OpenAI opens the Codex agent loop for inspection
OpenAI documented how Codex composes instructions, model calls, tools and tool results into a repeated agent loop. The engineering note also explains stateless Responses API calls, prompt-cache constraints, automatic compaction and support for OpenAI-compatible endpoints.
Why it matters: Agent quality depends on the harness around the model—tool schemas, state, context growth, caching and termination—not only the model selected in a picker.