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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

#551

Post by TurdF3rguson »

pdimitar wrote: post #342
coldtea wrote: post #295

Earlier quoted context omitted.

> 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. Your counter argument would be valid for a 2000 or a 2020 business decision about some tech stack. But the whole point of their product is that it supposedly nullifies such "business" concerns around the use of technology, by making it cheap and fast to build w…

> But the whole point of their product is that it supposedly nullifies such "business" concerns around the use of technology, by making it cheap and fast to build whatever you like automatically. This is a spectrum, it's not just 0% vs 100%. Even Fable frakked up a few things really badly in my professional work (though to its credit after a very detailed 2h chat it self-corrected and fixed all the blunders). I would…

It has to have been purely a marketing move. There's no other way that acquiring a software product for millions and then completely rewriting it makes any sense.

Re: Claude Code uses Bun written in Rust now

#552

Post by endospore »

daishi55 wrote: post #382
endospore wrote: post #358

Earlier quoted context omitted.

That's a stated goal, so nope. But releasing it before any cleanups is another story. I'd also like to inform you that - the current success metrics solely consist of their advertisement, my eyes looking at the code strongly suggest otherwise - the Bun team lacks knowledge to actually make it more idiomatic: none of the Bun team has written any Rust and relying on model knowledge is already proven insufficient. The r…

> the Bun team lacks knowledge to actually make it more idiomatic: none of the Bun team has written any Rust Do you know how silly this sounds? Good engineers can work in any language. At the top companies (of which Anthropic is certainly one) they don’t have “C++ engineers” and “rust engineers” they just have engineers.

> Good engineers can work in any language

Given that they are investing their time into actually learning the language before diving into this mess. No, unsafe Rust isn't supposed to be something you can handle without prior knowledge.

Jarred was even asking in public how to use traits without runtime overhead during the rewrite. And the blog post phrased it like a normal problem to encounter while doing a million lines port... That's two months after the initial rewrite so you can guess if they learned.

Re: Claude Code uses Bun written in Rust now

#553

Post by usef- »

mobelkh wrote: post #522
pdimitar wrote: post #342

Earlier quoted context omitted.

> But the whole point of their product is that it supposedly nullifies such "business" concerns around the use of technology, by making it cheap and fast to build whatever you like automatically. This is a spectrum, it's not just 0% vs 100%. Even Fable frakked up a few things really badly in my professional work (though to its credit after a very detailed 2h chat it self-corrected and fixed all the blunders). I would…

can't imagine rewriting a runtime to be easier than a coding harness

You might be surprised. Runtimes can be easier to test than UI for AI, and the bun rewrite was almost line-for-line translation, whereas CC likely uses many more libraries that cannot be ported directly.

The major open harnesses are in the ~800k lines of code size ( https://simonwillison.net/2026/Jul/15/grok-build/ ) and CC is the oldest and quite-likely largest judging by features.

Re: Claude Code uses Bun written in Rust now

#554

Post by cube00 »

pronik wrote: post #477
bmitc wrote: post #220

Earlier quoted context omitted.

The outrage was more how the rewrite was communicated and defended, which was objectively poor.

Which was completely bonkers -- it happened completely in the open, people were just outraged that something like this happens without asking them and not on a schedule they perceive as adequate. Most of them haven't really grasped what was happening and why. Many still don't.

> it happened completely in the open

You don't create PRs like this when you're working "completely in the open"

  test ci #30412
  No description provided.
https://web.archive.org/web/20260509235415/https://github.co...

Re: Claude Code uses Bun written in Rust now

#555

Post by pdimitar »

TurdF3rguson wrote: post #551
pdimitar wrote: post #342

Earlier quoted context omitted.

> But the whole point of their product is that it supposedly nullifies such "business" concerns around the use of technology, by making it cheap and fast to build whatever you like automatically. This is a spectrum, it's not just 0% vs 100%. Even Fable frakked up a few things really badly in my professional work (though to its credit after a very detailed 2h chat it self-corrected and fixed all the blunders). I would…

It has to have been purely a marketing move. There's no other way that acquiring a software product for millions and then completely rewriting it makes any sense.

Maybe it's one of the cases when techies call the shots?

I can understand people being more sold on Rust compared to Zig -- there are legitimate differences on the memory safety front and I can see how people will prefer one or the other.

Plus I believe Jarred (Bun's main dev) went out of his way to explain why he chose Rust over Zig in his blog post.

Of course the marketing angle might indeed be the explanation; not arguing against that, I simply followed with interest and Jarred seemed very reasonable to me.

Re: Claude Code uses Bun written in Rust now

#556

Post by endospore »

Tadpole9181 wrote: post #369
endospore wrote: post #234

Earlier quoted context omitted.

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 ~0.2…

> I'd remind you the current new version is not an improvement compared to the previous one, both in terms of correctness and maintainability. Except, you know, multiple real companies saying that it is and using it in production. And the fact it closed all known memory leaks. And that nobody has a really pointed to a single actual issue the new version introduced after two months of endless, ceaseless bitching. I'm…

> nobody has really pointed to a single actual issue

I personally know 3 categories of newly introduced issues spanning the code base with 200+ occurrences, that leads to undefined behavior and memory issues. Not doing any contributions to the code is my conscious decision, which includes not pointing to the specific problems in public.

It's nice to hear that people are doing the same.

Re: Claude Code uses Bun written in Rust now

#557

Post by zamadatix »

gchamonlive wrote: post #424
mrothroc wrote: post #372

Drilling into the original article where Jarred explained the reasoning behind the change, It's pretty clear that under zig the team was doing things by hand that are automatic in rust. Humans and agents share one thing: they are both non-deterministic. He talks about the issue of tracking memory lifecycles manually in zig so it can be explicitly freed. As expected, this leads to a long list of bugs where people miss…

> Rust does this automatically. It removes an entire class of errors from his backlog. Even with the huge amount of "unsafe" rust currently in bun? https://news.ycombinator.com/item?id=48967630

As opposed to every commit abd line of code being unsafe? They use a lot of C/C++ libraries with their own code so unsafe isn't even a a problem in most places they use it.

Re: Claude Code uses Bun written in Rust now

#558

Post by mixmastamyk »

hahahaa wrote: post #534
mixmastamyk wrote: post #479

Earlier quoted context omitted.

Stuff doable on a pentium 90, and before that, just slowly.

I wonder if those were fast enough nostalgia says yes but I bet it was dog slow if we tried today with 2026 lens.

My Turbo Pascal programs could calculate and print to the screen faster than could be read, on a 486. With DOS however.

I picked the P90 because I remember running Linux ~1.0 and/or NT on it, with real multitasking and services etc. Those soaked up many of the cycles.

Another story, around Y2k/Win2k was asked to write a C program to record all the files/sizes on the C: in to file. Then something would be installed, my program run again, and the results diffed. Despite thousands of files the program was done before the enter key came back up. Boss told me that he thought the program was cheating.

That’s just how fast modern computers are these days, I said. When running C code anyway. That perf is still available today, compare astral’s uv to slow competitors. Just takes some investment, though probably less when boosted by VC bionics.

Re: Claude Code uses Bun written in Rust now

#559

Post by godwinson__4-8 »

William_BB wrote: post #346
phinnaeus wrote: post #158

Earlier quoted context omitted.

> I'm a developer Then you must know that the hardest part of software development is getting clear requirements... > everyone on this planet could create a software for themselves exactly as how they wanted Even with perfect AGI this will never happen because people will never be able to express clear requirements

It's not even about requirements. It's about responsibility. Someone has to take responsibility for the code and the product. Someone has to hotfix a bug that's costing millions of dollars an hour and someone has to be blamed for a bug that's consting millions of dollars an hour.

Not really. The secret of our society is it wouldn't work if people with a controlling interest in things ever faced personal responsibility. Who got blamed for the Iraq war? How much did that cost? The theory is if you tried to blame someone then some future president wouldn't be able to declare war. You don't know how the law works apparently. Corporate law is pretty much the same. The economy doesn't boom if CEOs can actually be blamed for things. You don't blame a "someone" that's what incorporation does. If some code does a booboo just put a $ amount on it, withdraw the funds and move on. The media will focus on the humans because of the drama, but they are largely irrelevant. It doesn't come out of their pockets. A human is already window dressing in this status quo. A LLM has no reason to do the sorts of things that violate such legal protections, be it of a president (none really apply) or a CEO (good luck getting a CEO to take personal responsibility for anything short of outright fraud). If it suddenly goes haywire the law already has a concept of vis major.

The secret is individual humans are already largely irrelevant to our society. That's how we got here. The trend will only continue.

Re: Claude Code uses Bun written in Rust now

#560

Post by TurdF3rguson »

pdimitar wrote: post #555
TurdF3rguson wrote: post #551

Earlier quoted context omitted.

It has to have been purely a marketing move. There's no other way that acquiring a software product for millions and then completely rewriting it makes any sense.

Maybe it's one of the cases when techies call the shots? I can understand people being more sold on Rust compared to Zig -- there are legitimate differences on the memory safety front and I can see how people will prefer one or the other. Plus I believe Jarred (Bun's main dev) went out of his way to explain why he chose Rust over Zig in his blog post. Of course the marketing angle might indeed be the explanation; not…

The rewrite makes sense, it's all their dog food at that point. It's the acquisition that doesn't make sense. Why didn't Anthropic just rewrite it themselves?
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