During January 2026, Der focused on enhancing the pandas-dev/pandas repository by developing robust test coverage for the DataFrame.agg aggregation path. He implemented new tests using Python and data analysis techniques to verify that aggregation with np.size correctly computes column sizes, addressing potential edge cases and improving regression safety. His work emphasized thorough testing practices, contributing to the stability of continuous integration pipelines. Der collaborated across teams, aligning his efforts with ongoing issues and co-authoring changes to ensure consistency. The depth of his contribution lay in expanding core test coverage, leveraging his skills in Python and testing to improve reliability.