Although Toulmin models of argumentation are pervasive in composition textbooks, research on the model's use in writing classrooms has been scarce'typically limited to evaluating how students' essays align with the model's elements (claim, data, warrant, qualifier, rebuttal, backing) construed as objective standards. That approach discounts Toulmin's emphasis on context. In contrast, this study ... [Show full abstract] of a major university's summer composition program for high school students employs Wenger's notion of communities of practice and Bakhtin's notion of response to trace how classroom contexts mediate students' and teachers' understandings of a Toulmin model. The article presents a case study of a controversy that emerged when participants attempted to identify the main claim in one student's essay. The controversy arose, the analysis suggests, as participants positioned competing tacit and explicit representations of claims with/against other rhetorical terms (for example, thesis), variously interpreted the assigned tasks, and negotiated over tasks and texts.