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Experimental.WebVitalsAttribution Has No Effect On UseReportWebVitals

The experimental.webVitalsAttribution config option is accepted but the attribution data is never attached to metrics reported by useReportWebVitals because the code that consumed __NEXT_WEB_VITALS_ATTRIBUTION was removed. The hook currently imports only the non-attribution web-vitals build.

highConfidence 85%Next.js Affected V14.2.x Affected V15.0.0 Affected V16.3.0-Canary.89

Origin Analysis

The `experimental.webVitalsAttribution` option is injected as `__NEXT_WEB_VITALS_ATTRIBUTION` but nothing reads it after the performance relayer was removed in #64199. `useReportWebVitals` still imports the standard `next/dist/compiled/web-vitals` functions, which do not provide attribution; it never imports or uses the attribution build.

1. Clone https://github.com/dayongkr/webvitals-attribution-repro, then run `npm install && npm run dev` 2. Open http://localhost:3000 with the browser console open 3. Click anywhere on the page (flushes LCP/INP), then switch tabs once (flushes CLS) 4. Compare the `[useReportWebVitals]` logs with the `[web-vitals/attribution]` logs

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

Enabling the attribution build increases bundle size; static import of both modules may cause attribution module to be included for all users if tree-shaking fails. Additionally, this implementation enables attribution for all metrics when any are configured, not just the whitelisted ones. Rollback by reverting to `import * as webVitals from 'next/dist/compiled/web-vitals'` and removing the attribution import. Verify `process.env.__NEXT_WEB_VITALS_ATTRIBUTION` remains a valid JSON array; malformed JSON is caught but would silently disable attribution.