In this paper, I detail the ways in which a South African initiative focused on mental mathematics in the early grades (the Mental Starters Assessment Project — MSAP) can be considered as an intervention aligned with the idea of ambitious instructional practice. In building this argument, I take note of the fact that the materials associated with the MSAP initiative are relatively prescriptive in their format, a feature that has sometimes been argued to work against the goals of ambitious instructional practice. The reasons for considering the MSAP an ambitious instructional practice initiative is linked in the paper with the attention given in the materials to working across the strands of mathematical proficiency, with local conditions and cultures driving the relatively prescriptive format of materials provision.