Codex

Codex is OpenAI's coding-agent harness. This entry covers harness behavior rather than behavior of any underlying model.

Summary

Codex is OpenAI's coding-agent harness. This entry covers harness behavior rather than behavior of any underlying model.

Instruction files

AGENTS.md

Repository-level instruction files this harness reads. Treat their effective permissions as part of the setup.

Recommended

  • Put durable repository instructions in a scoped AGENTS.md.
  • Keep project instructions concrete, current, and compatible with repository tooling.
  • Include actual validation commands and boundaries the agent must preserve.

Avoid

  • Attributing instruction-file discovery to the model itself.
  • Storing stale or mutually conflicting rules in nested instruction files.

Context & tool use

Codex constructs an instruction chain from global and project-scoped files. More local project guidance can refine broader guidance.

Evidence

official

Sources