Background Excellence in clinical reasoning is one of the most important outcomes of medical education programs, but assessing learners' reasoning to inform corrective feedback is challenging and unstandardized. Methods The Society to Improve Diagnosis in Medicine formed a multi-specialty team of medical educators to develop the Assessment of Reasoning Tool (ART). This paper describes the tool development process. The tool was designed to facilitate clinical teachers' assessment of learners' oral presentation for competence in clinical reasoning and facilitate formative feedback. Reasoning frameworks (e.g. script theory), contemporary practice goals (e.g. high-value care [HVC]) and proposed error reduction strategies (e.g. metacognition) were used to guide the development of the tool. Results The ART is a behaviorally anchored, three-point scale assessing five domains of reasoning: (1) hypothesis-directed data gathering, (2) articulation of a problem representation, (3) formulation of a prioritized differential diagnosis, (4) diagnostic testing aligned with HVC principles and (5) metacognition. Instructional videos were created for faculty development for each domain, guided by principles of multimedia learning. Conclusions The ART is a theory-informed assessment tool that allows teachers to assess clinical reasoning and structure feedback conversations.