In collaborative open-source development, the rationale for code changes is often captured in commit messages, making them a rich source of valuable information. However, research on rationale in commit messages remains limited. In this paper, we present CoMRAT, a tool for analyzing rationale in commit messages. CoMRAT enables researchers to investigate rationale information in any Github module, and for developers to check the level of rationale in their commit messages. A preliminary evaluation suggests the tool’s usefulness and usability in both a research and development context.