This paper reports on the central role of disciplinary (engineering) criteria in the development of an ESP-based diagnostic writing task and rubric, used to identify entering undergraduate engineering students in need of academic support. In this mixed methods study, Phase 1 investigated the usefulness of a generic writing task and analytic rubric used for the diagnosis. Phase 2, informed by the results of Phase 1, focused on the development of an engineering writing task. The outcomes of the two phases were merged to develop an engineering ESP-based writing task and rubric, informed by a) the collaboration of language/writing experts and engineering stakeholders, and b) criteria, indigenously drawn from the engineering community of practice. The study supports an academic literacies approach in diagnostic assessment (rather than a generic, one-size- fits-all, ‘academic literacy’ approach), and suggests that the demands of university study are best viewed as the practices of disciplinary communities of practice. The paper provides evidence of the increased meaningfulness and usefulness of a disciplinary, ESP- based approach in diagnosing need for academic support.