This study focuses on computer-based formative assessment for supporting problem solving and reasoning in mathematics. To be able to assist students who find themselves in difficulties, the software suggested descriptions - diagnoses - of the encountered difficulty the students could choose from. Thereafter, the software provided metacognitive and heuristic feedback as suitable. The findings provide insight into how formative assessment can be designed to support students during their problem solving and indicate that diagnosis and feedback could be helpful for promoting students' creative reasoning. In other words, the feedback guided students in justifying, monitoring, and controlling their solution method.