One Human + One Agent = One Browser From Scratch

One Human + One Agent = One Browser From Scratch

2026-01-27

Just for the fun of it, I thought I'd embark on a week-long quest to
generate millions of tokens and millions of lines of source code to
create one basic browser that can render HTML and CSS (no JS tho), and
hopefully I could use this to receive even more VC investments.

But then I remembered that I have something even better: a human
brain! It is usually better than any machine at coordinating and
thinking through things, so let's see if we can hack something together,
one human brain and one LLM agent brain!

Demonstration of
one-agent-one-browser running with a bunch of different websites on
Linux/X11

The above might look like a simple .webm video, but it's actually a
highly sophisticated and advanced browser that was super hard to build,
encoded as pixels in a video file! Wowzers.

Day 1 - Starting out

For extra fun when building this, I set these requirements for myself
and the agent:

So with these things in mind, I set out on the journal to build a
browser "from scratch". I started with something really based, being
able to just render "Hello World". Then to be able to render some nested
tags. Added the ability of taking screenshots so the agent could use
that. Added specifications for HTML/CSS (which I think the agent never
used :| ), and tried to nail down the requirements for the agent to use.
Also started doing "regression" or "E2E" tests with the screenshotting
feature, so we could compare to some baseline images and so on. Added
the ability to click on links just for the fun of it.

After about a day together with Codex, I had something that could via
X11 and cURL, fetch and render websites when run, and the Cargo.lock is
empty. It was about 7500 lines long in total at that point, split across
files with all of them under 1000 lines long (which was a stated
requirement, so not a surprise).

Day 2 - Moving On

Second day I got annoyed by the tests spawning windows while I was
doing other stuff, so added a --headless flag too. Did some fixes for
resizing the window, various compatibility fixes, some performance
issues and improved the font/text rendering a bunch. Workflow was
basically to pick a website, share a screenshot of the website without
JavaScript, ask Codex to replicate it following our instructions. Most
of the time was the agent doing work by itself, and me checking in when
it notifies me it was done.

Day 3 - Polish &

Cross-platform (+ day 4)

Third day we made large changes, lots of new features and a bunch of
new features supported. More regression tests, fixing performance
issues, fixing crashes and whatnot. Also added scrolling because this is
a mother fucking browser, it has to be able to scroll. Added some debug
logs too because that'll look cool in the demonstration video above, and
also added support for the back button because it was annoying to start
from scratch if I clicked the wrong link while testing.

At the end of the third day we also added starting support for macOS,
and managed to get a window to open, and the tests to pass. Seems to
work OK :) Once we had that working, we also added Windows support,
basically the same process, just another platform after all.

Then the fourth day (whaaaat?) was basically polish, fixing CI for
all three platforms, making it pass and finally cutting a release based
on what got built in CI. Still all within 72 hours (3 days * 24 hours,
which obviously this is how you count days).

The results after ~3 days

(~70 hours)

And here it is, in all its glory, made in ~20K lines of code and
under 72 hours of total elapsed time from first commit to last:

You could try compiling it yourself (zero Rust dependencies, so it's
really fast :) ), or you can find binaries built on CI
here:https://github.com/embedding-shapes/one-agent-one-browser/releases

You can clone the repository, build it and try it out for yourself.
It's not great, I wouldn't even say it's good, but it works, and
demonstrates that one person with one agent can build a browser from
scratch.

This is what the "lines of code" count ended up being after all was
said and done, including support for three OSes:

Takeaways

If one person with one agent can produce equal or better results than
"hundreds of agents for weeks", then the answer to the question: "Can we
scale autonomous coding by throwing more agents at a problem?", probably
has a more pessimistic answer than some expected.

Built with