Bun Has Been Converted to Rust. Now What?

On May 14, PR #30412 merged into Bun's main branch: a little over a million lines of Rust, 6,755 commits, generated almost entirely by Claude Code agents over nine days. Anthropic, which acquired Bun in December, supplied the agents. The Zig implementation that powered Bun is gone. Jarred Sumner's own words - "we haven't been typing code ourselves for many months now" - are the part everyone quoted, and the part that turned a routine merge into a 685-point Hacker News thread with the PR itself split almost evenly between thumbs-up and thumbs-down.

The Rust rewrite passed 99.8% of the existing test suite. That number is enormous and significant, but let's be precise about what it actually says: it says that the new implementation behaves like the old one at the runtime's public interface. That's it. It does not say that the new implementation is safe, or better, or even good. Those are different claims.

The benchmarks are neutral-to-faster, and the binary shrank by a few megabytes (from a starting point around 93MB on Linux on x64, for example). If that were the whole story there would be nothing to add: it'd be a "good merge," I suppose, because "smaller" and "faster" while not losing test validation are "good attributes." But the actual stated reason is not one of those reasons and that suggests that there's more here than people are thinking about, blinded by fascination with Rust - a fascination I share - and with the interest in whether an LLM can accomplish something like this at all.