feat(app-router): filter skipped layouts from RSC responses and cached reads [3/6]

packages/vinext/src/entries/app-rsc-entry.ts

@@ -394,6 +394,8 @@ import {
 import {
   APP_INTERCEPTION_CONTEXT_KEY as __APP_INTERCEPTION_CONTEXT_KEY,
   createAppPayloadRouteId as __createAppPayloadRouteId,
+  parseSkipHeader as __parseSkipHeader,
+  X_VINEXT_ROUTER_SKIP_HEADER as __X_VINEXT_ROUTER_SKIP_HEADER,
 } from ${JSON.stringify(appElementsPath)};
 import {
   buildAppPageElements as __buildAppPageElements,
@@ -1420,6 +1422,10 @@ async function _handleRequest(request, __reqCtx, _mwCtx) {
   }
 
   const isRscRequest = pathname.endsWith(".rsc") || request.headers.get("accept")?.includes("text/x-component");
+  const __EMPTY_SKIP_LAYOUT_IDS = new Set();
+  const __skipLayoutIds = isRscRequest
+    ? __parseSkipHeader(request.headers.get(__X_VINEXT_ROUTER_SKIP_HEADER))
+    : __EMPTY_SKIP_LAYOUT_IDS;
   // Read mounted-slots header once at the handler scope and thread it through
   // every buildPageElements call site. Previously both the handler and
   // buildPageElements read and normalized it independently, which invited
@@ -2206,6 +2212,7 @@ async function _handleRequest(request, __reqCtx, _mwCtx) {
       isrSet: __isrSet,
       mountedSlotsHeader: __mountedSlotsHeader,
       revalidateSeconds,
+      skipIds: __skipLayoutIds,
       renderFreshPageForCache: async function() {
         // Re-render the page to produce fresh HTML + RSC data for the cache
         // Use an empty headers context for background regeneration — not the original
@@ -2420,6 +2427,7 @@ async function _handleRequest(request, __reqCtx, _mwCtx) {
     isForceStatic,
     isProduction: process.env.NODE_ENV === "production",
     isRscRequest,
+    supportsFilteredRscStream: false,
     isrDebug: __isrDebug,
     isrHtmlKey: __isrHtmlKey,
     isrRscKey: __isrRscKey,
@@ -2468,6 +2476,7 @@ async function _handleRequest(request, __reqCtx, _mwCtx) {
     },
     revalidateSeconds,
     mountedSlotsHeader: __mountedSlotsHeader,
+    requestedSkipLayoutIds: __skipLayoutIds,
     renderErrorBoundaryResponse(renderErr) {
       return renderErrorBoundaryPage(route, renderErr, isRscRequest, request, params, _scriptNonce);
     },

packages/vinext/src/server/app-elements.ts

@@ -101,6 +101,52 @@ export function resolveVisitedResponseInterceptionContext(
   return payloadInterceptionContext ?? requestInterceptionContext;
 }
 
+export const X_VINEXT_ROUTER_SKIP_HEADER = "X-Vinext-Router-Skip";
+export const X_VINEXT_MOUNTED_SLOTS_HEADER = "X-Vinext-Mounted-Slots";
+const MAX_SKIP_LAYOUT_IDS = 50;
+const EMPTY_SKIP_HEADER_IDS: ReadonlySet<string> = new Set<string>();
+
+export function parseSkipHeader(header: string | null): ReadonlySet<string> {
+  if (!header) return EMPTY_SKIP_HEADER_IDS;
+  const ids = new Set<string>();
+  for (const part of header.split(",")) {
+    const trimmed = part.trim();
+    if (trimmed.startsWith("layout:")) {
+      ids.add(trimmed);
+      if (ids.size >= MAX_SKIP_LAYOUT_IDS) {
+        break;
+      }
+    }
+  }
+  return ids.size > 0 ? ids : EMPTY_SKIP_HEADER_IDS;
+}
+
+const EMPTY_SKIP_DECISION: ReadonlySet<string> = new Set<string>();
+
+/**
+ * Pure: computes the authoritative set of layout ids that should be omitted
+ * from the outgoing payload. Defense-in-depth — an id is only included if the
+ * server independently classified it as `"s"` (static). Empty or missing
+ * `requested` yields a shared empty set so the hot path does not allocate.
+ *
+ * See `LayoutFlags` type docblock in this file for lifecycle.
+ */
+export function computeSkipDecision(
+  layoutFlags: LayoutFlags,
+  requested: ReadonlySet<string> | undefined,
+): ReadonlySet<string> {
+  if (!requested || requested.size === 0) {
+    return EMPTY_SKIP_DECISION;
+  }
+  const decision = new Set<string>();
+  for (const id of requested) {
+    if (layoutFlags[id] === "s") {
+      decision.add(id);
+    }
+  }
+  return decision;
+}
+
 export function normalizeAppElements(elements: AppWireElements): AppElements {
   let needsNormalization = false;
   for (const [key, value] of Object.entries(elements)) {

packages/vinext/src/server/app-page-cache.ts

@@ -1,5 +1,8 @@
 import type { CachedAppPageValue } from "../shims/cache.js";
 import { buildAppPageCacheValue, type ISRCacheEntry } from "./isr-cache.js";
+import { wrapRscBytesForResponse } from "./app-page-skip-filter.js";
+
+const EMPTY_SKIP_SET: ReadonlySet<string> = new Set<string>();
 
 type AppPageDebugLogger = (event: string, detail: string) => void;
 type AppPageCacheGetter = (key: string) => Promise<ISRCacheEntry | null>;
@@ -22,6 +25,7 @@ export type BuildAppPageCachedResponseOptions = {
   isRscRequest: boolean;
   mountedSlotsHeader?: string | null;
   revalidateSeconds: number;
+  skipIds?: ReadonlySet<string>;
 };
 
 export type ReadAppPageCacheResponseOptions = {
@@ -37,6 +41,7 @@ export type ReadAppPageCacheResponseOptions = {
   revalidateSeconds: number;
   renderFreshPageForCache: () => Promise<AppPageCacheRenderResult>;
   scheduleBackgroundRegeneration: AppPageBackgroundRegenerator;
+  skipIds?: ReadonlySet<string>;
 };
 
 export type FinalizeAppPageHtmlCacheResponseOptions = {
@@ -106,7 +111,9 @@ export function buildAppPageCachedResponse(
       rscHeaders["X-Vinext-Mounted-Slots"] = options.mountedSlotsHeader;
     }
 
-    return new Response(cachedValue.rscData, {
+    const body = wrapRscBytesForResponse(cachedValue.rscData, options.skipIds ?? EMPTY_SKIP_SET);
+
+    return new Response(body, {
       status,
       headers: rscHeaders,
     });
@@ -142,6 +149,7 @@ export async function readAppPageCacheResponse(
         isRscRequest: options.isRscRequest,
         mountedSlotsHeader: options.mountedSlotsHeader,
         revalidateSeconds: options.revalidateSeconds,
+        skipIds: options.skipIds,
       });
 
       if (hitResponse) {
@@ -198,6 +206,7 @@ export async function readAppPageCacheResponse(
         isRscRequest: options.isRscRequest,
         mountedSlotsHeader: options.mountedSlotsHeader,
         revalidateSeconds: options.revalidateSeconds,
+        skipIds: options.skipIds,
       });
 
       if (staleResponse) {

packages/vinext/src/server/app-page-render.ts

@@ -1,10 +1,15 @@
 import type { ReactNode } from "react";
 import type { CachedAppPageValue } from "../shims/cache.js";
-import { buildOutgoingAppPayload, type AppOutgoingElements } from "./app-elements.js";
+import {
+  buildOutgoingAppPayload,
+  computeSkipDecision,
+  type AppOutgoingElements,
+} from "./app-elements.js";
 import {
   finalizeAppPageHtmlCacheResponse,
   scheduleAppPageRscCacheWrite,
 } from "./app-page-cache.js";
+import { createSkipFilterTransform } from "./app-page-skip-filter.js";
 import {
   buildAppPageFontLinkHeader,
   resolveAppPageSpecialError,
@@ -32,6 +37,8 @@ import {
   type AppPageSsrHandler,
 } from "./app-page-stream.js";
 
+const EMPTY_SKIP_SET: ReadonlySet<string> = new Set<string>();
+
 type AppPageBoundaryOnError = (
   error: unknown,
   requestInfo: unknown,
@@ -68,6 +75,7 @@ export type RenderAppPageLifecycleOptions = {
   isForceStatic: boolean;
   isProduction: boolean;
   isRscRequest: boolean;
+  supportsFilteredRscStream?: boolean;
   isrDebug?: AppPageDebugLogger;
   isrHtmlKey: (pathname: string) => string;
   isrRscKey: (pathname: string, mountedSlotsHeader?: string | null) => string;
@@ -97,6 +105,7 @@ export type RenderAppPageLifecycleOptions = {
   waitUntil?: (promise: Promise<void>) => void;
   element: ReactNode | Readonly<Record<string, ReactNode>>;
   classification?: LayoutClassificationOptions | null;
+  requestedSkipLayoutIds?: ReadonlySet<string>;
 };
 
 function buildResponseTiming(
@@ -148,9 +157,9 @@ export async function renderAppPageLifecycle(
 
   const layoutFlags = preRenderResult.layoutFlags;
 
-  // Render the CANONICAL element. The outgoing payload carries per-layout
-  // static/dynamic flags under `__layoutFlags` so the client can later tell
-  // which layouts are safe to skip on subsequent navigations.
+  // Always render the CANONICAL element. Skip semantics are applied on the
+  // egress branch only so the cache branch receives full bytes regardless of
+  // the client's skip header. See `app-page-skip-filter.ts`.
   const outgoingElement = buildOutgoingAppPayload({
     element: options.element,
     layoutFlags,
@@ -172,9 +181,17 @@ export async function renderAppPageLifecycle(
       revalidateSeconds !== Infinity &&
       !options.isForceDynamic,
   );
-  const rscForResponse = rscCapture.responseStream;
   const isrRscDataPromise = rscCapture.capturedRscDataPromise;
 
+  const skipIds =
+    options.isRscRequest && (options.supportsFilteredRscStream ?? true)
+      ? computeSkipDecision(layoutFlags, options.requestedSkipLayoutIds)
+      : EMPTY_SKIP_SET;
+  const rscForResponse =
+    skipIds.size > 0
+      ? rscCapture.responseStream.pipeThrough(createSkipFilterTransform(skipIds))
+      : rscCapture.responseStream;
+
   if (options.isRscRequest) {
     const dynamicUsedDuringBuild = options.consumeDynamicUsage();
     const rscResponsePolicy = resolveAppPageRscResponsePolicy({
@@ -241,6 +258,9 @@ export async function renderAppPageLifecycle(
       return renderAppPageHtmlStream({
         fontData,
         navigationContext: options.getNavigationContext(),
+        // HTML SSR always consumes the canonical stream: skipIds is only
+        // non-empty for RSC requests, so the HTML path never sees filtered
+        // bytes even though it reuses the same `rscForResponse` handle.
         rscStream: rscForResponse,
         scriptNonce: options.scriptNonce,
         ssrHandler,