OpenAI Opens API Access to "Codex," a Natural-Language-to-Code AI: The Brain Behind GitHub Copilot
On August 10, 2021, OpenAI released “OpenAI Codex,” an AI that converts natural-language instructions into executable code, as a limited private beta via API. Built on GPT-3 and further trained on a huge volume of source code, the model had already been adopted about a month and a half earlier as the brain behind the “GitHub Copilot” technical preview that GitHub had announced. Positioned as a way to lower the barrier to entry for programming, this original Codex became the starting point for the “OpenAI Codex” brand that would be reborn in 2025.
Details
- Origins: Developed as a derivative of GPT-3, specialized in converting natural-language descriptions into executable code
- Supported languages: Most proficient in Python, but also supported more than ten programming languages including JavaScript, Go, Perl, PHP, Ruby, Swift, TypeScript, and Shell
- Performance: OpenAI reported that it solved 71.2% of problems on its published “HumanEval” benchmark (compared to 0% for the original GPT-3)
- Relationship to GitHub Copilot: The “GitHub Copilot” technical preview that GitHub announced on June 29, 2021 already used this Codex as its brain, meaning Copilot’s own public debut preceded the release of the Codex API itself
- Availability: Initially offered as a free private beta via API, with OpenAI stating it would “scale as quickly as possible while remaining safe”
- Design philosophy: Aimed at lowering the barrier of “you can write code, but figuring out how to use an API is hard,” the model was designed to excel at mapping tasks onto existing libraries and APIs
What happened next
The original Codex model remained the foundation for GitHub Copilot and other code-generation tools until March 2023, when OpenAI announced its retirement in favor of integration into the GPT-3.5 and GPT-4 model families. The “Codex” name disappeared for a time, only to be revived in 2025 as an entirely new cloud- and terminal-based coding agent.