This study aimed to investigate the use of an approach, where heuristics for mathematical problem solving is taught by the use of process level formative feedback (Hattie & Timperley, 2007; Shute, 2008). This means that (detailed, domain-specific, and situation-specific) feedback is provided on students’ attempts to solve mathematical problems, so that heuristics are taught not as general rules or in the abstract, and that the feedback only addresses those shortcomings that are identified in the specific situation, to avoid excessive and/or overwhelming information. Such an approach requires, however, that teachers are able to accurately assess students’ solutions, as well as their reasoning, and provide feedback that may aid the students in developing their problem-solving skills. In order to support the teachers, they were provided with a scoring rubric for mathematical reasoning, which could help them in assessing student performance, as well as supporting them in providing future-directed feedback (or “feedforward”) (Panadero & Jonsson, 2013; Smit & Birri, 2017). Specifically, this study investigated: (a) how the teachers provided feedback to the students with the support of a rubric, and (b) how students’ reasoning was affected by this feedback.