One-on-one diagnostic interviews with school students have been proposed as learning opportunities to acquire diagnostic competences. Moreover, role-play-based simulations have proved promising to foster interactive competences similar to diagnosis during early phases of teacher and medical education. Thus, we developed a role-play-based simulation of diagnostic interviews on the topic of decimal fractions for mathematics pre-service teachers. During the role-play, participants either take on the role of a sixth grader, a teacher interviewing a sixth grader, or an observer watching the interview. Based on cognitive labs addressing criteria such as authenticity and immersion in the teacher’s diagnostic task in the role-play, we analyze the feasibility of the chosen simulation approach to measure and foster mathematics pre-service teachers’ diagnostic competences.