Anthropic’s Bun Rust rewrite merged at speed of AI
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Anthropic’s Bun Rust rewrite merged at speed of AI
Version 1.3.14 of JavaScript toolkit released as last Zig version; a million lines of Rust code merged in gargantuan commit
Tim Anderson
Tim<br>Anderson
Published<br>thu 14 May 2026 // 14:01 UTC
A pull request with a Rust version of Anthropic’s Bun, a<br>JavaScript toolkit and runtime originally written in Zig, has been merged to<br>the main Bun repository. This comes just days after its author, Jared Sumner, said<br>"there's a very high chance all this code gets thrown out."<br>Sumner posted on X (formerly Twitter) five days ago that "99.8 percent of bun's pre-existing test suite passes on Linux x64f<br>glibc in the rust rewrite," a clue that what was initially described as an<br>experiment was likely to make it to production. Three days later, the Bun team<br>released version 1.3.14, with Sumner stating that if the Rust rewrite was<br>merged, "this would be the last version in Zig."
Creator Jarred Sumner presented Bun in a launch video
Today that merge took place,<br>adding more than one million lines of code. Sumner said it passes Bun's test<br>suite on all platforms, fixes some memory leaks, and shrinks the binary size by<br>between 3 and 8 MB.
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"Most importantly, we now have compiler-assisted tools<br>for catching and preventing memory bugs, which have cost the team an enormous<br>amount of development and debugging time over the years," he said in a<br>comment. Performance is either neutral or faster, he said, though the codebase<br>is "the same architecture, the same data structures." No async Rust<br>is used.
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Bun users have hit memory leak issues when deploying it as a<br>production runtime. According to Sumner,<br>"Rust won’t catch all of these - leaks from holding references too long<br>and anything that re-enters across the JS boundary are still on us. But a large<br>percentage of that list is use-after-free, double-free, and forgot-to-free-on-error-path,<br>and those become compile errors or automatic cleanup."
AI Slop? A pull request removing Zig source files from Bun was automatically flagged.
A second pull request, removing upwards of 600,000 lines of Zig code,<br>was automatically flagged by GitHub as "AI slop" and closed, but will<br>presumably reappear in some form.<br>The size of these commits makes them near-impossible for<br>humans to review. "What a nice reviewable little commit. I'm sure it will<br>not contain any bugs," said one comment on the Rust merge.<br>Although the idea of the Rust port has been well received, the<br>speed of the transition has taken the community by surprise. In normal circumstances,<br>porting a major project so quickly would be risky, but this has been accomplished<br>using AI tools. According to Sumner, it is "essentially the same codebase<br>ported to Rust."
Bun creator Jarred Sumner said that AI writes all the code
Asked whether the Rust version would be maintained mainly by<br>Anthropic’s Claude Code, Sumner said "this is already the status<br>quo; we haven’t been typing code ourselves for many months now. Even<br>pre-acquisition [by Anthropic] this was pretty much accurate."<br>Sumner was formerly a strong Zig advocate, but Zig’s no-AI<br>policy is at odds with the Bun team’s way of working, and recent versions of<br>Bun use a Zig fork with contributions that cannot be merged upstream, and which<br>Zig’s maintainers said would not be welcome regardless of the AI aspect.
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Version 1.3.14, the last one still to use Zig, adds a<br>built-in image processing API for decoding, transforming and encoding images.<br>It is designed as a drop-in replacement for the Sharp image processing library for Node.js. The new release also adds experimental<br>support for the HTTP/3 (QUIC) protocol in Bun’s integrated server. The full<br>release notes describe these and other new<br>features.<br>Is it possible to move this fast and not break things? Bun's<br>migration from Zig to Rust will be watched with interest by AI advocates and<br>sceptics alike. ®
javascript runtime<br>ai<br>zig<br>developer<br>rust<br>devops<br>bun<br>anthropic
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