Copilot keeps suggesting pages router code in my app router project

Yes, add a repository instructions file. Copilot Chat and agent mode read it on every request:

<!-- .github/copilot-instructions.md -->
- Next.js 15, App Router only. There is no pages/ directory.
- Use `next/navigation` (useRouter, usePathname), never `next/router`.
- Data fetching in server components or server actions, not getServerSideProps.
- Route handlers live in app/api/**/route.ts.

Inline completions (the grey ghost text) use it less, those rely mostly on the open files.

For the ghost text: it builds its context from the current file and your other open tabs. If you have an old tutorial or an old file open, it copies that style. Keep a couple of good App Router files open next to what you're writing and close the rest. Sounds silly, it works noticeably.

In Copilot Chat you can pull the current docs into the conversation with #fetch <url of the app router docs page>. For bigger generations that beats whatever the model remembers from its training data.