This project aimed to develop an engaging formative assessment that simulates the social and dialogic processes that are often missing in traditional instruction and assessment of argumentation. In this article, we describe the design of an argumentative reasoning task within a scenario-based assessment enhanced with game elements ("Seaball--Semester at Sea"). This "Seaball" task requires students to classify foods as "junk food" or not, by identifying relevant evidence to support their decisions and evaluating reasoning errors in others' arguments. Data from a small-scale study of 42 middle school students revealed that students performed significantly better on questions requiring selection of appropriate evidence than questions requiring identification of logical fallacies. Additionally, students' total scores were significantly correlated with external measures of argumentation skills, and with teacher's ratings of their argumentative writing skills. Survey indicated that students perceived the game-enhanced scenario-based assessment as engaging. Implications for formative assessment are discussed.