Leopoldo focused on improving the reliability of the tidy3d codebase by addressing concurrency issues in temporary directory creation. Working within the flexcompute/tidy3d repository, he replaced the use of os.mkdir with os.makedirs(..., exist_ok=True) in Python, ensuring thread-safe operations when multiple threads attempted to create the same directory. This change targeted a race condition that previously caused intermittent failures in parallel workflows, particularly those involving file system operations. By applying his skills in bug fixing and concurrency, Leopoldo enhanced the stability of simulations and tests that depend on local temporary storage, demonstrating careful attention to robust, concurrent code design.