What they did
The author was about to replace a five-year-old Mac that took over three minutes to boot. Before buying, he opened Claude Code in the Claude desktop app, left it on the setting that asks before every action, and told it to work out why the machine was slow while changing, deleting and installing nothing, and to explain each check in one sentence before running it. He approved eleven checks. Follow-up prompts asked for the ten largest folders in plain words with copies and backups flagged, a list of apps that launch at login with what each one does, and how he could recover anything before deleting it.
How it went
The disk was 96% full, with 4.8 GB free of 256. Claude Code pointed to 41 GB of backups from two phones he no longer owned, in a folder hidden by default, 23 login items, and a Downloads folder of about 2,300 files taking 9 GB. He turned off 17 of the login items and deleted the backups himself from System Settings after confirming his current phone backs up to iCloud. Free space rose to 52 GB and boot time fell to 48 seconds. He emptied the shopping basket.
Worth knowing
The agent never changed anything; every deletion was done by hand, which the author treats as the point. A friend in IT noted the Storage screen in macOS shows the same facts in two minutes; what the agent added was explaining each item. Tidying will not fix worn hardware. The post also sells the author's prompt library.