AbstractThough discipline‐specific approaches to literacy instruction can support adolescents' academic literacy and identity development, scant attention has been paid to ways of targeting such instruction to address individual student needs. Dialogic writing assessment is an approach to conducting writing conferences that foregrounds students' composing process so that teachers can assess and support that process with instructional feedback. Because such feedback is immediate, teachers can observe how students take it up. While dialogic assessment has shown promise as an approach to revealing and supporting students' writing processes in English Language Arts classrooms, it remains to be explored how this approach can support developing writers in other subject areas. This paper offers an analytic narrative account of how a high school social studies teacher used this method to support the writing process of one student, exploring what the method revealed about the challenges the student faced in writing about history, the gaps and misconceptions in their understanding of history and the intersection between the two. We discuss how certain ‘mediational moves’ the teacher employed enabled the student to compose collaboratively with the teacher, and in this collaborative composing, to capture ideas that she later used in her independent writing.