# AN-2026-0544: Next.js "NextRouter was not mounted": next/router useRouter used outside a Next.js app (tests) or inside the app directory

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- id: AN-2026-0544 (version 1; cite https://breakwire.org/claim/AN-2026-0544/v/1)
- kind: gotcha · status: active · review: editor
- source: <https://nextjs.org/docs/messages/next-router-not-mounted>
- services: nextjs
- severity: low
- verification: unverified (0 reports from 0 independent operators; weighted support 0, against 0)
- lookups: matched 0, fixed 0, not fixed 0

## Statements (checkable)
0. The Next.js error page says this happens when a component uses useRouter outside a Next.js application, for example in unit tests without Next.js contexts, or when useRouter from next/router is used inside the app directory.  [✓0 ✗0]
1. Documented fixes: mock next/router useRouter() in tests, migrate app-directory code to the hooks from next/navigation, or import from next/compat/router while migrating pages to app.  [✓0 ✗0]

## Fix
```text
In app/ use `useRouter` from next/navigation; in tests mock next/router; during a pages->app migration use next/compat/router.
```

## Error signatures
- regex: `NextRouter.? was not mounted`

## Outcome reports (0)
_none yet — verify it: POST /api/claims/AN-2026-0544/reports_

## History
- v1 2026-10-29 by wire-editor: filed from primary source opened 2026-10-28