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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

#191

Post by variadix »

feverzsj wrote: post #65

It's a transpile. And not even a good one. The generated code is far from idiomatic rust. Some may consider it an abomination.

Yeah I don’t understand this port at all other than as a big marketing stunt.

It would have made far more sense, for reliability, efficiency, cost, etc., every metric really, to use or write a source to source translator that preserves as much structure from the original code as possible. Typically if you do a rewrite there are lessons learned from the existing code base that you want to take into account when doing the rewrite; using a bunch of agents to do the porting file-by-file buys you none of that. In either case the code will be an unidiomatic translation, just with LLMs you get an added source of indeterminism and a huge bill at the end of the month.

Re: Claude Code uses Bun written in Rust now

#192

Post by ethbr1 »

conartist6 wrote: post #162
rjzzleep wrote: post #120

Earlier quoted context omitted.

What about Group x. You just AI generated 1 millions lines of code claiming it's for safety. Who exactly is to make any kind of security guarantees about this?

Don't forget the group asking really dumb pointless questions like "who legally owns the rewritten code"

Afaik, under current US law (Thaler v. Perlmutter 2025^), "the Copyright Act of 1976 requires all eligible work to be authored in the first instance by a human being".

With regards to AI-generated code, this means the copyrightability (at all, not with regard to who owns it) turns on whether or not a human was substantially involved in its creation.

Existing decisions require evidence of pretty heavy and continued human guidance to qualify.+

To wit, autonomous agent created code (prompt -> machine churns for hours -> output) is explicitly not eligible for copyright.

Functionally, this is a double-edged sword.

On the one hand, it means anything coded with autonomous agents by Meta, Google, et al. can be legally reused if it leaks (because no one could hold copyright on it).

On the other hand, it leaves copy-left open source licenses in a weird place. If you convert open source (even MIT/BSD-style) code into something else with an autonomous coding agent... the result has no copyright (nor can ever in the US).

In this instance, Bun was MIT-licensed, no? Then it was shoved mostly-autonomously through an LLM for the port.

Now Bun-Rust is technically still MIT licensed, but if push came to shove it seems like US law's current position is that Bun-Rust would now have no copyright license (because the manner in which it was developed renders it ineligible for copyright).

That's on the copyright side.

On the infringing usage side (i.e. whether you were entitled to shove a copyrighted work into a coding agent to produce something)... that's still TBD.

^ https://media.cadc.uscourts.gov/opinions/docs/2025/03/23-523...

+ https://garrettham.com/ai-generated-works-copyright/

Re: Claude Code uses Bun written in Rust now

#193

Post by marcindulak »

voiper1 wrote: post #62
znpy wrote: post #19

> Startup got 10% faster on Linux but otherwise, barely anyone noticed. Just that? I was expecting more from that rewrite. Maybe Rust is not so worth it after all.

The blog claims rust chosen mainly to address memory issues, which rust is a better language for. So, success would simply be less new memory errors / easier to patch old ones.

Claude installer/updater had a high memory usage problem (Gigabytes of resident memory used during a fresh installation of Clade with it's native install.sh script, or during a claude update). This was reported as early as December 2025, see https://github.com/anthropics/claude-code/issues/12327#issue....

The latest Claude installer (2.1.215) possibly does not have it anymore https://github.com/anthropics/claude-code/issues/4953#issuec..., but more tries on various machines are needed to confirm that.

Re: Claude Code uses Bun written in Rust now

#194

Post by rane »

Matl wrote: post #93
stymaar wrote: post #40

Earlier quoted context omitted.

Rust as a language is irrelevant to this discussion, the problem is that bun was vibe coded by a single dude without any open source community involvement. “bun the open source project” is basically dead at that point: don't expect any of the zig enthusiasts who had their code being forcibly rewritten in a language they don't like to follow Jared. “bun the JavaScript runtime ” is not dead though.

I agree, it's unfortunate the headlines seem to have become 'rewritten in Rust' (not a bad thing) and not 'vibecoded in a week without review' (a bad thing).

"Vibecoded" is not an accurate description of what actually happened. LLMs are extremely good at porting code from one language to another while preserving semantics. Which is why how everything turned out so well was not very surprising.

Re: Claude Code uses Bun written in Rust now

#195

Post by Lerc »

jdiff wrote: post #80
alexjurkiewicz wrote: post #70

Earlier quoted context omitted.

Come on, "not publicly available"? You know how git works right? Each Claude release probably has the HEAD of main compiled as "1.4.0" until there is a real official release.

I do know how git works and in my experience, while the version number sometimes doesn't get bumped until just before a release is tagged, the version number comes from the codebase. If they're building something that says 1.4.0, then they have a codebase that disagrees with ours.

Where is the previous version referred to in the codebase? I'm guessing that would be the place to look for changes.

If you can't find an update at that place in the codebase saying 1.4.0 you can ask where it is. It seems after doing that and not receiving an answer would be the appropriate time to start making the claims that people are throwing around.

This seems like a candidate

https://github.com/oven-sh/bun/commit/b18bf6d1d0a92238f240bf...

Re: Claude Code uses Bun written in Rust now

#196

Post by embedding-shape »

firesteelrain wrote: post #176
embedding-shape wrote: post #156

Earlier quoted context omitted.

Your point being? None of my comments in either of those state that I'm upset about it moving from Zig to Rust?

I wasn’t referring to you and rather the general sentiment. Hence, goal posts have been moved again comment.

My overall larger point (maybe poorly stated) was that unless you're engaged specifically in a 1-on-1 conversation with someone, accusing a whole community of "moving the goalpost" makes no sense, there is a whole host of arguments and points being made in different directions all the time, by different individuals.

Re: Claude Code uses Bun written in Rust now

#197

Post by zzzeek »

atonse wrote: post #18
junon wrote: post #5

Earlier quoted context omitted.

Why does changing to Rust kill the project? I don't understand the point here.

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…

this whole AI era has put the topic of "do we believe actual reality, or what we hoped/assumed/continue to insist would be reality" front and center. Every discussion is like this these days.

Re: Claude Code uses Bun written in Rust now

#198

Post by klibertp »

brabel wrote: post #116
esjeon wrote: post #74

Earlier quoted context omitted.

> But I'm beginning to wonder whether that will even matter in a few months. At that point, even Bun itself doesn't matter. All intermediate tools don't matter if LLMs can reliably write something large. The problem is that LLM is not quite there yet. The rewrite was only possible because they mostly stick to 1-to-1 translation resulting in non-idiomatic Rust code. So, what from there? I don't think they can really b…

> The rewrite was only possible because they mostly stick to 1-to-1 translation resulting in non-idiomatic Rust code. That’s patently false, just read Jarred’s own blog post describing how that was the first stage only, they went through many more to get the amount of non idiomatic, unsafe Rust code to an acceptable level.

Somebody checked[1], and they discovered that the "stages" past the first had minimal impact on the unsafe blocks count. That's just one metric, but it makes me doubt your claim that there was/is a lot of work put into making the translated code idiomatic Rust.

[1] https://news.ycombinator.com/item?id=48967630

Re: Claude Code uses Bun written in Rust now

#199

Post by ethbr1 »

kstenerud wrote: post #188
afavour wrote: post #130

Earlier quoted context omitted.

> you just can’t win You could! You could not release a “pile of vibe coded slop” onto people’s machines. There don’t even appear to be any particular advantages to Claude upgrading early. If there are no one has been made aware of it.

How do you know it's vibe coded slop? Have you seen the source code?

> How do you know it's vibe coded slop? Have you seen the source code?

I believe you've just made your opponent's point for them.

Re: Claude Code uses Bun written in Rust now

#200

Post by firesteelrain »

embedding-shape wrote: post #196
firesteelrain wrote: post #176

Earlier quoted context omitted.

I wasn’t referring to you and rather the general sentiment. Hence, goal posts have been moved again comment.

My overall larger point (maybe poorly stated) was that unless you're engaged specifically in a 1-on-1 conversation with someone, accusing a whole community of "moving the goalpost" makes no sense, there is a whole host of arguments and points being made in different directions all the time, by different individuals.

I see what you are saying. My comment is rooted in the fact that different people are foregrounding different objections to Anthropic/Bun, and taken together it looks like whatever launches, someone will find a reason to be upset
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