Codex

Codex is a cloud-based software engineering agent developed by OpenAI. The system, powered by the codex-1 model (an optimized version of OpenAI o3 for software engineering), is designed to handle multiple programming tasks in parallel. Codex runs in secure, isolated cloud environments preloaded with the user's repository.

The agent can perform actions such as implementing features, answering code-related questions, debugging, running tests, and proposing changes via pull requests. Each task is executed independently in a separate environment where Codex can read and edit files, and run commands including tests, linters, and type checkers.

Codex can be configured via AGENTS.md files placed in the repository, which serve as guides for navigating the codebase, running tests, and following project conventions. The agent produces verifiable evidence of its actions through terminal logs and test outputs.

As a complement, Codex CLI is an open-source, lightweight coding agent that runs in the local terminal. It uses a smaller variant of codex-1 (codex-mini-latest), optimized for low-latency code queries and edits.

During execution, Codex operates without internet access and is restricted to code provided explicitly via GitHub repositories and preinstalled dependencies configured by the user. The model was fine-tuned through reinforcement learning on real-world coding tasks.