The article describes the development of a digital tool for formative self-assessment in a design-based research study. The aim is to create a tool that allows students to become assessors themselves rather than having technology evaluate their answers. Thus, learners are provided with a list that implies information on typical misconceptions to check their solution to an open assessment task. This task tests the students' ability to draw a graph based on a given situation. Two case studies in form of task based interviews with sixteen-year-old students are described. The analysis leads to reconstruction of the learners' formative assessment processes by using a theoretical framework developed in the EU-project FaSMEd. The results show which formative assessment strategies students actively use when working with the digital tool and which functionalitites of the technology can be identified.