Assessment in stem education is a difficult activity. It requires teachers to have the competence of designing a competency assessment tool (cdcat). This article proposes the model of cdcat. The components and behavioral indicators of the cdcat were proposed based on analyzing the thinking process when designing an assessment tool. The model was built through a rigorous multi-step research process that ensures the value and reliability of the model. The pilot of 53 science pre-service teachers also showed that the model has a guiding effect on the design of the assessment tool in teaching science. Published under licence by iop publishing ltd.