Congcong Cai contributed to both the llvm/clangir and moonbitlang/moon repositories, focusing on compiler development and error handling using C++ and Rust. In llvm/clangir, Congcong enhanced CIR generation by refining base-class constructor handling during aggregate initialization and adding robust support for empty C++ unions, which improved code generation stability and portability. For moonbitlang/moon, Congcong stabilized the mod processing workflow by implementing defensive validation for MoonModJSON, ensuring that empty mod names are caught early and runtime panics are avoided. The work demonstrated careful attention to reliability, with added regression tests and improved error paths to reduce downstream failures.