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Next/Link Prefetches Across Routers In Mixed App/Pages Apps Causing 404S And Wasted RSC Requests

In mixed App Router and Pages Router applications, Pages Router links to App Router routes trigger erroneous `_next/data` prefetch requests that return 404, and App Router links to Pages Router routes send RSC prefetch requests that receive HTML documents instead of Flight data.

highConfidence 85%Next.js Affected V16.3.6 Affected V16.4.0-Canary.51

Origin Analysis

The Pages Router `prefetch()` method lacks an early return after the client router filter (`_bfl`) matches a cross-router transition, causing it to continue issuing `_next/data` requests. The App Router `Link` component does not distinguish Pages Router targets and sends RSC prefetch requests indiscriminately.

1. Build and start a Next.js app with one App Router page (`app/dashboard`) and Pages Router routes (`pages/about.js`, `pages/[[...slug]].js`). 2. Open a Pages route (e.g., `/about`) with Network tab, hover the link to `/dashboard`; observe 404 for `/_next/data/<buildId>/dashboard.json?slug=dashboard`. 3. Open the App route `/dashboard`, hover link to `/about`; observe a prefetch request `/about?_rsc=...` returning HTML instead of Flight data.

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 only addresses Part A (Pages Router to App Router). Part B (App Router to Pages Router) still requires a similar guard in the App Router Link component to avoid RSC prefetch requests to Pages routes. Without Part B fix, users will still experience wasted HTML downloads. Rollback: remove the added `return` statement and restore the original comment to revert to previous behavior. Ensure no side effects from skipped prefetch in rare cases where `_bfl` might match but the navigation is not actually hard (e.g., custom client router filters). Test thoroughly in mixed apps with custom routing logic.