Bun Drops Zig for Rust — Thanks to Claude AI

Bun, the JavaScript runtime that first promised to replace Node.js, NPM, and even bundlers like esbuild, just got a radical makeover. The open-source project, snapped up by AI giant Anthropic late in 2025, has ditched its entire Zig codebase. Now? It's all Rust.

And guess who did the heavy lifting? Anthropic's own language model, Claude. This isn't just a code update; it's a new chapter in how software gets built. Or maybe, how it could get built.

The Rust Shift

Word started leaking out in late April: a new branch popped up, clearly showing Claude busy converting Bun's entire codebase. From Zig to Rust. Developers were skeptical, naturally. Bun founder Jarred Sumner tried to calm everyone down on May 5th. "This whole thread is an overreaction," he posted. "We haven’t committed to rewriting."

But they didn't give up. Not at all. By May 14th, the changes were merged into the main branch. We're talking 2,188 files altered, a million lines of code rewritten, and 4,000 lines deleted. Bun is now fully Rust.

Community Weighs In

This move, as you might expect, blew up the developer community forums. Some folks cheered the switch to Rust, praising its safety features — especially those tools designed to catch and prevent memory bugs. But then there were the others, pointing fingers at the heavy use of the unsafe keyword in the new code. Kinda defeats Rust's safety advantages, doesn't it?

This Rust version of Bun? It's still pretty rough. Issues that never showed up in the Zig version are now popping up on GitHub. No official version number for the Rust build yet, but that's just a matter of time. If you're keen to stick with the old Zig, better lock your version at 1.3.14. It might just be the last of its kind.

The AI Rewrite's Price Tag

Think about it: an entire project of this scale, migrated solely by a language model. It's unprecedented in software development. We don't know the exact number of tokens Claude chewed through. But honestly, figuring that out would give us a real sense of what these AI-driven projects actually cost.

AI isn't just changing how we write code; it's rewriting the rules.

What This Means for You

If you're a Bun developer, this Rust shift could mean better performance and enhanced safety. But you'll need to stay on top of the latest developments. Using Bun in your projects? Test the Rust version. Thoroughly. Before you even think about pushing it to production.

Still Plenty of Questions

Long-term, what are the implications of AI doing these massive rewrites on software quality? How will the community tackle the bugs and issues that crop up from this transition? And yeah, what's the real cost of using AI for such a huge task?

Why It Matters

This Bun project is a peek at the future. Claude's transformation of the codebase underscores the raw potential of AI models to handle massive migrations. It's a glimpse into a world where AI plays a central role in, well, everything. Good luck, developers. You're gonna need it.