BrowserCat MCP Server

Instructions

Implementation Reference

• index.ts:304-348 (handler)

  Handler function for 'browsercat_evaluate' tool. Evaluates the provided JavaScript script in the browser page, temporarily overrides console methods to capture logs, restores console, and returns the result with captured console output.

  case "browsercat_evaluate":
    try {
      // Set up console log capture
      await page.evaluate(() => {
        window.mcpHelper = {
          logs: [],
          originalConsole: { ...console },
        };
  
        ['log', 'info', 'warn', 'error'].forEach(method => {
          (console as any)[method] = (...args: any[]) => {
            window.mcpHelper.logs.push(`[${method}] ${args.join(' ')}`);
            (window.mcpHelper.originalConsole as any)[method](...args);
          };
        });
      });
  
      const result = await page.evaluate(args.script);
  
      // Restore original console and get captured logs
      const logs = await page.evaluate(() => {
        Object.assign(console, window.mcpHelper.originalConsole);
        const logs = window.mcpHelper.logs;
        delete (window as any).mcpHelper;
        return logs;
      });
  
      return {
        content: [
          {
            type: "text",
            text: `Execution result:\n${JSON.stringify(result, null, 2)}\n\nConsole output:\n${logs.join('\n')}`,
          },
        ],
        isError: false,
      };
    } catch (error) {
      return {
        content: [{
          type: "text",
          text: `Script execution failed: ${(error as Error).message}`,
        }],
        isError: true,
      };
    }

• index.ts:100-110 (schema)

  Tool definition including name, description, and input schema for 'browsercat_evaluate'.

  {
    name: "browsercat_evaluate",
    description: "Execute JavaScript in the browser console",
    inputSchema: {
      type: "object",
      properties: {
        script: { type: "string", description: "JavaScript code to execute" },
      },
      required: ["script"],
    },
  },

• index.ts:421-423 (registration)

  MCP server request handler for listing tools, which includes 'browsercat_evaluate' from the TOOLS array.

  server.setRequestHandler(ListToolsRequestSchema, async () => ({
    tools: TOOLS,
  }));

• index.ts:425-427 (registration)

  MCP server request handler for calling tools, which dispatches to handleToolCall based on the tool name 'browsercat_evaluate'.

  server.setRequestHandler(CallToolRequestSchema, async (request) =>
    handleToolCall(request.params.name, request.params.arguments ?? {})
  );