cloudflare/vinext

Bot/crawler detection does not block fallback HTML on prerender

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#1,543 opened on May 22, 2026

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Description

This issue was created by an agent analysing CI failures from the Next.js Deploy Suite (vinext main vs Next.js v16.2.6, 2026-05-22).

Problem

When fallback: true static params encounter a crawler bot user-agent, Next.js renders the page synchronously instead of serving the fallback shell. vinext serves the fallback HTML containing Loading... to crawlers, which is bad for SEO.

Expected substring: not "Loading..."; received fallback shell

Estimated Impact

~1 test failures across the deploy suite.

Affected Test Suites

  • test/e2e/prerender-crawler.test.ts

Recommendation

  1. Reproduce first in vinext's own test suite. Request a fallback: true page with a Googlebot UA and assert the body does not contain the fallback text.

  2. Switch to synchronous render for known crawler UAs. When the UA matches the Next.js bot list and the page would otherwise serve a fallback shell, render synchronously instead.

Part of #1328.

Contributor guide

Research direction
Reproduce the failing test by requesting a `fallback: true` page with a Googlebot user agent and assert the response body does not contain fallback text. Then modify the prerender logic to detect crawler user agents and synchronously render the page instead of serving the fallback shell.
Tech stack
typescriptreactnextjsvite
Domain
backend
Issue type
Bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-2 days
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
Active
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
Clear
Prerequisites
JavaScriptNext.js
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
45