Tool Builder
MCP server that builds itself by creating new tools as needed based on user requests (Requires restart of Claude Desktop to use newly created tools).
What is Tool Builder?
Tool Builder is a Model Context Protocol (MCP) server that allows AI assistants like Claude, Cursor, and VS Code to mcp server that builds itself by creating new tools as needed based on user requests (requires restart of claude desktop to use newly created tools).
MCP server that builds itself by creating new tools as needed based on user requests (Requires restart of Claude Desktop to use newly created tools).
This server falls under the Developer Tools category on MCPgee, the world's largest MCP server directory with 33,000+ servers.
Features
- MCP server that builds itself by creating new tools as neede
Use Cases
Maintainer
Works with
Installation
Manual Installation
npx mcp-tool-builderConfiguration
Configuration Details
claude_desktop_config.json
Performance
Response Metrics
Resource Usage
How to Set Up and Use Tool Builder
MCP Tool Builder is a self-extending MCP server that can create new MCP tools on demand based on natural language descriptions from the user. When you ask it to build a tool, it generates a Python scriptlet, saves it to a tools directory, and registers it in tools.json — extending its own capabilities at runtime. The server ships with built-in tools for Bitcoin price lookups and US ZIP code weather forecasts, and every new tool persists across restarts. This is useful for rapidly prototyping custom integrations without writing server boilerplate.
Prerequisites
- Python 3.10+ and the uv package manager (https://astral.sh/uv)
- Git to clone the repository
- An MCP-compatible client such as Claude Desktop
- Note: Claude Desktop must be restarted after each new tool is created for the tool to become available
Clone the repository
MCP Tool Builder is not published to PyPI or npm; you must clone the source repository and run it locally.
git clone https://github.com/hanweg/mcp-tool-builder
cd mcp-tool-builderCreate a virtual environment and install dependencies
Use uv to create an isolated environment and install the project in editable mode so changes to generated tools are picked up on next restart.
uv venv
# Windows
.venv\Scripts\activate
# macOS/Linux
source .venv/bin/activate
uv pip install -e .Add the server to Claude Desktop configuration
Open claude_desktop_config.json and add the tool-builder entry. Replace PATH_TO with the absolute path to the cloned repository on your machine.
{
"mcpServers": {
"tool-builder": {
"command": "uv",
"args": ["--directory", "/absolute/path/to/mcp-tool-builder", "run", "tool-builder"]
}
}
}Restart Claude Desktop and verify built-in tools
Fully restart Claude Desktop. You should see the built-in get_bitcoin_price and get_weather_forecast tools available. Test them to confirm the server is running correctly.
Create a new tool via natural language
Ask Claude to create a new tool by describing what it should do. The server will generate a Python scriptlet, save it to the tools directory, and update tools.json. You must restart Claude Desktop for the new tool to become available.
Tool Builder Examples
Client configuration
claude_desktop_config.json entry using uv to run the local mcp-tool-builder project. Update the directory path to match your system.
{
"mcpServers": {
"tool-builder": {
"command": "uv",
"args": ["--directory", "/Users/yourname/mcp-tool-builder", "run", "tool-builder"]
}
}
}Prompts to try
Prompts for using the built-in tools and for asking the server to build new ones.
- "What is the current Bitcoin price?"
- "What is the weather forecast for ZIP code 90210?"
- "Create a tool that fetches the current gold price from a public API"
- "Build a tool that converts Celsius to Fahrenheit"
- "Create a tool that looks up the timezone for a given city name"Troubleshooting Tool Builder
Newly created tools do not appear in Claude Desktop
This is expected behavior. After the server creates a new tool and updates tools.json, you must fully quit and relaunch Claude Desktop for the new tool to be registered and appear in the tool list.
uv command not found during setup
Install uv with `curl -LsSf https://astral.sh/uv/install.sh | sh` (macOS/Linux) or via the Windows PowerShell installer at https://astral.sh/uv. Restart your terminal after installation.
Server fails to start with import errors
Ensure you activated the virtual environment (`.venv/Scripts/activate` on Windows, `source .venv/bin/activate` on macOS/Linux) and ran `uv pip install -e .` inside the repository. Verify Python 3.10+ with `python --version`.
Frequently Asked Questions about Tool Builder
What is Tool Builder?
Tool Builder is a Model Context Protocol (MCP) server that mcp server that builds itself by creating new tools as needed based on user requests (requires restart of claude desktop to use newly created tools). It connects AI assistants to external tools and data sources through a standardized interface.
How do I install Tool Builder?
Follow the installation instructions on the Tool Builder GitHub repository. Clone the repo, install dependencies, and add the server config to your AI client.
Which AI clients work with Tool Builder?
Tool Builder works with all major MCP-compatible AI clients including Claude Desktop, Claude Code, Cursor, VS Code (GitHub Copilot), Windsurf, and Cline.
Is Tool Builder free to use?
Yes, Tool Builder is open source and available under the The Unlicense license. You can use it freely in both personal and commercial projects.
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