This paper evaluates the effectiveness of penetration testing (pentesting) using generative artificial intelligence (GenAI) tools (Claude Opus, GPT-4, Copilot). Experiments were conducted in a virtual environment across each stage of the Penetration Testing Execution Standard (PTES). While GenAI tools cannot automate the entire pentest process, they significantly improve the efficiency and effectiveness of specific tasks. Claude Opus, in particular, outperformed other tools.