Codex Dev is a free add-on for Chrome designed for web developers looking to test responsive designs across multiple devices simultaneously. This tool allows users to view their websites in a variety of screen sizes, including popular mobile and desktop formats, all within a single interface. Key features include the ability to test multiple breakpoints, a comprehensive library of devices, and a synchronized scrolling function for an efficient testing experience.

The tool supports a wide array of devices such as the iPhone 14 Pro, iPad Pro, and various desktop resolutions, with options for custom sizes as well. Codex Dev emphasizes privacy by storing data locally and not using external servers for analytics. Its modern interface and powerful controls make it suitable for frontend developers, QA engineers, and designers, ensuring an optimal workflow for anyone building mobile-first websites.