fix: static not-found missing in prerender manifest

packages/next/src/build/index.ts

@@ -3423,7 +3423,7 @@ export default async function build(
             return staticGenerationSpan
               .traceChild('move-exported-app-not-found-')
               .traceAsyncFn(async () => {
-                const orig = path.join(
+                const underscoreNotFoundHtml = path.join(
                   distDir,
                   'server',
                   'app',
@@ -3433,9 +3433,10 @@ export default async function build(
                   .join('pages', '404.html')
                   .replace(/\\/g, '/')
 
-                if (existsSync(orig)) {
+                // If the _not-found.html exists, use it instead of the pages 404.html
+                if (existsSync(underscoreNotFoundHtml)) {
                   await fs.copyFile(
-                    orig,
+                    underscoreNotFoundHtml,
                     path.join(distDir, 'server', updatedRelativeDest)
                   )
 
@@ -3636,6 +3637,23 @@ export default async function build(
           await writeManifest(pagesManifestPath, pagesManifest)
         })
 
+        const hasStaticAppRouterNotFound = existsSync(
+          path.join(distDir, 'server', 'app', '_not-found.html')
+        )
+        // If the _not-found.html exists, add /_not-found to the prerender manifest
+        if (hasStaticAppRouterNotFound) {
+          prerenderManifest.routes['/_not-found'] = {
+            initialRevalidateSeconds: false,
+            initialExpireSeconds: undefined,
+            experimentalPPR: undefined,
+            renderingMode: undefined,
+            srcRoute: null,
+            dataRoute: null,
+            prefetchDataRoute: null,
+            allowHeader: ALLOWED_HEADERS,
+          }
+        }
+
         // As we may have modified the dynamicRoutes, we need to sort the
         // dynamic routes by page.
         routesManifest.dynamicRoutes = sortSortableRouteObjects(

packages/next/src/export/index.ts

@@ -762,6 +762,10 @@ async function exportAppImpl(
   if (!options.buildExport && prerenderManifest) {
     await Promise.all(
       Object.keys(prerenderManifest.routes).map(async (unnormalizedRoute) => {
+        // Skip handling /_not-found route, it will copy the 404.html file later
+        if (unnormalizedRoute === '/_not-found') {
+          return
+        }
         const { srcRoute } = prerenderManifest!.routes[unnormalizedRoute]
         const appPageName = mapAppRouteToPage.get(srcRoute || '')
         const pageName = appPageName || srcRoute || unnormalizedRoute

test/e2e/app-dir/app-static/app-static.test.ts

@@ -987,6 +987,19 @@ describe('app-dir static/dynamic handling', () => {
            "initialRevalidateSeconds": false,
            "srcRoute": "/",
          },
+         "/_not-found": {
+           "allowHeader": [
+             "host",
+             "x-matched-path",
+             "x-prerender-revalidate",
+             "x-prerender-revalidate-if-generated",
+             "x-next-revalidated-tags",
+             "x-next-revalidate-tag-token",
+           ],
+           "dataRoute": "/_not-found.rsc",
+           "initialRevalidateSeconds": false,
+           "srcRoute": null,
+         },
          "/api/large-data": {
            "allowHeader": [
              "host",

test/e2e/app-dir/cache-components-dynamic-imports/cache-components-dynamic-imports.test.ts

@@ -39,6 +39,7 @@ describe('async imports in cacheComponents', () => {
 
       expect(prerenderedRoutes).toMatchInlineSnapshot(`
        [
+         "/_not-found",
          "/inside-render/client/async-module",
          "/inside-render/client/sync-module",
          "/inside-render/route-handler/async-module",

test/e2e/app-dir/metadata-static-generation/metadata-static-generation.test.ts

@@ -24,6 +24,7 @@ const isPPREnabled = process.env.__NEXT_EXPERIMENTAL_PPR === 'true'
         const staticRoutes = prerenderManifest.routes
         expect(Object.keys(staticRoutes).sort()).toEqual([
           '/',
+          '_not-found',
           '/suspenseful/static',
         ])
       })

test/e2e/app-dir/metadata-streaming-static-generation/metadata-streaming-static-generation.test.ts

@@ -22,6 +22,7 @@ const isPPREnabled = process.env.__NEXT_EXPERIMENTAL_PPR === 'true'
         const staticRoutes = prerenderManifest.routes
         expect(Object.keys(staticRoutes).sort()).toEqual([
           '/',
+          '_not-found',
           '/slow/static',
           '/suspenseful/static',
         ])