Programmers using bug-finding tools often review their reported warnings one by one. Based on the insight that identifying recurring themes and relationships can enhance the cognitive process of sensemaking, we propose SWIRL, which supports interpreting tool-generated warnings through interactive, customized summarization. With active feedback, SWIRL derives summary rules for grouping of related warnings on the fly. As users mark warnings as interesting or uninteresting, SWIRL’s rule inference algorithm surfaces common characteristics, highlighting structural similarities in containment, subtyping, invoked methods, accessed fields, and expressions. We demonstrate SWIRL on real-world warnings generated from Infer and SpotBugs on two mature Java projects. In a within-subject user study with 14 participants, users articulated root causes for similar uninteresting warnings with more confidence when using SWIRL, compared to the baseline that lists individual warnings without customized summary rules. Among participants, we observed significant individual variation in desired grouping, reinforcing the need for individualized sensemaking. The simulation we conducted shows that SWIRL’s rule-level feedback expedites sensemaking—requiring only 11.8 interactions on average to align all inferred rules with a simulated user’s labels when combined with instance-level feedback, compared to 17.8 interactions when using instance-level feedback alone. Our evaluation suggests that SWIRL’s active learning-based summarization can enhance the sensemaking process of tool-generated warnings.