vercel · unstubbable · May 19, 2026 · May 13, 2026
diff --git a/packages/next/src/shared/lib/router/router.ts b/packages/next/src/shared/lib/router/router.ts
@@ -227,9 +227,14 @@ function getMiddlewareData<T extends FetchDataOutput>(
           const parsedSource = getNextPathnameInfo(
             parseRelativeUrl(source).pathname,
             {
-              nextConfig: process.env.__NEXT_HAS_REWRITES
-                ? undefined
-                : nextConfig,
+              // Pass basePath (and trailingSlash) so the basePath prefix is
+              // stripped before the `_next/data/` check, but omit `i18n` so the
+              // locale prefix is preserved here — the rewrite resolver below
+              // handles the locale-prefixed `as`.
+              nextConfig: {
+                basePath: nextConfig.basePath,
+                trailingSlash: nextConfig.trailingSlash,
+              },
               parseData: true,
             }
           )

diff --git a/...re-dynamic-basepath-matcher-rewrites/middleware-dynamic-basepath-matcher-rewrites.test.ts b/...re-dynamic-basepath-matcher-rewrites/middleware-dynamic-basepath-matcher-rewrites.test.ts
@@ -0,0 +1,19 @@
+import { nextTestSetup } from 'e2e-utils'
+import { retry } from 'next-test-utils'
+
+describe('middleware-dynamic-basepath-matcher-rewrites', () => {
+  const { next } = nextTestSetup({
+    files: __dirname,
+  })
+
+  it('preserves router.query on client-side navigation to a catch-all page', async () => {
+    const browser = await next.browser('/docs')
+    await browser.elementById('catchall-link').click()
+
+    await retry(async () => {
+      expect(await browser.elementById('page-title').text()).toBe('CatchAll')
+    })
+
+    expect(await browser.elementById('query-path').text()).toBe('["first"]')
+  })
+})
diff --git a/test/e2e/middleware-dynamic-basepath-matcher-rewrites/middleware.js b/test/e2e/middleware-dynamic-basepath-matcher-rewrites/middleware.js
@@ -0,0 +1,9 @@
+import { NextResponse } from 'next/server'
+
+export default function middleware() {
+  return NextResponse.next()
+}
+
+export const config = {
+  matcher: '/:path*',
+}
diff --git a/test/e2e/middleware-dynamic-basepath-matcher-rewrites/next.config.js b/test/e2e/middleware-dynamic-basepath-matcher-rewrites/next.config.js
@@ -0,0 +1,8 @@
+module.exports = {
+  basePath: '/docs',
+  async rewrites() {
+    // Any non-empty rewrites array sets __NEXT_HAS_REWRITES=true; this entry
+    // only exists to flip that build flag, it isn't expected to match.
+    return [{ source: '/never-matched-rewrite', destination: '/never' }]
+  },
+}
diff --git a/test/e2e/middleware-dynamic-basepath-matcher-rewrites/pages/[...path].tsx b/test/e2e/middleware-dynamic-basepath-matcher-rewrites/pages/[...path].tsx
@@ -0,0 +1,15 @@
+import { useRouter } from 'next/router'
+
+export default function CatchAll() {
+  const router = useRouter()
+  return (
+    <div>
+      <h1 id="page-title">CatchAll</h1>
+      <p id="query-path">{JSON.stringify(router.query.path)}</p>
+    </div>
+  )
+}
+
+export async function getServerSideProps() {
+  return { props: {} }
+}
diff --git a/test/e2e/middleware-dynamic-basepath-matcher-rewrites/pages/index.tsx b/test/e2e/middleware-dynamic-basepath-matcher-rewrites/pages/index.tsx
@@ -0,0 +1,9 @@
+import Link from 'next/link'
+
+export default function Home() {
+  return (
+    <Link href="/first" id="catchall-link">
+      Go to catchall
+    </Link>
+  )
+}