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This is an Advanced Topic that explains the technical architecture of the MCP-B Extension. If you’re new to the extension, start with the Extension Overview.
The MCP-B browser extension serves as a development and testing tool that collects WebMCP servers from browser tabs and provides an interface for AI agents to interact with registered tools.

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MCP-B Extension Components

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Component Breakdown

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Background Service Worker

The background service worker is the central hub that:
  • Aggregates tools from all open tabs
  • Manages connections to content scripts and UI components
  • Routes tool calls to the appropriate tab/context
  • Maintains state across page navigation
  • Bridges to native MCP servers via native messaging

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Content Scripts

Content scripts are injected into web pages to:
  • Establish communication with page MCP servers via Tab Transport
  • Forward tool registrations to the background worker
  • Execute tool calls in the page context
  • Inject userscripts for development/testing
  • Monitor page lifecycle and tool availability

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Sidebar/Popup UI

The extension UI provides:
  • Tool browser - View all available tools across tabs
  • Agent interface - Chat with AI agents using WebMCP tools
  • Debugging tools - Inspect tool calls and responses
  • Settings - Configure extension behavior and permissions
  • Userscript management - Install and manage userscripts

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Native Host Bridge

The native host bridge enables:
  • Local MCP servers - Connect to filesystem, database, and system tools
  • Desktop integration - Access local applications and resources
  • Performance - Run compute-intensive operations locally
  • Privacy - Keep sensitive data on the local machine
See Native Host Setup for configuration instructions.

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Communication Flow

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Tool Discovery

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Tool Execution

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Multi-Tab Tool Aggregation

One of the extension’s key features is aggregating tools from multiple tabs: All tools from all tabs are available to AI agents simultaneously, enabling cross-site workflows.

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Userscript Support

The extension can inject userscripts into web pages to add WebMCP functionality to sites that don’t natively support it:

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Userscript Capabilities

  • Add tools to any website - Expose website functionality as MCP tools
  • DOM manipulation - Interact with page elements
  • API integration - Make authenticated requests using page session
  • Custom workflows - Automate multi-step processes

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Example Userscript

// ==UserScript==
// @name         GitHub WebMCP Tools
// @match        https://github.com/*
// @grant        none
// ==/UserScript==

if (navigator.modelContext) {
  navigator.modelContext.registerTool({
    name: "github_create_issue",
    description: "Create a new GitHub issue in the current repository",
    inputSchema: {
      type: "object",
      properties: {
        title: { type: "string" },
        body: { type: "string" }
      },
      required: ["title"]
    },
    async execute({ title, body }) {
      // Use GitHub's existing UI/API to create issue
      // Implementation details...
      return {
        content: [{ type: "text", text: `Issue created: ${title}` }]
      };
    }
  });
}
See Managing Userscripts for more details.

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Extension Permissions

The MCP-B extension requests minimal permissions:
  • activeTab - Access the current tab’s page content
  • storage - Store user preferences and settings
  • nativeMessaging - Connect to local MCP servers (optional)
  • webRequest (optional) - Debug network requests
The extension follows the principle of least privilege and only requests permissions necessary for its core functionality.

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Development Mode

The extension includes features specifically for developers:
  • Tool inspection - View tool schemas and test executions
  • Console logging - Debug tool calls and responses
  • Hot reload - Automatically refresh when page tools change
  • Error reporting - Detailed error messages for failed tool calls

Extension Guide

Complete extension user guide

Userscript Management

Installing and managing userscripts

Native Host Setup

Configure native MCP server bridge

Transports

Understanding extension transport