Frequent diagnostics and feedback regarding individual learning processes are central to formative assessment, and successful implementation of this in heterogeneous inclusive education is a major issue. To gain insights into the current implementation of diagnostics and feedback in inclusive classrooms, we conducted semi-structured expert interviews with six mathematics teachers. The research questions were as follows: (1) How do teachers diagnose learning and performance in inclusive mathematics education? (2) What feedback practices do teachers apply in inclusive mathematics classrooms? and (3) What challenges do teachers face regarding their inclusive everyday instruction in diagnostics and feedback? The interview responses were analysed using qualitative content analysis, and the results indicated that (1) dialogues and written responses were the most commonly used diagnostic strategies; (2) immediate feedback within lessons focused more on learning processes or learning activities, while delayed feedback outside of lessons aimed at learning goals and respective strategies; and (3) teachers perceived the time costs of diagnostics and feedback in inclusive education and the fact that many students do not engage with written feedback as challenges. The implications of implementing formative assessment in inclusive education and the need to research how engaging elaborated feedback looks like will be discussed.