Codex on your phone, from an open-source app

Codex is the agent Happier launches in App Server mode by default, and the useful consequence is a refusal: if that mode is unavailable for a new session the launch fails instead of dropping quietly to something weaker, so ACP or MCP are choices you make on purpose rather than fallbacks that happen to you behind a session that then behaves differently from the last one.

What is at stake in that choice is written down rather than felt. On the default runtime a Codex conversation can be forked, rolled back to an earlier point, and asked whether a usage limit has cleared yet; pick the MCP or ACP runtime instead and the release drops all three, which is exactly why a quiet downgrade would be the wrong kindness. Codex also keeps a vendor TUI you can be handed back — happier attach gives you the real Codex interface on the session already running, and while that terminal holds it, anything you send from the app waits in the queue instead of arriving underneath your cursor.

It is also one of the two agents whose account can change mid-session. Connect a Codex subscription or an OpenAI key once, and a session that runs into a limit on one of them can be carried onto another profile in the same group instead of being abandoned — at most once a turn, and three times in a session hour, so it is a recovery rather than a rotation. The sign-in itself stays the vendor’s: the provider page runs codex login on the computer you chose, and the credential it writes stays there.

Codex CLI is OpenAI’s terminal coding agent, shipped as a release binary from the openai/codex repository. happier codex starts that binary in the repository you point it at — same approval prompts, same working directory, same account as running it yourself.

What changes is where you can read it from. The transcript, the file diffs and the approval requests appear in the Happier app on a phone, in a browser tab, or in the desktop app, end-to-end encrypted before they leave your computer.

The iOS and Android apps, the desktop app for macOS, Windows and Linux, and app.happier.dev are all clients onto the Codex CLI process running on your computer — not read-only mirrors. Answer a permission request, send the next instruction, browse the repository, read the diff. The transcript is end-to-end encrypted before it leaves your computer, so the server carrying it holds ciphertext.

Happier installs on the computer that already holds your repository, and happier codex starts Codex CLI there as an ordinary subprocess, in the directory you point it at, signed in the way you signed it in. There is no gateway between the agent and OpenAI, and no copy of your source anywhere else.

Happier looks for codex on your PATH first and runs the copy you installed. If there is not one, it can download the openai/codex release binary into ~/.happier/tools/providers/codex/ and keep it there. Your own install always takes priority.

OpenAI’s own page for Codex CLI is at github.com/openai/codex. Happier does not mirror it, because a copy of someone else’s install instructions is a copy that goes stale.

Configuration reference for the Happier side — Codex CLI in the Happier docs.

Codex CLI is one of the agents Happier can point at a credential you connected once instead of at whatever that computer is logged into. It accepts a Codex subscription or an OpenAI API key.

Connected services are keyed by the credential, not by the agent, so the same Codex subscription is selectable for every agent that can take it, on every computer you have connected — which is also what makes a pool of your own accounts useful across more than one of them. The credential is encrypted and decrypted on your own devices; the docs page carries the per-profile rules, because which kinds of profile each agent accepts is exactly the sort of detail that moves between releases.

Codex CLI is one of 13 command-line coding agents Happier runs, and they share one session list, one permission inbox, one set of keyboard shortcuts and one MCP configuration.

You define an MCP server once and Codex CLI receives it as native MCP inventory, next to whatever MCP servers it already had of its own.

That matters most on the days you are running more than one: sessions from different vendors side by side, and a notification tap opening the session that raised the request rather than the app’s front door. The full list is here.

Typing happier codex in a terminal starts the session right where you are standing, and the same session is in the app at the same time. What you read in that shell is Happier’s own display of the session — the transcript, the permission prompts, the tool output — rather than Codex CLI’s interface. Nobody has to give up the terminal to get a phone client.

When you want Codex CLI’s own TUI, happier attach <session-id> on the computer that owns the session hands it to you inside tmux, with the history already there. On the default runtime that hand-off is exclusive: while the terminal has the session, a message you send from the app waits in the queue rather than being typed underneath you, and it goes through when you hand control back.

The attach guide lists the cases instead of promising all of them.

One command on the computer that holds your code, then happier codex in the repository you want to work in.

Walkthroughs are in the guides. All of it is MIT-licensed, and the relay is one you can run yourself.