How students use generative AI is a configuration of distinct behaviors, not one skill, yet scoring typically collapses it onto a single proficiency axis, conflating process with product. We develop a multidimensional Bayesian IRT model with rater effects and post-hoc Varimax-permutation identification, surfacing three substantive dimensions: prompting effort, content delegation, AI-delivered citations. Domain familiarity shifts students toward more active engagement and away from delegation.