Codex: an Apache-2.0 core, one agent in four form factors, and the reference harness for OpenAI models
Codex is the coding agent from OpenAI, and in 2026 it is not one product but four entry points onto one core: the CLI in your terminal, an IDE extension (officially for VS Code, Cursor and Windsurf), a desktop app launched with a single codex app command, and Codex Web running tasks remotely at chatgpt.com/codex. The openai/codex repository is Apache-2.0 and stood at 126,918 stars / 19,819 forks when fetched on 2026-09-28 - one of the few frontier-vendor coding harnesses whose harness is itself open source, so context assembly, tool ordering and approval gating can all be read. Safety is a first-class citizen: docs/ carries sandbox.md, execpolicy.md, exec.md, agents_md.md, skills.md and slash_commands.md, where execpolicy turns which commands may run unattended into an auditable, versionable, org-distributable policy instead of model goodwill, exec lets Codex run in CI, scripts and batch jobs rather than only waiting in a terminal, and AGENTS.md has become a de facto cross-tool standard. Four install channels (install.sh, install.ps1, npm, brew) and two billing models: usage included in ChatGPT plans versus per-token API keys, a cost question to settle before choosing. Codex is also the reference harness for OpenAI models - the Cursor Terminal-Bench 2.0 footnote states that OpenAI model scores use the Simple Codex harness, and the vendor-published Terminal-Bench 4.0 reading of 58.18% is already carried on the GPT-6 Astra card because it measures the model rather than the shell, so this entry does not claim it. Boundaries: models are closed and data egress cannot be avoided; developers.openai.com returns 403 to automated fetching, so the fact layer rests on the repository README/docs and the GitHub API. The card metric keeps only the verifiable fact layer (license plus stars). Confidence C (vendor-claim).