Responsive Tester
Preview any page in multiple synced viewports, interact with or edit one screen and every screen follows.
Add to ChromeChange It Once, See It Land Everywhere
Every device size sits side by side. One edit shows up on every screen the moment you make it.
Every Screen Follows Your Click
Open a menu and it opens on every device, so the mobile drawer and the desktop dropdown show up in the same glance.
Edit CSS With Every Breakpoint Watching
Bump a font weight and catch the orphan it creates three breakpoints down, because you edit and preview every screen at the same time.
Overflows Flag Themselves
Accidental horizontal scroll gets marked the moment it appears, on any viewport, before a user ever finds it.
Synced Viewports. See phones, tablets, and desktops at once, in a row or wrapped into a grid.
Interaction Sync. Clicks, hovers, typing, and navigation mirror across every viewport in real time.
Scroll Sync. Scroll one frame and every frame follows, including scroll areas inside the page.
Live CSS Editing. Edit an element's rules and watch the change apply instantly in every viewport, media queries included.
Site Breakpoints. Reads the breakpoints from the page's own stylesheets and builds matching viewports in one click.
Breakpoint Inspector. Hover any element to see which of its styles change at every breakpoint.
Device Presets. Add viewports from popular phone, tablet, and desktop presets, or define custom sizes.
Mobile Spoofing. Phone and tablet viewports identify as touch devices, so touch menus, carousels, and components behave like they do on device.
Workspaces. Save viewport sets as named workspaces and switch between projects in one click.
Rotate & Resize. Flip any viewport to landscape or drag its edges to test the widths in between.
Screenshots. Capture a clean screenshot of any viewport in one click.
Overflow Warnings. Any viewport whose content spills past its width gets flagged automatically.
Dark Mode Switches. Force light or dark on any single viewport and check both schemes side by side.
MCP Server. Connect Claude Code or Cursor and let your agent launch pages, audit breakpoints, and edit CSS.
Broad Web Support. Test live sites, localhost, staging, and pages behind login, including sites that normally refuse to be embedded.
Connect Responsive Tester to Claude Code, Cursor, Windsurf, and any MCP-compatible client. Your agent can launch any page, test every breakpoint at once, and fix what breaks.
npx -y @responsive-tester/mcp
Available in Responsive Tester Pro
Preview across every breakpoint
Sync and edit across every breakpoint
Add to Chrome to start your 7-day Pro trial, no payment details needed.
Yes. The core preview features are free: synced viewports, scroll sync, device presets, rotate and resize, zoom, and screenshots. Pro adds interaction sync, the breakpoint inspector, live CSS editing, and change tracking with CSS export.
Every install starts a 7-day Pro trial automatically, no card required. When the trial ends, the extension reverts to the free features. No auto-charge, no surprise. Upgrade any time to keep Pro.
One-time. Pay once, use it forever. No subscription, no auto-renew.
Yes. Each license is good for 3 activations, so you can use Pro across your work and personal computers.
Sign in to gumroad.com with the email you bought with to retrieve your key, or email violetsmyster@gmail.com for help.
All payments are processed by Gumroad. Your card and billing details are stored on Gumroad’s servers; Responsive Tester never sees them.
A 30-day money-back guarantee is included. Email violetsmyster@gmail.com within 30 days for a full refund.
For any billing, licensing, or product question, email violetsmyster@gmail.com.