VTool-R1: VLMs Learn to Think with Images via Reinforcement Learning on Multimodal Tool Use - Wu, Mingyuan; Yang, Jingcheng; Jiang, Jize; Li, Meitang; Yan, Kaizhuo; Yu, Hanchao; Zhang, Minjia; Zhai, ChengXiang; Nahrstedt, Klara (International Conference on Learning Representations, 2026)