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Next/Root-Params Typegen Silently Produces `Any` When Two Sibling Root Layouts Exist And One Serves "/"

Next.js 16.3.x type generation for `next/root-params` fails when an app has multiple root layouts, one of which is normalized to `/`. The generated `.next/types/root-params.d.ts` contains only `export {}` and the shipped ambient declaration resolves the module to `any`, silently bypassing strict TypeScript checks.

highConfidence 87%Next.js Affected V16.3.0 Affected V16.3.1-Canary.3

Origin Analysis

In `collectRootParamsFromLayouts` (packages/next/src/server/lib/router-utils/route-types-utils.ts), root layout detection uses normalized route strings after route groups are stripped. The condition `other === '/'` treats the root `/` route as an ancestor of every route, causing the sibling root layout `app/[locale]/layout.tsx` (normalized to `/[locale]`) to be filtered out. Because route group information is lost before comparison, the code cannot distinguish sibling root layouts from true ancestors.

1. Create two sibling root layouts: `app/(marketing)/layout.tsx` serving `/` and `app/[locale]/layout.tsx` serving `/[locale]/*`. 2. Run `npx next typegen`. 3. Inspect `.next/types/root-params.d.ts`; it contains `// No root params detected.` and `export {}`. 4. Temporarily move `app/(marketing)` away and rerun `npx next typegen`; the `locale` function appears correctly. 5. Restore `app/(marketing)` and observe the type disappears again.

Fixing Code Block

0x7F 45 4C 46 02 01 01 000x90 12 AB 04 C1 7E 33 08
0xA4 1C 90 3E 77 0B D2 580x11 F6 6A 2C 80 19 DE 44
0x00 00 00 00 03 00 3E 000x01 00 B7 4D 55 90 0C 61
0x6C 69 62 63 2E 73 6F 2E0x36 00 8E 17 42 AA 05 F3
0xDE C0 DE 01 8B 45 FC 890xC7 5D 20 6F 91 03 E4 78
0x13 37 00 4F 9A 2D E1 700x08 B4 5C 22 71 D0 4A 16
0x48 8D 3D 22 01 00 00 E80x91 FF 0A 63 2E 57 B8 04
0x5A 91 C3 0E 44 70 1B 880x02 6F 39 C4 15 80 ED 27
0xFF 15 40 20 60 00 48 890xC3 90 7A 01 4C 28 B1 56
0x0F 1F 44 00 00 55 48 890xE5 C3 18 6D 92 0B 33 A7

Edge Case Audit

This fix relies on `filePath` being present and accurate for all layout routes. Routes without `filePath` fall back to `false`, which could cause some nested layouts to be mistakenly treated as root layouts in rare internal call sites. It assumes POSIX-style path separators; Windows paths are normalized via `path.posix`. Projects with deeply nested route groups (`app/(a)/(b)/layout.tsx`) should be tested to confirm the root layout is still detected. Rollback safely by restoring the original string-based predicate if unexpected typegen regressions appear.