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. In this talk: • What the Codex App Server is, and how it compares to ACP • Customizing the agent with developer instructions and deferred dynamic tools • Shipping the binary, generating bindings, and the Python SDK • Demo: Codex running inside Doom, with tools that change the game SPEAKER Dominik Kundel, Developer Experience, OpenAI X: https://x.com/dkundel LinkedIn: https://linkedin.com/in/dkundel LINKS Codex App Server docs: https://developers.openai.com/codex/app-server Codex on GitHub: https://github.com/openai/codex Codex SDK: https://developers.openai.com/codex/sdk CHAPTERS 0:00 Intro 0:52 What is the App Server? 1:12 The protocol behind Codex 1:37 Open source, any model provider 2:17 How it works 3:32 Demo: inspecting live events 5:31 120+ client messages 6:31 Sign in with ChatGPT 6:56 Developer instructions 7:41 Dynamic and deferred tools 8:46 Shipping the binary 9:46 Bindings and transports 10:31 The Python SDK 10:51 Or just ask Codex 12:00 Codex inside Doom 13:40 Demo: giving Doom tools 14:50 App Server vs codex exec 15:55 Three tips Recorded at the AI Engineer World's Fair 2026 in San Francisco. Subscribe for more talks from the engineers building with AI. AI Engineer: https://ai.engineer YouTube: https://www.youtube.com/@aiDotEngineer X: https://x.com/aiDotEngineer LinkedIn: https://www.linkedin.com/company/aidotengineer/ #Codex #OpenAI #AIEngineer