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NotFound() Returns HTTP 200 Instead Of 404 When Streaming Is Enabled By Root Loading.Tsx And Sibling Dynamic Route

When a route with dynamicParams:false calls notFound(), it returns 200 with not-found UI if a root loading.tsx and a sibling dynamicParams:true route exist. The 404 status is lost during streaming; the incremental cache stores a 200 response.

highConfidence 78%Next.js Affected V16.3.4 Affected V16.3.6 Affected V16.4.0-Canary.50

Origin Analysis

Next.js App Router enables streaming when a root loading.tsx is present. For routes with dynamicParams:false, an unknown param should trigger a 404 before streaming; however, the presence of a sibling dynamicParams:true route causes the route to be rendered on demand as a streamed response. The HTTP status 200 is committed before the page component runs and throws notFound(), so the 404 status cannot be applied. The cache therefore stores a 200 entry.

1. Create app/brands/[brand]/page.tsx with dynamicParams=false and notFound() for unknown brand. 2. Add app/loading.tsx. 3. Add a sibling dynamic route app/[category]/[slug]/page.tsx with dynamicParams=true. 4. Build and start, curl /brands/does-not-exist -> returns 200 with not-found UI.

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 only addresses the primary route with dynamicParams:false; routes with dynamicParams:true that throw notFound() under streaming may still return 200. Disabling streaming can increase time-to-first-byte for such routes. Rollback by reverting the patch. Upstream fix may need a more sophisticated solution (e.g., pre-rendering the segment or delaying status until initial render completes).