A Claude session has been rewriting an app in Swift for over two weeks
Speaking at Y Combinator's Startup School 2026, Claude Code lead Boris Cherny said the real skill now isn't prompt engineering. It's handing Claude a task slightly beyond what it can comfortably do, and giving the model a way to check its own work as it goes.
His test case: a Claude Tag session (Claude running inside Slack) wired up to spin a macOS virtual machine through GitHub, with access to an empty repo for a Swift build of the desktop app. The prompt was to rewrite the Electron app in Swift, run the Electron version in the Mac VM, take screenshots, compare them pixel by pixel, and not stop until it's done.
As of the interview, the job had been running a bit over two weeks, 14 to 15 days, and hadn't finished. In the interview breakdown, launching the current app on a Mac took 30 seconds, the last 15 of them on a spinning wait cursor.
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