This paper discusses the conception of an online formative assessment tool in the field of functional thinking and focuses on the first cycle of a design-based research project. This tool is based on a preceding qualitative research project in an Austrian grade 7 class for examining students' conceptions as well as possible influences of designed digital worksheets on their conceptions. Research results led to considerations on how to implement the digital tasks in regular teaching for diagnosing and enhancing students' conceptual development concerning functions. New developments in online classroom collaboration such as GeoGebra Classroom enable the implementation of the digital tasks for formative assessment. In this paper, we present digital worksheets that focus on one particular task addressing a graph-as-picture error, summarize relevant research results from the preceding study, and outline the conception of an online formative assessment tool.