Discursive formative assessment (FA) practices during instruction have grown in importance in the education literature as a way of improving teaching and learning. This study involves three years of design-based research as part of an educational project. We examine lower secondary school teachers’ qualitative Informal Formative assessment (IFA) practices during inquiry-based Science, Mathematics and Technology-Engineering (STEM) lessons. In-service teachers (N = 9) were filmed during a whole-classroom discussion session with respect to students’ alternative solutions or predictions in the context of problem-solving or investigating a natural phenomenon. The analysed discussions in the project’s third year lasted 15–30 minutes. The ESRU (Eliciting, Student response, Recognising and Using phases) model is adapted to analyse IFA practices, while also considering the role played by students. Inquiry dimensions (e.g. conceptual and epistemic) were used to examine the inquiries given priority in these discussions. The results show frequent (>30%) ESRU complete-cycle implementations by most teachers, which is associated with effective IFA practices. Frequent complete-cycle implementation led to teachers eliciting the same students at different moments of the session with longer consecutive cycles. The dimensions the teachers implement are more epistemic than conceptual. Moreover, students took charge of several ESRU phases.