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 decision and rationale sentences rationale in commit messages. CoMRAT enables a) researchers to produce metrics and analyses on rationale information in any Github module, and b) developers to check the amount of rationale in their commit messages. A preliminary evaluation suggests the tool's usefulness and usability in both these research and development contexts.