Unify appShells flag with Partial Prefetching

packages/next/src/build/define-env.ts

@@ -392,7 +392,6 @@ export function getDefineEnv({
       config.experimental.optimisticRouting ?? false,
     'process.env.__NEXT_INSTRUMENTATION_CLIENT_ROUTER_TRANSITION_EVENTS':
       config.experimental.instrumentationClientRouterTransitionEvents ?? false,
-    'process.env.__NEXT_APP_SHELLS': config.experimental.appShells ?? false,
     'process.env.__NEXT_VARY_PARAMS': config.experimental.varyParams ?? false,
     'process.env.__NEXT_EXPOSE_TESTING_API':
       dev || config.experimental.exposeTestingApiInProductionBuild === true,

packages/next/src/build/templates/app-page.ts

@@ -929,7 +929,6 @@ export async function handler(
             useCacheTimeout: nextConfig.experimental.useCacheTimeout,
             cachedNavigations:
               nextConfig.experimental.cachedNavigations ?? false,
-            appShells: nextConfig.experimental.appShells,
             clientTraceMetadata:
               nextConfig.experimental.clientTraceMetadata || ([] as any),
             clientParamParsingOrigins:

packages/next/src/build/templates/edge-ssr-app.ts

@@ -179,7 +179,6 @@ async function requestHandler(
         serverComponentsHmrCancellation: false,
         useCacheTimeout: nextConfig.experimental.useCacheTimeout,
         cachedNavigations: nextConfig.experimental.cachedNavigations ?? false,
-        appShells: nextConfig.experimental.appShells,
         clientTraceMetadata:
           nextConfig.experimental.clientTraceMetadata || ([] as any),
         clientParamParsingOrigins:

packages/next/src/client/components/segment-cache/cache.ts

@@ -2629,10 +2629,11 @@ export async function fetchSegmentPrefetchesUsingDynamicRequest(
     // staleAt that corresponds to whatever payload the spawned entries get
     // filled with below.
     let staleAtForSpawnedEntries = staleAt
-    if (!process.env.__NEXT_APP_SHELLS || cacheData === null) {
-      // NOTE: cacheData is always set when Cached Navigations is enabled, and
-      // therefore when App Shells is enabled. This null check can be removed
-      // when those flags fully land.
+    if (cacheData === null) {
+      // No shell can be extracted without cache metadata (only present when
+      // Cached Navigations is enabled). For routes without a distinct App Shell
+      // the extraction below is a no-op anyway (`resolveShellStageData` returns
+      // null), so this just short-circuits that case.
       serverDataThatSatisfiesSpawnedEntries = serverData
     } else {
       const shellStageData = await resolveShellStageData(

packages/next/src/client/components/segment-cache/scheduler.ts

@@ -10,6 +10,7 @@ import {
 import { matchSegment } from '../match-segments'
 import {
   readOrCreateRouteCacheEntry,
+  readRouteCacheEntry,
   readOrCreateSegmentCacheEntry,
   fetchRouteOnCacheMiss,
   fetchSegmentsOnCacheMiss,
@@ -575,9 +576,24 @@ function processQueueInMicrotask() {
         continue
       case PrefetchTaskExitStatus.Done:
         if (task.phase === PrefetchPhase.RouteTree) {
-          // Finished prefetching the route tree. If App Shells are enabled,
-          // run the Shell phase next; otherwise go straight to Speculative.
-          task.phase = process.env.__NEXT_APP_SHELLS
+          // Finished prefetching the route tree. The two-phase (Shell then
+          // Speculative) flow only applies to routes that have opted into
+          // Partial Prefetching — either globally via the `partialPrefetching`
+          // config or per segment (`instant`, `prefetch: 'partial'`,
+          // `'unstable_eager'`, or `'allow-runtime'`), all surfaced as the
+          // `SubtreeHasPartialPrefetching` hint on the route tree. Every other
+          // route skips the Shell phase and goes straight to Speculative.
+          //
+          // The route entry is fulfilled at this point (the RouteTree phase
+          // just completed), so its prefetch hints are available.
+          const route = readRouteCacheEntry(now, task.key)
+          const routeHasPartialPrefetching =
+            route !== null &&
+            route.status === EntryStatus.Fulfilled &&
+            (route.tree.prefetchHints &
+              PrefetchHint.SubtreeHasPartialPrefetching) !==
+              0
+          task.phase = routeHasPartialPrefetching
             ? PrefetchPhase.Shell
             : PrefetchPhase.Speculative
           heapResift(taskHeap, task)
@@ -2051,17 +2067,13 @@ export function subtreeHasSpeculativePrefetch(
   fetchStrategy: FetchStrategy,
   prefetchHints: number
 ): boolean {
-  if (!process.env.__NEXT_APP_SHELLS) {
-    // When App Shells is disabled, all prefetches implicitly include the
-    // speculative (non-shell) part of the target.
-    return true
-  }
-
   return (
     // Check if this is a "full" prefetch (<Link prefetch={true}>).
     fetchStrategy === FetchStrategy.Full ||
     // Check if something in this subtree is configured to be eagerly
-    // prefetched at the route level.
+    // prefetched at the route level. Segments that don't opt into Partial
+    // Prefetching are marked eager, so a route without any Partial Prefetching
+    // still speculatively prefetches everything.
     (prefetchHints & PrefetchHint.SubtreeHasEagerPrefetch) !== 0
   )
 }

packages/next/src/export/index.ts

@@ -519,7 +519,6 @@ async function exportAppImpl(
       authInterrupts: !!nextConfig.experimental.authInterrupts,
       useCacheTimeout: nextConfig.experimental.useCacheTimeout,
       cachedNavigations: nextConfig.experimental.cachedNavigations ?? false,
-      appShells: nextConfig.experimental.appShells,
       maxPostponedStateSizeBytes: parseMaxPostponedStateSize(
         nextConfig.experimental.maxPostponedStateSize
       ),