Browser JavaScript Evaluator

Name: Browser JavaScript Evaluator
Author: matdmiller

@matdmiller

About Browser JavaScript Evaluator

This is a reference design for a MCP server that hosts a web page that connects back to the server via sse and allows Claude to execute javascript on the page.

Basic information

Config

Add this server to your MCP-compatible client using the configuration below.

{
  "mcpServers": {
    "mcp_py_sse_browser_control": {
      "command": "python",
      "args": [
        "web_server.py"
      ]
    }
  }
}

Tools

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We auto-extract tools from the README. The maintainer can list them under a ## Tools heading to populate this section.

Overview

What is Browser JavaScript Evaluator?

Browser JavaScript Evaluator is an MCP (Model Control Protocol) server that lets you execute arbitrary JavaScript code in a browser from Python. It uses a web server with SSE (Server-Sent Events) and a REST API to relay code to a connected browser and return results.

How to use Browser JavaScript Evaluator?

Add the server to your Claude Desktop configuration using uv pointing to the repository directory and the browser_server.py script. After launching Claude, open a browser to http://127.0.0.1:8000 to connect. The server provides two tools: execute_javascript (run any JavaScript) and add_numbers (add two numbers via browser execution).

Key features of Browser JavaScript Evaluator

Executes arbitrary JavaScript in a connected browser
Provides two MCP tools: execute_javascript and add_numbers
Uses SSE for real-time communication with the browser
Returns results synchronously via REST API
Web server and MCP server are split into separate components

Use cases of Browser JavaScript Evaluator

Perform browser‑based calculations from an MCP client (e.g., add_numbers)
Run custom JavaScript to manipulate or inspect a live browser page
Test and debug JavaScript code in a real browser environment
Automate browser interactions within an AI‑assisted workflow

FAQ from Browser JavaScript Evaluator

What does Browser JavaScript Evaluator do?

It allows you to execute JavaScript code in a browser from Python via MCP. A web server sends code to a connected browser using SSE, and the browser returns results.

How do I install and run it?

Add the server to your Claude config with the uv command pointing to the repository, then launch Claude. Open a browser at http://127.0.0.1:8000 to connect. Alternatively, run python web_server.py to start only the web server.

What MCP tools are available?

Two tools: execute_javascript (runs any JavaScript) and add_numbers (adds two numbers using JavaScript).

Is there a security warning?

Yes. The server can execute arbitrary JavaScript in your browser, which is potentially dangerous. Use at your own risk and be aware of the implications.

What are the dependencies or runtime requirements?

The server runs with Python and uses the uv package manager. A browser must be open and connected to the web server (default at port 8000) for code execution to work.