Attempts to reform mathematics teaching in the past decades have endorsed the view that mathematical reasoning and strategic competence are central learning goals. The affordances of technology for creating learning environments that nurture mathematical reasoning have been developed and investigated for the past decades. Recent efforts bring diagnostic tools from cognitive research to technological platforms that automatically assess students’ work to provide immediate feedback. Such feedback is used for assessment, to support conceptual adaptive learning, and to help teachers with formative assessment in the classroom. Yet, automatic feedback has usually been limited to tasks that require procedural interactions that do not necessarily offer teachers opportunities to assess various dimensions of conceptual understanding. In the past few years, across design-based research cycles, we developed and tested an innovative online assessment approach and supporting platform (STEP) that provides meaningful immediate feedback. We used STEP to study the affordances of example-eliciting tasks (EETs) and articulate design principles for an online example-based formative assessment environment. The article focuses on the rationale and principles of the development and of the design-based research. I analyze cognitive aspects of the conceptual online feedback and of our attempts to articulate the potential of technology for making EETs less demanding for teachers to manage.