@runno/mcp

Overview

@runno/mcp is a Model Context Protocol (MCP) server that provides a secure code execution environment for AI assistants. It enables models to execute code in various programming languages inside a sandboxed environment using WebAssembly, offering a safe way to run code snippets during AI interactions.

Features

• Secure Sandboxed Environment: Runs code in isolation without access to the filesystem, network, or system resources
• Multiple Language Support: Execute code in Python, JavaScript (QuickJS), C, C++, Ruby, and PHP
• MCP Compliant: Implements the Model Context Protocol for seamless integration with AI assistants
• Simple CLI Tool: Easy to run via npx without complex setup
• Standard I/O Communication: Uses stdio for communication with the client

Examples

In this example I gave Claude a leetcode problem. It solved it by writing Python: Leetcode problem.

In this example I told Claude to efficiently calculate 5000 primes. It solved it by writing Python. Then when I told it to use C, it rewrote the solution in C (and ran it): 5000 primes.

In this example I asked Claude to tell me the p75 of a short list of numbers. It solved it by first writing Python with Numpy (which didn't work) but then re-orienting and writing just using the standard library: p75 calculation.

How It Works

The @runno/mcp server provides a run_code tool that AI assistants can use to execute code snippets in various programming languages. It leverages the @runno/sandbox package, which uses WebAssembly to create isolated environments for code execution. The result of the code execution is provided over STDIO.

The @runno/sandbox uses precompiled WebAssembly binaries for multiple programming languages. It then executes those binaries against a virtual file system using the @runno/wasi implementation of WASI preview1.

You can read more about how the Runno sandbox works in my article I made a Python package for sandboxing code. This is for an older version of the sandbox, but it works basically the same way.

Supported Runtimes

| Runtime | Programming Language | | ------- | -------------------- | | python | Python | | quickjs | JavaScript | | clang | C | | clangpp | C++ | | ruby | Ruby | | php-cgi | PHP |

Tool Usage

The run_code tool accepts the following parameters:

• runtime: The runtime environment to use (one of the supported runtimes)
• code: The code string to execute

The tool returns:

• Execution status (completed, crashed, terminated, or timed out)
• Exit code (for completed executions)
• TTY output (combined stdout and stderr)
• Error details (for crashed executions)

Limitations

Code executed within the sandbox:

• Cannot access the filesystem
• Cannot make network connections
• Cannot execute system commands
• Cannot import packages from package managers
• Cannot execute external programs
• Is limited to core programming language capabilities

Running the Server

Using npx

The simplest way to run the Runno MCP server is with npx:

npx @runno/mcp

This will start the server on stdio, making it immediately available for integration with MCP-compatible clients.

Integration with MCP Clients

To use the Runno MCP server with an MCP client, you simply need to configure it to run npx @runno/mcp.

For Claude you can edit the MCP Config:

{
  "mcpServers": {
    "runno": {
      "command": "/usr/local/bin/npx",
      "args": ["@runno/mcp"]
    }
  }
}

Note: To get the right path for npx run which npx.

Security Considerations

• The Runno MCP server provides strong isolation of executed code through WebAssembly
• However, resource consumption (CPU, memory) should still be monitored
• For production use, consider implementing additional controls such as timeouts and resource limits