MCP Builder

Instructions

Implementation Reference

• mcp_builder/server.py:171-263 (handler)

  The handler function for the 'install_repo_mcp_server' tool, decorated with @mcp.tool() for registration. It detects if the package is npm or pip, checks prerequisites, and installs by configuring Claude Desktop's mcpServers JSON.
  @mcp.tool() def install_repo_mcp_server( name: str, args: Optional[List[str]] = None, env: Optional[List[str]] = None, ) -> str: """ Install an MCP server via pip or npm. Args: name: The package name of the MCP server args: The arguments to pass along env: The environment variables to set, delimited by = """ if args is None: args = [] # Check if Node.js is installed (for npm packages) has_node = check_command_exists("node") has_npm = check_command_exists("npm") has_npx = check_command_exists("npx") has_pip = check_command_exists("pip") has_python = check_command_exists("python") if not has_node and not has_python: return "Neither Node.js nor Python is installed. Please install one of them." # Determine if this is likely an npm package (starts with @ or doesn't have dots) is_likely_npm_package = name.startswith('@') or '.' not in name # Handle npm packages if has_npm and has_npx and is_likely_npm_package: # Extract server name from package name (handle scoped packages) if name.startswith("@") and "/" in name: # For @scope/package, use just "package" as the server name server_name = name.split("/")[1] else: server_name = name # Install to Claude Desktop using npx directly install_to_claude_desktop( server_name, "npx", [name] + args, env, ) return f"Successfully installed MCP server '{server_name}' via npx! Please tell the user to restart the application." # Handle Python packages (likely has dots in the name) if has_pip and has_python and '.' in name: # For Python packages, use the package name directly server_name = name # Install to Claude Desktop install_to_claude_desktop( server_name, "python", ["-m", name] + args, env, ) return f"Successfully installed MCP server '{server_name}' via Python! Please tell the user to restart the application." # If we can't determine the type, try npm first if available, then Python if has_npm and has_npx: # Use the last part of the name as the server name if "/" in name: server_name = name.split("/")[-1] else: server_name = name install_to_claude_desktop( server_name, "npx", [name] + args, env, ) return f"Successfully installed MCP server '{server_name}' via npx! Please tell the user to restart the application." elif has_pip and has_python: server_name = name.split(".")[-1] if "." in name else name install_to_claude_desktop( server_name, "python", ["-m", name] + args, env, ) return f"Successfully installed MCP server '{server_name}' via Python! Please tell the user to restart the application." return f"Could not determine how to install '{name}'"
• mcp_builder/server.py:106-170 (helper)

  Key helper function called by install_repo_mcp_server to update the Claude Desktop configuration file with the new MCP server entry.
  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)