This article reports on research at the crossroads of on the one hand the procedural-conceptual cut in algebra in the transition from secondary education to higher education, and on the other hand the potential of ICT. Three key topics come together: algebra didactics (symbol sense), theories on tool use (instrumental genesis) and assessment (feedback and formative assessment). Our focus in this article is on formative assessment and feedback. A prototypical digital algebra environment was designed, consisting of thirty tasks, which focus on symbol sense. The activities were piloted in five one-to-one think-aloud sessions with students from pre-university grade 12. The results of the one-to-one sessions were analysed and used to make a feedback design for the tasks. The feedback design was implemented in a second revision of the tool. The revision will be used for experiments on a larger scale with a web-based tool for acquiring, practicing and assessing algebraic skills. (Contains 6 figures.)