We present preliminary evidence on the impact of a NLP-based writing feedback tool, Write-On with Cambi! on students’ argumentative writing. Students were randomly assigned to receive access to the tool or not, and their essay scores were compared across three rubric dimensions; estimated effect sizes (Cohen’s d) ranged from 0.25 to 0.26 (with notable variation in the average treatment effect across classrooms). To characterize and compare the groups’ writing processes, we implemented an algorithm that classified each revision as Appended (new text added to the end), Surface-level (minor within-text corrections to conventions), or Substantive (larger within-text changes or additions). We interpret within-text edits (Surface-level or Substantive) as potential markers of metacognitive engagement in revision, and note that these within-text edits are more common in students who had access to the tool. Together, these pilot analyses serve as a first step in testing the tool’s theory of action.