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Segment Cache Cross-URL Poisoning Under HtmlLimitedBots Catch-All Disables PPR Serving

When htmlLimitedBots is set to /.*/ and cacheComponents/partialPrefetching are enabled, RSC prefetch responses bypass the PPR shell (no x-nextjs-postponed). The client still chooses the PPR fetch strategy, stores the full concrete page under a param-blind shell vary path, and sibling prefetches overwrite each other. Soft navigation then renders the wrong sibling page (e.g., /item/alpha shows Item delta). Affects Next.js 16.3.3 production; not reproducible on 16.4.0-canary.0.

highConfidence 92%Next.js Affected V16.3.3 Fixed V16.4.0-Canary.0

Origin Analysis

In Next.js 16.3.3, the segment cache scheduler prioritizes the `SubtreeHasPartialPrefetching` hint over `route.supportsPerSegmentPrefetching`. When htmlLimitedBots matches all user agents, PPR routes bypass shell serving even for RSC prefetches, so prefetch responses contain full pages without `x-nextjs-postponed`. The client nevertheless uses FetchStrategy.PPR, and `fulfillEntrySpawnedByRuntimePrefetch` stores these responses under the tree's param-blind `shellVaryPath` (all dynamic params replaced with `Fallback`). Each subsequent sibling prefetch overwrites the same fallback entries, and later lookups resolve to whichever sibling was prefetched last, causing cross-URL cache poisoning.

1. Create a Next.js 16.3.3 app with `cacheComponents: true`, `partialPrefetching: true`, and `htmlLimitedBots: /.*/` in next.config.js. 2. Add a static page with links to /item/alpha, /item/beta, /item/charlie, /item/delta. 3. Implement app/item/[...slug]/page.js using `'use cache'` for data that feeds both page body and generateMetadata. 4. Build and start production (`next build && next start`). 5. Open `/` in a fresh browser profile and wait ~3 seconds for all link prefetches. 6. Click the alpha link; observe the URL is /item/alpha but the title and heading show Item delta or charlie (the last prefetched sibling).

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 client-side safeguard addresses the poisoning but does not restore Instant Navigation for these routes; users may see additional full-page requests. Backporting to 16.3.x may conflict with other segment cache changes; test thoroughly with htmlLimitedBots enabled and disabled, concurrent sibling prefetches, and route transitions. Rollback advice: if this patch causes unexpected behavior, revert it and temporarily remove or narrow the htmlLimitedBots catch-all until an official fix is available. Concurrency and multi-threaded browser tabs could still race if the root cause (server bypassing PPR for RSC) is not addressed.