TL;DR

Unity CLI and Unity Plugin for Codex are two Unity products that show up together in a Codex session. The CLI is a terminal binary: install Editors, add modules, create and open projects, sign in, set up version control. The plugin loads Unity Skills into Codex, so a request can follow current Unity practice. Installing one does not stand in for the other.

Unity CLI

Use the Unity command-line interface (CLI) is Unity’s own page. The CLI is experimental. It is a standalone unity binary, so a machine can install Editors and open projects without the Unity Hub desktop app. CI jobs, scripts, and a person at a terminal all run the same commands. Output can be JSON or TSV, and the process returns an exit code.

Common tasks on that page: unity install, unity install-modules, unity editors, unity open, unity projects create, unity auth login, unity vcs setup. Controlling an open Editor from the CLI is a further setup, the Unity Pipeline package, which that page lists separately.

Unity Plugin for Codex

Unity’s plugin for Codex adds Unity Skills to Codex. A Skill is written guidance for one Unity job, so the session does not rebuild that guidance from old posts. Install from the marketplace:

codex plugin marketplace add Unity-Technologies/unity-agent-plugin
codex plugin add unity@unity-agent-plugin

A new session lists the Skills under /unity:. codex plugin list shows unity installed and enabled. The directory entry is Unity Plugin in Codex, current release 0.1.6-beta. Install steps on this site: Unity Plugin for Codex: install, connect, and call a Skill. What each Skill fits: A detailed introduction to Unity Skills in Codex.

Where the two meet

The plugin includes a Skill named unity-cli. That Skill tells Codex how to call the CLI. The CLI binary still has to be on the machine. A person can run unity open with no Codex session at all. A Codex session with the plugin still needs the CLI when the task is “install this Editor” or “open this project.” The CLI operates Unity from a terminal. The plugin is the Codex-side Skills. unity-cli is the Skill that connects those two.