Rivalcfg is a Python 3 library and command-line tool for configuring SteelSeries gaming mice (Rival, Sensei, Prime, Aerox, Kana, Kinzu, etc.) on Linux, macOS, and Windows. It communicates with devices via HID (using hidapi) using reverse-engineered HID report protocols. The system provides a clean device profile dictionary schema for each mouse, a plugin-like handler system for different value types (choice, range, RGB color, button mapping, gradients, etc.), and a CLI that dynamically generates argparse options from the device profiles. It also includes persistent settings storage in JSON, udev rule management for Linux, and test doubles (FakeDevice, FakeMouseSettings) for testing without physical hardware. The project is community-driven, with new devices added frequently through reverse engineering.