OpenAI releases open-source "Codex CLI" for the terminal — first step in reviving the "Codex" brand
On April 16, 2025, alongside the announcement of its new models “o3” and “o4-mini,” OpenAI released “Codex CLI,” a lightweight open-source coding agent that runs right in your local terminal. Carrying the name “Codex,” a service that had been discontinued back in 2023, this autonomous agent writes and edits code and executes commands on its own, marking the first step in the revival of what would later expand into the cloud-based “OpenAI Codex.”
Details
- Overview: A lightweight coding agent used from a local terminal, which OpenAI described as “a transparent, minimal open-source project” and “a small step toward an agentic software engineer”
- Capabilities: Given natural-language instructions, it can write code, edit code, run commands, and manipulate files, all completed within the user’s local development environment
- Models announced alongside it: The reasoning model “o3” and the cheaper, faster “o4-mini” were announced the same day; Codex CLI can use these models
- Open-sourcing: Published its source code on GitHub, available for anyone to inspect or modify
- New Codex Open Source Fund: Also established a $1 million fund, distributed in $25,000 increments, aimed at providing API credits to open-source projects
What happened next
Codex CLI became the starting point for reviving the “Codex” brand — discontinued two years earlier — not as a code-specific model but as an “autonomously operating coding agent.” About a month later, in May 2025, “Codex,” which handles tasks in parallel in the cloud, was announced as a research preview within ChatGPT, and the “OpenAI Codex” brand went on to expand in earnest around these two pillars: a local CLI and a cloud-based agent.