Sass MQ is a Sass library that provides a sophisticated mq() mixin for generating em-based media queries with named breakpoints. It converts pixel values to ems for better zoom compatibility, handles inclusive/exclusive boundary conditions properly, and supports complex media query compositions. The library includes utility functions for unit conversion and breakpoint lookup, plus a developer tool for visualizing active breakpoints. Built with modern Sass module system practices, it maintains compatibility with current Dart Sass versions while offering a clean, reusable API for responsive design.