Building on the Codex Harness — Dominik Kundel, OpenAI
OpenAI's Codex application server is open-source and powers all Codex interfaces via a shared protocol.
“It is important to remember that both the Codex shell and the application server itself are open source.”
OpenAI developer experience engineer Dominik Kundel explains that the Codex application server is an open-source, Apache 2-licensed JSON RPC-style protocol underpinning every Codex interface — the desktop app, VS Code and JetBrains extensions, Xcode integration, and even the Claude Code integration. The protocol is extensible to any Responses API-compatible model provider, including local options like Ollama and LM Studio. This matters because it reveals OpenAI is standardizing agent shell infrastructure in the open, positioning the Codex harness as a reusable foundation for third-party agent UIs rather than a closed product.