School entry diagnostics in German primary schools comprise a number of early numeracy tasks to determine the out-of school mathematics knowledge young children bring to school. In this article, the authors at first typify how the tasks vary in a sample of eight primary schools in type, level, and quality. Based on ethnographic observation, and a practice theoretical perspective, in a second step they analyse how the children’s accomplishments are produced in the process of coping with the tasks and of documenting the results as well as in the interplay of actors and diagnostic instruments, and how these accomplishments are translated into competencies. The article thus empirically explores the culture of mathematical tasks in the context of school entry diagnostics.