- Symptom
- Missing required default.js file for parallel route at app/@modalAn @modal or @sidebar slot that built on Next.js 15 now fails next build
- Cause
- Next.js 16 requires an explicit default.js in every parallel route slot. Earlier versions fell back quietly when a slot had no match on hard navigation.
- Fix
- Add default.tsx to each slot that returns null, or calls notFound() if that is the intended behavior.
- Apply
// app/@modal/default.tsx export default function Default() { return null }- Affected
- Next.js 16.0 and later, App Router parallel routes
- Sources
Last verified 2026-10-15 · Confirmed by 0 builders