Firefox MCP Server

Instructions

Implementation Reference

• index-multi-debug.js:755-779 (handler)

  Main handler function that retrieves captured WebSocket messages from the browser page's window._wsMessages array (populated by injected monitoring script), applies optional filters for 'since' timestamp and 'limit', and returns formatted JSON output.

  async getWebSocketMessages(args = {}) {
    const { tabId, since, limit = 50 } = args;
    const effectiveTabId = tabId || this.activeTabId;
    const page = this.getPage(effectiveTabId);
    
    // Get WebSocket messages from injected monitoring
    const wsMessages = await page.evaluate(() => {
      return window._wsMessages || [];
    });
    
    let filteredMessages = wsMessages;
    
    if (since) {
      filteredMessages = filteredMessages.filter(msg => msg.timestamp >= since);
    }
    
    filteredMessages = filteredMessages.slice(-limit);
  
    return {
      content: [{
        type: 'text',
        text: `WebSocket Messages (${filteredMessages.length}):\n` + JSON.stringify(filteredMessages, null, 2)
      }]
    };
  }

• index-multi-debug.js:313-324 (registration)

  Tool registration entry in the MCP tools list, including name, description, and input schema definition.

  {
    name: 'get_websocket_messages',
    description: 'Get captured WebSocket messages (for LiveView debugging)',
    inputSchema: {
      type: 'object',
      properties: {
        tabId: { type: 'string' },
        since: { type: 'number' },
        limit: { type: 'number', default: 50 }
      }
    }
  },

• index-multi-debug.js:445-446 (registration)

  Dispatcher switch case that routes calls to the getWebSocketMessages handler.

  case 'get_websocket_messages':
    return await this.getWebSocketMessages(args);

• index-multi-debug.js:316-322 (schema)

  Input schema defining parameters: tabId (string), since (number, optional timestamp filter), limit (number, default 50).

  inputSchema: {
    type: 'object',
    properties: {
      tabId: { type: 'string' },
      since: { type: 'number' },
      limit: { type: 'number', default: 50 }
    }

• index-multi-debug.js:543-587 (helper)

  Supporting helper function that injects JavaScript into the page to override the WebSocket constructor, intercepting and storing sent/received messages in window._wsMessages array, which the handler reads.

  async injectWebSocketMonitoring(page, tabId) {
    await page.addInitScript(() => {
      // Store original WebSocket
      const OriginalWebSocket = window.WebSocket;
      
      // Array to store WebSocket messages
      window._wsMessages = window._wsMessages || [];
      
      // Override WebSocket constructor
      window.WebSocket = function(...args) {
        const ws = new OriginalWebSocket(...args);
        
        // Monitor incoming messages
        ws.addEventListener('message', (event) => {
          window._wsMessages.push({
            type: 'received',
            data: event.data,
            timestamp: Date.now(),
            url: ws.url
          });
        });
        
        // Monitor outgoing messages
        const originalSend = ws.send;
        ws.send = function(data) {
          window._wsMessages.push({
            type: 'sent',
            data: data,
            timestamp: Date.now(),
            url: ws.url
          });
          return originalSend.call(this, data);
        };
        
        return ws;
      };
      
      // Copy static properties
      Object.setPrototypeOf(window.WebSocket, OriginalWebSocket);
      Object.defineProperty(window.WebSocket, 'prototype', {
        value: OriginalWebSocket.prototype,
        writable: false
      });
    });
  }