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Server HTML For NotFound() Is An Empty Shell Without Root Layout And Lang Attribute

When a page calls notFound(), Next.js serves a minimal error shell (`__next_error__`) in the server HTML, omitting the root layout, `<html lang>` attribute, and the nearest not-found UI. This breaks accessibility, no-JS clients, and dynamic root layouts, and is inconsistent with the hydrated result. Affects App Router across multiple Next.js versions.

highConfidence 85%Next.js Affected V14.2.33 Affected V15.5.4 Affected V16.2.11 Affected V16.3.7 Affected V16.5.0-Canary.2

Origin Analysis

Next.js treats notFound() thrown during server rendering as a top-level error and renders a dedicated error shell without including the root layout or nearest not-found boundary. The global-not-found feature only covers unmatched URLs, so programmatic notFound bypasses the layout tree.

1. Create an App Router project with root layout (including `lang`), root not-found.tsx, and a dynamic page that calls notFound() for certain params. 2. Build and start the server. 3. `curl` a missing route: the HTML is `<html id="__next_error__">` with empty body, no lang, no layout, no not-found output. 4. Open the same route in a browser: after hydration the correct layout and not-found UI appear.

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 change may affect streaming, caching, and error boundary behavior. Thorough testing is required for dynamic routes, ISR, and concurrent rendering. Potential issues: double rendering of layout if not-found component itself throws, or infinite loop if notFound is called again in not-found. Defensive measure: ensure a flag to prevent re-entry and fallback to original shell if an error occurs during not-found rendering. Rollback: revert to the previous commit or disable the new behavior behind a flag if regressions appear.