bun-runner-mcp

An MCP (Model Context Protocol) server that executes TypeScript/JavaScript code in a sandboxed Bun environment with permission-based security controls.

Features

• Sandboxed Execution: Run TypeScript/JavaScript code in an isolated environment

• Permission System: Fine-grained control over HTTP requests, file access, and environment variables

• Code Snippets: Save and reuse code snippets across sessions with dependency resolution

• Web Management UI: Browser-based interface for managing environment variables and viewing snippets

• Two Execution Modes:

  • Preload (default): Uses Bun's preload feature for runtime sandboxing

  • Container: Uses Apple Containers for VM-level isolation (macOS 26+)

• HTTP Proxy: All network requests are routed through a permission-checking proxy

Related MCP server: MCP Server Playground

Quick Start

Installation

git clone https://github.com/timoconnellaus/bun-runner-mcp.git
cd bun-runner-mcp
bun install

Claude Desktop Configuration

Add to ~/Library/Application Support/Claude/claude_desktop_config.json:

Standard Mode (Preload Sandbox):

{
  "mcpServers": {
    "bun-runner": {
      "command": "bun",
      "args": ["run", "/path/to/bun-runner-mcp/src/mcp/server.ts"]
    }
  }
}

Container Mode (Apple Containers - Recommended for untrusted code):

{
  "mcpServers": {
    "bun-runner": {
      "command": "bun",
      "args": ["run", "/path/to/bun-runner-mcp/src/mcp/server.ts"],
      "env": {
        "EXECUTION_MODE": "container"
      }
    }
  }
}

Note: Container mode requires macOS 26 (Tahoe) or later with Apple Containers installed.

Running Manually

# Standard mode
bun run start

# Container mode
EXECUTION_MODE=container bun run start

# Development with watch
bun run dev

MCP Tools

run_code

Execute TypeScript/JavaScript code in the sandbox.

{
  "code": "console.log('Hello, world!')",
  "timeout": 30000
}

grant_permission

Grant a permission for the current session.

HTTP Permission:

{
  "permission": {
    "type": "http",
    "host": "api.example.com",
    "description": "Access example API"
  }
}

File Permission:

{
  "permission": {
    "type": "file",
    "path": "/tmp/data/*",
    "operations": ["read", "write"],
    "description": "Access temp files"
  }
}

Environment Variable Permission:

{
  "permission": {
    "type": "env",
    "variables": ["API_KEY", "SECRET_*"],
    "description": "Access API keys"
  }
}

list_permissions

List all currently granted permissions.

revoke_permission

Revoke a previously granted permission.

save_snippet

Save a reusable code snippet. Snippets must include a JSDoc @description tag.

{
  "name": "fetch-json",
  "code": "/** @description Fetches JSON from a URL */\nexport async function fetchJson(url: string) {\n  const res = await fetch(url);\n  return res.json();\n}"
}

list_snippets

List all saved snippets with their names and descriptions.

get_snippet

Get the full code and metadata for a saved snippet.

delete_snippet

Delete a saved snippet.

list_env_vars

List available environment variable names (values are hidden for security).

get_web_ui_url

Get the URL for the web management interface. The AI can use this to direct users to the browser UI.

Code Snippets

Snippets are reusable code blocks that persist across sessions. They're stored in ~/.bun-runner-mcp/snippets/.

Using Snippets in Code

Reference snippets in your code using the @use-snippet directive:

// @use-snippet: fetch-json
// @use-snippet: format-date

const data = await fetchJson('https://api.example.com/data');
console.log(formatDate(data.timestamp));

The snippet code is automatically inlined before execution. Snippets can depend on other snippets, and circular dependencies are detected.

Snippet Requirements

• Must include a JSDoc comment with @description tag

• Name must be alphanumeric with hyphens/underscores

• Should export functions for reuse

Environment Variables

Environment variables can be configured for use in executed code:

Configuration Sources

1. MCP Config: Pass variables with BUN_ prefix in your MCP config:

   {
     "env": {
       "BUN_API_KEY": "your-api-key",
       "BUN_DEBUG": "true"
     }
   }

   The BUN_ prefix is stripped when accessed in code (e.g., process.env.API_KEY).

2. Env File: Create ~/.bun-runner-mcp/.bun-runner-env:

   API_KEY=your-api-key
   DATABASE_URL=postgres://localhost/db

File variables take precedence over MCP config variables.

Hot Reload

The env file is watched for changes. When modified, variables are automatically reloaded (and containers restarted if in container mode).

Web Management UI

A browser-based interface is available at http://localhost:9999 for:

• Environment Variables: Add, edit, and delete environment variables

• Code Snippets: View saved snippets and their code

The web UI is built automatically when the server starts using Bun's native bundler.

Execution Modes

Preload Mode (Default)

Uses Bun's preload feature to intercept and sandbox network requests. All HTTP requests are routed through a local proxy server (port 9999) that enforces permissions.

Container Mode (Apple Containers)

For stronger isolation, use Apple Containers which provides VM-level isolation. This is the recommended mode for untrusted code execution.

Requirements

• macOS 26 (Tahoe) or later

• Apple Containers CLI (container command) installed

• Internet connection for initial image pull

How It Works

1. Lazy Initialization: The container is created on first code execution, not at startup

2. Session Persistence: The same container is reused for all executions within a session

3. Auto-Cleanup: Container is automatically stopped when the MCP server exits

4. Image Management: Uses oven/bun:alpine from Docker Hub, automatically pulled on first use

Container Specifications

Features

• VM-Level Isolation: Code runs in a fully isolated virtual machine

• Isolated Filesystem: No access to host filesystem

• Network Isolation: Network access is controlled by the container runtime

• Package Support: npm packages are automatically installed via Bun

• TypeScript Support: Full TypeScript execution with type checking via tsserver

Verifying Container CLI

Check if Apple Containers is available:

container --version

List available images:

container image list

Troubleshooting

Container CLI not found:

• Ensure you're running macOS 26 (Tahoe) or later

• The container CLI should be available at /usr/bin/container

Image pull fails:

• Check your internet connection

• Verify Docker Hub is accessible

• Try manually: container image pull docker.io/oven/bun:alpine

Container won't start:

• Check system resources (memory, disk space)

• Look for error messages in stderr output

• Ensure no conflicting containers are running

Architecture

┌─────────────────┐     ┌──────────────────┐
│   MCP Client    │────▶│   MCP Server     │
│  (Claude, etc)  │     │  (stdio transport)│
└─────────────────┘     └────────┬─────────┘
                                 │
                    ┌────────────┴────────────┐
                    │                         │
              ┌─────▼─────┐           ┌───────▼───────┐
              │  Preload  │           │   Container   │
              │  Sandbox  │           │    (Apple)    │
              └─────┬─────┘           └───────────────┘
                    │
              ┌─────▼─────┐
              │HTTP Proxy │
              │ (port 9999)│
              └───────────┘

License

MIT