Building Codex with Tibo Sottiaux

Brought to You By: • turbopuffer – a vector and full-text search engine built on object storage. It’s fast, cheap, and extremely scalable • Antithesis – verify your system’s correctness without human review or traditional integration tests – and avoid bugs or outages. • Entire – Git hosting, rebuilt for the agentic era. Every agent session, prompt  and tool calls: stored in your repo. — Tibo Sottiaux is one of the engineers who created Codex, and today, he heads up the Core Products & Platform org at OpenAI which also includes Codex. He’s also one of the most public faces of Codex due to his frequent – and generous – usage reset announcements, like this one yesterday. In this episode of the Pragmatic Engineer Podcast, Tibo and I discuss how Codex was built and continues to be iterated upon. We explore why the Codex CLI is written in Rust and was released as open source, how the harness and models have evolved, and why Codex supports models from multiple providers. Tibo also shares details about how the OpenAI team uses Codex throughout the software development lifecycle, including code reviews, maintenance, and system rearchitecture. We look into how AI is lowering the cost of changing code – and some interesting side effects of this – the merger of ChatGPT and Codex, and also how Tibo uses the tools in his own work. — Timestamps 00:00 Intro 07:21 Working at Google 12:41 What drew Tibo to OpenAI 15:19 The early days of Codex 18:20 Why Codex was built in Rust 21:15 Why Codex is open source 25:50 Codex plays nice with other models: why? 32:09 How the harness works 36:44 Harness and model improvements 41:19 The SDLC behind Codex 46:39 Code reviews at Codex 52:09 Maintenance and architecture 56:43 How AI tools expand what engineers can do 1:02:30 The Merge: ChatGPT + Codex 1:07:16 How Tibo uses Codex and ChatGPT 1:10:44 Advice for engineers who want to work in AI — The Pragmatic Engineer deepdives relevant for this episode: • How Codex is built • How Claude Code is built • How Cursor was built • What is "loop engineering?” • How Uber uses AI for development: inside look • Why Ramp built its own in-house coding agent, Inspect • “I ship code I don’t read”: with Peter Steinberger, the creator of OpenClaw — Production and marketing by ⁠⁠⁠⁠⁠⁠⁠⁠https://penname.co/⁠⁠⁠⁠⁠⁠⁠⁠. For inquiries about sponsoring the podcast, email [email protected]. Get full access to The Pragmatic Engineer at newsletter.pragmaticengineer.com/subscribe