Cursor adds 'use client' to every file, is that actually a problem?

It matters a bit, not a lot, for a small site. What you lose:

• Everything under a "use client" file ships as JavaScript to the browser, including big libraries (markdown renderers, date libs) that could stay on the server.
• You can't fetch data directly in those components (DB calls, secrets), so you end up with extra API routes.

The cheap fix: "use client" only needs to be on the entry of an interactive part. Children imported by a client component are client code automatically. So usually you can delete it from most files and keep it on the 5–10 components that really have state or onClick.

Tell Cursor: "when you need client features, make a small client component and import it into the server component. Don't add 'use client' to pages or layouts."

Counterpoint, so you can prioritize: if the site is fast and your Lighthouse is fine, this is a cleanup for a rainy afternoon, not an emergency. Fix it when you're touching those files anyway or when you notice bundle size / data-fetching pain. Don't let the agent do a big "server components migration" in one go, that's how you get a broken week.

If you want numbers before deciding: @next/bundle-analyzer shows what's in your client JS. When the big blocks are libraries you only use for rendering (markdown, syntax highlight), moving those to server components is the quick win.