Codex

Codex is OpenAI’s first dedicated code-generation model, announced in August 2021 as a GPT-3 descendant fine-tuned on billions of lines of public source code from GitHub. The largest published version had 12 billion parameters and a 4,096-token context window, and it solved 28.8% of problems on OpenAI’s HumanEval benchmark on the first try, rising to 77.5% when allowed 100 sampled attempts per problem. Codex is what powered the original GitHub Copilot when Copilot launched in technical preview in 2021, marking one of the first large-scale deployments of an LLM as a coding assistant rather than a chat or completion demo. OpenAI made it available through a private beta API for a few years before retiring the standalone Codex API in March 2023, folding its capabilities into successor GPT-3.5 and GPT-4 family models. The name lives on today as the brand for OpenAI’s newer agentic coding products, but the original Codex model itself is no longer accessible through the API.