We study the difficulties that teachers experience in learning to use Cabri in their teaching, as several studies have revealed, in order to improve teacher’s training. We use a triple theoretical frame, with the Anthropological Theory of Didactics (Chevallard), the Instrumental Approach (Rabardel) and the Theory of Situations (Brousseau). We have implemented a long ingeniery, with an intensive course about geometric problem solving with Cabri, a period of coached practice, an intensive cours about teaching with Cabri, and a period of coached practice. We show that cabri’s new ostensive objects require a praxeological reconstruction to assimilate the dynamics and the complexity and richness of relations among objects, and that despite a good assimilation of a mathematical praxeology, teachers experience difficulties in practising a didactic praxeology based on validation.