Worked on the jellyfin/jellyfin repository to enhance the reliability and concurrency of the subtitle parsing workflow. Addressed a thread-safety issue in the SubtitleEditParser by ensuring a new SubtitleFormat instance was created for each Parse call, effectively eliminating shared state and preventing race conditions during concurrent subtitle parsing. This backend development effort, implemented in C# with a focus on multithreading, stabilized the subtitle subsystem and reduced parsing errors, particularly when handling large subtitle datasets in multi-threaded environments. The work improved overall system stability and user experience by making subtitle parsing more robust under concurrent workloads without introducing new features.