Topics›Industry and Market Signal›How OpenAI Built Codex: Rust-First Harness Design and 'Have You Asked Codex?' as Default Onboarding
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How OpenAI Built Codex: Rust-First Harness Design and 'Have You Asked Codex?' as Default Onboarding
The Pragmatic Engineer·September 9, 2026
New
Summary
Tibo Sottiaux (original Codex engineer, now heads OpenAI's Core Products & Platform) explains Codex CLI is written in Rust (not Python/TS) for millions of concurrent cloud instances, why open-sourcing Codex enabled multi-model-provider support (vs. Claude Code's Anthropic lock-in), and that the harness always stays 'slightly ahead' of the model so guardrails shrink as models improve. At OpenAI, Codex is the default onboarding move for new hires.
Why this signal
Concrete harness-design philosophy and org-adoption tactics ('have you asked Codex?') directly relevant to end-to-end AI-accelerated dev at enterprise scale.
Original email
The Pragmatic Engineer · 9/9/2026
Building Codex with Tibo Sottiaux