Codex, Behind the Harness — Dominik Kundel, OpenAI
Aug 10, 2026 · 20:55
Dominik Kundel, an OpenAI engineer, explains the internals of the Codex agent harness, which is open source under Apache 2 and written in Rust. He details how context construction caps available skills at 2% of the context window and uses deferred tools with tool search to manage size and cost. For actions, Codex relies on an apply patch tool for file edits, a shell tool with ripgrep, and sandboxes: seatbelt on macOS, bubblewrap on Linux, and a custom open-source Windows sandbox. To reduce approval fatigue, an auto-review subagent with read-only permissions judges actions against user authorization and risk taxonomies. Speed improvements come from websocket mode in the responses API, which sends only changed items instead of full state, crucial when GPT 5.3 Codex Spark hit 1,000 tokens per second on Cerebras. Long-horizon goals work via a continuation prompt until the model calls an update goal tool, favoring concrete verifiable objectives, and auto compaction maintains performance…
