Worked on the facebook/pyrefly repository to enhance backend stability by addressing a critical edge-case bug in error handling. Focused on preventing crashes when the NotImplemented singleton is mistakenly invoked as a function, the solution introduced a clear NotCallable error, improving the API’s robustness and user experience. The approach involved updating both Rust and Python codebases, expanding test coverage, and ensuring all related checks passed across cross-language test suites. Emphasized test-driven development and code review best practices, resulting in safer runtime behavior and faster debugging. Demonstrated proficiency in Python, Rust, backend development, and comprehensive testing throughout the development cycle.