One Human + One Agent = One Browser From Scratch (https://emsh.cat/one-human-one-agent-one-browser/)

embedding-shapes was so infuriated (https://emsh.cat/cursor-implied-success-without-evidence/) by the hype around Cursor’s FastRender browser project (https://simonwillison.net/2026/Jan/23/fastrender/) - thousands of parallel agents producing ~1.6 million lines of Rust - that they were inspired to take a go at building a web browser using coding agents themselves.

The result is one-agent-one-browser (https://github.com/embedding-shapes/one-agent-one-browser) and it’s really impressive. Over three days they drove a single Codex CLI agent to build 20,000 lines of Rust that successfully renders HTML+CSS with no Rust crate dependencies at all - though it does (reasonably) use Windows, macOS and Linux system frameworks for image and text rendering.

I installed the 1MB macOS binary release (https://github.com/embedding-shapes/one-agent-one-browser/releases/tag/0.1.0) and ran it against my blog:

chmod 755 ~/Downloads/one-agent-one-browser-macOS-ARM64 ~/Downloads/one-agent-one-browser-macOS-ARM64 https://simonwillison.net/

Here’s the result:

It even rendered my SVG feed subscription icon! A PNG image is missing from the page, which looks like an intermittent bug (there’s code to render PNGs).

The code is pretty readable too - here’s the flexbox implementation (https://github.com/embedding-shapes/one-agent-one-browser/blob/0.1.0/src/layout/flex.rs).

I had thought that “build a web browser” was the ideal prompt to really stretch the capabilities of coding agents - and that it would take sophisticated multi-agent harnesses (as seen in the Cursor project) and millions of lines of code to achieve.

Turns out one agent driven by a talented engineer, three days and 20,000 lines of Rust is enough to get a very solid basic renderer working!

I’m going to upgrade my prediction for 2029 (https://simonwillison.net/2026/Jan/8/llm-predictions-for-2026/#3-years-someone-will-build-a-new-browser-using-mainly-ai-assisted-coding-and-it-won-t-even-be-a-surprise): I think we’re going to get a production-grade web browser built by a small team using AI assistance by then.

Via Show Hacker News (https://news.ycombinator.com/item?id=46779522)

Tags: browsers (https://simonwillison.net/tags/browsers), predictions (https://simonwillison.net/tags/predictions), ai (https://simonwillison.net/tags/ai), rust (https://simonwillison.net/tags/rust), generative-ai (https://simonwillison.net/tags/generative-ai), llms (https://simonwillison.net/tags/llms), ai-assisted-programming (https://simonwillison.net/tags/ai-assisted-programming), coding-agents (https://simonwillison.net/tags/coding-agents), codex-cli (https://simonwillison.net/tags/codex-cli), browser-challenge (https://simonwillison.net/tags/browser-challenge)