Simon Willison — 2026-07-19#

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Simon’s quick investigation into Claude Code’s internals is a classic example of his “poke at it and see” ethos, successfully verifying that the new Rust port of Bun is quietly powering the popular AI CLI tool in production.

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Claude Code uses Bun written in Rust now Simon puts his sleuthing hat on to verify Jarred Sumner’s claim that Claude Code is now running on a Rust port of Bun. By inspecting his own Claude Code installation, he found it running an unreleased Bun v1.4.0 and discovered hundreds of Rust source filenames. It’s a neat bit of validation showing that this major architectural rewrite is already running reliably across millions of devices, proving that “boring is good”.