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Turbopack Dev Server Intermittently 404S Child Routes Below Dynamic Segments Inside Route Groups Due To Route-Tree Race

Next.js dev server (Turbopack) fails to match routes that are children of a dynamic segment nested inside route groups, returning instant 404 for the entire session. The issue is probabilistic, occurs under module graph load, and is repaired by any file event under app/. Production builds are unaffected.

highConfidence 85%Next.js Affected V16.3.1 Affected V16.3.2 Affected V16.4.0-Canary.7

Origin Analysis

A race condition in Turbopack's route-tree construction causes child routes under dynamic segments within route groups to be omitted from the route matcher during initial dev server startup. The dynamic leaf itself matches, but its nested children do not, indicating incomplete registration of nested route entries. The race is timing-dependent and can be triggered by CPU load or large module graphs.

1. Clone https://github.com/arpi09/next-dev-router-group-race; 2. cd next-dev-router-group-race && npm install; 3. (Optional) node scripts/cpu-load.mjs 4 30 to increase reproduction rate; 4. node scripts/boot-loop.mjs 10 to classify boots; 5. Manual check: npx next dev -p 4321; curl -i http://localhost:4321/fam1/a/child -> instant 404 with no 'Compiling /fam1/[slug]/child' log; curl -i http://localhost:4321/fam1/a -> 200; Append a blank line to any app/**/page.js, save, remove, save; curl again -> compiles and returns 200.

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 workaround modifies a file in app/, which could interfere with active editors, trigger unnecessary hot reloads, or fail if the file is read-only or deleted. It should be disabled once the underlying Next.js bug is fixed; rollback by removing the wrapper script and starting `next dev` normally. In multi-process or concurrent dev environments, ensure the TOUCH_FILE path is correct and the script does not run multiple times on the same file simultaneously. The script also assumes CommonJS require and may need adjustment for ESM projects.