Start a Codex session with: 
happy codex

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Prerequisites

Install the Codex CLI and ensure you have a valid OpenAI API key configured. You can connect your OpenAI credentials through Happy: 
happy connect codex

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Model selection

Codex supports several models with different capability levels. Set the model when starting a session — model selection is handled through the mobile app or session configuration. Available models include:
  • gpt-5.4 — Latest, most capable
  • gpt-5.3-codex — Optimized for code
  • gpt-5.2-codex — Previous generation
  • gpt-5.1-codex-max — Maximum reasoning
  • gpt-5.1-codex-mini — Lightweight, fast

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How it works

Under the hood, Happy starts the Codex app-server and communicates via JSON-RPC. The mobile app and voice features work identically to Claude mode — you can send messages, approve permissions, and monitor progress from your phone.