nextjs - 💡(How to fix) Fix App Router: route groups can share prefetch cache key and affect back/forward restore [1 comments, 2 participants]

Link to the code that reproduces this issue

(Will add repro repo or test case in follow-up if needed.)

To Reproduce

1. Use an App Router app with route groups (e.g. (marketing) and (shop)) that can resolve to the same pathname segments.
2. Navigate between pages so prefetch entries are created.
3. Trigger back/forward navigation where the router restores from cache.
4. Observe that cache reuse can cross route-group boundaries.

Current vs. Expected behavior

• Current: route cache/prefetch lookup can collide across entries that share the same URL-derived key but belong to different parallel route slots/route groups, leading to stale or incorrect restoration on browser back/forward.
• Expected: cache entries should be isolated by parallel route slot so back/forward restores the correct tree per group/slot context.

Provide environment information

npx envinfo --system --binaries --browsers --npmPackages 'next,react,react-dom'

Which area(s) are affected? (Select all that apply)

App Router

Which stage(s) are affected? (Select all that apply)

next dev (local) next build (local) next start (local)

Additional context

During investigation, parallelSlot was added to RouteCacheKey, but initially it did not affect cache behavior because keying still used vary-path generation that ignored this field in read paths. A robust fix appears to require routing parallelSlot through vary-path creation and all cache read/write sites so the effective cache key changes (not just the TS type). If useful, I can follow up with a focused regression test covering:

• prefetch in one route group
• back/forward restore in another group with same URL shape
• assertion that the restored route tree matches the correct parallel slot context