GPT-5-Codex
OpenAI's version of GPT-5 optimized for agentic coding inside Codex. RL-trained on real-world coding tasks, available via Codex CLI, IDE extension, cloud, and the Responses API. The Codex model line has since iterated to GPT-5.3-Codex.
What it is
GPT-5-Codex is a GPT-5 variant fine-tuned for agentic coding in Codex or similar environments. Like its predecessor codex-1, it was trained with reinforcement learning on real-world coding tasks across many environments to produce code that mirrors human style and pull-request preferences, follow instructions precisely, and iteratively run tests until they pass. It is exposed through the OpenAI Responses API only, and the underlying snapshot is updated regularly. Source: OpenAI, https://openai.com/index/gpt-5-system-card-addendum-gpt-5-codex/, 2025 Source: OpenAI API docs, https://developers.openai.com/api/docs/models/gpt-5-codex, 2026-05-31
How developers reach it
Codex (the product around the model) runs in several surfaces:
- Codex CLI - terminal-based agentic coding.
- IDE extension - in-editor agent, including VS Code-family editors.
- Cloud - Codex web, GitHub integration, and the ChatGPT mobile app for delegating tasks.
Safety is handled at both layers: model-level training against harmful tasks and prompt injection, plus product-level agent sandboxing and configurable network access so the agent's blast radius is bounded. Source: OpenAI, https://openai.com/index/gpt-5-system-card-addendum-gpt-5-codex/, 2025
Where it fits Kevin's stack
GPT-5-Codex is the OpenAI counterpart to Cursor's Composer (Cursor 2.0 and Composer) and the Claude Code harness (Everything Claude Code - Agent Harness Performance System) - a coding agent rather than a chat model. It is also a first-class alternative agent in security tooling: Security and Review Skills runs against either Claude Opus or the GPT-5.x line at maximum reasoning. As an OpenAI model it slots directly into the OpenAI Agents SDK for tool-using agents. The Codex family has continued to advance (GPT-5.3-Codex is the current top coding snapshot), so treat "GPT-5-Codex" as the line, not a frozen checkpoint. Source: OpenAI API docs, https://developers.openai.com/api/docs/models/gpt-5-codex, 2026-05-31
Timeline
- 2025 | OpenAI shipped GPT-5-Codex with an addendum to the GPT-5 system card: RL-trained for agentic coding, available via Codex CLI, IDE extension, cloud, and Responses API, with agent sandboxing and configurable network access. Source: OpenAI, https://openai.com/index/gpt-5-system-card-addendum-gpt-5-codex/, 2025
- 2026-05-31 | Tool page created. The Codex coding-model line has iterated to GPT-5.3-Codex; GPT-5-Codex remains Responses-API-only with regularly updated snapshots. Source: OpenAI API docs, https://developers.openai.com/api/docs/models/gpt-5-codex, 2026-05-31