Domagoj enhanced the user experience of the Nix REPL in the NixOS/nix repository by redesigning the continuation prompt to improve clarity for incomplete expressions. Using C++ and focusing on command-line interface and REPL development, Domagoj replaced previously invisible spaces with a visible prompt, directly addressing user confusion during multi-line input. The implementation followed clean commit practices and adhered to open source contribution guidelines, resulting in a well-documented, review-ready patch. This targeted feature improved onboarding and daily productivity for Nix users and developers, demonstrating thoughtful UX-focused design and effective collaboration within a major open source project over a one-month period.