Building on the Codex Harness — Dominik Kundel, OpenAI
Building your own agent harness is daunting. So use the one that already powers Codex.
Dominik Kundel from OpenAI's Codex developer experience team explains the Codex App Server, the open-source JSON-RPC protocol that wraps the Codex harness and powers the Codex app, the IDE extensions, and third-party clients in Xcode, JetBrains and more. Using an inspector he built, he shows the real events flowing between the Codex app and the server, then covers what you can control: more than 120 client messages, sign in with ChatGPT so users bring their own subscription, developer instructions layered on top of the system prompt, dynamic and deferred tools, and per-thread model and sandbox settings. He explains how to ship the binary, generate bindings, and use the Python SDK, when to use codex exec instead, and demos Codex running inside Doom.
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