OpenAI Codex CLI is an open-source (Apache 2.0), Rust-built terminal coding agent. It has over 72,000 GitHub stars. It pairs GPT-5.4’s 272K default context window, which you can push to 1M tokens, with OS-level sandboxing. That sandbox runs on Apple Seatbelt on macOS and Landlock plus seccomp on Linux. Here is the key point: Codex CLI is the only major AI coding agent that enforces security at the kernel level, not through application-layer hooks. With codex exec for CI pipelines, MCP client and server support, and a GitHub Action for PR review, it is the most infrastructure-ready rival to Claude Code in 2026.