Dominik Kundel from OpenAI's Codex team explains the Codex App Server, the open-source JSON-RPC protocol that wraps the Codex harness and powers the Codex app, IDE extensions, and third-party clients. He covers live protocol events, more than 120 client messages, ChatGPT sign-in, developer instructions, deferred dynamic tools, shipping and binding generation, the Python SDK, and a demo of Codex running inside Doom.
Once GPT 5.3 Codex Spark achieved 1000 tokens/sec on Cerebras, network latency superseded inference as the bottleneck for agents. This talk details how the Codex harness addresses this and other agentic challenges through innovations like WebSocket mode for stateful context, deferred tools for efficient context construction, robust sandboxing (Seatbelt, Bubblewrap, custom Windows solution), and an auto-review subagent to mitigate approval fatigue while ensuring security. It also covers structured actions via 'apply patch' for file edits, shell tools for system interaction, and sophisticated long-horizon goal management, with most distinct features exposed through the open Responses API.