Responsively App is an Electron-based desktop application for responsive web development testing. It allows developers to preview a website across multiple device simulators simultaneously (phones, tablets, laptops) with synchronized interactions such as scrolling, clicking, and form input mirroring via BrowserSync. The application includes a built-in MCP (Model Context Protocol) server that exposes automation tools (navigate, screenshot, click, read_page) for AI coding agents like Claude Code and Cursor. It also provides design overlays, color blindness simulation, ruler/guide systems, and screenshot capture. The codebase is structured as a monorepo with three packages: the desktop app (Electron + React + Redux), a browser extension (Manifest V3) to open URLs in the app, and an MCP CLI package published to npm. The architecture follows a main-process/renderer split with IPC communication, extensive E2E tests (Playwright), and a device frame system with pixel-accurate bezel artwork generated from upstream sources.