Worked on the hapifhir/hapi-fhir repository to upgrade the HAPI FHIR Core library to version 6.5.17, focusing on parser adjustments for contained resources and aligning DSTU3 validation. Addressed test failures by refining expectations and correcting error messages across the Structure, R4, and R4 JPA test suites, ensuring compatibility with the new core behavior. Utilized Java and YAML to implement these changes, emphasizing robust testing and careful refactoring. The work improved validation workflows, reduced flaky tests in continuous integration, and streamlined the upgrade process for clients, ultimately enhancing interoperability and lowering maintenance overhead for FHIR-based Java applications.