The following research focuses on Game-Based Learning (GBL) for assessment through the lens of designing and validating a stealth assessment for the calculus game Variant: Limits™. The process of using Evidence-Centered Design (ECD) to create a valid assessment is highlighted through the development of a competency model and scoring rubrics. A sample of college students enrolled in a Calculus 1 course played the game outside of class during a 2-week period. Comparing the results of students' in-game and external assessments, researchers examined the validity of the stealth assessment measure. The stealth assessment was significantly correlated with the external measures and found to be a valid assessment of students' overall calculus knowledge as defined by the competency model.