Codex
openai/codex
OpenAI's terminal coding agent.
Codex is OpenAI's open-source, sandboxed terminal coding agent with multi-provider support — the reference implementation of an 'official CLI that ships code', and one half of the Claude-Code-vs-Codex duopoly that defines default harness expectations.
In the news
OpenAI open-sources Symphony, its codex orchestration harness
OpenAI releases Symphony — open-source orchestration that turns project work into isolated, autonomous coding-agent implementation runs and surfaces the ones that pass, so teams manage outcomes instead of supervising sessions. Big Tech keeps raising the floor on what counts as a baseline harness.
HarnessTax: harness choice swings agent cost more than model choice
Arena.ai's HarnessTax study tests 21 model–harness combinations across Claude Code, Codex CLI and Pi, finding harness choice can significantly change cost even at similar task success rates. Weeks earlier, SWE-bench Pro analysis showed swapping harnesses moved pass@1 more than model upgrades (23%→52% on GLM-5.2) — and that harness rankings barely transfer across models.
xAI open-sources Grok Build, its coding agent harness
xAI open-sources Grok Build — a Rust coding agent harness and fullscreen TUI around Grok models — unusually early for a first-party tool. It joins Claude Code, Codex and Gemini CLI as the fourth major first-party harness, now with a fourth open one.
GPT-5 arrives with a focus on agentic coding
OpenAI launches GPT-5, touting major gains for coding agents and reduced 'thinking' friction in workflows like Cursor, Copilot and ChatGPT — a model release explicitly aimed at the harness layer.
Related projects
superpowers
obra/superpowers
An agentic skills framework and software methodology that works.
superpowers is the most-adopted skills pack in harness engineering: skills, instincts, memory, security and research-first workflows for Claude Code and 13 other harnesses (Codex, Cursor, opencode, Gemini CLI and more). Treats harness engineering itself as the discipline.
DeepSeek Harness
deepseek-ai/deepseek-harness
Everything is a Plugin — DeepSeek's open agent harness.
DeepSeek Harness is the breakout open-source harness of 2026: an architecture where everything — tools, subagents, even the loop itself — is a plugin. A Desktop app for macOS and Windows followed in October 2026, and the project passed 240k stars in under three months.
opencode
anomalyco/opencode
The open source coding agent — terminal-first, model-agnostic.
opencode is a blazingly popular terminal coding agent: a language-server-backed TUI that works with any provider, scriptable and hackable. Its growth made it the open counterweight to Claude Code.