What does it mean to be "good-at-math," and how is it determined? Cobb et al. (2009) defined the normative identity of mathematics classrooms as the obligations that students must meet to be considered good-at-math. Obligations are negotiated between teachers and students through series of bids. Normative identities reveal distributions of agency and authority within classrooms, which affect learning opportunities for students. Traditionally, mathematics teachers held the predominance of agency and authority in classrooms. Research supports shifting toward more equitable teaching and learning (e.g., National Council of Teachers of Mathematics, 2018). Clear examples of enacting and supporting changes are helpful. This article shares how sixth-grade students and their teacher co-constructed good-at-math to invite and obligate students to become active agents in mathematical argumentation.