Codex is OpenAI’s AI agent. You can use it in three ways: as a CLI in the terminal, as a desktop app, or as the ChatGPT/Codex extension in VS Code, which the welcome page of the Application Platform extension links to directly. The setup app installs the CLI and the desktop variant during developer setup.
Starting
For the CLI, change into the project folder and run codex. As with every agent, open the parent project folder so that all repositories and the shared configuration are visible.
cd ~/Documents/application-platform/<organization-slug>/<project-slug>
codex
Preconfiguration
Unlike Claude Code, Codex needs no instruction file of its own; it reads AGENTS.md directly. In the project folder it is a symlink to local-configuration/AGENTS.md, accompanied by the .codex/ directory with Codex-specific configuration from the same repository. Codex therefore knows the platform conventions from the start: applications start via ap run-local, .gitlab-ci.yml and env/*.generated.env are managed by the platform, and custom values belong in env/*.custom.env. Because AGENTS.md applies to all agents, your own additions for Codex go in the same place; how that works is described in Your own rules for AI agents.
Agent plugin through the marketplace
Codex manages plugins through marketplaces and the /plugins dialog. First register the plugin’s marketplace:
codex plugin marketplace add doppelt-digital/application-platform-plugin
Then open /plugins in Codex, select Application Platform and install it. The plugin connects the MCP server application-platform, which runs through ap mcp serve, and provides the skills for sign-in, project creation, import and local start. If the ap CLI is not signed in yet, run ap login and restart Codex. All other installation routes, the list of MCP tools and the typical failure cases are described in CLI and agent plugin.
A prompt such as “Create a new Application Platform project with a Flutter app and a FastAPI backend and clone it.” is enough from then on; the flow behind it is described in Create a new project.