ScreenSync: free multi-screen responsive QA with MCP

I’m building ScreenSync, a Chrome extension for testing responsive websites across mobile, tablet, and desktop viewports simultaneously. It mirrors scroll, clicks, and form inputs, and includes screenshots, annotations, WebM recording, breakpoint inspection, localhost/staging support, and an MCP bridge for Claude, Cursor, Gemini, and Windsurf.

It is free to use, requires no account or sign-up, and runs locally in the browser. I’m using it in real software development projects and updating it continuously. I’d love honest feedback from frontend developers and QA testers on device coverage, responsive edge cases, and AI-assisted testing workflows.

Demo:

Website: https://responsivetester.info/

Chrome extension: https://chromewebstore.google.com/detail/responsive-tester-multi-s/phaafbmdcomffcacnipggkejflpbmnki