FastRender is an experimental web browser written almost entirely by autonomous coding agents, produced as the test case for an experiment at Cursor into how far a large fleet of concurrently running coding agents could be pushed. Cursor’s own description, as quoted by Simon Willison, says it pointed the system at the goal of building a web browser from scratch, and that the agents ran for close to a week, writing over a million lines of code across 1,000 files. The source code is published on GitHub.
The agents were organised with planners and sub-planners creating tasks, workers executing them, and a judge agent deciding at the end of each cycle whether the project was complete — a structure Willison likened to the way Claude Code uses sub-agents, and which is described in more detail under Planner-Worker Model.
Capabilities
Willison built and ran FastRender on macOS following the build instructions in the repository’s README, and got a working browser window. His screenshots of google.com and his own blog show pages that were legible and mostly correct but with obvious rendering glitches, such as unstyled buttons, a garbled tab name and a misplaced decorative quotation mark; he took those glitches as evidence that the project was not simply wrapping an existing rendering engine. The repository includes various WhatWG and CSS-WG specifications as Git submodules, which he described as a smart way of making sure the agents had the reference material they might need.
Adoption & Ecosystem
The initial announcement was met with skepticism, particularly once it emerged that the project’s GitHub Actions CI was failing and the repository had no build instructions; build instructions were added shortly afterwards. Willison, who had predicted that someone would build a full web browser mostly with AI assistance by 2029, wrote that FastRender was close to the quality of result he had in mind, while not expecting projects of this kind to compete with Chrome, Firefox or WebKit any time soon. He noted it was the second attempt he had seen within two weeks at building a full browser with AI-assisted coding.