Worked on the AdguardTeam/HostlistsRegistry repository, focusing on backend development using JavaScript. Addressed a critical reliability issue by implementing a defensive validation for the filterKey parameter, ensuring it is non-empty before processing. This targeted bug fix reduced the risk of runtime errors caused by null or undefined keys, directly improving the robustness and reliability of downstream consumers. The solution was delivered as a precise, minimal change to minimize API impact and production risk. Demonstrated careful attention to compile-time safety and disciplined engineering practices, emphasizing robust input validation and maintainability within a core registry module over the course of the month.