This paper investigates how mathematics teachers plan lessons with a recently implemented Danish learning platform designed to support teachers in planning lessons in line with a recent objective-oriented curriculum. Drawing on data from observations of and interviews with teachers, three mathematics teachers' joint planning of a lesson in geometry with a learning platform called Meebook is analyzed using the instrumental approach. It is concluded that the interface in Meebook orients the teachers work toward what the students should do rather than what they should learn, although the latter is a key intention behind the implementation of the platform. It is also concluded that when the teachers succeed in using learning objectives actively in their planning, the objectives support the teachers to design lessons that correspond with their intentions. The paper concludes with a discussion of the dialectics between learning objectives and planned activities.