During December 2025, Matteo Favetti focused on enhancing the reliability of core expression evaluation in the LimeSurvey/LimeSurvey repository. He addressed a critical bug in the JavaScript Expression Manager, specifically correcting the substr function to handle cases where the start parameter is zero. This fix ensured that string operations within dynamic surveys and form calculations produced accurate and predictable results, directly improving end-user experience. Matteo applied his debugging expertise and deep understanding of JavaScript to resolve this edge-case, demonstrating careful attention to detail. The work reflected a targeted, in-depth approach to maintaining correctness and stability in complex survey logic.