TL;DR:

Bun is a very large and very influential open-source project. It is being migrated from the easier-to-read Zig programming language to harder-to-read but memory-safe Rust. This is done almost entirely by the AI tool Claude Code. The migration may become one of the earliest major case studies of human control over a significant software project becoming increasingly indirect--both due to the nature of the programming language itself and LLM-generated code being quite convoluted.

What happened?

On May 14 2026, a Rust version of Bun was merged to the "main" branch of the Bun repository[1].

Bun was originally written in Zig, by humans. Then they started to use AI coding tools, likely with an increased intensity after their acquisition by Anthropic in December 2025[2].

Jared Sumner, Bun's creator, posted that the Zig version will be discontinued in favor of the Rust version.

Why is this a big deal?

As far as I'm aware, this is the first example of a hand-written large open-source software project entirely transitioning to LLM-written code.

According to what Jarred Sumner says on X, they haven't been typing code themselves, even before the acquisition--in other words, Claude was already Bun's de facto primary coder.