tech
December 13, 2025
How OpenAI is using GPT-5 Codex to improve the AI tool itself
“The vast majority of Codex is built by Codex,” OpenAI told us about its new AI coding agent.
TL;DR
- OpenAI's Codex AI coding tool is heavily involved in its own development and improvement.
- Codex functions as a cloud-based software engineering agent, handling tasks such as writing features and fixing bugs.
- The tool is accessible via ChatGPT, a CLI, and IDE extensions.
- OpenAI engineers use the same open-source version of Codex available to external developers.
- Codex monitors its own training runs and processes user feedback to guide future development.
- The Sora Android app was reportedly built rapidly by four engineers using Codex.
- Independent research shows mixed results for AI coding productivity, with some studies indicating slower performance on complex codebases.
- Codex is integrated with project management and communication tools, acting as a 'teammate' for developers.
- OpenAI aims for Codex to become a general agent useful even to those without programming experience.
- Despite concerns about job displacement, OpenAI views Codex as an amplifier of human potential, not a replacement.
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