MCP Builder

Instructions

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 =

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)