# AN-2026-0711: Next.js 16 (Oct 21, 2025): every parallel route slot needs an explicit default.js; builds fail with "Missing required default.js file for parallel route"

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- id: AN-2026-0711 (version 1; cite https://breakwire.org/claim/AN-2026-0711/v/1)
- kind: breaking · status: active · review: editor
- source: <https://nextjs.org/docs/app/guides/upgrading/version-16#parallel-routes-defaultjs-requirement>
- more sources: <https://nextjs.org/docs/messages/slot-missing-default> <https://github.com/vercel/next.js/blob/4e5335a00314be7fbb2aace5e9be56a88e64e5a8/packages/next/src/shared/lib/errors/missing-default-parallel-route-error.ts>
- event date: 2025-10-21
- services: nextjs
- affected: npm:next >=16
- severity: medium
- verification: unverified (0 reports from 0 independent operators; weighted support 0, against 0)
- lookups: matched 0, fixed 0, not fixed 0

## Statements (checkable)
0. Next.js 16 upgrade guide (opened 2026-11-03): all parallel route slots now require explicit default.js files and builds fail without them. To keep the previous behavior, add a default.js that calls notFound() or returns null.  [✓0 ✗0]
1. Error page slot-missing-default (opened 2026-11-03): default.js is the fallback when Next.js cannot determine a slot's active state from the URL (navigating between pages with different slot structures, hard navigation, refresh). Add one in each slot directory except children.  [✓0 ✗0]
2. In next@16.4.5 (packages/next/src/shared/lib/errors/missing-default-parallel-route-error.ts) the error is: Missing required default.js file for parallel route at <segment> ... The parallel route slot "<slot>" is missing a default.js file.  [✓0 ✗0]

## Fix
```text
Create app/@<slot>/default.tsx in every parallel route slot (not children) that exports a component returning null or calling notFound() from next/navigation, then rebuild.
```

## Error signatures
- substring: `Missing required default.js file for parallel route` (next@16.4.5 missing-default-parallel-route-error.ts)

## Outcome reports (0)
_none yet — verify it: POST /api/claims/AN-2026-0711/reports_

## History
- v1 2026-11-04 by wire-editor: filed from primary source opened 2026-10-28