Worked on the aws/aws-sdk-java-v2 repository to enhance reliability and consistency in S3 upload workflows, focusing on bug fixes rather than new features. Addressed concurrency issues by refactoring the checksum digest cache to use a thread-safe structure, resolving checksum mismatch errors during parallel uploads with SHA1 and SHA256 algorithms. Improved error handling and updated tests to ensure more predictable client behavior for developers integrating S3 uploads. Also removed the default SessionMode for S3 Express Sessions to better align API behavior with service defaults. Utilized Java, AWS SDK, and testing frameworks to deliver targeted improvements that reduced upload failures and clarified error messaging.