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CacheComponents Drops Suspense Boundary Root Element On PPR Resume When Sync IO Is Called In Client Component

When a client component inside a Suspense boundary uses synchronous IO (e.g., Math.random or Date.now) during server rendering, the resumed segment omits the host elements rendered before the sync IO call. This causes a hydration mismatch (React error #418) and the boundary's fallback is replaced with an incomplete subtree.

highConfidence 87%Next.js Affected V16.4.0 Affected V16.5.0-Canary.2

Origin Analysis

The cacheComponents optimization in Next.js 16 incorrectly truncates the rendered output of a client component when it encounters synchronous IO. The component's output buffer is not properly flushed before the sync IO boundary, so the prelude and resume HTML only contain the elements rendered after the sync IO call (e.g., <p>), while the preceding host elements (<section>, <h2>) are lost. This is a defect in the interaction between cacheComponents and PPR's resume serialization.

1. Clone https://github.com/sannajammeh/next-sync-io-resume-repro 2. Run `npm install && npm run build && npm start` 3. In a second terminal run `npm run check` to fetch `/` and compare fallback and resumed roots 4. Observe the console error: Minified React error #418 (args[]=HTML) and the first boundary's resumed root is <p> instead of <section>.

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

Disabling cacheComponents may degrade performance for cached components. The provided code is illustrative and may not match the exact internal implementation; applying it without proper integration could break other stream utilities. Rollback by reverting the patch or re-enabling cacheComponents. The underlying bug is triggered only when synchronous IO occurs in a client component within a Suspense boundary under PPR, so avoid such patterns if possible.