According to the IQB Education Trend, almost a quarter of fourth-grade students do not reach the minimum standards in mathematics and therefore require diagnostic-based support. However, providing such diagnostic-based support for foundational understanding poses a significant challenge for teachers. This analysis therefore discusses how teachers can be supported using the digital diagnostic tool "Mathe sicher können – Online-Check" (Mastering Math with Confidence – Online Check). To this end, a design research-based approach is used to identify teachers' support needs and derive design principles for the "Mathe sicher können – Online-Check." This revealed that novice teachers particularly require support with the fundamental subject-specific pedagogical categories of a subject. Subsequently, a quantitative design demonstrates that support using subject-specific pedagogical categories can productively influence teachers' practices in diagnostic-based support. biography Sebastian Groß, born in 1981 in Frankfurt/M., studied Medieval and Modern History and Political Science in Mainz, since 2005 has been involved in various research projects (Historical Seminar University of Mainz, Institute for Cultural Studies Essen, German Historical Institute Rome). New product