All harnesses
Codex CLI
OpenAI
OpenAI's open-source terminal agent for the GPT-5.5 and o-series models. Released in 2025 as the company's answer to Claude Code, with a similar tool-use loop, sandboxed code execution, and a plug-in model that aligns with the OpenAI Apps SDK. The MIT-licensed source on GitHub is the reference for how OpenAI thinks about agentic coding harnesses.
Type
cli
License
Open source
Model story
OpenAI models only
Vendor
OpenAI
Leaderboard Placements
| Benchmark | Best base model | Score | Rank |
|---|---|---|---|
| SWE-bench Verified | GPT-5.5 | 72.8 | #2 / 15 |
| Terminal-Bench | GPT-5.5 | 48.2 | #2 / 13 |
| Aider Polyglot | GPT-5.5 | 82.1 | #3 / 7 |
| SWE-Lancer | GPT-5.5 | 39.6 | #2 / 5 |
Distribution
Open-source CLI. Install via npm or build from source. MIT license.
Model Story
OpenAI models only. Defaults to GPT-5.5; o3 and o4-mini selectable for cheaper or higher-reasoning routing.
Pricing
Free harness; you pay for the underlying OpenAI API tokens.
Who It's For
Engineers who want a terminal agent on the OpenAI stack and prefer a transparent, MIT-licensed implementation they can fork and extend.
Notable Features
- Sandboxed shell and code execution
- OpenAI Apps SDK plug-ins
- Open MIT license
- Auto-approve mode for unattended runs
- Native streaming tool-call UI
Vendor site for Codex CLI:https://github.com/openai/codex