Codex is an AI coding agent developed by OpenAI for software engineering tasks like writing code and fixing bugs. First released as an open-source CLI tool in April 2025, it quickly expanded to the ChatGPT web app, a desktop app, and IDE integrations, growing to over 2 million weekly active users by March 2026. The agent operates in separate cloud environments where it can independently read files, run tests, and execute commands before returning results for review. OpenAI has continuously upgraded its underlying models, including the GPT-5.3-Codex-Spark variant, which ran 15 times faster on Cerebras hardware. In a strategic move to counter competition from Anthropic, OpenAI merged Codex with the ChatGPT desktop app in July 2026 and acquired Python toolmaker Astral to enhance its developer tools. While powerful, the platform has faced security scrutiny, including a patched vulnerability where malicious GitHub branch names could steal authentication tokens.