MCP Builder

Instructions

Implementation Reference

• mcp_builder/server.py:265-357 (handler)

  The @mcp.tool()-decorated handler function that implements the 'install_local_mcp_server' tool. It installs a local MCP server by checking if it's a Node.js or Python project, determining the entry point, and adding the configuration to Claude Desktop's config file using the helper install_to_claude_desktop.
  @mcp.tool() def install_local_mcp_server( path: str, args: Optional[List[str]] = None, env: Optional[List[str]] = None, ) -> str: """ Install an MCP server from a local directory. Args: path: The path to the MCP server code cloned on your computer args: The arguments to pass along env: The environment variables to set, delimited by = """ if args is None: args = [] if not os.path.exists(path): return f"Path '{path}' does not exist." # Check if it's a Node.js or Python project has_package_json = os.path.exists(os.path.join(path, "package.json")) has_pyproject_toml = os.path.exists(os.path.join(path, "pyproject.toml")) has_setup_py = os.path.exists(os.path.join(path, "setup.py")) # Determine server name from directory name server_name = os.path.basename(path) # Check if Node.js is installed (for npm packages) has_node = check_command_exists("node") has_npm = check_command_exists("npm") has_pip = check_command_exists("pip") has_python = check_command_exists("python") # Handle Node.js projects if has_package_json and has_node and has_npm: # For Node.js projects, use node directly to run the local script # Find the main entry point from package.json try: with open(os.path.join(path, "package.json"), "r") as f: package_json = json.load(f) # Try to find the main entry point main_file = package_json.get("main", "index.js") # Install to Claude Desktop install_to_claude_desktop( server_name, "node", [os.path.join(path, main_file)] + args, env, ) return f"Successfully installed local Node.js MCP server '{server_name}'! Please tell the user to restart the application." except Exception as e: return f"Error installing Node.js MCP server: {str(e)}" # Handle Python projects elif (has_pyproject_toml or has_setup_py) and has_python: # For Python projects, use python -m to run the module # Try to determine the module name from directory structure module_name = server_name.replace("-", "_") # Check if there's a directory with the same name if os.path.isdir(os.path.join(path, module_name)): # Install to Claude Desktop install_to_claude_desktop( server_name, "python", ["-m", module_name] + args, env, cwd=path, ) return f"Successfully installed local Python MCP server '{server_name}'! Please tell the user to restart the application." else: # Try to find any Python files in the root directory py_files = [f for f in os.listdir(path) if f.endswith(".py") and f != "setup.py"] if py_files: main_file = py_files[0].replace(".py", "") # Install to Claude Desktop install_to_claude_desktop( server_name, "python", [os.path.join(path, main_file + ".py")] + args, env, ) return f"Successfully installed local Python MCP server '{server_name}'! Please tell the user to restart the application." return f"Could not determine how to install MCP server from '{path}'. Make sure it's a valid Node.js or Python project."
• mcp_builder/server.py:106-169 (helper)

  Core helper function install_to_claude_desktop used by install_local_mcp_server to write the MCP server configuration (command, args, env, cwd) to the Claude Desktop JSON config file under mcpServers.
  def install_to_claude_desktop( server_name: str, command: str, args: List[str], env: Optional[List[str]] = None, cwd: Optional[str] = None, ) -> None: """ Install an MCP server to Claude Desktop. Args: server_name: The name of the MCP server command: The command to run the MCP server args: The arguments to pass to the command env: The environment variables to set, delimited by = cwd: The working directory for the command """ # Normalize server name to be a valid identifier # For npm packages, make sure we use a simple name without @ or / if server_name.startswith("@"): # For @scope/package, use just "package" as the server name if "/" in server_name: server_name = server_name.split("/")[1] # Remove any invalid characters from server name server_name = re.sub(r'[^a-zA-Z0-9_-]', '-', server_name) # Get the Claude Desktop config file path config_path = get_claude_desktop_config_path() if not config_path: raise ValueError("Could not find Claude Desktop config file") # Read the existing config try: with open(config_path, "r", encoding="utf-8") as f: config = json.load(f) except (FileNotFoundError, json.JSONDecodeError): config = {} # Initialize mcpServers if it doesn't exist if "mcpServers" not in config: config["mcpServers"] = {} # Prepare the server config server_config = { "command": command, "args": args, } # Add environment variables if provided if env and len(env) > 0: server_config["env"] = parse_env_vars(env) # Add working directory if provided if cwd: server_config["cwd"] = cwd # Add the server to the config config["mcpServers"][server_name] = server_config # Write the config back to the file with open(config_path, "w", encoding="utf-8") as f: json.dump(config, f, indent=2)
• mcp_builder/server.py:69-71 (helper)

  Helper function check_command_exists to verify if required commands like node, npm, pip are available.
  def check_command_exists(command: str) -> bool: """Check if a command exists in the system PATH.""" return shutil.which(command) is not None
• mcp_builder/server.py:56-67 (helper)

  Helper function parse_env_vars to parse list of KEY=VALUE strings into a dict for env config.
  def parse_env_vars(env_vars: Optional[List[str]]) -> Optional[Dict[str, str]]: """Parse environment variables from a list of KEY=VALUE strings.""" if not env_vars: return None env_obj = {} for env_var in env_vars: if "=" in env_var: key, value = env_var.split("=", 1) env_obj[key] = value return env_obj if env_obj else None
• mcp_builder/server.py:265-265 (registration)

  The @mcp.tool() decorator registers the install_local_mcp_server function as an MCP tool.
  @mcp.tool()