T The implementation of the core tax administration system (coretax) by the directorate general of taxes since 2025 represents a strategic step in reforming indonesia\\\'s tax administration, aimed at integrating all taxation services into a single platform. this report aims to describe and analyze the implementation of the coretax application in the process of reporting the monthly tax return (spt) for income tax article 21 (pph pasal 21) for the december 2025 tax period at pt kkn, to identify the obstacles encountered, and to elaborate the measures taken to address them. the study was developed based on direct experience during an internship, employing a qualitative descriptive method through participatory observation, documentation of work processes, and a literature review of applicable tax regulations. the findings indicate that the implementation of coretax was carried out through eight interrelated stages, ranging from data collection to evaluation, with a distinctive feature in the december period requiring an annual reconciliation between the average effective rate (ter) mechanism and the progressive tax rate under article 17. the obstacles identified include technical system issues, user competency gaps, and data integration challenges between the internal payroll system and coretax. the mitigation measures undertaken by the company included rescheduling system access, implementing multi-layer data verification, enhancing user competency through training and consultation, and standardizing data conversion procedures. overall, coretax has contributed positively to the company\\\'s formal and material tax compliance, although the initial adaptation process required considerable time and effort.