fix: avoid /index cache-key collision for root dynamic routes

packages/next/src/server/route-modules/pages/pages-handler.ts

@@ -26,6 +26,7 @@ import {
   type CacheControl,
 } from '../../lib/cache-control'
 import { normalizeRepeatedSlashes } from '../../../shared/lib/utils'
+import { isDynamicRoute } from '../../../shared/lib/router/utils'
 import { getRedirectStatus } from '../../../lib/redirect-status'
 import {
   CACHE_ONE_YEAR_SECONDS,
@@ -179,7 +180,11 @@ export const getHandler = ({
       }
 
       // ensure /index and / is normalized to one key
-      cacheKey = cacheKey === '/index' ? '/' : cacheKey
+      // Only applies for non-dynamic routes — dynamic routes with slug=index
+      // must keep /index to avoid cache-key collision with the root page.
+      if (cacheKey === '/index' && !isDynamicRoute(srcPage)) {
+        cacheKey = '/'
+      }
     }
 
     if (hasStaticPaths && !isDraftMode) {

packages/next/src/server/route-modules/route-module.ts

@@ -1033,9 +1033,10 @@ export abstract class RouteModule<
       )
     }
 
-    if (resolvedPathname === '/index') {
-      resolvedPathname = '/'
-    }
+    resolvedPathname = normalizeResolvedPathname(
+      resolvedPathname,
+      normalizedSrcPage
+    )
 
     if (
       res &&
@@ -1171,3 +1172,20 @@ export abstract class RouteModule<
     return cacheEntry
   }
 }
+
+/**
+ * Normalizes resolvedPathname for ISR cache key usage.
+ *
+ * Only normalizes /index to / when the source page is not a dynamic route.
+ * For dynamic routes (e.g., [slug] with slug="index"), resolvedPathname
+ * must remain /index to avoid colliding with the / page's ISR cache key.
+ */
+export function normalizeResolvedPathname(
+  resolvedPathname: string,
+  normalizedSrcPage: string
+): string {
+  if (resolvedPathname === '/index' && !isDynamicRoute(normalizedSrcPage)) {
+    resolvedPathname = '/'
+  }
+  return resolvedPathname
+}

test/unit/normalize-resolved-pathname.test.ts

@@ -0,0 +1,28 @@
+/* eslint-env jest */
+import { normalizeResolvedPathname } from 'next/dist/server/route-modules/route-module'
+
+describe('normalizeResolvedPathname', () => {
+  it('should normalize /index to / for static root page', () => {
+    expect(normalizeResolvedPathname('/index', '/')).toBe('/')
+  })
+
+  it('should NOT normalize /index to / for [slug] dynamic route', () => {
+    expect(normalizeResolvedPathname('/index', '/[slug]')).toBe('/index')
+  })
+
+  it('should NOT normalize /index to / for catch-all [...slug] route', () => {
+    expect(normalizeResolvedPathname('/index', '/[...slug]')).toBe('/index')
+  })
+
+  it('should NOT normalize /index to / for optional catch-all [[...slug]] route', () => {
+    expect(normalizeResolvedPathname('/index', '/[[...slug]]')).toBe('/index')
+  })
+
+  it('should pass through other pathnames unchanged', () => {
+    expect(normalizeResolvedPathname('/about', '/about')).toBe('/about')
+    expect(normalizeResolvedPathname('/posts/123', '/posts/[id]')).toBe(
+      '/posts/123'
+    )
+    expect(normalizeResolvedPathname('/', '/')).toBe('/')
+  })
+})