6 ms·

Wilson Lin on FastRender: a browser built by parallel agents

73 points by lumpa 9mo ago

yeasku 9mo ago
[dead]
WD-42 9mo ago
Is this the project announced a week or two ago by an AI company claiming they had built a browser but it turned out to be a crappy wrapper around Servo that didn’t even build? Or is this another one? I thought it was Anthropic but this says Cursor.
- thunderbong 9mo ago
  The first paragraph of the article - > Last week Cursor published Scaling long-running autonomous coding, an article describing their research efforts into coordinating large numbers of autonomous coding agents. One of the projects mentioned in the article was FastRender, a web browser they built from scratch using their agent swarms. I wanted to learn more so I asked Wilson Lin, the engineer behind FastRender, if we could record a conversation about the project. That 47 minute video is now available on YouTube. I’ve included some of the highlights below.
- comex 9mo ago
  It is the same project, but my impression is that HN exaggerated many of the issues with it. For example: - They did eventually get it to build. Unknown to me: were the agents working on it able to build it, or were they blindly writing code? The codebase can't have been _that_ broken since it didn't take long for them to get it buildable, and they'd produced demo screenshots before that. - It had a dependency on QuickJS, but also a homegrown JS implementation; apparently (according to this post) QuickJS was intended as a placeholder. I have no idea which, if either, ended up getting used, though I suspect it may not even matter for the static screenshots they were showing off (the sites may not have required JS to show that). - Some of the dependencies (like Skia and HarfBuzz) are libraries that other browsers also depend on and are not part of browser projects themselves. - Other dependencies probably shouldn't have been used, but they only represent a fraction of what a browser has to do. However… What I don't know, and seemingly nobody else knows, is how functional the rest of the codebase is. It's apparently very slow and fails to render most websites. But is this more like "lots of bugs, but a solid basis", or is it more like "cargo-culted slop; even the stuff that works only works by chance"? I hope someone investigates.
  - simonw 9mo ago
    > were the agents working on it able to build it, or were they blindly writing code? The project was able to build the whole time, and the agents were constantly compiling it using the Rust compiler and fixing any compile errors as they occurred. The GitHub CI builds were failing, and when they first opened the repo people incorrectly assumed that meant the code didn't compile at all. The biggest problem with the repo when they first released it was that there were no build instructions for end-users, so it was hard to try out. They fixed that within 24 hours of the initial release. > What I don't know, and seemingly nobody else knows, is how functional the rest of the codebase is. It's functional enough to render web pages - you can build it and run it yourself to see that, I have some screenshots from trying it out here: https://simonwillison.net/2026/Jan/19/scaling-long-running-autonomous-coding/ https://simonwillison.net/2026/Jan/19/scaling-long-running-a... That said, it's very much intended as a research project into running parallel coding agents as opposed to a serious browser project that's intended for end users. At the end of my post I compare it to "hello world" - I think "build a browser" may be the "hello world" of massively parallel coding agent systems, which I find quite amusing.
    - felipeerias 9mo ago
      If you were looking for a good long-term AI benchmark, “build me a Web browser” should last you for a while.
    - beepbooptheory 9mo ago
      I hear you but still just struggle what we are supposed to take from this. If I worked for McDonalds and came up with a way to make 1000 bad hamburgers in the time it takes for them to currently make 10 good ones, no one would be that impressed. "Hello world" is self-justifying, you know it when you see it, and it is what it is because it shows something unambiguous and impossible to mistake.
      - deleted 9mo ago
        [deleted]
      - 9mo ago
Ronsenshi 9mo ago
"Your scientists were so preoccupied with whether or not they could, they didn’t stop to think if they should." I'm curious what is the energy/environmental/financial impact of this "research" effort of cobbling together a browser based on AI model that had been trained on freely available source code of existing browsers. I can't imagine this browser being used outside of tinkering or curiosity toy - so the purpose of the research is just to see whether you can run absurd amount of agents simultaneously and produce something that somewhat works?
- sealeck 9mo ago
  I'd love to see what happens if you hook this renderer up to AFL++...
- deleted 9mo ago
  [deleted]
- sebzim4500 9mo ago
  >I can't imagine this browser being used outside of tinkering or curiosity toy - so the purpose of the research is just to see whether you can run absurd amount of agents simultaneously and produce something that somewhat works? Yes but this is a very interesting question IMO
benatkin 9mo ago
I'm going to propose a law for these AI orchestration systems based on Greenspun's Tenth Law: > Any sufficiently complicated AI orchestration system contains an ad hoc, informally-specified, bug-ridden, slow implementation of half of Gas Town.
- polotics 9mo ago
  that statement is a bit early no?
- danpalmer 9mo ago
  Isn't it the other way around, Gas Town is an ad hoc, informally specified, bug ridden, slow implementation of other AI orchestration systems.
nurettin 9mo ago
I welcome living in this absurd time where monkeys with typewriters producing a work of Shakespeare has become a reality.
- trhway 9mo ago
  It took 2M years for the monkeys to produce typewriters and Shakespeare. Now the task is to make monkeys which can do the same in many orders of magnitude shorter time.
terabytest 9mo ago
> FastRender may not be a production-ready browser, but it represents over a million lines of Rust code, written in a few weeks, that can already render real web pages to a usable degree I feel that we continue to miss the forest for the trees. Writing (or generating) a million lines of code in Rust should not count as an achievement in and of itself. What matters is whether those lines build, function as expected (especially in edge cases) and perform decently. As far as I can tell, AI has not been demonstrated to be useful yet at those three things.
- ksynwa 9mo ago
  Line count also becomes less useful of a metric because LLM generated code tends to be unnecessarily verbose.
  - agumonkey 9mo ago
    Makes me wonder what would happen if you introduce cyclomatic complexity constraints in your agents.md file.
    - electroglyph 9mo ago
      well, first the model has to actually follow the instructions in agents.md =)
- mejutoco 9mo ago
  100%. An equivalent situation would be: Company X does not have a production-ready product, but they have thousands of employees. I guess it could be a strange flex about funding but in general it would be a bad signal.
- bflesch 9mo ago
  SLOC was a bad indicator 20 years ago and it is today. Don't tell them - once they realize it's a red flag for us they will use some other metric, because they fight for our attention.
joduplessis 9mo ago
I wonder if we're heading to a situation where agent written code will function as something distinct, like bytecode.
embedding-shape 9mo ago
simonw, I find it almost shocking how you had the chance to talk directly with the engineer who built this, and even when he directly says things that contradict what Cursor's own CEO said, you didn't push back a single iota. Is the takeaway here that it's fine for a CEO to claim "it even has a custom JS VM!" on Twitter/X, then afterwards the engineer explains: "The JavaScript engine isn’t working yet" and "the agents decided to pause it", and this is all OK? Not a single pushback about this very obvious contradiction? This is just one example of many, and again, since it seems to be repeated: no, no one thinks this was supposed to rival Chrome, what a trite way of trying to change the narrative. I understand you don't want to spook future potential interviewees, but damn if that didn't feel like you suddenly are trying to defend Cursor here, instead of being curious about what actually happened. It doesn't feel curious, it feels like we're all giving up the fight against unneeded hype, exaggeration and degradation of quality. What happened with balanced perspectives, where we don't just take people for their words, and when we notice something is off, we bring it up? On a separate note, I actually emailed Wilson Lin too, asking if I could ask questions about it. While he initially accepted, I never actually received any answers. I'm glad to you were able to get someone from Cursor to clarify a bit at least, even though we're still just scratching the surface. I just wish we had a bit more integrity in the ecosystem and community I guess.
- sebzim4500 9mo ago
  Surely this would be push back to the CEO, not the engineer? The engineer is presumably the one telling the truth.
  - embedding-shape 9mo ago
    Either of them. If the CEO refuses to answer, you ask others. If you get a chance to talk with them, you ask them about it. Just ignoring the elephant in the room and hoping that the unclear details gets forgotten helps no one except Cursor here.
- simonw 9mo ago
  Honestly, grilling him about what the CEO had tweeted didn't even cross my mind. I wanted to get to the truth of what had actually been built and how. If that contradicts what the CEO said then great, the truth is now out there - anyone is free to call that out and use my video as ammunition. I just had a look to see what Michael Truell had said about the project, here it is: https://x.com/mntruell/status/2011562190286045552 https://x.com/mntruell/status/2011562190286045552 > We built a browser with GPT-5.2 in Cursor. It ran uninterrupted for one week. > It's 3M+ lines of code across thousands of files. The rendering engine is from-scratch in Rust with HTML parsing, CSS cascade, layout, text shaping, paint, and a custom JS VM. > It kind of works! It still has issues and is of course very far from Webkit/Chromium parity, but we were astonished that simple websites render quickly and largely correctly. This doesn't strike me as the world's most dishonest tweet, though it exaggerates what was achieved. There IS a JS VM in there but it's feature-flagged off. The from-scratch is misleading because there are libraries handling certain aspects - most notably Taffy - which we discussed in the interview. I just ran "cloc" and to my surprise it counted 3,036,403 (I had thought the 3M was an exaggeration) though only 1,658,651 of that was Rust. "It kind of works" is a fair assessment IMO! I don't think "Let's talk about your CEO exaggerating what you built on Twitter" would have added much to the interview. I did make sure to go over the controversies I thought were material to the project, which is why I dug into the dependencies and talked about QuickJS and Taffy.
lifis 9mo ago
Why not help the Servo project instead of just making a proof of concept?
- simonw 9mo ago
  Servo have a strict policy against AI contributions: https://github.com/servo/book/blob/dd34e931be2fc8d555b38bcf1d0553f77c46acaa/src/contributing/getting-started.md#ai-contributions https://github.com/servo/book/blob/dd34e931be2fc8d555b38bcf1...
pooploop64 9mo ago
The reaction to this would have been different two or three years ago but it looks extremely lame when you open January 2026 hackernews and this is the kind of thing a tech company is trying to persuade you into thinking is exciting or useful. You've heard about what people are doing in the medical industry. Using AI to accelerate diagnosis and analysis of biological material. In astronomy it's showing us things that no human had ever seen before. You hear about all these things changing the world at large and the smaller worlds of individual people and families. Then you look at the actual IT industry and we've got... some premade libraries duct taped together into a crappy browser that barely works. Of course when the value of this is compared to the cost, the response is that it's fine because it was never actually intended to be useful in the first place. Well we're actually a step ahead of you there. The phrase "high on their own supply" describes all the people involved in this very well. I assure you we understand the goal of this project perfectly. It just wasn't a good, worthy, or even interesting goal. The immense amount of resources that went into this should have gone into something better. That's all there is to it.

73 points by lumpa 9mo ago

[dead]

WD-42 9mo ago

Is this the project announced a week or two ago by an AI company claiming they had built a browser but it turned out to be a crappy wrapper around Servo that didn’t even build? Or is this another one? I thought it was Anthropic but this says Cursor.

thunderbong 9mo ago
The first paragraph of the article - > Last week Cursor published Scaling long-running autonomous coding, an article describing their research efforts into coordinating large numbers of autonomous coding agents. One of the projects mentioned in the article was FastRender, a web browser they built from scratch using their agent swarms. I wanted to learn more so I asked Wilson Lin, the engineer behind FastRender, if we could record a conversation about the project. That 47 minute video is now available on YouTube. I’ve included some of the highlights below.
comex 9mo ago
It is the same project, but my impression is that HN exaggerated many of the issues with it. For example: - They did eventually get it to build. Unknown to me: were the agents working on it able to build it, or were they blindly writing code? The codebase can't have been _that_ broken since it didn't take long for them to get it buildable, and they'd produced demo screenshots before that. - It had a dependency on QuickJS, but also a homegrown JS implementation; apparently (according to this post) QuickJS was intended as a placeholder. I have no idea which, if either, ended up getting used, though I suspect it may not even matter for the static screenshots they were showing off (the sites may not have required JS to show that). - Some of the dependencies (like Skia and HarfBuzz) are libraries that other browsers also depend on and are not part of browser projects themselves. - Other dependencies probably shouldn't have been used, but they only represent a fraction of what a browser has to do. However… What I don't know, and seemingly nobody else knows, is how functional the rest of the codebase is. It's apparently very slow and fails to render most websites. But is this more like "lots of bugs, but a solid basis", or is it more like "cargo-culted slop; even the stuff that works only works by chance"? I hope someone investigates.
- simonw 9mo ago
  > were the agents working on it able to build it, or were they blindly writing code? The project was able to build the whole time, and the agents were constantly compiling it using the Rust compiler and fixing any compile errors as they occurred. The GitHub CI builds were failing, and when they first opened the repo people incorrectly assumed that meant the code didn't compile at all. The biggest problem with the repo when they first released it was that there were no build instructions for end-users, so it was hard to try out. They fixed that within 24 hours of the initial release. > What I don't know, and seemingly nobody else knows, is how functional the rest of the codebase is. It's functional enough to render web pages - you can build it and run it yourself to see that, I have some screenshots from trying it out here: https://simonwillison.net/2026/Jan/19/scaling-long-running-autonomous-coding/ https://simonwillison.net/2026/Jan/19/scaling-long-running-a... That said, it's very much intended as a research project into running parallel coding agents as opposed to a serious browser project that's intended for end users. At the end of my post I compare it to "hello world" - I think "build a browser" may be the "hello world" of massively parallel coding agent systems, which I find quite amusing.
  - felipeerias 9mo ago
    If you were looking for a good long-term AI benchmark, “build me a Web browser” should last you for a while.
  - beepbooptheory 9mo ago
    I hear you but still just struggle what we are supposed to take from this. If I worked for McDonalds and came up with a way to make 1000 bad hamburgers in the time it takes for them to currently make 10 good ones, no one would be that impressed. "Hello world" is self-justifying, you know it when you see it, and it is what it is because it shows something unambiguous and impossible to mistake.
    - deleted 9mo ago
      [deleted]
    - 9mo ago

Ronsenshi 9mo ago

"Your scientists were so preoccupied with whether or not they could, they didn’t stop to think if they should." I'm curious what is the energy/environmental/financial impact of this "research" effort of cobbling together a browser based on AI model that had been trained on freely available source code of existing browsers. I can't imagine this browser being used outside of tinkering or curiosity toy - so the purpose of the research is just to see whether you can run absurd amount of agents simultaneously and produce something that somewhat works?

sealeck 9mo ago
I'd love to see what happens if you hook this renderer up to AFL++...
deleted 9mo ago
[deleted]
sebzim4500 9mo ago
>I can't imagine this browser being used outside of tinkering or curiosity toy - so the purpose of the research is just to see whether you can run absurd amount of agents simultaneously and produce something that somewhat works? Yes but this is a very interesting question IMO

benatkin 9mo ago

I'm going to propose a law for these AI orchestration systems based on Greenspun's Tenth Law: > Any sufficiently complicated AI orchestration system contains an ad hoc, informally-specified, bug-ridden, slow implementation of half of Gas Town.

polotics 9mo ago
that statement is a bit early no?
danpalmer 9mo ago
Isn't it the other way around, Gas Town is an ad hoc, informally specified, bug ridden, slow implementation of other AI orchestration systems.

nurettin 9mo ago

I welcome living in this absurd time where monkeys with typewriters producing a work of Shakespeare has become a reality.

trhway 9mo ago
It took 2M years for the monkeys to produce typewriters and Shakespeare. Now the task is to make monkeys which can do the same in many orders of magnitude shorter time.

terabytest 9mo ago

> FastRender may not be a production-ready browser, but it represents over a million lines of Rust code, written in a few weeks, that can already render real web pages to a usable degree I feel that we continue to miss the forest for the trees. Writing (or generating) a million lines of code in Rust should not count as an achievement in and of itself. What matters is whether those lines build, function as expected (especially in edge cases) and perform decently. As far as I can tell, AI has not been demonstrated to be useful yet at those three things.

ksynwa 9mo ago
Line count also becomes less useful of a metric because LLM generated code tends to be unnecessarily verbose.
- agumonkey 9mo ago
  Makes me wonder what would happen if you introduce cyclomatic complexity constraints in your agents.md file.
  - electroglyph 9mo ago
    well, first the model has to actually follow the instructions in agents.md =)
mejutoco 9mo ago
100%. An equivalent situation would be: Company X does not have a production-ready product, but they have thousands of employees. I guess it could be a strange flex about funding but in general it would be a bad signal.
bflesch 9mo ago
SLOC was a bad indicator 20 years ago and it is today. Don't tell them - once they realize it's a red flag for us they will use some other metric, because they fight for our attention.

joduplessis 9mo ago

I wonder if we're heading to a situation where agent written code will function as something distinct, like bytecode.

embedding-shape 9mo ago

simonw, I find it almost shocking how you had the chance to talk directly with the engineer who built this, and even when he directly says things that contradict what Cursor's own CEO said, you didn't push back a single iota. Is the takeaway here that it's fine for a CEO to claim "it even has a custom JS VM!" on Twitter/X, then afterwards the engineer explains: "The JavaScript engine isn’t working yet" and "the agents decided to pause it", and this is all OK? Not a single pushback about this very obvious contradiction? This is just one example of many, and again, since it seems to be repeated: no, no one thinks this was supposed to rival Chrome, what a trite way of trying to change the narrative. I understand you don't want to spook future potential interviewees, but damn if that didn't feel like you suddenly are trying to defend Cursor here, instead of being curious about what actually happened. It doesn't feel curious, it feels like we're all giving up the fight against unneeded hype, exaggeration and degradation of quality. What happened with balanced perspectives, where we don't just take people for their words, and when we notice something is off, we bring it up? On a separate note, I actually emailed Wilson Lin too, asking if I could ask questions about it. While he initially accepted, I never actually received any answers. I'm glad to you were able to get someone from Cursor to clarify a bit at least, even though we're still just scratching the surface. I just wish we had a bit more integrity in the ecosystem and community I guess.

sebzim4500 9mo ago
Surely this would be push back to the CEO, not the engineer? The engineer is presumably the one telling the truth.
- embedding-shape 9mo ago
  Either of them. If the CEO refuses to answer, you ask others. If you get a chance to talk with them, you ask them about it. Just ignoring the elephant in the room and hoping that the unclear details gets forgotten helps no one except Cursor here.
simonw 9mo ago
Honestly, grilling him about what the CEO had tweeted didn't even cross my mind. I wanted to get to the truth of what had actually been built and how. If that contradicts what the CEO said then great, the truth is now out there - anyone is free to call that out and use my video as ammunition. I just had a look to see what Michael Truell had said about the project, here it is: https://x.com/mntruell/status/2011562190286045552 https://x.com/mntruell/status/2011562190286045552 > We built a browser with GPT-5.2 in Cursor. It ran uninterrupted for one week. > It's 3M+ lines of code across thousands of files. The rendering engine is from-scratch in Rust with HTML parsing, CSS cascade, layout, text shaping, paint, and a custom JS VM. > It kind of works! It still has issues and is of course very far from Webkit/Chromium parity, but we were astonished that simple websites render quickly and largely correctly. This doesn't strike me as the world's most dishonest tweet, though it exaggerates what was achieved. There IS a JS VM in there but it's feature-flagged off. The from-scratch is misleading because there are libraries handling certain aspects - most notably Taffy - which we discussed in the interview. I just ran "cloc" and to my surprise it counted 3,036,403 (I had thought the 3M was an exaggeration) though only 1,658,651 of that was Rust. "It kind of works" is a fair assessment IMO! I don't think "Let's talk about your CEO exaggerating what you built on Twitter" would have added much to the interview. I did make sure to go over the controversies I thought were material to the project, which is why I dug into the dependencies and talked about QuickJS and Taffy.

lifis 9mo ago

Why not help the Servo project instead of just making a proof of concept?

simonw 9mo ago
Servo have a strict policy against AI contributions: https://github.com/servo/book/blob/dd34e931be2fc8d555b38bcf1d0553f77c46acaa/src/contributing/getting-started.md#ai-contributions https://github.com/servo/book/blob/dd34e931be2fc8d555b38bcf1...

pooploop64 9mo ago

The reaction to this would have been different two or three years ago but it looks extremely lame when you open January 2026 hackernews and this is the kind of thing a tech company is trying to persuade you into thinking is exciting or useful. You've heard about what people are doing in the medical industry. Using AI to accelerate diagnosis and analysis of biological material. In astronomy it's showing us things that no human had ever seen before. You hear about all these things changing the world at large and the smaller worlds of individual people and families. Then you look at the actual IT industry and we've got... some premade libraries duct taped together into a crappy browser that barely works. Of course when the value of this is compared to the cost, the response is that it's fine because it was never actually intended to be useful in the first place. Well we're actually a step ahead of you there. The phrase "high on their own supply" describes all the people involved in this very well. I assure you we understand the goal of this project perfectly. It just wasn't a good, worthy, or even interesting goal. The immense amount of resources that went into this should have gone into something better. That's all there is to it.