validate API contracts. Addressed multiple bugs, such as issues in post service logic and naming inconsistencies, resulting in a more stable release and improved integration for frontend and downstream services.