Codex is OpenAI’s coding agent: you describe a change and it works through your code to make it. It is in the news because OpenAI’s new GPT-6 Luna model reached free ChatGPT users in October 2026, and the Codex pricing page now lists a free tier.
Is it really free? Yes, for light use. OpenAI’s Codex pricing page says the ChatGPT Free plan includes Codex for “quick coding tasks” with GPT-6 Luna in the desktop app, subject to rollout. It gives no fixed free limit, and the cheapest paid plan is Go at $8 a month.