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Next/Dynamic Manifest ID Collision Between Pages And App Routers Breaks Pages Hydration

When a module using next/dynamic is imported by both Pages Router and App Router, ReactLoadablePlugin writes the App Router webpack module id into react-loadable-manifest.json, causing Pages Router client bundle to request the wrong dynamic chunk id and fail hydration with React error #418.

highConfidence 95%Next.js Affected V16.3.2 Affected V16.4.0-Canary.21

Origin Analysis

ReactLoadablePlugin keys manifest entries by `${importer} -> ${request}` without considering webpack layer. A module imported by both routers exists twice in the client compilation (pages-dir-browser and app-pages-browser) with different module ids. Both copies are walked under the same key, and whichever is processed last overwrites the entry. In typical builds the App Router copy is processed last, so the manifest contains the App Router module id, which the Pages Router client cannot use.

1. Clone https://github.com/korhan/next-loadable-layer-repro 2. Run `bun install` or `npm install` 3. Run `bun run build` (uses `next build --webpack`) 4. Run `bun run check` to compare manifest ids 5. Run `bun run start` and open http://localhost:3400/ with browser console open; observe React #418 error on pages router route

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 assumes the App Router will never need entries from react-loadable-manifest.json. If future Next.js versions change that behavior, this filter would cause missing manifest entries for App Router dynamic imports. Additionally, the layer name 'app-pages-browser' may change in future webpack layer schemas. Rollback recommendation: revert this patch and instead make the manifest key layer-aware (e.g., include layer in the key) or maintain separate manifests per router, then merge only the Pages Router entries.