Responsive Tester MCP Server

Drive Responsive Tester from your terminal AI. This package is a local bridge: your AI client (Claude Code, Cursor, Continue, or any MCP client) speaks MCP over stdio to this process, and the Responsive Tester Chrome extension connects to it over a localhost WebSocket (port 8732). No browser data leaves your machine.

Requires a Responsive Tester Pro license, Node.js 18+, and an MCP client.

Setup

Claude Code:

claude mcp add responsive-tester -- npx -y @responsive-tester/mcp

Cursor, Windsurf, Cline, Continue (mcp.json):

{
  "mcpServers": {
    "responsive-tester": {
      "command": "npx",
      "args": ["-y", "@responsive-tester/mcp"]
    }
  }
}

Then connect from the extension: open Responsive Tester on any page and click the MCP pill in the bottom-left corner, then Connect. The icon fills in when the bridge is live.

What the agent can do

• launch a URL in a new tab and activate the multi-viewport shell with no user action
• Read the same element in every viewport at once with compare_across_viewports
• Get a width-segmented audit of what an element actually does at each breakpoint with get_element_audit
• Inspect the full cascade with get_matched_rules, then edit it with apply_styles and add_rule (edits land in the user's Changes panel and are undoable)
• Manage viewports: add, remove, rotate, per-frame light/dark scheme, site breakpoints mode
• screenshot_viewport for visual checks

Full documentation: https://www.responsive-tester.app/docs/mcp.html