The Changing Craft of Programming with John Crepezzi

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.

You can find the transcript for this episode on our website.

Some links to topics that came up in the discussion:

• QBasic
• Pirates of Silicon Valley
• Hoogle — the Haskell API search engine that inspired the original code-search project
• GitHub merge queue and Codespaces
• Sorbet — gradual typing for Ruby; Hack — Meta’s typed PHP
• Active Record
• Bubblewrap and passt/pasta — sandboxing tools
• seccomp BPF
• Claude Code and OpenAI Codex CLI
• Model Context Protocol and Agent Skills
• Perfetto and magic-trace
• Antithesis
• Bonsai — Jane Street’s OCaml web UI library
• Signals & Threads: Language Design with Leo White