Rubrics, commonly used in L2 writing programs, not only facilitate teacher grading of student writing, but also serve the purpose of providing feedback to improve learners' texts. While the potential of rubrics to support writing development more generally has been discussed, realizing that potential through specific pedagogical practices remains under-researched. Grounded in Vygotskian theory, this study conceptualizes rubrics as a mediating tool for diagnosing and promoting learner writing abilities. We report data concerning two focal participants from a larger study of Dynamic Assessment (DA) in an English academic writing program. A three-step DA procedure was followed: learners independently produced a draft, reviewed it alongside a mediator who offered prompts, feedback, and guidance, and then independently revised their writing. Both the pre- and post-mediation drafts were rated according to a writing rubric, and comparisons of these ratings highlighted learner responsiveness to mediation. In addition, the mediator annotated the rubric during interaction to reflect the extent of mediation offered and particular areas of learner struggle. Together, the ratings and annotations provided a profile of learner abilities -- developed and emerging. From a Vygotskian perspective, the latter provides the appropriate focus of instruction. Implications for use of this approach in intact classrooms are discussed.