vercel · acdlite · Jul 22, 2026 · Jul 22, 2026
diff --git a/packages/next/src/build/adapter/build-complete.ts b/packages/next/src/build/adapter/build-complete.ts
@@ -1572,6 +1572,9 @@ export async function handleBuildComplete({
           )?.routeKeys || {}
         const allowQuery = Object.values(routeKeys)
         const partialFallback =
+          // Partial fallback shells are only emitted when Partial Prefetching
+          // is enabled in the app's Next.js config.
+          Boolean(config.partialPrefetching) &&
           isAppPage &&
           remainingPrerenderableParams !== undefined &&
           remainingPrerenderableParams.length > 0 &&

diff --git a/...ction/app-dir/adapter-partial-fallback-disabled/adapter-partial-fallback-disabled.test.ts b/...ction/app-dir/adapter-partial-fallback-disabled/adapter-partial-fallback-disabled.test.ts
@@ -0,0 +1,41 @@
+import { nextTestSetup } from 'e2e-utils'
+import type { NextAdapter } from 'next'
+
+describe('adapter-partial-fallback-disabled', () => {
+  const { next } = nextTestSetup({
+    files: __dirname,
+  })
+
+  it('should not emit partial fallback metadata when partialPrefetching is disabled', async () => {
+    const { outputs }: Parameters<NextAdapter['onBuildComplete']>[0] =
+      await next.readJSON('build-complete.json')
+
+    const withGspPrerender = outputs.prerenders.find(
+      (output) => output.pathname === '/with-gsp/[slug]'
+    )
+    const withGspRouteTreePrerender = outputs.prerenders.find(
+      (output) =>
+        output.pathname === '/with-gsp/[slug].segments/_tree.segment.rsc'
+    )
+    const withGspOtherSegmentPrerenders = outputs.prerenders.filter(
+      (output) =>
+        output.pathname.startsWith('/with-gsp/[slug].segments/') &&
+        output.pathname !== '/with-gsp/[slug].segments/_tree.segment.rsc'
+    )
+
+    expect(withGspPrerender).toBeDefined()
+    expect(withGspRouteTreePrerender).toBeDefined()
+    expect(withGspOtherSegmentPrerenders.length).toBeGreaterThan(0)
+
+    // Without `partialPrefetching` enabled, the route that would otherwise
+    // qualify for a partial fallback shell must not emit the partialFallback
+    // config, and its fallback shell collapses to the shared shell (empty
+    // allowQuery).
+    expect(withGspPrerender.config.partialFallback).toBeUndefined()
+    expect(withGspPrerender.config.allowQuery).toEqual([])
+    expect(withGspRouteTreePrerender.config.partialFallback).toBeUndefined()
+    for (const output of withGspOtherSegmentPrerenders) {
+      expect(output.config.partialFallback).toBeUndefined()
+    }
+  })
+})
diff --git a/test/production/app-dir/adapter-partial-fallback-disabled/app/layout.tsx b/test/production/app-dir/adapter-partial-fallback-disabled/app/layout.tsx
@@ -0,0 +1,8 @@
+import { ReactNode } from 'react'
+export default function Root({ children }: { children: ReactNode }) {
+  return (
+    <html>
+      <body>{children}</body>
+    </html>
+  )
+}
diff --git a/test/production/app-dir/adapter-partial-fallback-disabled/app/with-gsp/[slug]/loading.tsx b/test/production/app-dir/adapter-partial-fallback-disabled/app/with-gsp/[slug]/loading.tsx
@@ -0,0 +1,3 @@
+export default function Loading() {
+  return <div>loading...</div>
+}
diff --git a/test/production/app-dir/adapter-partial-fallback-disabled/app/with-gsp/[slug]/page.tsx b/test/production/app-dir/adapter-partial-fallback-disabled/app/with-gsp/[slug]/page.tsx
@@ -0,0 +1,27 @@
+import { Suspense } from 'react'
+
+async function Dynamic() {
+  await new Promise((resolve) => setTimeout(resolve, 1000))
+  return <div>Custom Data</div>
+}
+
+export function generateStaticParams() {
+  return [{ slug: 'one' }]
+}
+
+export default async function Page({
+  params,
+}: {
+  params: Promise<{ slug: string }>
+}) {
+  const { slug } = await params
+
+  return (
+    <div>
+      <div>{slug}</div>
+      <Suspense fallback={<div>Loading...</div>}>
+        <Dynamic />
+      </Suspense>
+    </div>
+  )
+}
diff --git a/test/production/app-dir/adapter-partial-fallback-disabled/my-adapter.mjs b/test/production/app-dir/adapter-partial-fallback-disabled/my-adapter.mjs
@@ -0,0 +1,9 @@
+import fs from 'fs/promises'
+
+/** @type {import('next').NextAdapter } */
+export default {
+  name: 'adapter-partial-fallback-disabled',
+  async onBuildComplete(ctx) {
+    await fs.writeFile('build-complete.json', JSON.stringify(ctx, null, 2))
+  },
+}
diff --git a/test/production/app-dir/adapter-partial-fallback-disabled/next.config.js b/test/production/app-dir/adapter-partial-fallback-disabled/next.config.js
@@ -0,0 +1,9 @@
+/**
+ * @type {import('next').NextConfig}
+ */
+const nextConfig = {
+  cacheComponents: true,
+  adapterPath: require.resolve('./my-adapter.mjs'),
+}
+
+module.exports = nextConfig
diff --git a/test/production/app-dir/adapter-partial-fallback/next.config.js b/test/production/app-dir/adapter-partial-fallback/next.config.js
@@ -3,6 +3,7 @@
  */
 const nextConfig = {
   cacheComponents: true,
+  partialPrefetching: true,
   adapterPath: require.resolve('./my-adapter.mjs'),
 }