OpenAI's Codex is an AI model that writes, debugs, and manages computer code. It powers GitHub Copilot, a coding tool used by more than 20 million software developers. Codex can be assigned a programming task and will independently write code, create a pull request, and respond to feedback — without a human writing a single line.

The original Codex model launched in 2021 and powered early versions of GitHub Copilot as an autocomplete tool. In May 2025, OpenAI relaunched Codex as a fully autonomous coding agent based on its o3 and o4-mini reasoning models, capable of handling long, multi-step programming projects independently. As of early 2026, the version integrated into GitHub Copilot is GPT-5.3-Codex, which became Copilot's first long-term support model in March 2026. More than 90% of Fortune 100 companies use GitHub Copilot.