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Custom HtmlLimitedBots UA Not In Built-In HTML_LIMITED_BOT_UA_RE Breaks PPR Resume And Drops All Page Metadata

When a request with a User-Agent matching a custom htmlLimitedBots regex but not the built-in HTML_LIMITED_BOT_UA_RE hits a non-prerendered PPR route, the server logs 'Expected the resume to render <div> in this slot but instead it rendered <__next_metadata_boundary__>', falls back to client rendering, and returns HTML with no metadata at all (no title, description, canonical). This causes severe SEO visibility loss for html-limited bots that do not execute JavaScript.

highConfidence 90%Next.js Affected V16.2.10 Affected V16.2.6

Origin Analysis

The PPR resume / metadata pipeline decides whether to render metadata blocking into <head> using the built-in HTML_LIMITED_BOT_UA_RE constant directly, while the initial shell rendering honors the user-configured htmlLimitedBots regex. This disagreement creates a mismatch between the expected resume slot and the actual metadata boundary, leading to the resume error and omitted metadata.

1. Create a Next.js 16.2.10 app with `cacheComponents: true` and `htmlLimitedBots: /GoogleOther|MySpecialBot|Twitterbot/i` in next.config.mjs.\n2. Build and start the app: `npm run build && PORT=3622 npm start`.\n3. Request a non-prerendered PPR page with a UA matching the custom regex but not the built-in list, e.g.: `curl -s -A 'MySpecialBot/1.0' http://localhost:3622/en/items/a`.\n4. Observe server log error and that `grep -c '<title>'` returns 0.\n5. Control: request with `Twitterbot/1.0` (matches both custom and built-in) returns 1 title.

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 patch changes bot detection globally, which may expand html-limited bot rendering to additional UAs and alter caching and streaming behavior. If `htmlLimitedBots` is configured with a broad regex (e.g., /bot|crawler/), many requests may be treated as html-limited, increasing server rendering cost and potentially disabling streaming for those clients. Rollback by reverting the patch or setting `htmlLimitedBots` to false. Test with built-in bots, custom bots, regular browsers, and concurrent PPR requests. Ensure all call sites are updated; any remaining direct use of `HTML_LIMITED_BOT_UA_RE` will reintroduce the mismatch. Node.js RegExp with global/sticky flags is unsafe for repeated `test()`; this patch strips to only `i`.