This article reports on results from a design research project that implements a formative assessment practice with support from a classroom response system (CRS). Cumulatively building on earlier research and drawing on iteratively generated data from secondary schools in Sweden, the article elaborates on design principles and task types for constructing CRS tasks aimed at generating mathematical classroom discussions. The study generated three design principles, six task types and 31 empirically evaluated tasks useful for developing and using CRS tasks in classroom practices. The results are discussed in relation to earlier research, methodological considerations and the cultural context of Sweden.