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#rust#zig#code#rewrite#bun#more#language#claude#llm#better

Discussion (134 Comments)Read Original on HackerNews

didibus•about 1 hour ago
Article did a decent job of showing discipline and care and human involvement to assert the automated rewrite was done diligently, as best as it can be when using AI for it. I does make me feel a bit more comfortable about it.

As an aside, I don't know why anyone would not want to use a memory-safe (and possibly race-safe) language in 2026. Rust gives you that in a performant package, so if you are turned off by GCs and immutability for performance reasons, you still have the option to use Rust.

I can understand when you need the absolute best performance and you decide to drop to down to C++, and I also relate with just personal preference, but beyond those it seems a no brainer to me.

leecommamichael•44 minutes ago
> As an aside, I don't know why anyone would not want to use a memory-safe (and possibly race-safe) language in 2026.

The rust compiler is very slow. The best way to speed it up appears to be organizing a codebase in many crates. This is not preferable ergonomics to many. Beside that, for many problems, a garbage collector eliminates a large amount of defects (including the ones stated in the article) without any added friction, whereas Rust asks that you think in terms of ownership. This is not preferable ergonomics to many.

I realize what I'm saying above, while true, doesn't give a clear example. Many gamedevs would rather iterate with a language that is lower friction, not only because game code is finnicky (like frontend UI code) but because the build process can be unique. Many gamedevs prefer to iterate with hot-reloading, and asking them to use a slower compiler is asking them to accept greater latency in that cycle.

I do not claim that these reasons apply to everyone.

zamalek•8 minutes ago
Game engines are typically in two languages, one for the engine itself and one for scripting. That even goes for Unity: in Unity, C# is a significantly more powerful than average scripting language (for lack of a better term), but the engine itself is still C++.

That's not to say that you couldn't write a commercial game engine with something like C# that stands shoulder-to-shoulder with unity and unreal, but it doesn't seem like anyone has attempted to do so. Maybe it's the decompilation fear.

Also, it would continue to make sense to use a scripting language alongside Rust.

gpm•38 minutes ago
The comment you're replying to wasn't arguing rust > GCed languages (e.g. C# or whatever game dev language you are thinking of). It was arguing rust > non-GC non-safe languages (e.g. zig).
leecommamichael•13 minutes ago
I see that now, thanks. There's a lot to say here, especially with other approaches to memory management. My overall goal was to give them some context that wasn't their own.
kibwen•11 minutes ago
> The best way to speed it up appears to be organizing a codebase in many crates.

A "crate" in Rust is the unit of compilation. In C, a file is the unit of compilation. Rust just lets you have a compilation unit that's composed of more than one file (without having to resort to C-style textual inclusion). But if you want, you can certainly have one-file-per-crate, just like you would in C. And what's nice about that is that crates forbid circular dependencies, which trivially enables coarse-grained parallelism in the build system. So yes, organizing a large codebase into crates is the best way to achieve parallelism, but that isn't something to be deplored (and strictly controlling circular dependencies is useful for comprehending large codebases in general).

dabinat•about 1 hour ago
> This Rust rewrite would've taken a team of engineers with full-context on the codebase a year of work. With 1 engineer using Fable & closely monitoring Claude Code, we went from start to 100% of the test suite passing on all platforms in 11 days.

This is impressive from a technological standpoint, but it does gloss over the fact that it would have cost $165k in tokens were Bun not part of Anthropic.

The comparison here isn’t completely fair - it would take a small team a year to port it if they spent $0 extra on it.

I’d be interested to see a comparison between spending $165k in 11 days on Claude vs splitting that between 50 people over 11 days for a line-by-line rewrite of the Zig code. I suspect Claude might be faster and therefore cheaper, but maybe not by a lot.

jeremyloy_wt•32 minutes ago
They napkin math is fairly easy to do. One human works around 250 days per year, and if we assume Bay Area salaries we could assume ~300k/y conservatively for a fully loaded cost.

$1200 per day.

Your estimation is 50*11 days so $660,000. That’s 4x what Claude cost.

That’s assuming that you actually get those 50 people to work without blockers, stepping on each other, or other coordination issues. The coordination complexity alone is astounding.

I don’t like it necessarily, but Claude wins here, easily. It’s not close.

piskov•28 minutes ago
Unless you hire smart people from EU and what have you (especially ex-USSR)

Which takes us to a point of future US dev salaries if this thing with agents gets better more and more

slopinthebag•23 minutes ago
Why assume the upper level salary here? Using senior level developers making astronomical salaries for what is a mechanical line-by-line port would be a poor financial decision.

What does the math look like with 25 devs making ~100k and doing it in 22 days? I’m sure you could find a reasonable combination which costs less. And if you’re already paying the devs the salary, it’s basically free (minus the opportunity cost of them not working on other things).

Philpax•about 1 hour ago
I think it'd take you at least eleven days to meaningfully coordinate 50 people!
tekacs•about 1 hour ago
I feel like a core difference is that the AI implementor can get cheaper/faster (and indeed _uniformly_ better), whereas it would be very difficult for the same humans to do so.

Even if this is not the right answer today, it can at the very least serve as a herald of a possible future, no?

Jenk•about 1 hour ago
$165k won't get you far on salaried engineers. There's every chance that 1 engineer, assuming Anthropic employs them, is on $500k or more. Assuming average of $336k in that pool of 50 engineers, then for 11 days for 50 engineers you've spent $710k[0].

Salary info: https://www.levels.fyi/companies/anthropic/salaries/software...

[0]The maths I used (posting because I'm tired and prone to mistakes):

    $336,000 / 260 (working days of the year) = ~$1,292.
    $1,292 * 11 * 50 = ~$710,769
Philpax•about 3 hours ago
Without commenting on Bun itself as a project, or the nature of the rewrite, it can't be good for Zig that a naive rewrite away from it fixed memory leaks, improved stability, shrunk binary size by 20%, and improved performance by 5%.
simonw•about 1 hour ago
I don't think it's care to categorize this as "a naive rewrite away from [Zig]" - Jarred has been immersed in this project for five years, got to benefit from everything he learned along the way and spent $165,000 of tokens on the most advanced coding LLM anyone has access to.

I expect if he'd spent $165,000 running Fable against the Zig version he could have got a 5% performance improvement, too.

Philpax•about 1 hour ago
Oh, I have no doubt that they could have extracted those gains from Zig! My point is more that, from a relatively naive line-to-line port, they were able to claim these benefits without much effort.

It's not great for Zig if you have to put in more work to end up at the same place efficiency-wise, especially for a language marketed at people who like to get the most out of their metal.

malisper•21 minutes ago
I can confirm a naive rewrite won't make things faster. I've been working on rewriting Postgres in Rust. I rewrote things function by function similar to how Jarred did. Even though the new Rust code mapped closely with the previous C code, it was 8x slower. This was due to myriad of reasons. For example naively converting a C union into a Rust enum can be slower because Rust stores a tag with the enum, while C unions do not.

I've been working on a new rewrite that's focused on beating Postgres on performance. As of this morning I got to 100% of the tests passing and have meaningful performance gains over Postgres.

bielok•about 2 hours ago
I would guess that people looking to use Zig understand that those are project concerns and not language concerns.
Zakis1•about 2 hours ago
The stability gains are a direct language concern as mentioned throughout the article.
h14h•23 minutes ago
While it's easy to look at it that way on the surface, from reading the blog post, it sounds like a big part of it may just be the nature of Bun as a project.
throwaway27448•about 2 hours ago
True, but rewrites often allow for this sort of benefit in themselves. It's possible rewriting it in zig would have yielded some of the same improvements.
pdpi•about 2 hours ago
A sophisticated rewrite? Sure. This was a naive like-for-like rewrite, though.
lifthrasiir•about 2 hours ago
The same concern applies to every GC language, so it's not necessarily bad for Zig. Bun can have been grown too large for Zig to be effective, while moderately sized projects may still greatly benefit from Zig.
saghm•about 1 hour ago
I thought Zig was supposed to be a C replacement (as in, it doesn't actually provide full safety in the way that Rust or a GC language would)?
lifthrasiir•about 1 hour ago
Oh, yeah that might be confusing. I meant "you can say the same thing for GC language if that's true, which isn't necessarily true, so that must be false".

More precisely speaking: GC languages are said to delay memory problems far beyond the horizon, which is often unreachable throughout the project's history. Zig can be a similar case.

pyrolistical•about 2 hours ago
Yeah but they turned it into something unreadable. Call it a skill issue if you wish.

I just haven’t found another language that just makes sense. Zig doesn’t hide anything from you

kgeist•about 2 hours ago
>they turned it into something unreadable

Did you compare the code before/after? It's a mechanical line-by-line port, and most of the code is identical to the old version, just with Rust syntax. They have an example in the blog post.

lifthrasiir•about 2 hours ago
The article explicitly mentions the maintainability as a foremost concern.
silver_silver•about 1 hour ago
People say a lot of things, especially when they have a vested interest in a positive outcome. Bun has been fully vibe coded into another language. There’s no way in hell it’s maintainable. Go read any analysis of the Claude Code leak for proof.
baby•27 minutes ago
I find Rust more readable than zig
frollogaston•about 1 hour ago
I pay attention when someone makes a hard decision based on a hard-learned lesson. It's like, most who choose to use an ORM just heard of it or want to avoid learning SQL, everyone who removes an ORM learned firsthand horrors.
geon•about 2 hours ago
Wouldn't the same improvements have been made in zig if they instructed the agents to improve instead of rewrite?
gpm•about 1 hour ago
Maybe they'd get the same numeric improvements and bug fixes today (or maybe not, or maybe they'd get even more since the LLM isn't spending time rewriting correct code).

But they wouldn't get a change to the structural issues that created the issues in the first place. They'd end up "ke[eping] fixing these kinds of bugs one-off in perpetuity".

Zakis1•about 2 hours ago
But how would you verify that the agents have written memory safe code? Rust's borrowchecker is a lot faster and actually verifiably safe compared to asking an LLM to fix the safety issues that the Zig version had.
rq1•about 2 hours ago
From a PL Theory perspective, Zig is vibe-coded.

Not sure why people use it.

pier25•8 minutes ago
Personally I don't care that they used AI to rewrite Bun to Rust. Even if 1.4 is not good enough it will probably get better over time.

What has pushed me back to Node is seeing how amateurish the transition has been handled.

- No LTS support for the Zig version regarding CVEs etc.

- Huge bugs like the 3MB memory leak mentioned in the blog post abandoned in the Zig version to basically force people into the Rust version to fix their apps in production.

- Zero involvement with the Bun community about such a major decision. One day it was "stop the drama I'm just playing with this" and a couple of days later "yolo merged to main".

Jarred basically keeps operating as if he was a lone hacker working on his personal project.

sashank_1509•25 minutes ago
I think the important thing is this is much cheaper than hiring a software engineering team. They could have hired me for 200k and I could not do this in a year. I do not have the context, and I do not know Zig or Rust, perhaps I could pick it up in a month, but I would be extremely slow.

Forgetting all the predictions about singularity etc, at the very least AI as it is now, is going to make it very hard to justify hiring a SWE for 200k. I will say, at the very top for a software heavy company like Google or Anthropic, they will still hire excellent engineers to create new software that AI is not very good at.

But for companies where software is simply a cost center. Like Walmart, or Target, companies that were already outsourcing software development, or using cheap H1bs, now they have the alternative of AI which is much better than even hiring an average software engineer for 200k. This is a sea change in the job market, it’s going to have a pretty big effect as it is right now. US has around 1.6 Million software developers, this number is going to get cut drastically, the very top, say an L6 quality in FAANG will be fine, the average in a no name Bank, or the guy building the website for McDonalds is out, he needs to learn something else or he’ll end up without a job soon.

I would not have predicted this a year ago, now it seems clear that this will happen. Just shows how much of a sea change we have witnessed just like that.

jameskraus•20 minutes ago
It's funny, I see the opposite and I would only trust a senior engineer with conducting such a wide-reaching change. I would be more likely to hire a senior engineer who might now be able to effect such change.
galleywest200•12 minutes ago
If you already employed the engineers the extra cost would have been $0.
theLiminator•about 2 hours ago
That's the power of a strong test suite. LLMs excel when you have verifiable rewards. I imagine we'll get a lot more rewritten in rust projects in the future. Rust is also an ideal target for such rewrites as it offers a lot of verification (via its type system) and is low overhead with zero-gc. There's less and less reason to use GC'd languages in the agentic coding era.

I think Rust is a locally optimal target for LLM coding, we might see a better language in the future, but I think Rust will dominate for quite some time.

lifthrasiir•about 2 hours ago
> There's less and less reason to use GC'd languages in the agentic coding era.

Faster iteration, maybe? Rust's safety guarantee isn't exactly free (while still being very excellent) and does affect iteration time. I have a private project (>300K LoC) that has been translated from Python to TypeScript and the reason we couldn't use Rust was definitely the iteration time.

talloaktrees•about 1 hour ago
I like using Odin with LLMs for this. it's a simple statically typed language with no GC and very fast compile
gpm•about 2 hours ago
Eh... rust's safety isn't free, but not having it and wasting time on "oh I forgot to change this call site" also isn't free. On the whole I'd say the safety assists in iteration time.

What costs rust in iteration time in my opinion is the low level (by default) nature of it. There's a faster-to-iterate language that has yet to be created which is rust but we sacrifice performance (and memory fiddling ergonomics for the odd person who does that) so we don't have to worry about things like whether a variable is stack or heap allocated. Which is in the direction of a GCed language but retains the mutable-xor-aliasable semantics.

Between rust and current GCed languages though... I guess I agree with "maybe" in both directions.

theLiminator•about 2 hours ago
Maybe something like Hylo? But personally I don't see anything displacing rust for the next few years, as I think there's enough rust in the training data for it to be the best "serious" language for agentic systems-level development.

It's really the only systems language in its exact niche.

lifthrasiir•about 1 hour ago
The project in question needed lots of near-instant human judgements and the iteration loop had to be extremely tight. Maybe Rust should be reconsidered once it gets stabilized enough, but not right now.
YuechenLi•about 2 hours ago
>Combined with the Rust rewrite, ICU changes, and identical code folding, Bun's binary size shrinks by ~20% on Linux & Windows.

People who are surprised by this probably has not seen what Zig code actually looks like. Zig's explicitness and lack of abstraction have a real cost that it is basically one of the most verbose programming languages I've ever seen, it's somehow even more verbose than Go. Basic features of modern languages like pattern matching and generics, and as you can see, having to manually clean up everything means that if you forget once, it's a memory leak. Having SOME abstraction is actually good if it prevents you from making mistakes.

Ironically, Zig is a programming language that's probably best written by LLMs, since they can actually tolerate the verbosity.

benced•about 2 hours ago
Not a compiler expert - shouldn't language verbosity and binary size be, at best, very loosely related?
YuechenLi•about 1 hour ago
Fair point, I phrased that too broadly, and you are right about the loose correlation.

What I was gesturing at, badly, was more that Zig’s low-abstraction / explicit-by-default syntax tends to have you write more boilerplate-y code in general that are more annoying to write and maintain, while not buying you enough over a language with better tooling and ecosystem and compiler optimization like Rust.

steveklabnik•about 1 hour ago
I don't think you can draw the conclusion that source length and binary size are correlated. For example, in Rust:

    #[derive(Copy, Clone)]
    enum Expr {
        Int(i32),
        Add(i32, i32),
        Neg(i32),
    }
    
    fn eval(expr: Expr) -> i32 {
        match expr {
            Expr::Int(x) => x,
            Expr::Add(a, b) => a + b,
            Expr::Neg(x) => -x,
        }
    }
Rust's enums can carry data. You can write the same thing in C, but because it does not have the enum feature, you have to do it yourself. They're sometimes called "tagged unions" for a reason, you use a union + a tag when doing it by hand:

    #include <stdint.h>
    
    typedef enum {
        EXPR_INT,
        EXPR_ADD,
        EXPR_NEG,
    } ExprTag;
    
    typedef struct {
        ExprTag tag;
        union {
            struct {
                int32_t value;
            } Int;
    
            struct {
                int32_t left;
                int32_t right;
            } Add;
    
            struct {
                int32_t value;
            } Neg;
        };
    } Expr;
    
    int32_t eval(Expr expr) {
        switch (expr.tag) {
            case EXPR_INT:
                return expr.Int.value;
    
            case EXPR_ADD:
                return expr.Add.left + expr.Add.right;
    
            case EXPR_NEG:
                return -expr.Neg.value;
        }
    
        __builtin_unreachable();
    }
I haven't actually compiled this, but it should compile to almost the exact same, if not literally the exact same, machine code. Yet one is way more verbose than the other.
pavon•about 1 hour ago
I think you are saying the same thing as benced - just because Zig source code is verbose is no reason to assume the binary should be larger.
14113•about 1 hour ago
It required a little bit of messing with optimisation settings and library generation in Rust, but they emit very very similar x86-64 assembly:

https://godbolt.org/z/89W4srz4d

surajrmal•43 minutes ago
Why? Python is terse but has large binaries because of the runtime overhead. C++ is fairly verbose but can make useful binaries in double digit kib.
giancarlostoro•about 2 hours ago
> Ironically, Zig is a programming language that's probably best written by LLMs, since they can tolerate actually tolerate the verbosity.

Rust in my opinion feels the same.

pjjpo•about 1 hour ago
I have found LLMs struggle with Rust's constraints - they are optimized to produce code that passes the tests, not necessarily good code. So instead of working out lifetimes and borrowing, it will be happy to copy a buffer many times without thought. This means I have to still go through line by line to review and often rewrite either by hand or with another LLM iteration.

There may be some prompting that can help with this but I suspect there is a fundamental tension between writing working code vs good code in LLMs. Go is popular for being simple, making it easy to jump in and write something fast and stable - minimizing the gap between working and good code probably helps out the LLMs a lot.

afavour•about 1 hour ago
I don’t feel the verbosity with Rust. Haven’t written it in a while but now in the LLM era I’m looking forward to saying “sort out the lifetime errors for me”.
i_am_a_peasant•about 1 hour ago
I trust a lot more Rust code generated by an LLM than anything else ngl.
sroussey•about 2 hours ago
Agree. But just because it feels the same doesn’t mean it compiles the same.
honeycrispy•about 2 hours ago
That can often depend on how you write it.
tapirl•36 minutes ago
Zig is indeed verbose in some aspects, but not overall. For example, its `try error-union` syntax eliminates a lot of boilerplate code.

The main reason why Zig is verbose in some aspects is the main goal of Zig is program performance. It is a worthy tradeoff.

typ•about 1 hour ago
Abstraction doesn't necessarily lead to a smaller binary. Much of the bloat in modern software is indeed due to (bad) abstractions.
reinitctxoffset•28 minutes ago
The twenty percent quoted is referring to the size of the compiled artifact (one assumes ELF or Mach-O).

Whether or not a language is verbose or obscure is very much about your coordinate system. Not unlike safety.

I think C is a reasonable zero for both things.

Zig is more succinct and safer than C while still being comparably ergonomic. Rust is (mostly) safer and more succinct than Zig while being dramatically less ergonomic (take it up with Wadler memory chads, no one likes affine types).

I like lean4, which is dramatically safer, more succinct, and more ergonomic than Rust.

But I can see why some would say it's a bit too succinct.

bsder•about 1 hour ago
Naive was 4-9% on the initial pass.

Also note that the larger percentages were against already smaller binaries. That smells like there was a single large constant number that got saved somewhere rather than general improvements.

> After that initial shrinkage, the team explored more opportunities for binary size reduction using linker optimizations like Identical Code Folding, removing unused data from ICU, and lazily decompressing small parts of libicu with a zstd dictionary on-demand.

I'd be VERY interested in seeing what the individual effects of those parts were.

anonyonoor•6 minutes ago
I think we're finally getting to see a glimpse of the future. People and LLMs, working together. (And doing it really well.)

It's pretty exciting.

suby•8 minutes ago
I'm so jaded at this point. The AI translation from Bun to Rust doesn't bother me, I think it's interesting, but that this blog was so clearly written by LLM's is offputting for some reason. I think after having to interact with LLM's for much of the day, it's exhausting to read LLM speak in so many things I see online. It feels almost disrespectful to the reader. It's written from a first person perspective, but Jarred did not write these words.

I was looking forward to this blog post too, but in retrospect I don't know why. I could have had an LLM generate a hypothetical of what this blog post might have looked like and it would have probably been able to get close.

I feel like we've replaced unique voices on the internet with the same style / author, which might be more tolerable if the breathless LLM writing style wasn't so jarring. Contrary to the amount of times "But honestly" or "genuinely" is mentioned, nothing about having your LLM speak for you feels honest or genuine.

I know it's not cool to leave responses like this, but I'm really tired of all of this at this point. The ironic thing too is that it might actually be better to have LLM written text be so distinct so that you can still pick out when a human has actually authored something. Again, this is a blog post from Anthropic about having an AI translate 500k+ lines of code in 11 days, so I guess my disappointment is my fault for expecting otherwise.

hansvm•about 2 hours ago
Every time I've rewritten a major project I've made it smaller and faster while fixing all the major bugs and most of the minor ones. My current team has had similar experiences. I'd be curious to see what a Zig -> Zig rewrite of the same magnitude would have done for quality.
thevinter•about 3 hours ago
One thing that I found interesting is that most of the discourse surrounding the topic happened with the assumption that the rewrite was happening with an Opus-like model, and not with Fable. Those assumptions, at least partially, were used as arguments against the fact that the rewrite was feasible and/or a good idea.

Clearly the model itself doesn't completely change the narrative, but at least as a note to myself, I would like to be more careful with assuming the capabilities of the models used internally by Anthropic and affiliated orgs.

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nzoschke•about 1 hour ago
> Historically, rewrites are a terrible idea.

This changed for me over the last 5 years.

The first scenario was joining a company where a software product barely worked. We did the traditional incremental refactoring / rewriting, but eventually learned how rotten the core was that rewriting from first principles was the best path forward.

The lesson learned here is that the conventional wisdom probably only applies to rewriting complex but working systems.

Then multiple scenarios in the agentic coding age. Between day jobs and hobbies I've reproduced major chunks of complicated software like Salesforce, Gmail, Pioneer Rekordbox with very lean teams.

Much like the blog post, the trick is to get an excellent verification loop with a compiler, linter, and test harness / test suite around the core behaviors.

It's feeling more and more that designing and implementing comprehensive test harnesses is the real work, once you have that let the LLM cook.

Buttons840•about 1 hour ago
I've always felt [0] the people who created Bun had, as their first and foremost goal, a desire to use Zig--and that's great, I like Zig, I like when people build things their own way.

However, I've been skeptical of using Bun, because I want a project whose first and foremost goal is to build good tools that achieve the objectives of the project.

It reminds me of asking game developers: Do you want to build a game, or do you want to build a game engine? Building a game engine is fine, but if you're goal is to make a game, then building an engine is a poor way of achieving your goals.

Likewise, I've wondered if the creators of Bun wanted to build better JavaScript tools, or if they wanted to use Zig.

[0]: https://news.ycombinator.com/item?id=35970044

ameliaquining•about 1 hour ago
So does that mean the rewrite made you less skeptical?
Buttons840•about 1 hour ago
Yeah, I guess. Now it appears to be a project run by Anthropic and I'm sure the real focus is on making money--which is still slightly different than having the focus be on making the best tool.
bel8•about 2 hours ago
> Claude Code v2.1.181 (released June 17th) and later use the Rust port of Bun.

It seems the reports of Bun's death have been greatly exaggerated.

achristmascarl•about 1 hour ago
I was fairly skeptical about the rewrite when news about it first started going around, and I still don't plan on switching anything to use the Bun rewrite anytime soon, but I appreciate how detailed and well-written the blog post is; it also seems to be primarily human-authored, in my opinion, which is refreshing.

The most significant revelation for me was that Claude Code has been using the rewrite without much fanfare since June 17th.

ianm218•about 2 hours ago
Inspired by this project I ported most of Valkey to Rust here valdr.dev .

The coolest outcome was being able to run a redis comparible store on an a cloudflare durable object so you do I.e. rate limiting for free with little infra.

duhhhhh1212•about 2 hours ago
Where is the cost breakdown? I feel like this would be the easiest number to determine and write in this post. It's hard to believe that there have been no problems/downsides since the port.
gpm•about 2 hours ago
> Where is the cost breakdown?

From the article

> Pre-merge, this took 5.9 billion uncached input tokens, 690 million output tokens, and 72 billion cached input token reads — around $165,000 at API pricing

> It's hard to believe that there have been no problems/downsides since the port.

A significant portion of the article was dedicated to the 19 regressions they've found. Starting here: https://bun.com/blog/bun-in-rust#porting-mistakes

BearOso•about 1 hour ago
I posted on an older article that I thought it probably cost half a million in API pricing. 165k USD is a lot lower. I wonder what the actual compute cost was. When this first hit the news, Opus 4.7 was brand new and required 6x the compute power per user token vs 4.6. The article says they were using Fable, which is way more expensive.
duhhhhh1212•about 2 hours ago
Thanks!! Those are solid numbers but confusing. He reported input, output, and cached input token reads but not cache writes/cached creation input tokens? Maybe cache writes aren't a thing internally?
didibus•about 1 hour ago
> to exhaustively come up with reasons why the changes create bugs or do not work

My biggest issue currently, is I can't seem to get a code review that's about the simplicity of the code, and no /simplify ain't it. Removing certain bugs and generally working seems to be doing alright, especially if it's following either an example code (like in the Bun rewrite case) or a well defined "spec" of how to proceed.

frollogaston•about 1 hour ago
"I used a pre-release version of Claude Fable 5 for much of the Rust rewrite."

It'd be interesting if Anthropic became a general software company just because they have access to models that aren't yet released, possibly export-banned.

minimaxir•about 3 hours ago
Adding bespoke animations via Claude Code to the blog post is definitely thematic. It's unclear if they're useful data visualizations as they take a bit of time to parse, but they're neat.
giancarlostoro•about 2 hours ago
So I kept hearing that the author did this purely because Anthropic wanted a PR story, but reading this entire very well written post, with meticulous detail, what say you now? I never thought it made any sense for him to do this just because Anthropic asked him to. Sometimes you find yourself fighting the stack you're currently using, and another stack (or programming language) looks like it would alleviate a lot. LLM was just another tool in his toolbelt. I had already ported projects that were old and abandoned before using Claude Code, so I knew it was possible.
nozzlegear•about 2 hours ago
> what say you now?

I think that when you have a $165,000 hammer, all of your problems begin to look a lot like nails.

giancarlostoro•about 2 hours ago
I've done rewrites like this, maybe it wasn't Zig to Rust, but I have been able to rewrite sizable projects, from C# to Rust before. I incorporated a similar strategy, have Claude Opus review the codebase, write a spec, then have Claude implement it, while reviewing the spec, and using the codebase as fallback and gospel over the spec. That said, it's not the entire story here as I said, there was a lot of thought put into it, it it had not been done with Claude, I have a feeling he might have started an "experimental" version of Bun in Rust instead, as many developers have done in the past before LLMs.
benced•about 1 hour ago
I would guess the cost to do this with humans would be _at least_ $1.5M in compensation alone (I'm thinking three 500k/year Bay Area engineers) so this is already an order of magnitude cheaper.

Is it worth $165K? I'm less sure of that but it's honestly a moot point - this will get to 5 then 4 digits of cost pretty fast.

BearOso•about 1 hour ago
I think putting it in terms of API pricing is oversimplifying disingenuously. Anthropic still hasn't pulled the rug out from under us, so I'm sure it cost a great deal of money once everything comes together, likely surpassing 1.5M. Summarily, they got the result faster, which a group of engineers couldn't do, but at a greater expense.
johnfn•about 1 hour ago
So much of the discourse around this on HN is nonsensical, and I fully agree with you. It's patently absurd that Anthropic would demand him to rewrite Bun into Rust; it's equally absurd that they would demand any sort of stunt at all when Anthropic already pulled off the biggest stunt with Bun: running Claude Code on it. And why on earth would you cannibalize the runtime of your golden goose?
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pier25•about 1 hour ago
> Compiler errors are a better feedback loop than a style guide

So essentially this whole re-write was about making Bun LLM compatible.

incognito124•about 1 hour ago
> In Bun v1.3.14, every build leaks about 3 MB, forever

I'm sorry but that is insane, how was this never fixed before the rewrite?

pier25•24 minutes ago
I've been impacted by a couple of bugs in Bun.SQL and lo and behold these were only fixed for 1.4. Presumably Claude could have fixed those in the Zig version but the Bun team decided to not do that.

Furthermore, there's no mention of an LTS plan for the Zig version. It seems that if a CVE is discovered in the future, Bun users will no have no option than to update to the Rust port.

This is not how you run a project that others depend on and enough for me to not touch Bun ever again.

dgacmu•6 minutes ago
I'm a little puzzled: Why should you care? The language in which Bun is written isn't part of its API, if you will. You care that you have something that does various javascripty things according to a particular spec of what it's supposed to do. If a bug is fixed in 1.4.x it's fixed, why should it matter, really, if that's in Zig or Rust?
tangenter•about 1 hour ago
Should we brace for another front page Zig donation announcement? A fast follow with a “Why Zig?” penance piece, replete with anecdotes about how it is the only true way to express oneself?
Barrin92•22 minutes ago
the thing I don't understand about this, given that the goal was a line-by-line transpilation, and the author had already transpiled it once from Go to Zig, why not write an actual transpiler? A problem is as complex as the smallest program required to solve it, and having an LLM, which doesn't produce deterministic output churn through almost 200 grand when you only need to write a deterministic program maybe 5% of that size seems like not a great way to go about this
SergeAx•about 1 hour ago
I still think that generating a Zig-Rust transpiler would be a better approach, given all the LLM quirks, including the ability to just /goal the model with binary-identical LLVM bytecode.

However, an open-sourced tool like that would've greatly harmed the Zig ecosystem and community.

leecommamichael•41 minutes ago
Go famously used machine translation to remove dependency from C. It's a nice way to retain structural familiarity with the target language. I imagine they could've saved a large portion of that $165,000 using this route. Hard to say for certain, though. You wouldn't want to scope that transpiler at "being able to transpile all programs generally," and so scoping the project does become a serious task.
ivanjermakov•about 1 hour ago
> would've greatly harmed the Zig ecosystem and community

People looking to abandon the ship first chance are unlikely to contribute much to the ecosystem and community.

rvz•about 3 hours ago
As expected [0] [1], this was a clear advertisement / marketing opportunity of Anthropic's Fable model on rewriting Bun (which powers Claude Code) from Zig into Rust.

Something that would have taken hundreds of developers now took 1 developer with Fable.

Now Claude, rewrite Claude Code from TypeScript to Rust. Make absolutely zero mistakes.

[0] https://news.ycombinator.com/item?id=48073893

[1] https://news.ycombinator.com/item?id=48240829

steveklabnik•about 3 hours ago
EDIT: the parent has effectively deleted their original comment

> There are a lot of ways to do a terrible job of this. For example, prompting Claude "Rewrite Bun in Rust. Don't make any mistakes." and then praying it would work is not what I did.

rvz•about 3 hours ago
*whoosh* goes the joke.
steveklabnik•about 2 hours ago
Hacker News does not have a meme-y culture, and this post takes this topic pretty seriously and is technically interesting.

It's not so much that I missed that it was a joke, I just don't think that it really added to the discussion.

What you've edited it to is a much better comment.

merb•about 2 hours ago
1 Developer with a 200k budget for tools.
classicposter•about 2 hours ago
This slop rewrite introduced new vulnerabilities and regressions.
wilkystyle•about 2 hours ago
Care to elaborate?
dfabulich•about 2 hours ago
This blog post further undermines my trust in Jarred.

He makes it sound like Claude did a fantastic Rust rewrite, and "the work continues."

But when the Rust port merged to main, the state of the code was very, very bad. There were 13,000 instances of `unsafe`, no Miri tests at all, and, sure enough, it exposed UB in safe Rust. https://github.com/oven-sh/bun/issues/30719

Observers could see this coming from a mile away, objected strongly to using AI to RIIR before the code merged. Rather than incorporate feedback and get the code ready for production, Jarred gaslit us all, right here on HN. https://news.ycombinator.com/item?id=48019226

Just 9 days before he merged the Rust rewrite to the main branch, Jarred wrote:

> This whole thread is an overreaction. 302 comments about code that does not work. We haven’t committed to rewriting. There’s a very high chance all this code gets thrown out completely.

It's plausible that Bun's Rust rewrite is now in much better shape than it was in May. But a blog post like this would have been a place to apologize, to accept that it was a very bumpy rollout, to acknowledge that public messaging was extremely poor, and to earn back our trust.

As it stands, I guess I'll have to run my own tests to try to evaluate whether Bun 1.4 is ready for prime time, because I just can't trust Jarred to give us a straight answer.

Georgelemental•about 2 hours ago
Pre-release code had bugs that were fixed before the release? Why is that a problem? That's the point of having a testing and release process
pier25•about 1 hour ago
what about new bugs introduced after the rewrite?
simonw•about 1 hour ago
> But when the Rust port merged to main, the state of the code was very, very bad. There were 13,000 instances of `unsafe`, no Miri tests at all, and, sure enough, it exposed UB in safe Rust.

I mean yeah, that's what this whole post is about. It's about the process of going from that original state to something that's now shipping in production.

benced•about 2 hours ago
> We haven’t committed to rewriting. There’s a very high chance all this code gets thrown out completely.

God forbid an engineer express uncertainty.

Atotalnoob•about 2 hours ago
Uncertainty is one thing, but a high chance means it’s 51% or higher to me.

Based on that, the bun rewrite messaging was fairly misleading.

wccrawford•about 1 hour ago
That was their estimate at the time, based off the information they had. You can't ask more of someone than that.

Either they estimated poorly, or it ended up the lesser portion of their estimate after all. After all, unless the estimate is 100%, there's always a chance it'll fall into the other portion.

jen20•about 1 hour ago
To understand your error, consider that in the month leading up to the 2016 US presidential election, the widely-accepted probabilities were between 70% (Five-Thirty-Eight) and 90% (Reuters) in favour of Clinton.
himata4113•about 1 hour ago
They didn't mention the cost of this. Assuming mythos was somewhat involved I'd extrapolate this as: 128 x20 max accounts needed which comes at $25.6k or over 75k in api costs. For 75k you can hire a team of engineers that would produce a better result with sematic conversion and other tricks used in porting from language A to language B at the cost of maybe taking 1 month instead of 10 days.

I will be a lot more excited when this is possible with <10k of api costs.

fps-hero•about 1 hour ago
> Pre-merge, this took 5.9 billion uncached input tokens, 690 million output tokens, and 72 billion cached input token reads — around $165,000 at API pricing. By hand, I think this would've taken 3 engineers with full context on the codebase about a year, during which time we wouldn't be able to improve Node.js compatibility, fix bugs, fix security issues or implement new features. We never would've done that. The realistic alternative was to do nothing and keep fixing the bugs at the top of this post forever.
erichocean•about 1 hour ago
Even at $165K, it's worth it to have a better base to build on top of—especially since it didn't take a year's worth of time for three programmers.
JavierFlores09•27 minutes ago
I don't think the realistic alternative here was “hire a team for a month and get a better semantic conversion”

For a rewrite of this size, the expensive part is deep understanding of the underlying system in order to preserve behavior while keeping performance, and above all that not freezing product work while doing it. Adding more engineers would just end up in managerial burden and review bottlenecks, to say the least.

So even assuming the API cost estimate is high, I don't buy the “just hire engineers for a month” take. A team unfamiliar with the codebase would probably spend a large chunk of that month just building context and deciding how not to break everything. A team familiar with the codebase is even more valuable doing product work, bug fixes, and review of the existing codebase.

So, in short, I do agree with the simple fact that this is still too expensive for most projects, but not with the idea that “a small team would trivially do better in a month”.

incognito124•about 1 hour ago
It states $165k in the article