Worked on the modal-client repository to enhance backend reliability and user experience, focusing on error handling and test stability. Addressed sandbox lifecycle issues by implementing graceful handling of NotFoundError when executing commands on finished sandboxes, providing clearer messaging for users. Improved test reliability across Windows environments by increasing startup timeouts, introducing asyncio-based synchronization for input slots, and adjusting timer granularity for ephemeral resource tests. These changes reduced test flakiness and accelerated engineering feedback loops. The work leveraged Python, asynchronous programming, and robust testing practices, demonstrating a methodical approach to backend development and quality assurance within a short project timeframe.