Building on the Codex Harness — Dominik Kundel, OpenAI

This post introduces the Codex App Server, an open-source JSON-RPC protocol that acts as a harness for the Codex AI model. It details how this server powers various clients (Codex app, IDE extensions) and allows for deep customization. Key contributions include: exposing over 120 client messages for control, enabling 'Sign in with ChatGPT' for user subscriptions, layering developer instructions on top of system prompts, implementing dynamic and deferred tools, and configuring per-thread model and sandbox settings. The talk also covers practical aspects like shipping the binary, generating bindings, and using the Python SDK, with a demonstration of Codex running within the game Doom, equipped with custom tools.