The OpenAI Codex model itself is not open source, as it remains proprietary technology developed and maintained by OpenAI. The underlying codex-1 model, which powers the current version of Codex, is based on OpenAI’s o3 architecture and represents significant proprietary research and development that OpenAI has not released publicly. Users cannot access the model weights, training code, or internal architecture details that would be available with a truly open source AI model. This is consistent with OpenAI’s general approach to their most advanced models, which are typically made available through API access and integrated products rather than open source releases.
However, there is an important distinction between the AI model and the tools that interact with it. The Codex CLI tool, which provides command-line access to Codex functionality, is indeed open source and available on GitHub under the OpenAI organization. This CLI tool allows developers to interact with the Codex service from their terminal and includes features for task management, code execution in sandboxed environments, and integration with local development workflows. The open source CLI provides transparency into how the tool works, allows community contributions for improvements and bug fixes, and enables developers to customize the interface to meet their specific needs. Developers can examine the code, submit pull requests, and even fork the project if they want to create modified versions.
The open source nature of the CLI tool means that while the core AI capabilities remain proprietary, the tooling and user interface components are available for community inspection and contribution. This hybrid approach allows OpenAI to maintain control over the expensive and complex AI model while enabling community participation in the development of user-facing tools. For developers interested in contributing to the Codex ecosystem, the CLI tool represents the primary avenue for participation, whether through bug reports, feature requests, or code contributions. This model reflects a broader trend in AI development where companies maintain proprietary models while open sourcing the tooling and interfaces that make those models accessible to developers.