OpenAI lifts lid on Codex engine with technical deep dive
The company has published the first in a new series of technical posts explaining how its Codex command-line tool works, detailing the agent loop, prompt handling and performance choices that underpin the software development assistant.
OpenAI has published a technical post setting out how the agent loop behind its Codex command-line interface operates, offering a detailed look at the mechanics that power the company’s software development tools.
The post, the first in a planned series, focuses on the Codex “harness”, which OpenAI describes as the core execution loop shared across its Codex products. Codex CLI is designed to run locally on a user’s machine and to generate software changes directly, rather than operating solely as a cloud-based service.
According to the company, the tool builds prompts by combining inputs assigned different roles, including system, developer, user and assistant. These prompts are then sent as HTTP requests to the Responses application programming interface, which handles model inference.
The endpoint can be configured in several ways, including through a ChatGPT login, an application programming interface key, or local model hosting using open-source runtimes. It can also connect to cloud providers such as Microsoft Azure.
The post also highlights design choices around performance and data handling. As conversations grow longer, prompts can approach the limits of a model’s context window.
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To manage this, Codex uses prompt caching and conversation compaction to reduce computational cost. OpenAI said it has opted to keep requests stateless, supporting zero data retention configurations, even if that increases the likelihood of cache misses.
The company said the series aims to give developers a clearer understanding of how Codex behaves in practice and how to integrate it into their own workflows.
The Recap
- OpenAI explained the Codex CLI agent loop mechanics.
- Codex builds prompts from system, developer, user and assistant roles.
- Codex will compact conversations when token counts exceed thresholds.
