DIY Tools MCP Server

A Model Context Protocol (MCP) server that allows you to create custom tools/functions at runtime in any programming language and expose them to Claude or other MCP clients.

Overview

The DIY Tools MCP server enables you to dynamically add custom tools without needing to write a full MCP server. Simply provide the function code, parameters schema, and the server handles the rest - validation, execution, persistence, and MCP protocol integration.

This server bridges the gap between simple function definitions and the MCP protocol, making it easy to extend Claude's capabilities with custom tools written in Python, JavaScript, Bash, Ruby, or TypeScript.

Features

• Dynamic Tool Registration: Add new tools at runtime without restarting the server
• Multi-Language Support: Write functions in Python, JavaScript, Bash, and more
• File-Based Functions: Define functions in separate files for better maintainability
• Automatic Validation: Functions are validated for syntax before registration
• Security Validation: Comprehensive security checks for file-based functions
• Persistence: Registered tools are saved and automatically loaded on server restart
• Type Safety: Full JSON Schema validation for function parameters
• Error Handling: Comprehensive error messages and timeout protection
• Source Code Viewer: Built-in tool to inspect function source code

Installation

# Clone the repository
git clone https://github.com/yourusername/diy-tools-mcp.git
cd diy-tools-mcp

# Install dependencies
npm install

# Build the project
npm run build

Usage

Starting the Server

# Start the server
npm start

# Or for development with auto-reload
npm run dev

Adding Tools

The server provides four built-in tools:

1. add_tool - Register a new custom function
2. remove_tool - Remove a registered function
3. list_tools - List all available custom tools
4. view_source - View the source code of a registered tool

Example: Adding a Python Tool

{
  "name": "calculate_factorial",
  "description": "Calculate the factorial of a number",
  "language": "python",
  "code": "def main(n):\n    if n <= 1:\n        return 1\n    return n * main(n - 1)",
  "parameters": {
    "type": "object",
    "properties": {
      "n": {
        "type": "integer",
        "description": "The number to calculate factorial for",
        "minimum": 0
      }
    },
    "required": ["n"]
  },
  "returns": "The factorial of the input number"
}

Example: Adding a JavaScript Tool

{
  "name": "format_date",
  "description": "Format a date string",
  "language": "javascript",
  "code": "function main({ date, format }) {\n  const d = new Date(date);\n  if (format === 'short') {\n    return d.toLocaleDateString();\n  }\n  return d.toLocaleString();\n}",
  "parameters": {
    "type": "object",
    "properties": {
      "date": {
        "type": "string",
        "description": "Date string to format"
      },
      "format": {
        "type": "string",
        "enum": ["short", "long"],
        "default": "long"
      }
    },
    "required": ["date"]
  }
}

Example: Adding a Bash Tool

{
  "name": "system_info",
  "description": "Get basic system information",
  "language": "bash",
  "code": "main() {\n  echo '{\"os\": \"'$(uname -s)'\", \"kernel\": \"'$(uname -r)'\", \"arch\": \"'$(uname -m)'\"}'\n}",
  "parameters": {
    "type": "object",
    "properties": {}
  }
}

File-Based Functions

You can now define functions in separate files instead of inline code. This is useful for:

• Complex functions that are easier to maintain in dedicated files
• Functions you want to version control separately
• Reusing existing code without modification

Example: Adding a Python Function from File

1. Create your function file my_function.py:

from datetime import datetime

def main(name, age):
    """
    Generate a personalized greeting.
    """
    return {
        "greeting": f"Hello {name}!",
        "message": f"You are {age} years old.",
        "timestamp": datetime.now().isoformat()
    }

2. Register the function:

{
  "name": "personalized_greeting",
  "description": "Generate a personalized greeting with timestamp",
  "language": "python",
  "codePath": "./my_function.py",
  "parameters": {
    "type": "object",
    "properties": {
      "name": {
        "type": "string",
        "description": "Person's name"
      },
      "age": {
        "type": "integer",
        "description": "Person's age"
      }
    },
    "required": ["name", "age"]
  }
}

Example: Adding a JavaScript Function from File

1. Create your function file data_processor.js:

async function main({ data, format }) {
  // Process data based on format
  if (format === 'csv') {
    return processCSV(data);
  } else if (format === 'json') {
    return processJSON(data);
  }
  throw new Error(`Unsupported format: ${format}`);
}

function processCSV(data) {
  // CSV processing logic
  return { processed: true, format: 'csv', rows: data.split('\n').length };
}

function processJSON(data) {
  // JSON processing logic
  const parsed = JSON.parse(data);
  return { processed: true, format: 'json', keys: Object.keys(parsed) };
}

module.exports = { main };

2. Register the function:

{
  "name": "data_processor",
  "description": "Process data in various formats",
  "language": "javascript",
  "codePath": "./data_processor.js",
  "parameters": {
    "type": "object",
    "properties": {
      "data": {
        "type": "string",
        "description": "Raw data to process"
      },
      "format": {
        "type": "string",
        "enum": ["csv", "json"],
        "description": "Data format"
      }
    },
    "required": ["data", "format"]
  }
}

Configurable Entry Points (New in v1.2.0)

You can now specify any function name as the entry point, not just main. This allows you to:

• Use existing code without renaming functions
• Share a single file between multiple tools with different entry points
• Better organize related functions

Example: Multiple Tools from One File

1. Create a file with multiple functions math_utils.py:

def calculate_tax(income, tax_rate):
    """Calculate tax amount."""
    return {
        "tax_amount": income * tax_rate,
        "net_income": income * (1 - tax_rate)
    }

def compound_interest(principal, rate, time):
    """Calculate compound interest."""
    amount = principal * (1 + rate) ** time
    return {
        "principal": principal,
        "interest": amount - principal,
        "total": amount
    }

2. Register multiple tools using different entry points:

// Tax calculator tool
{
  "name": "tax_calculator",
  "description": "Calculate tax and net income",
  "language": "python",
  "codePath": "./math_utils.py",
  "entryPoint": "calculate_tax",  // Specify which function to use
  "parameters": {
    "type": "object",
    "properties": {
      "income": { "type": "number" },
      "tax_rate": { "type": "number" }
    },
    "required": ["income", "tax_rate"]
  }
}

// Interest calculator tool
{
  "name": "interest_calculator",
  "description": "Calculate compound interest",
  "language": "python",
  "codePath": "./math_utils.py",
  "entryPoint": "compound_interest",  // Different function from same file
  "parameters": {
    "type": "object",
    "properties": {
      "principal": { "type": "number" },
      "rate": { "type": "number" },
      "time": { "type": "number" }
    },
    "required": ["principal", "rate", "time"]
  }
}

If no entryPoint is specified, the system defaults to looking for a function named main for backward compatibility.

Viewing Function Source Code

Use the view_source tool to inspect any registered function:

{
  "name": "view_source",
  "arguments": {
    "name": "data_processor",
    "verbose": true
  }
}

The verbose option includes full metadata about the tool in addition to the source code.

Supported Languages

• Python (python) - Requires Python 3.x
• JavaScript (javascript or node) - Requires Node.js
• Bash (bash) - Requires Bash shell
• TypeScript (typescript) - Transpiled and run as JavaScript
• Ruby (ruby) - Requires Ruby

Function Requirements

Python Functions

• Must define a main function that accepts keyword arguments
• Should return JSON-serializable data
• Example:

  def main(x, y):
      return x + y
  

JavaScript Functions

• Must define a main function (regular or async)
• Receives parameters as a single object
• Example:

  function main({ x, y }) {
    return x + y;
  }
  

Bash Functions

• Must define a main function
• Receives JSON arguments as first parameter
• Should output JSON to stdout
• Example:

  main() {
    # Parse JSON args if needed
    echo '{"result": "success"}'
  }
  

Ruby Functions

• Must define a main method that accepts keyword arguments
• Should return JSON-serializable data
• Example:

  def main(name:, age:)
    { greeting: "Hello #{name}, you are #{age} years old!" }
  end
  

Configuration

Timeout Settings

You can specify a timeout for each function (in milliseconds):

{
  "timeout": 5000 // 5 seconds
}

Maximum timeout is 300000ms (5 minutes).

Dependencies

For Python functions, you can specify required packages:

{
  "dependencies": ["numpy", "pandas"]
}

Note: Automatic dependency installation is not yet implemented.

Error Handling

The server provides detailed error messages for:

• Syntax errors in function code
• Invalid parameter schemas
• Runtime execution errors
• Timeout violations
• Missing dependencies

Storage

Functions are stored in the functions/ directory as JSON files. Each file contains:

• Function specification
• Unique ID
• Creation and update timestamps

Development

# Run in development mode with auto-reload
npm run dev

# Run tests
npm test

# Build for production
npm run build

# Clean build artifacts
npm run clean

Security Considerations

File-Based Functions Security

When using file-based functions, the server implements multiple security layers:

1. Path Traversal Protection: Prevents access to files outside the intended directories
2. Symbolic Link Detection: Blocks symbolic links to prevent unauthorized file access
3. System Directory Protection: Restricts access to critical system directories including:
   • /etc, /usr/bin, /System (macOS), C:\Windows
   • User-specific sensitive directories (~/.ssh, ~/.aws, etc.)
4. File Size Limits: Files are limited to 10MB to prevent resource exhaustion
5. Dangerous Pattern Detection: Scans for potentially malicious code patterns:
   • eval() and exec() calls
   • Dynamic imports and requires
   • Dangerous shell commands (rm -rf /, etc.)
   • Subprocess calls with shell=True
6. File Extension Validation: Only allows appropriate extensions for each language
7. Main Function Requirement: Enforces that all functions have a proper main entry point

General Security Notes

• Functions run with the same