--- title: How is the Bun Rewrite in Rust Going? | daily.dev description: A skeptical analysis of Bun's claimed Rust rewrite, which Anthropic funded at $165,000 in API costs over 11 days. Six weeks after the rewrite was merged to... canonical: https://daily.dev/posts/how-is-the-bun-rewrite-in-rust-going--gh9gwt6nl twitter:card: summary_large_image twitter:site: @dailydotdev og:type: website og:site_name: daily.dev og:title: How is the Bun Rewrite in Rust Going? | daily.dev og:description: A skeptical analysis of Bun's claimed Rust rewrite, which Anthropic funded at $165,000 in API costs over 11 days. Six weeks after the rewrite was merged to... og:url: https://daily.dev/posts/how-is-the-bun-rewrite-in-rust-going--gh9gwt6nl og:image: https://api.daily.dev/og/posts/Gh9gwt6NL.png og:image:alt: How is the Bun Rewrite in Rust Going? og:image:width: 1200 og:image:height: 630 og:locale: en --- > ## Documentation Index > Fetch the complete documentation index at: https://daily.dev/llms.txt > Use this file to discover all available pages before exploring further. # How is the Bun Rewrite in Rust Going? **[Hacker News](https://daily.dev/sources/hn)** · 5 min read · 0 upvotes · 0 comments ## Summary A skeptical analysis of Bun's claimed Rust rewrite, which Anthropic funded at $165,000 in API costs over 11 days. Six weeks after the rewrite was merged to main, there is still no release tag — the longest gap in Bun's release history. The number of open AI-generated pull requests from 'robobun' nearly doubled in 18 days (from 1,277 to 2,475), CI/CD pipelines are running continuously, and Anthropic employees are directly contributing code. The author argues the true cost far exceeds the stated $165k figure and urges skepticism toward AI capability claims that support large company valuations, noting that similar high-profile AI coding projects (Anthropic's C compiler, Cursor's FastRender browser) have gone dormant. ## Full article daily.dev links to this article rather than hosting it. Read it at the original source: ## Community take How the wider developer community reacted, aggregated from 1 discussion and 141 comments across hackernews (as of 2026-07-27). **TL;DR:** The HN community is broadly skeptical that Bun's Rust rewrite is truly "done" — no public release has shipped in two months, the codebase is acknowledged to be non-idiomatic Rust full of unsafe blocks, and many see the whole effort as primarily an Anthropic marketing stunt, though some note it is already running in Claude Code for millions of users. **Sentiment:** 20% positive · 45% mixed · 35% skeptical **The case for** - The Rust version has been silently serving millions of Claude Code users for over a month without anyone noticing, which is a meaningful real-world validation. - The $165k–$800k cost is considered trivial for a company of Anthropic's scale, making the experiment low-risk financially. - LLM-assisted translation is seen as a reasonable head-start, especially when the source code itself serves as a detailed spec. **The pushback** - No public release has shipped in two months, suggesting the rewrite is far from production-stable for the broader Bun ecosystem. - The resulting Rust code is explicitly non-idiomatic, heavy with unsafe blocks, and reportedly introduces unsoundness/UB — arguably less safe than the Zig it replaced. - Many commenters view the entire exercise as an Anthropic marketing stunt rather than a genuine engineering improvement. - The original Zig code was itself a non-idiomatic port from Go, so the Rust version inherits layers of accumulated tech debt. - Anthropic has shown little transparency about progress, costs, or remaining work, fueling distrust. - Claude Code's usage of Bun may exercise only a narrow slice of the API surface, making it a weak proof of correctness for the full runtime. **By community** - hackernews (skeptical): Commenters are broadly doubtful the rewrite is complete or safe, framing the lack of a release and non-idiomatic code as red flags, while a minority defend the Claude Code deployment as meaningful validation. **Hottest debate:** Whether shipping the Rust build silently inside Claude Code constitutes meaningful proof the rewrite is production-ready, or is just a narrow, low-coverage test that proves little about the broader Bun runtime. **Open questions** - When will Bun v1.4 (Rust) actually ship as a stable public release? - How much of the Bun/Node API surface does Claude Code actually exercise, and what remains untested? - Will Anthropic publish a full retrospective on costs, unsound code, and remaining unsafe usage? - Will the team ever refactor the codebase toward idiomatic Rust, or will it remain a direct Zig-to-Rust translation indefinitely? **Highlights** > Rust contains the ability to do everything that C can, if you use `unsafe`. And a file-by-file rewrite in Rust from another language usually involves keeping the ABI and API between files very C-like (and unsafe). In practice this means that the result of a first pass this way has all the memory safety of C code, but with worse readability because Rust makes unsafe things less ergonomic. To actually get the safety benefits of Rust in a real way you have to rework those files to not treat eachother as C code. This is the interesting and the difficult part of a rewrite in Rust, and one that an unsupervised LLM rewrite is probably not even going to attempt. I don't think this latter stage has actually happened with Bun's codebase. The result of the Rust rewrite in Bun's case is actually less safe than the Zig it is replacing, especially because (I am told) a lot of the new Rust code is unsound (introducing UB). > — [TazeTSchnitzel on hackernews](https://news.ycombinator.com/item?id=49068906) > The Bun author himself writes that the port is not idiomatic Rust: Do the rewrite that looks like we transpiled our Zig code to Rust. We can gradually refactor it to reduce unsafe usage and look more like idiomatic Rust after Bun v1.4 ships. (from: https://bun.com/blog/bun-in-rust) The original version of Bun was already a line-by-line (manual) port of esbuild from Go to Zig (also mentioned in this post), so the Zig code already wasn't "Zig-idiomatic" and apparently riddled with problems. That doesn't inspire much confidende in the Rust LLM-translated Rust version tbh. > — [flohofwoe on hackernews](https://news.ycombinator.com/item?id=49068985) > I really do not understand how software developer think anymore. Using a LLM to translate a project in a short time, is by itself incredible. Just like one-shot whatever office clone. But what makes software is not the fast creation of a "product" but that actual development of its features. Figuring out how everything needs to work together, fixing the bugs, and the o so boring UI work. I have used LLMs to create stuff like word clones just for fun. It was a disaster. Sure, it had the basic functionality. But the moment you started with page structure (harder then one non-stop scrolling page), tables, images, rotating, and so many details that make up just the basics of word. Not even the extended functionality. You see every LLM just fall on its face. Sure, i can clone sqlite from c to rust. Hell, i may even get it to do all the tests 100%. But there is a 99% chance that the clone will be slower, as it lacks the years of optimizations from the original language. There will be new bugs because of the language changeover. There is a need for future support and fixes. People threat software like its something it is not. But unlike the past where your clients question your sanity for charging 100k for a piece of software. Not understanding its not just about writing the code. Now those expectation are even more pushed forwards, because of articles like this. I constantly see software being published on reddit that does X, Y, Z only for the authors to abandon it as fast as they vibe coded it. Because fixing bugs is NOT sexy. Even with a LLM at your fingertips. Dealing with nagging users, is not sexy. Dealing with security issues, is NOT sexy. Dealing with data structure / databases, especially as your system changes ... you get the point. Not understanding to the core the software you wrote, is going to exploded in your face. This is why these stupid "we rewrote X into Z with a LLM in Y days" mean nothing. Its one thing to get a head start using this trick, its another to actually learn the code of your rewrite. And dedicated the time into maintaining the port, growing it, fixing it. This is where a lot of software fails. But now this crap is out there, instead of the maintained version of Zig, now we have a unmaintained Rust version that clouded the airwaves because if people now search for it, those articles "X in Z days" will pop up. What have we become ... > — [benjiro29 on hackernews · 7 comments](https://news.ycombinator.com/item?id=49068644) > Claude Code has a huge userbase. This is an impressive result so soon after the rewrite. This is an impressive milestone for their rewrite. There are a lot of comments trying to downplay this as being unimpressive for some reason, but I can’t see them as anything other than sour grapes because the rewrite hasn’t crashed and burned like they were hoping. > — [Aurornis on hackernews](https://news.ycombinator.com/item?id=49069402) > It's just a marketing stunt. They've got the ad for Claude-powered code rewrites, and that worked beautifully. Everyone got the message: spend big bucks with Anthropic to get rid of whatever codebase bugs you - please, think of the IPO! If it was about Claude Code itself, they could have rewritten it. They keep saying they don't even write code any more, so it shouldn't even matter what language it's in. > — [pornel on hackernews](https://news.ycombinator.com/item?id=49069222) **Source threads** - [hackernews](https://news.ycombinator.com/item?id=49067854) · 62 points · 141 comments --- Tags: [#open-source](https://daily.dev/tags/open-source), [#rust](https://daily.dev/tags/rust), [#ai-coding](https://daily.dev/tags/ai-coding), [#anthropic](https://daily.dev/tags/anthropic), [#bun](https://daily.dev/tags/bun) [View this post on daily.dev](https://daily.dev/posts/how-is-the-bun-rewrite-in-rust-going--gh9gwt6nl) ```json {"@context":"https://schema.org","@graph":[{"@type":"Organization","@id":"https://daily.dev/#organization","name":"daily.dev","url":"https://daily.dev","logo":{"@type":"ImageObject","url":"https://daily.dev/apple-touch-icon.png","width":180,"height":180},"sameAs":["https://twitter.com/dailydotdev","https://github.com/dailydotdev","https://www.linkedin.com/company/daily-dev-ltd"]},{"@type":"WebSite","@id":"https://daily.dev/#website","url":"https://daily.dev","name":"daily.dev","publisher":{"@id":"https://daily.dev/#organization"},"potentialAction":{"@type":"SearchAction","target":{"@type":"EntryPoint","urlTemplate":"https://daily.dev/search?q={search_term_string}"},"query-input":"required name=search_term_string"}}]} {"@context":"https://schema.org","@type":"TechArticle","headline":"How is the Bun Rewrite in Rust Going?","url":"https://daily.dev/posts/how-is-the-bun-rewrite-in-rust-going--gh9gwt6nl","mainEntityOfPage":{"@type":"WebPage","@id":"https://daily.dev/posts/how-is-the-bun-rewrite-in-rust-going--gh9gwt6nl"},"datePublished":"2026-07-27T12:01:18.927Z","dateModified":"2026-07-27T13:47:53.130Z","description":"A skeptical analysis of Bun's claimed Rust rewrite, which Anthropic funded at $165,000 in API costs over 11 days. 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