Codex, Behind the Harness — Dominik Kundel, OpenAI
OpenAI's Codex agent harness is open source, MIT/Apache 2 licensed, and written in Rust.
“the Codex harness and everything I'm showing you is actually open source. Uh it's MIT it's Apache 2 license and the harness is written in Rust.”
OpenAI engineer Dominik Kundel walked AI Engineer World's Fair attendees through the internal architecture of the Codex coding agent, revealing that the harness is fully open source under MIT/Apache 2 licenses and built in Rust. The system uses two protocols: an app server protocol for UI-to-harness communication and the Responses API for harness-to-inference communication, both designed for an open ecosystem so third parties can build custom UIs or agents on top. This is a meaningful signal for the developer tooling space as it gives the community a production-grade agentic coding scaffold to fork and extend.