code · pull · Aug 21, 2026 · Aug 21, 2026
diff --git a/packages/next/src/client/route-params.ts b/packages/next/src/client/route-params.ts
@@ -11,6 +11,7 @@ import {
   NEXT_RSC_UNION_QUERY,
 } from './components/app-router-headers'
 import { hasBasePath } from './has-base-path'
+import { normalizePathTrailingSlash } from './normalize-trailing-slash'
 import { removeBasePath } from './remove-base-path'
 import type {
   NormalizedPathname,
@@ -225,9 +226,15 @@ export function urlToUrlWithoutFlightMarker(url: URL): URL {
       urlWithoutFlightParameters.pathname.endsWith('.txt')
     ) {
       const { pathname } = urlWithoutFlightParameters
-      const length = pathname.endsWith('/index.txt') ? 10 : 4
-      // Slice off `/index.txt` or `.txt` from the end of the pathname
-      urlWithoutFlightParameters.pathname = pathname.slice(0, -length)
+      // Undo the marker appended in `fetchServerResponse`, which is keyed on
+      // whether the requested pathname ended with a slash: `index.txt` for
+      // `/foo/`, `.txt` for `/foo`. Slicing off only `index.txt` keeps that
+      // slash, then `normalizePathTrailingSlash` applies the configured
+      // policy so we don't hand-roll a second one here.
+      const length = pathname.endsWith('/index.txt') ? 9 : 4
+      urlWithoutFlightParameters.pathname = normalizePathTrailingSlash(
+        pathname.slice(0, -length)
+      )
     }
   }
   return urlWithoutFlightParameters

diff --git a/test/cache-components-tests-manifest.json b/test/cache-components-tests-manifest.json
@@ -264,6 +264,7 @@
       "test/e2e/app-dir/server-source-maps/server-source-maps.test.ts",
       "test/e2e/app-dir/set-cookies/set-cookies.test.ts",
       "test/e2e/app-dir/shallow-routing/shallow-routing.test.ts",
+      "test/e2e/app-dir/static-export-skew-trailing-slash/static-export-skew-trailing-slash.test.ts",
       "test/e2e/app-dir/static-generation-status/index.test.ts",
       "test/e2e/app-dir/static-shell-debugging/static-shell-debugging.test.ts",
       "test/e2e/app-dir/taint/process-taint.test.ts",

diff --git a/test/e2e/app-dir/static-export-skew-trailing-slash/app/layout.tsx b/test/e2e/app-dir/static-export-skew-trailing-slash/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/e2e/app-dir/static-export-skew-trailing-slash/app/page.tsx b/test/e2e/app-dir/static-export-skew-trailing-slash/app/page.tsx
@@ -0,0 +1,12 @@
+import Link from 'next/link'
+
+export default function Page() {
+  return (
+    <main>
+      <h1>Home page</h1>
+      <Link id="target-link" href="/target/" prefetch={false}>
+        Target page
+      </Link>
+    </main>
+  )
+}
diff --git a/test/e2e/app-dir/static-export-skew-trailing-slash/app/target/page.tsx b/test/e2e/app-dir/static-export-skew-trailing-slash/app/target/page.tsx
@@ -0,0 +1,3 @@
+export default function TargetPage() {
+  return <h1 id="target-page">Target page</h1>
+}
diff --git a/test/e2e/app-dir/static-export-skew-trailing-slash/next.config.js b/test/e2e/app-dir/static-export-skew-trailing-slash/next.config.js
@@ -0,0 +1,10 @@
+/**
+ * @type {import('next').NextConfig}
+ */
+const nextConfig = {
+  output: 'export',
+  trailingSlash: true,
+  generateBuildId: async () => 'current-build-id',
+}
+
+module.exports = nextConfig
diff --git a/test/e2e/app-dir/static-export-skew-trailing-slash/server.mjs b/test/e2e/app-dir/static-export-skew-trailing-slash/server.mjs
@@ -0,0 +1,19 @@
+import { createReadStream } from 'node:fs'
+import { createServer } from 'node:http'
+import { join } from 'node:path'
+import handler from 'serve-handler'
+
+export function createExportServer(outDir, requests) {
+  return createServer((request, response) => {
+    const { pathname } = new URL(request.url, 'http://localhost')
+    requests.push(pathname)
+
+    if (pathname === '/target' || pathname === '/target/') {
+      response.writeHead(200, { 'Content-Type': 'text/html; charset=utf-8' })
+      createReadStream(join(outDir, 'target/index.html')).pipe(response)
+      return
+    }
+
+    return handler(request, response, { public: outDir })
+  })
+}
diff --git a/test/e2e/app-dir/static-export-skew-trailing-slash/static-export-skew-trailing-slash.test.ts b/test/e2e/app-dir/static-export-skew-trailing-slash/static-export-skew-trailing-slash.test.ts
@@ -0,0 +1,64 @@
+import type { Server } from 'node:http'
+import { readFileSync, writeFileSync } from 'node:fs'
+import { join } from 'node:path'
+import { findPort, retry } from 'next-test-utils'
+import { isNextStart, nextTestSetup } from 'e2e-utils'
+import { createExportServer } from './server.mjs'
+
+describe('static-export-skew-trailing-slash', () => {
+  if (!isNextStart) {
+    test('build test should not run during dev test run', () => {})
+    return
+  }
+
+  const { next } = nextTestSetup({
+    files: __dirname,
+    skipStart: true,
+    disableAutoSkewProtection: true,
+  })
+
+  let port: number
+  let server: Server
+  const requests: string[] = []
+
+  beforeAll(async () => {
+    await next.build()
+
+    const targetFlightPath = join(next.testDir, 'out/target/index.txt')
+    const targetFlight = readFileSync(targetFlightPath, 'utf8')
+    const currentBuildId = '"b":"current-build-id"'
+    if (!targetFlight.includes(currentBuildId)) {
+      throw new Error('Could not find the current build ID in target RSC data')
+    }
+    writeFileSync(
+      targetFlightPath,
+      targetFlight.replace(currentBuildId, '"b":"foreign-build-id"')
+    )
+
+    port = await findPort()
+    server = createExportServer(join(next.testDir, 'out'), requests)
+    server.listen(port)
+  })
+
+  afterAll(() => {
+    server?.close()
+  })
+
+  it('preserves the trailing slash during an MPA fallback', async () => {
+    const browser = await next.browser('/', { baseUrl: port })
+
+    await browser.elementById('target-link').click()
+    await browser.waitForElementByCss('#target-page')
+
+    await retry(async () => {
+      expect(new URL(await browser.url()).pathname).toBe('/target/')
+    })
+
+    expect(requests).toContain('/target/index.txt')
+    expect(
+      requests.filter(
+        (pathname) => pathname === '/target' || pathname === '/target/'
+      )
+    ).toEqual(['/target/'])
+  })
+})