Simon Willison’s Weblog

Scaling long-running autonomous coding. Wilson Lin at Cursor has been doing some experiments to see how far you can push a large fleet of "autonomous" coding agents:

This post describes what we've learned from running hundreds of concurrent agents on a single project, coordinating their work, and watching them write over a million lines of code and trillions of tokens.

They ended up running planners and sub-planners to create tasks, then having workers execute on those tasks - similar to how Claude Code uses sub-agents. Each cycle ended with a judge agent deciding if the project was completed or not.

In my predictions for 2026 the other day I said that by 2029:

I think somebody will have built a full web browser mostly using AI assistance, and it won’t even be surprising. Rolling a new web browser is one of the most complicated software projects I can imagine[...] the cheat code is the conformance suites. If there are existing tests that it’ll get so much easier.

I may have been off by three years, because Cursor chose "building a web browser from scratch" as their test case for their agent swarm approach:

To test this system, we pointed it at an ambitious goal: building a web browser from scratch. The agents ran for close to a week, writing over 1 million lines of code across 1,000 files. You can explore the source code on GitHub.

But how well did they do? Their initial announcement a couple of days ago was met with unsurprising skepticism, especially when it became apparent that their GitHub Actions CI was failing and there were no build instructions in the repo.

It looks like they addressed that within the past 24 hours. The latest README includes build instructions which I followed on macOS like this:

cd /tmp
git clone https://github.com/wilsonzlin/fastrender
cd fastrender
git submodule update --init vendor/ecma-rs
cargo run --release --features browser_ui --bin browser

This got me a working browser window! Here are screenshots I took of google.com and my own website:

Honestly those are very impressive! You can tell they're not just wrapping an existing rendering engine because of those very obvious rendering glitches, but the pages are legible and look mostly correct.

The FastRender repo even uses Git submodules to include various WhatWG and CSS-WG specifications in the repo, which is a smart way to make sure the agents have access to the reference materials that they might need.

This is the second attempt I've seen at building a full web browser using AI-assisted coding in the past two weeks - the first was HiWave browser, a new browser engine in Rust first announced in this Reddit thread.

When I made my 2029 prediction this is more-or-less the quality of result I had in mind. I don't think we'll see projects of this nature compete with Chrome or Firefox or WebKit any time soon but I have to admit I'm very surprised to see something this capable emerge so quickly.

Update 23rd January 2026: I recorded a 47 minute conversation with Wilson about this project and published it on YouTube. Here's the video and accompanying highlights.

# 19th January 2026, 5:12 am / browsers, ai, generative-ai, llms, ai-assisted-programming, coding-agents, cursor, parallel-agents, conformance-suites, browser-challenge

Open Responses (via) This is the standardization effort I've most wanted in the world of LLMs: a vendor-neutral specification for the JSON API that clients can use to talk to hosted LLMs.

Open Responses aims to provide exactly that as a documented standard, derived from OpenAI's Responses API.

I was hoping for one based on their older Chat Completions API since so many other products have cloned the already, but basing it on Responses does make sense since that API was designed with the feature of more recent models - such as reasoning traces - baked into the design.

What's certainly notable is the list of launch partners. OpenRouter alone means we can expect to be able to use this protocol with almost every existing model, and Hugging Face, LM Studio, vLLM, Ollama and Vercel cover a huge portion of the common tools used to serve models.

For protocols like this I really want to see a comprehensive, language-independent conformance test site. Open Responses has a subset of that - the official repository includes src/lib/compliance-tests.ts which can be used to exercise a server implementation, and is available as a React app on the official site that can be pointed at any implementation served via CORS.

What's missing is the equivalent for clients. I plan to spin up my own client library for this in Python and I'd really like to be able to run that against a conformance suite designed to check that my client correctly handles all of the details.

# 15th January 2026, 11:56 pm / json, standards, ai, openai, generative-ai, llms, openrouter, conformance-suites

A Software Library with No Code. Provocative experiment from Drew Breunig, who designed a new library for time formatting ("3 hours ago" kind of thing) called "whenwords" that has no code at all, just a carefully written specification, an AGENTS.md and a collection of conformance tests in a YAML file.

Pass that to your coding agent of choice, tell it what language you need and it will write it for you on demand!

This meshes nearly with my recent interest in conformance suites. If you publish good enough language-independent tests it's pretty astonishing how far today's coding agents can take you!

# 10th January 2026, 11:41 pm / testing, ai, generative-ai, llms, ai-assisted-programming, drew-breunig, coding-agents, conformance-suites

twitter-text-conformance (via) This is a neat idea: Twitter have released open source libraries for parsing standard tweet syntax in Ruby and Java, but they’ve also released a set of YAML unit tests aimed at anyone who wants to implement the same parsing logic in other languages.

# 6th February 2010, 3:39 pm / java, ruby, testing, twitter, yaml, conformance-suites