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Claude Code uses Bun written in Rust now

simonwillison.net

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Re: Claude Code uses Bun written in Rust now

#231

Post by doctorpangloss »

layer8 wrote: post #100
atonse wrote: post #18

Earlier quoted context omitted.

It’s made absolutely no negative difference, as we’ve seen in the real world in the last 60 days since the merge. I feel weird having to defend reality; reality being that it was merged nearly 2 months ago and tons of people have had their pitchforks out without a shred of actual evidence that this made bun worse in any measurable way. But they still insist it was a mistake. I’ve never met Jared or the bun team but I…

> who cares what language a JS toolset is written in? Anthropic, apparently, not the least because they blanket-closed all issues submitted before the rewrite. If the language were irrelevant, it shouldn’t matter for the existing unfixed issues.

imagine being the leadership of the one of most valuable US private companies in the world. then you have to make something called a "pull request" that gets "reviewed" by someone named "Jarred" to make improvements to your stuff. do you see how that is untenable?

the language is relevant for political not technical reasons. you could say, blanket closing the Issues, getting rid of this thing called Jarred from your stack, etc., is really an attack on the GitHub and the GitHub Agitator Lifestyles, which is why so many commenters on Hacker News actually care.

Re: Claude Code uses Bun written in Rust now

#232

Post by whateveracct »

kburman wrote: post #9

Honestly, I initially thought rewriting an entire codebase with AI would be a huge mistake. After reading this, I'm starting to think I was wrong. If projects like Bun can be substantially rewritten and shipped to millions of users, it suggests we're entering a very different phase of software development. Today's AI-generated rewrites may not produce code that humans would consider high quality or maintainable. But…

> Honestly, I initially thought rewriting an entire codebase with AI would be a huge mistake. After reading this, I'm starting to think I was wrong.

> If projects like Bun can be substantially rewritten and shipped to millions of users, it suggests we're entering a very different phase of software development.

exactly. this wasn't a technical project. this was a marketing stunt that worked on you.

Re: Claude Code uses Bun written in Rust now

#233

Post by johnfn »

weakfish wrote: post #212

Maybe I’m taking crazy pills, but I’m still stuck on “why the hell does a TUI need to run in terminal React by way of JavaScript” The fact that Anthropic felt the need to buy a runtime so they could make their TUI better speaks more to the quality of engineering than anything else IMO. If rewrites are so easy, why not rewrite CC in a native language? Would’ve been a hell of a lot cheaper.

Why would rewriting Claude code, an app which probably has 30-40 (I might be significantly underestimating) extremely active contributors be easier than rewriting Bun, which has fewer contributors and almost certainly also less lines of code?

Re: Claude Code uses Bun written in Rust now

#234

Post by endospore »

Tadpole9181 wrote: post #217
endospore wrote: post #207

Earlier quoted context omitted.

Reviewing is meaningless while they are still keeping the 10433 (sorry it has become 10503 since last week) unsafe blocks, most unsound and none encapsulated. Any review would get to the simple conclusion that this should not be released before all the obvious bads are sorted out.

This feels like such an absurd, bad faith take I keep hearing. In Zig, every single memory operation is unsafe. And Bun must interface with C code that has no safe interface, necessitating a ton of boundary-level unsafe behaviors. There's too much, sure, but can we at least be honest and reasonable?

My conclusion was formed in my two months long tracking of the repo activities. They have done absolutely nothing in that front. (Well, to be precise they tried to fix exactly one thing that was pointed out but that's it)

> must interface with C code that has no safe interface

Yeah so the sane first step is to create encapsulated, safe interface for them, especially in a project like this. Deno for instance have ~0.2x as many unsafes.

And mind you if you haven't read the code, the vast majority of unsafe blocks in bun are for raw pointer access to local (Rust) objects because their ownership was a mess both before and after the rewrite. Also funnily enough a lot of the access patterns are wrong (in the Rust sense), leading to hundreds of new undefined behaviors.

> be honest and reasonable

Well, well. Talking about dishonest and unreasonable behavior, why is bun releasing a new version before solving any of those glaring issues? I'd remind you the current new version is not an improvement compared to the previous one, both in terms of correctness and maintainability.

Re: Claude Code uses Bun written in Rust now

#235

Post by K0nserv »

bitwize wrote: post #201
petcat wrote: post #12

Earlier quoted context omitted.

I think the Zig people are really just worried that maybe Zig itself is a DOA language before it has even reached 1.0 because it doesn't offer enough over C for any serious use and their flagship project has now abandoned it.

Zig is a DOA language for other reasons though. The reason why Rust enthusiasts act so morally superior is because they are: > Rob Pike, an unsafe guy, may insist that a small language and garbage collection are enough. But the typesafe visionary Grzegorz Wielbodłąński understands the deeper truth: every invalid state permitted by a compiler is a tiny act of civilizational sabotage. https://x.com/typememetics/status/…

> There is no sense pretending anymore that languages with as many gotchas as C and Zig are "simple". Quite simply, they are not. The complexities lie in what's left unspoken, unaddressed, and of course undefined; failure to comprehend these completely can be catastrophic.

This is where I’ve arrived too. After nearly 20 years of programming I feel done with leaving foot guns lying around. Languages that are full of them, but “fine” as long as you take extreme care are not interesting to me. I’ve started calling this “simple as long as you avoid all the foot guns” idea the sword juggling fallacy[0].

It’s too bad about Zig because it has many nice ideas, in particular allocators, that I hope Rust or some future language will adopt.

0: https://hugotunius.se/2026/06/15/the-sword-juggling-fallacy....

Re: Claude Code uses Bun written in Rust now

#236

Post by switz »

weakfish wrote: post #212

Maybe I’m taking crazy pills, but I’m still stuck on “why the hell does a TUI need to run in terminal React by way of JavaScript” The fact that Anthropic felt the need to buy a runtime so they could make their TUI better speaks more to the quality of engineering than anything else IMO. If rewrites are so easy, why not rewrite CC in a native language? Would’ve been a hell of a lot cheaper.

It largely works and it's a massive business success. This is the classic engineer asking the 'why this technology?' to what amounts to a business question.

They chose it early on, it works, and it makes obscene amounts of revenue. End of story. That doesn't mean it was the "greatest" choice, or has a perfect technical architecture.

Rewrites are never easy, even the bun rewrite. But a non-UI developer tool with a rigid API surface contract (and associated tests) will always be easier to trust after a rewrite than a partially tested UI tool with ambiguous functionality.

Re: Claude Code uses Bun written in Rust now

#237

Post by [deleted] »

weakfish wrote: post #212

Maybe I’m taking crazy pills, but I’m still stuck on “why the hell does a TUI need to run in terminal React by way of JavaScript” The fact that Anthropic felt the need to buy a runtime so they could make their TUI better speaks more to the quality of engineering than anything else IMO. If rewrites are so easy, why not rewrite CC in a native language? Would’ve been a hell of a lot cheaper.

[deleted]

Re: Claude Code uses Bun written in Rust now

#238

Post by MitziMoto »

Aeveus wrote: post #175

Anecdotal, but I’ve been getting segfaults in Claude Code. I run inside Kitty tabs, and the entire tab becomes unusable when this happens. No response to inputs at all from that point. When this happens, a link shows up to report the issue. It’s not clickable (likely due to the segfault), and perhaps more important: it’s encoded, so you can’t see what you would be sending in your report. Hope it gets better.

Something similar has been happening to me with Ghostty, but there is no error message or link and it only happens when Claude uses it's interactive questions interface.

It becomes completely unresponsive to any input except scrolling. I can't select options or even cancel out.

(I am in no way implying this is related to Bun)

Re: Claude Code uses Bun written in Rust now

#239

Post by voidhorse »

weakfish wrote: post #212

Maybe I’m taking crazy pills, but I’m still stuck on “why the hell does a TUI need to run in terminal React by way of JavaScript” The fact that Anthropic felt the need to buy a runtime so they could make their TUI better speaks more to the quality of engineering than anything else IMO. If rewrites are so easy, why not rewrite CC in a native language? Would’ve been a hell of a lot cheaper.

Probably because claude suggested some kind of wack react based setup early on (because react dominates the training data) and it's be blasphemy worthy of termination for the Anthropic employees to question the sacred pronouncements of the llm.

Re: Claude Code uses Bun written in Rust now

#240

Post by hmokiguess »

pi runs in TypeScript and I like it, the target language of the tool, for me, matters only such that I can extend it. I like writing extensions, plugins, things, in TypeScript so I'm cool with that!

Claude Code is closed source, doesn't let me extend it beyond hooks, and these I write in bash anyways.

That all said, why should I bother about this change? Feels like a nothing burger to me, as an end user. This should matter more for those that are internal to Claude Code and Bun developing it.

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