Codex CLI: Setup, Sandbox Modes, and Verifying Its Output

Codex CLI is OpenAI’s Apache-2.0-licensed, Rust-based terminal coding agent that works with local repositories to inspect code, edit files, run commands, use MCP servers, delegate tasks to cloud sandboxes, and operate non-interactively in CI through `codex exec`. The guide emphasizes its rapid release cadence and recommends pinning versions in CI, using `/init` to create project instructions, and understanding the separate roles of `sandbox_mode`, which controls technical access, and `approval_policy`, which controls when user confirmation is required. It describes `AGENTS.md` as a layered instruction system that combines global and directory-specific guidance up to a 32 KiB limit, while MCP configuration extends Codex with local or remote external tools. For automation, `codex exec` defaults to read-only operation, sends progress to stderr and final results to stdout, and supports options for sandbox permissions, ephemeral sessions, and configuration isolation. Although Codex can review diffs and run tests, the guide argues that user-facing changes require runtime verification, recommending Kane CLI from TestMu AI to test real browser behavior and provide evidence such as screenshots, DOM state, and network results. Codex is presented as most suitable for bounded, mechanically verifiable tasks such as dependency updates, codemods, migrations, release notes, and repository analysis, while subjective design, unmeasured performance work, and taste-driven refactoring still require substantial human judgment.

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