<p>Teachers’ ability to assess pupils’ knowledge and skills is a crucial component of their daily responsibilities, as accurate diagnoses influence both the support provided to pupils and their learning outcomes. Traditionally, diagnosing pupils’ skills is an individual task, with little consideration given to the potential of collaborative diagnosis. However, collaboration may offer significant benefits, particularly when diagnosing cross-domain skills, such as scientific reasoning, where teachers from different subjects can contribute observations from different classroom situations and integrate their perspectives into a joint diagnosis. Drawing on parallels with the medical field, where collaborative diagnoses have already been shown to improve diagnostic accuracy, potential advantages may also occur in an educational context. Nonetheless, collaboration also comes with difficulties: Distributed information is seldom shared, and collaboration can be ineffective. Thus, using video-based simulations, this study investigates how collaboration, distribution of information, and collaboration support, by means of collaboration scripts, influence diagnostic accuracy – potentially via the application of collaborative diagnostic activities – consequently contributing to the development of diagnostic competences in pre-service teachers. The results indicate that pre-service teachers’ collaborative diagnostic accuracy does not per se exceed individual diagnostic accuracy, but that collaboration support can significantly enhance the added value of collaborative diagnosis. The reason for this seems to be increased information sharing. Thus, this study suggests that successful collaborative diagnoses require collaboration support and that collaboration scripts can help overcome the challenges of collaboration. Future research should explore collaborative diagnosis in different contexts and for other (cross-domain) skills.</p>