OpenAI open-sourced the Codex harness: what you actually got
OpenAI just made the engine under Codex an Apache-2.0 platform — codex exec, the SDK, and the JSON-RPC app-server. The pitch is one number: harness changes alone took the same model from 13.3% to 38.3% on ARC-AGI-3 with 6x fewer tokens.
Backgrind Blog10 min read