> The post claims they were fuzzing their Zig code, while during our calls the whole Bun team told us that they were not fuzzing anything

If you read the post again this sentence is not removed and Sumner did not refute that

I’m puzzled by how many people seem to be convinced (deluded?) into believing that their productivity has been multiplied and costs have become fractional to build things, why don’t I see any of that productivity gain or cost reduction out in the world? What has become cheaper or better engineered? If you believe posts like thsi, we should be living in a golden age of prosperity, when it seems that aside from getting better lots of products and companies seem to just be getting worse? Like seriously, to a normal person, putting aside from the benefits of using LLM’s directly for the LLM user, what things in the world have gotten better thanks to this abundance and oversupply of “intelligence” that is supposedly mutiplying people’s productivity?

You have a very fitting username

The borrow checker is a good tool (and it makes Rust objectively better than Zig to me), but it unfortunately doesn't prevent writing bad code

An issue with Bun is that it interfaces with a C++ JS engine and it needs unsafe. In this case, the best practice is to write a safe binding to encapsulate this external dependency (that's why in Rust we have -sys crates with raw unsafe bindings, and other crates with a safe interface on top), and then write your business logic entirely in safe Rust

However, the Rust port of Bun didn't follow such best practices (perhaps with good SDD practices it could, not sure about that). The resulting code has literally thousands of unsafe blocks. It also contains plenty of UB. The port already costed hundreds of thousands of dollars. It's unclear if Mythos/Fable is able to refactor it further to remove unsafe usage without introducing further UB, and how much it will cost

(here It's important to note something. Rust UB is in some abstract sense harder to deal with than C or Zig UB, because it also needs to uphold the guarantees of safe Rust. If you get to write your business logic in safe Rust that's a good deal, but the price of that is that your unsafe code has extra responsibilities)

> The whole point of the borrow checker is to make it impossible to write wrong code.

> In 2026 we need guarantees that we can't produce invalid code.

Rust doesn't provide either of those guarantees.

If I were to rephrase your sentiment for accuracy: Rust disallows certain coding patterns. Certain classes of bugs can only appear in those coding patterns.

IOW, Rust disallows $FOO which is a superset of "specific class of errors". This means that while Rust prevents specific bugs, as a side-effect it will also prevent some correct code.

Rust requires discipline too. I can go around using Arc, Rc and .clone() everywhere without upsetting the borrow checker, I can use let mut a bunch and pretend if, match, etc. aren't expressions. This results in worse code, and Rust didn't stop me.

The borrow checker prevents a set of errors from being possible, but it doesn't prevent bad code from being written.

Most bugs are logic bugs which rust does nothing to help you with.

There's so much good stuff in this post.

Can't help to think of a recent HN post about most AI-generated projects being abandoned within months. Why?

Because value of a project is not in the code produced. It's in the amount of battle-testing that code has seen.

Battle-tested, mature code > fresh rewrite.

Existing Zig codebase has seen X amount of battle-testing. Rust rewrite: 0 (except -I'm assuming- passing test suites). Also:

"this was a port to unsafe Rust, allowing a literal file-by-file migration to minimize risk"

How is that better than the Zig codebase you started with?

Now if that's further migrated to safe Rust, put into production & gathered feedback from lots of users, yes then you have something. As it is, the impressive bit is do such a big rewrite & result seems to work ok. Are Bun users happy with this?

To me it reads like Bun was forked. Will the Zig version survive? Will the Rust one? Both? All options ok.

Edit: and fwiw, I don't think Zig community should get triggered on any of this. It says nothing about how suitable Zig is or isn't for project xyz, and Zig community is big enough to carry their own project & applications besides Bun.

I stand with Andrew.

As someone who's been following Sumner's work closely for years, Kelley's accusations are very much true even if unkind. While the results are useful and cool, it a wankfluencer op from start to finish. I dare you to refute thus.

And I say all this as someone who does agentic development 8hrs a day and someone who always pestered my team to opt for Rust and Deno instead of Node. Call a spade a spade, the rewrite was poorly justified and one in a long lines of successful psyops Dario and co. cooked and delivered.

Now, would Andrew's message have been better received if it had better "decorum"? Maybe. But I'm glad he stayed honest to himself instead and didn't have a PR team ghostwrite his thoughts. You have to appreciate that.

Did we read the same Anthropic and Andrew Kelly's posts? Anthropic is not in the programming language market; their post about rewriting Bun in Rust is full of technical details that led to improving the end product for their users. Zig's response is a sour opinion piece full of personal attacks.

For context, I'm using Codex and have no interest in either Zig or Rust, so just observing this drama from the sidelines.

I think like most people, I don’t have a problem with Andrew “calling a spade a spade,” even if I find his reasoning motivated. The bigger problem with the post is that it talks out of both ends of the mouth: it’s clearly meant as a personal attack, but also insists that it isn’t.

When I read the post, my first thought was that I wouldn’t want to build things in Zig, because any technical decision I make, good or bad, might subject me to this kind of article from their BDFL. I can’t conceive of the leadership of the Python or Rust or any other community I’ve ever worked with doing something like that.

What I have been missing in all this debate is substance. I don't care that Bun was ported to Rust; I don't care that Andrew wrote a hit piece about it; I don't care that Anthropic sells shovels in the gold rush.

What I do care about is technical details. Jared shared some motivation as to why they ported to Rust, and I think they look valid (even if provided with sparse evidence). But I have not seen any sort of refutation from Andrew that these are not actually issues or how they should be solved in Zig canonically. I'd really like to see an exploration of these arguments, specifically pertaining to the Zig code as it was written for Bun.

Those problems are partly attributable to Zig itself. If the project accepted AI-assisted contributions, this controversy might never have happened. Completely shutting out one side of the industry and taking a dogmatic position is not helpful, especially for a pre-1.0 language that is still relatively immature.

Now the Zig community is attacking one of the largest and best-known codebases ever built with the language. Anthropic’s motives are clearly promotional, but even so, I would not choose Zig for any project.

> Management eagerly approved the Rust rewrite option because it was a great marketing opportunity to showcase their new Fable model, Anthropic already uses Rust, and Zig is openly against using Anthropic’s products.

Yeah, don't discount how powerful "marketing" is to management/executives, and also don't discount how absolutely ridiculously petty people can be, especially people who end up like CEOs and similar, requires a particular person. I can definitively see reason #1 and #3 from that to basically already set in stone that Bun had to be rewritten in Rust.

| how their AI was powerful enough to do this rewrite (even though it was not powerful enough to catch a use-after-free)

This, I don't think enough people pointed this out. We have been sold the "coding is solved" and "software engineering is over" shit by Anthropic folks for a while now. The discourse should have revolved around claude's capabilities and instead it became a Jared vs Andrew thing.

Thank you. I was left confused after people praised the Bun to Rust blog post eventhough it contained very actual technical substance. No clear evaluation of options, very biased report on impact, missing figures. It absolutely didn't feel like an engineering blog post.

Maybe people were more interested in the agentic part more than the actual rationale for the port in the first place, because it was very disappointing from a technical standpoint.

I really don't understand what Andrew Kelly hopes to achieve here. Even the non-programmers at r/programming who usually piles on any type of anti-AI posts called it out.

I'm trying to not let this affect my thoughts about Zig as a language but it's hard.

I really don't understand what's the big deal here. Anthropic converted Bun from Zig to Rust using Fable and used that for marketing, but do people blindly trust them? Also isn't Zig still unstable and from that perspective regardless of how they did it, wouldn't it make sense to migrate it to a stable language?

> From my perspective, Anthropic is the party we need to hold accountable here.

It's insane that Ray Myers thinks that Anthropic did anything related to this port that requires holding them accountable; the fact that he used those words makes me want to prevent him from having any influence over AI policy.

Anthropic migrated Bun from Zig to Rust, they probably tried writing it in Zig using AI and ran into issues because there isn't enough Zig training data. A year ago, most LLMs couldn' t code reliably in Rust, But were fluent in Python, C, and web tech.

This paragraph is written by AI. I did not notice it earlier in the piece:

> The piece about the migration process is very cool, with details that are reusable. No complaints, I think that’s the real contribution here. I particularly like the honesty in explaining that this was a port to unsafe Rust, allowing a literal file-by-file migration to minimize risk, paving the way for redesign in future steps. That’s a sensible move explained well.

I think two things can be true at once....It was obviously a great marketing story for anthropic but that doesn't automatically mean that engineering work had no value. Companies have always turned interesting technical projects into marketing.

> Anthropic is actively campaigning to end software engineering

While this may be slightly overstated, my take on this is that AI progress should have us upgrade our view of the human brain rather than the opposite.

Wrote about it the other day:

https://livingsystems.substack.com/p/ai-progress-should-upgr...

Oh, they're converting Bun to unsafe rust. That's easy, but useless. That's the sort of thing c2rust does - transpile to an low level language which is unsafe Rust with a set of functions that unsafely emulate C pointer semantics. You don't need an LLM for that.

There's no point. It's not doing the job DARPA's TRACTOR program is trying to do - translate C to safe Rust. I've used c2rust. It works, but what you get out is like compiler output, not something you can work on.

Anything that can be written in Rust will be written in Rust.

I'm using Rust as a placeholder for a language with excellent performance and safety characteristics. It may not be a good thing, but there's no way around it.

I remember the uber db migration post and I can't help but wonder if the tone of these conversations would be different for bun if AI wasn't involved.

https://www.uber.com/us/en/blog/postgres-to-mysql-migration/

I think Zig being a new and a low-resource language (not enough training data) is also one of the reasons Bun decided to use Rust as large language models will simply not be as good no matter how good the model is.

Great Article. I appreciate the amount of detail you went into here to give everyone a fair hearing. Will there be a recording of your talk at the software should work conference next week? Thanks!

I think Anthropic is putting too much time and energy into marketing (and politics) while competitors are catching up on the engineering side.

But what do I know, maybe your CTO bought this and now wants to fire half of the dev team and use Claude to convert your COBOL codebase to Rust...

This post is ultimately complaining about a build step consisting of an automatic translation from technically irrelevant language A to technically slightly less, but still substantially irrelevant language B.

I don't care what Anthropic says and what the CEOs buy, I care about my own output and the recent models give me 2x uplift easily, I'd say 10x in single digit percentage of the time and I know I'm not using them to their full potential. We're living in an era of mass produced software. You can still be an artisan in this era, but you have to be aware.

The thing is - is it a self fulfilling prophecy?

We already know junior hiring an are down. And how many people are now excited to learn to code compared to 5years ago?

How many of those excited people are ACTUALLY learning to code and not just learning to prompt?

LLMs/agents will take over (or at least dominate) software dev even if they don’t get any better because humans will just get old and there’ll be no new humans who know how to do it.

> But I'm glad he stayed honest to himself instead and didn't have a PR team ghostwrite his thoughts.

If there's one thing I learned in this debacle is "I should spend 1-2 days and send to a close friend before hitting publish on a firey reply." The way Andrew rephrased the closing section is the kind of thing I should publish on the first edit in similar scenarios.

It was poorly justified up-front but frankly I did not need the examples to understand Jarred’s frustration.

What I found helpful were his explanation of how the interaction with JavaScript’s garbage collector created unique challenges for Bun. Andrew did not address this point at all. It was also helpful to understand how the test suite covered the new code - many people had assumed the tests were also vibe translated and couldn’t be trusted. Andrew pretends he didn’t understand that tests don’t catch all bugs, which is true of all software including Zig.

To me this whole exchange is mostly the typical “memory unsafe languages lead to time consuming and disruptive defects that can be almost entirely prevented with a choice of programming languages” versus the “git gud and don’t write bugs” response.

Layer on the AI tension, Anthropic’s involvement and Andrew’s classic Linus impersonation and of course its viral.

> psyops Dario and co. cooked and delivered

Does that include Claude itself? Half joking aside, my main concern remains that both OpenAI and Anthropic practice this fear mongering as strategy at the executive level because they have to. A corporation is a different beast. Not human, exactly. Not AI either. What happens when a corporation starts being informed by the AI it is building?

If you want a lens into master craft corporate human slung bullshit, read this: https://www.theverge.com/podcast/944138/microsoft-ai-ceo-mus...

In this, not only did the interviewer admit he knew Suleyman had written about the issues he talks about in his rebuttals, he then says he's only asking (the hard question) because the "audience" wants it.

Suleyman's response talks about taking "accountability for the things that we build" and the "types of problems that we choose to work on," it highlights the fundamental flaw in the cloud-rented AI model.

In that ecosystem, "we" means a handful of corporate executives deciding what tools the rest of the world gets to use, how they operate, and what data they extract. All based on the profitability of the corporation. It is the absolute antithesis of a sovereign architecture where execution happens locally and the user dictates the terms of the system.

They are coming for open models. It's time to harden the gates.

Watch out, a tech bro is going to quote the HN rules to you and say you're being a bad bad boy.

> Anthropic is not in the programming language market; their post about rewriting Bun in Rust is full of technical details that led to improving the end product for their users

Anthropic absolutely is in the programming language market. If/since AI makes rewrites to certain languages relatively easy, a success story will tie the given language(s) to the given AI company.

Rust may have a tremendous success in the future, because it's much easier to write it with AI (ignoring for a moment whether that's really a good thing). The implication is that Anthropic has a stake in Rust's success.

Also, to be kept in mind that devs advertising successfull rewrites often hide some aspects that are unfavorable to the narrative; typically, how bad was the code before the rewrite), although there are other (significant) aspects that have been omitted.

> Zig's response is a sour opinion piece full of personal attacks.

I take you haven't read Andrew Kelley's article (here: https://andrewkelley.me/post/my-thoughts-bun-rust-rewrite.ht...).

Summary:

- Jarred has written Bun with very bad engineering standards

- Jarred has managed public relations very poorly (e.g. ghosting the Zig foundation)

- When they rewrote the project to Rust, and described Zig as poor choice, there has been a negative fallout for Zig

- The ZSF is obviously upset because of the poor publicity

This is summarized at the end of the post:

> Zig users who knew next to none of these facts and have only the surface level understanding that an ex-Zig-user is getting trashed by the language creator. Such people might reasonably worry that might happen to them

As a matter of fact, I also believed the same after reading's Bun's post. This is undeserved though, and that's what Kelley explains.

There's definitely a personal attack somewhat, and this is addressed in the last (added later) section.

Yeah, exactly. It's weird that Zig even responded to that. Imagining that your studio switched from Unity to Unreal and Unity proceeded to release a hit piece attacking your codebase quality and workplace environment.

It's the old craftsmen vs industry issue, Andrew comes from the craftsman tradition that prefers all other people developing to also be proud craftsmen.

What Java, JS, Python and C# all did to conquer the industry from a C++ dominance was to provide safety harnesses for less "perfectionist" workers to fumble around without causing a mess, to write C and C++ in an increasingly hostile world we realized you needed a lot of craftsmanship, the performance benefits outdid and kept the latter languages relevant for a long while.

Still, the performance/predictability penalty didn't give way so Rust (and Swift) came into play. They don't have as many unpredictable performance characteristics as the previously "safe" GC languages but still provided more or less the same guarantees (in some ways perhaps even better for Rust).

The brilliance of the Rust ideas did start a bit of a cambrian explosion of languages in that niche, most of them however targeting a bit more of a craftsman position than Rust (that came out of distinct industry needs).

The problem as the article illustrates, in car terms.

If Java,JS,etc are mostly "regular safe cars" and C/C++ a two wheel motorcycle.

Rust is perhaps a rally car (fast but still a car so occupants inside are well protected) whilst Zig really is a quadbike or open wheel cart, not as unsafe as a two wheel bike since you won't slide for the smallest oil/ice patch but flipping over is still dangerous as hell.

And that takes us to the crux, so many developers who love the craft and perfection (and don't live under- or perhaps care of- financial constraints) think that "good careful" developers is all that's needed and don't see dangerous language designs as a problem.

I'm an older developer, and given that I can write "good careful code", but 90% of the time it's also a matter of time and financial constraints so I wouldn't trust mine (or anyone elses for that matter) code written under those "industry" conditions.

I think Zig has a lot of nice perks, but it was obvious from day 1 that it's very much for people that love their hacking freedom over writing code for todays hostile world.

I'm a heavy Rust user who doesn't like Zig all that much.

I browsed through the Bun code following Kelly's post, and decided to have Codex replace all my Bun usage with Deno.

The results improving the end user experience didn’t have much to do with the rewrite. Improvements in binary size and speed could be had with similar efforts on the Zig codebase. They spent extra effort to get those metrics to look good to sell the rewrite.

The memory safety aspects could be discussed. Arguably they could have had equally good memory safety by employing AI, tests and fuzzing (the Zig integrated fuzzer that the Zig team suggested they use, not just the high level fuzzing they were doing)

For this kind of project I do think using Rust is a good idea. At the very least because a project like Bun probably can benefit from a more mature language.

But I also think Andrew’s perspective of this process has been essential to understand what happened here, and though he could have been nicer with his word selection in a couple of places (he doesn’t have the clout of Linus Torvalds to get away with it), what he wrote absolutely needed to be said. I find it annoying that people dismiss it as personal attacks. If being a bad manager is the direct cause of a poor working relationship and bad engineering results, pointing it out is not a personal attacks. It’s essential context for understanding what happened.

> Anthropic is not in the programming language market

Arguing that a product that sells itself as improving programmer productivity by writing the code for you has no stakes in "the programming language market" because it doesn't sell a programming language of its own is impressively shortsighted. Especially when the leader of a programming language has openly stated their dislike of vibecoding, critisized the industry, and the language project itself rejects PRs made with the product being sold.

Yeah I don't understand these myopic takes.

Jarred's post about Bun-Zig-Rust post was technical and polite.

Andrew's post in response was anything but that.

> Anthropic is not in the programming language market;

No, they're intentionally in all the programming language markets.

Did we read the same Anthropic and Andrew Kelly's posts?

Anthropic's posts were sanctimonious, self-serving, tone-concealed delegitimisation of Zig. Kelly's post was a strategically poor but sincere individual understandably frustrated at this concealed attack, expressing his honest feelings about the situation.

Yeah and Bun and Zig are not competing in anyway as well. Zig blog post has been updated as well recently btw.

> Zig's response is a sour opinion piece full of personal attacks.

Not sure; it has some elements of personal issues, but they're followed by a rationale from the author.

Honestly, seeing the project lead (Andrew Kelly) take a stand against poor engineering practices without any equivocation makes me more inclined to want to use Zig - their values (in this regard, at least) align with mine.

He also substantiates what many of us are saying to all these "Very Senior Chief Engineer with 40 years experience" who are boasting of 10x productivity: these people aren't reading the code they are generating, and were producing slop even without AI.

You mean Anthropic has no agenda on its own? That seems a very biased analysis here. The response by Zig could be flawed (speculation, I have not reached this conclusion yet) but I don't see how this offsets Anthropic wanting to promote its AI slop here in any way, shape or form.

Stating you use Codex does not add any meaningful information to the case.

This is not just a random blog post or technical decision, it’s literally a trillion dollar company’s marketing department deciding to attack and slander Zig.

Insane that people + tokens are slobbering to rush to Clownthropic’s defense when the whole migration and subsequent blog post was just sour grapes about the Zig project’s no slop policy.

And this is a warning shot from Anthropic intended to have a chilling effect, we have tens of billions of dollars at our disposal and if you take any stance we don’t like that undermines our narrative we will fuck with your shit and throw billions of dollars of muscle at rewriting you or trying to make you irrelevant.