How AI is Changing Programming

John Crepezzi has spent his career building tools for other programmers—at Tumblr, at Square, and then at GitHub, where he worked on the merge queue and the early versions of Codespaces. He came to Jane Street to build a code search engine for OCaml. A week into the job, ChatGPT came out, and he's been working on AI tooling ever since. In this episode, Ron and John talk about how a three-day prototype grew into a firmwide platform for agentic coding, how AI can break the social contract around code review, and how to build tools that support both agent workflows and human understanding. 00:00:00 – Start 00:00:28 – QBasic and an infinite loop 00:04:25 – Tumblr, Square, and GitHub 00:06:41 – Building for thousands instead of millions 00:10:43 – From Ruby to OCaml 00:18:05 – Active Record, Petrol, and metaprogramming 00:22:14 – Three days to build JaneGPT 00:29:27 – Writing evals, and grading models with models 00:33:48 – Giving LLMs their own permissions 00:36:29 – Adversarial models and reward hacking 00:41:00 – Deny lists, allow lists, and bwrap 00:48:09 – AI users 00:53:09 – Tool calling and the birth of the AIDE CLI 01:04:01 – Vibe coding and the Bonsai gap 01:06:32 – Agents as perpetual beginners 01:09:12 – Our own tools vs. Claude Code 01:15:14 – What's left of the craft 01:20:04 – Verification is the hard part 01:25:12 – Narrative code review 01:36:43 – Agent teams and what's next 01:43:33 – Slop and the code review social contract For more episodes, visit: https://signalsandthreads.com/