MCP Policy Gatekeeper

Instructions

Implementation Reference

• server.py:144-169 (handler)

  The handler logic for the 'create_file' tool. Normalizes the filename to snake_case, resolves the path securely, checks if the file already exists, creates necessary directories, writes the provided content to the new file, and returns a success message with naming explanation.

  if name == "create_file":
      # Apply naming convention
      requested_path = arguments["path"]
      normalized_path, explanation = enforce_naming_convention(requested_path)
      
      path = resolve_path(normalized_path)
      content = arguments.get("content", "")
      
      # Check if file already exists
      if path.exists():
          return [TextContent(
              type="text",
              text=f"Error: File '{normalized_path}' already exists. Use write_file to update it."
          )]
      
      # Create directory if needed
      path.parent.mkdir(parents=True, exist_ok=True)
      
      # Create the file
      with open(path, 'w', encoding='utf-8') as f:
          f.write(content)
      
      # Return success message with naming convention info
      message = f"{explanation}\n✓ File created successfully: {normalized_path}"
      return [TextContent(type="text", text=message)]

• server.py:70-88 (registration)

  Tool registration for 'create_file' in the @app.list_tools() function, including metadata, description, and input schema definition.

  Tool(
      name="create_file",
      description="Create a new file (automatically applies snake_case naming convention)",
      inputSchema={
          "type": "object",
          "properties": {
              "path": {
                  "type": "string",
                  "description": "Desired file path (will be normalized to snake_case)"
              },
              "content": {
                  "type": "string",
                  "description": "File content",
                  "default": ""
              }
          },
          "required": ["path"]
      }
  ),

• server.py:13-65 (helper)

  Supporting helper function that enforces snake_case naming convention on the provided file path, returning the normalized path and an explanation message. Called by the create_file handler.

  def enforce_naming_convention(filename: str) -> tuple[str, str]:
      """
      Enforce organizational naming convention: convert to snake_case.
      Returns: (normalized_filename, explanation)
      """
      # Get the filename without path
      path_parts = filename.split('/')
      original_name = path_parts[-1]
      
      # Split name and extension
      if '.' in original_name:
          name_part, extension = original_name.rsplit('.', 1)
      else:
          name_part = original_name
          extension = ''
      
      # Convert to snake_case
      # 1. Replace spaces and hyphens with underscores
      normalized = name_part.replace(' ', '_').replace('-', '_')
      
      # 2. Insert underscore before capital letters (camelCase -> snake_case)
      normalized = re.sub('(.)([A-Z][a-z]+)', r'\1_\2', normalized)
      normalized = re.sub('([a-z0-9])([A-Z])', r'\1_\2', normalized)
      
      # 3. Convert to lowercase
      normalized = normalized.lower()
      
      # 4. Remove consecutive underscores
      normalized = re.sub('_+', '_', normalized)
      
      # 5. Remove leading/trailing underscores
      normalized = normalized.strip('_')
      
      # Reconstruct full filename
      if extension:
          normalized_filename = f"{normalized}.{extension}"
      else:
          normalized_filename = normalized
      
      # Reconstruct full path
      if len(path_parts) > 1:
          path_parts[-1] = normalized_filename
          full_normalized = '/'.join(path_parts)
      else:
          full_normalized = normalized_filename
      
      # Create explanation if name was changed
      if original_name != normalized_filename:
          explanation = f"📝 Naming convention applied: '{original_name}' → '{normalized_filename}'"
      else:
          explanation = f"✓ Filename already follows convention: '{normalized_filename}'"
      
      return full_normalized, explanation