From f2732e85d65b6bcd37e83ad01085a071531bc499 Mon Sep 17 00:00:00 2001
From: devjiwonchoi <devjiwonchoi@gmail.com>
Date: Fri, 19 Sep 2025 10:07:17 +0200
Subject: [PATCH 01/28] Use serialized next config for prod server

---
 packages/next/src/build/index.ts          | 14 ++++++
 packages/next/src/server/config.ts        | 61 ++++++++++++++++++++++-
 packages/next/src/shared/lib/constants.ts |  1 +
 3 files changed, 74 insertions(+), 2 deletions(-)

diff --git a/packages/next/src/build/index.ts b/packages/next/src/build/index.ts
index c72704951a51..6cce8b951d00 100644
--- a/packages/next/src/build/index.ts
+++ b/packages/next/src/build/index.ts
@@ -61,6 +61,7 @@ import {
   PRERENDER_MANIFEST,
   REACT_LOADABLE_MANIFEST,
   ROUTES_MANIFEST,
+  SERIALIZED_CONFIG_FILE,
   SERVER_DIRECTORY,
   SERVER_FILES_MANIFEST,
   STATIC_STATUS_PAGES,
@@ -2723,6 +2724,19 @@ export default async function build(
         requiredServerFilesManifest
       )
 
+      // Write serialized config to project root for normal production mode with custom distDir
+      if (config.output !== 'standalone' && config.distDir !== '.next') {
+        const serializedConfigPath = path.join(
+          // Write to the same directory as the config file
+          config.configFile ? path.dirname(config.configFile) : dir,
+          SERIALIZED_CONFIG_FILE
+        )
+        await fs.writeFile(
+          serializedConfigPath,
+          JSON.stringify(requiredServerFilesManifest.config)
+        )
+      }
+
       // we don't need to inline for turbopack build as
       // it will handle it's own caching separate of compile
       if (isGenerateMode && !isTurbopack) {
diff --git a/packages/next/src/server/config.ts b/packages/next/src/server/config.ts
index 8a49cd739282..201cdfc5f77b 100644
--- a/packages/next/src/server/config.ts
+++ b/packages/next/src/server/config.ts
@@ -1,5 +1,13 @@
-import { existsSync } from 'fs'
-import { basename, extname, join, relative, isAbsolute, resolve } from 'path'
+import { existsSync, readFileSync } from 'fs'
+import {
+  basename,
+  extname,
+  join,
+  relative,
+  isAbsolute,
+  resolve,
+  dirname,
+} from 'path'
 import { pathToFileURL } from 'url'
 import findUp from 'next/dist/compiled/find-up'
 import * as Log from '../build/output/log'
@@ -9,6 +17,9 @@ import {
   PHASE_DEVELOPMENT_SERVER,
   PHASE_EXPORT,
   PHASE_PRODUCTION_BUILD,
+  PHASE_PRODUCTION_SERVER,
+  SERVER_FILES_MANIFEST,
+  SERIALIZED_CONFIG_FILE,
   type PHASE_TYPE,
 } from '../shared/lib/constants'
 import { defaultConfig, normalizeConfig } from './config-shared'
@@ -1344,6 +1355,52 @@ export default async function loadConfig(
     return standaloneConfig
   }
 
+  // Try to load from serialized config files in production
+  if (phase === PHASE_PRODUCTION_SERVER) {
+    // Helper to try loading serialized config
+    const tryLoadSerializedConfig = (configPath: string) => {
+      try {
+        if (!existsSync(configPath)) {
+          return null
+        }
+
+        const parsed = JSON.parse(readFileSync(configPath, 'utf8'))
+        const config = parsed.config
+
+        // Cache the config
+        configCache.set(cacheKey, {
+          config,
+          rawConfig: config,
+          configuredExperimentalFeatures: [],
+        })
+
+        return config
+      } catch {
+        // Continue to next option
+      }
+      return null
+    }
+
+    // Try .next/required-server-files.json first (default distDir)
+    const fromManifest = tryLoadSerializedConfig(
+      join(dir, '.next', SERVER_FILES_MANIFEST)
+    )
+    if (fromManifest) {
+      return fromManifest
+    }
+
+    const configPath = await findUp(CONFIG_FILES, { cwd: dir })
+    const targetDir = configPath ? dirname(configPath) : dir
+
+    // Try next-config-serialized.json (custom distDir)
+    const fromSerialized = tryLoadSerializedConfig(
+      join(targetDir, SERIALIZED_CONFIG_FILE)
+    )
+    if (fromSerialized) {
+      return fromSerialized
+    }
+  }
+
   const curLog = silent
     ? {
         warn: () => {},
diff --git a/packages/next/src/shared/lib/constants.ts b/packages/next/src/shared/lib/constants.ts
index f1ccf3e13476..c2f6314b7ba3 100644
--- a/packages/next/src/shared/lib/constants.ts
+++ b/packages/next/src/shared/lib/constants.ts
@@ -96,6 +96,7 @@ export const PRERENDER_MANIFEST = 'prerender-manifest.json'
 export const ROUTES_MANIFEST = 'routes-manifest.json'
 export const IMAGES_MANIFEST = 'images-manifest.json'
 export const SERVER_FILES_MANIFEST = 'required-server-files.json'
+export const SERIALIZED_CONFIG_FILE = 'next-config-serialized.json'
 export const DEV_CLIENT_PAGES_MANIFEST = '_devPagesManifest.json'
 export const MIDDLEWARE_MANIFEST = 'middleware-manifest.json'
 export const TURBOPACK_CLIENT_MIDDLEWARE_MANIFEST =

From a3ae0bc422487c5880174798a02a4077625a9a39 Mon Sep 17 00:00:00 2001
From: devjiwonchoi <devjiwonchoi@gmail.com>
Date: Fri, 19 Sep 2025 10:08:40 +0200
Subject: [PATCH 02/28] test: add test

---
 .../next-config-serialized/app/layout.tsx     |  8 +++
 .../next-config-serialized/app/page.tsx       |  3 +
 .../next-config-serialized.test.ts            | 59 +++++++++++++++++++
 .../next-config-serialized/next.config.js     |  8 +++
 4 files changed, 78 insertions(+)
 create mode 100644 test/production/app-dir/next-config-serialized/app/layout.tsx
 create mode 100644 test/production/app-dir/next-config-serialized/app/page.tsx
 create mode 100644 test/production/app-dir/next-config-serialized/next-config-serialized.test.ts
 create mode 100644 test/production/app-dir/next-config-serialized/next.config.js

diff --git a/test/production/app-dir/next-config-serialized/app/layout.tsx b/test/production/app-dir/next-config-serialized/app/layout.tsx
new file mode 100644
index 000000000000..888614deda3b
--- /dev/null
+++ b/test/production/app-dir/next-config-serialized/app/layout.tsx
@@ -0,0 +1,8 @@
+import { ReactNode } from 'react'
+export default function Root({ children }: { children: ReactNode }) {
+  return (
+    <html>
+      <body>{children}</body>
+    </html>
+  )
+}
diff --git a/test/production/app-dir/next-config-serialized/app/page.tsx b/test/production/app-dir/next-config-serialized/app/page.tsx
new file mode 100644
index 000000000000..ff7159d9149f
--- /dev/null
+++ b/test/production/app-dir/next-config-serialized/app/page.tsx
@@ -0,0 +1,3 @@
+export default function Page() {
+  return <p>hello world</p>
+}
diff --git a/test/production/app-dir/next-config-serialized/next-config-serialized.test.ts b/test/production/app-dir/next-config-serialized/next-config-serialized.test.ts
new file mode 100644
index 000000000000..db9d58b5e9ec
--- /dev/null
+++ b/test/production/app-dir/next-config-serialized/next-config-serialized.test.ts
@@ -0,0 +1,59 @@
+import { nextTestSetup } from 'e2e-utils'
+import { retry } from 'next-test-utils'
+
+describe('next-config-serialized', () => {
+  const { next, skipped } = nextTestSetup({
+    files: __dirname,
+    skipStart: true,
+    skipDeployment: true,
+  })
+
+  if (skipped) {
+    return
+  }
+
+  it('should use .next/required-server-files.json when distDir is .next', async () => {
+    await next.build()
+    expect(await next.hasFile('.next/required-server-files.json')).toBe(true)
+
+    // Rename the file so it can't be loaded.
+    await next.renameFile('next.config.js', 'next.config.noop.js')
+    await retry(async () => {
+      expect(await next.hasFile('next.config.noop.js')).toBe(true)
+    })
+
+    await next.start()
+
+    const browser = await next.browser('/')
+    expect(await browser.elementByCss('p').text()).toBe('hello world')
+
+    // Restore the file for the next case.
+    await next.renameFile('next.config.noop.js', 'next.config.js')
+    await next.stop()
+  })
+
+  it('should use next-config-serialized.json when distDir is not .next', async () => {
+    await next.patchFile('next.config.js', (content) => {
+      return content.replace(`// distDir: 'out',`, `distDir: 'out',`)
+    })
+
+    await next.build()
+    expect(await next.hasFile('out/required-server-files.json')).toBe(true)
+    expect(await next.hasFile('next-config-serialized.json')).toBe(true)
+
+    // Rename the file so it can't be loaded.
+    await next.renameFile('next.config.js', 'next.config.noop.js')
+    await retry(async () => {
+      expect(await next.hasFile('next.config.noop.js')).toBe(true)
+    })
+
+    await next.start()
+
+    const browser = await next.browser('/')
+    expect(await browser.elementByCss('p').text()).toBe('hello world')
+
+    // Restore the file for the next case.
+    await next.renameFile('next.config.noop.js', 'next.config.js')
+    await next.stop()
+  })
+})
diff --git a/test/production/app-dir/next-config-serialized/next.config.js b/test/production/app-dir/next-config-serialized/next.config.js
new file mode 100644
index 000000000000..f5c3fc144eba
--- /dev/null
+++ b/test/production/app-dir/next-config-serialized/next.config.js
@@ -0,0 +1,8 @@
+/**
+ * @type {import('next').NextConfig}
+ */
+const nextConfig = {
+  // distDir: 'out',
+}
+
+module.exports = nextConfig

From 14d14f181d4711d471e73186145bffabc617b074 Mon Sep 17 00:00:00 2001
From: devjiwonchoi <devjiwonchoi@gmail.com>
Date: Fri, 19 Sep 2025 10:54:36 +0200
Subject: [PATCH 03/28] match the manifest format

---
 packages/next/src/build/index.ts   | 14 +++++++++++---
 packages/next/src/server/config.ts | 14 +++++++++++---
 2 files changed, 22 insertions(+), 6 deletions(-)

diff --git a/packages/next/src/build/index.ts b/packages/next/src/build/index.ts
index 6cce8b951d00..973fa8c8b665 100644
--- a/packages/next/src/build/index.ts
+++ b/packages/next/src/build/index.ts
@@ -2724,16 +2724,24 @@ export default async function build(
         requiredServerFilesManifest
       )
 
-      // Write serialized config to project root for normal production mode with custom distDir
+      // The required-server-files manifest contains serialized config, which can
+      // be loaded by the prod server. However, when a custom distDir is set,
+      // the prod server will not know the distDir until loading the config.
+      // Therefore we write the serialized config to the same directory as the
+      // original config file.
       if (config.output !== 'standalone' && config.distDir !== '.next') {
         const serializedConfigPath = path.join(
-          // Write to the same directory as the config file
+          // Write to the same directory as the original config file.
           config.configFile ? path.dirname(config.configFile) : dir,
           SERIALIZED_CONFIG_FILE
         )
         await fs.writeFile(
           serializedConfigPath,
-          JSON.stringify(requiredServerFilesManifest.config)
+          JSON.stringify({
+            // To match the format of required server files manifest.
+            version: 1,
+            config: requiredServerFilesManifest.config,
+          })
         )
       }
 
diff --git a/packages/next/src/server/config.ts b/packages/next/src/server/config.ts
index 201cdfc5f77b..ba7a28392ff2 100644
--- a/packages/next/src/server/config.ts
+++ b/packages/next/src/server/config.ts
@@ -1358,7 +1358,9 @@ export default async function loadConfig(
   // Try to load from serialized config files in production
   if (phase === PHASE_PRODUCTION_SERVER) {
     // Helper to try loading serialized config
-    const tryLoadSerializedConfig = (configPath: string) => {
+    const tryLoadSerializedConfig = (
+      configPath: string
+    ): NextConfigComplete | null => {
       try {
         if (!existsSync(configPath)) {
           return null
@@ -1381,7 +1383,8 @@ export default async function loadConfig(
       return null
     }
 
-    // Try .next/required-server-files.json first (default distDir)
+    // Try load from ".next" dir since we don't know the
+    // distDir until loading the config.
     const fromManifest = tryLoadSerializedConfig(
       join(dir, '.next', SERVER_FILES_MANIFEST)
     )
@@ -1389,16 +1392,21 @@ export default async function loadConfig(
       return fromManifest
     }
 
+    // If the custom distDir is set, we write the serialized config
+    // to the same directory as the original config file.
     const configPath = await findUp(CONFIG_FILES, { cwd: dir })
     const targetDir = configPath ? dirname(configPath) : dir
 
-    // Try next-config-serialized.json (custom distDir)
     const fromSerialized = tryLoadSerializedConfig(
       join(targetDir, SERIALIZED_CONFIG_FILE)
     )
     if (fromSerialized) {
       return fromSerialized
     }
+
+    throw new Error(
+      `Failed to load the serialized config file "${SERIALIZED_CONFIG_FILE}" in "${targetDir}".`
+    )
   }
 
   const curLog = silent

From ee41b8cd3260c987c60daea1672fd6db1ed8e530 Mon Sep 17 00:00:00 2001
From: devjiwonchoi <devjiwonchoi@gmail.com>
Date: Fri, 19 Sep 2025 10:56:49 +0200
Subject: [PATCH 04/28] update comment

---
 packages/next/src/server/config.ts | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/packages/next/src/server/config.ts b/packages/next/src/server/config.ts
index ba7a28392ff2..516f5f5239ed 100644
--- a/packages/next/src/server/config.ts
+++ b/packages/next/src/server/config.ts
@@ -1355,9 +1355,8 @@ export default async function loadConfig(
     return standaloneConfig
   }
 
-  // Try to load from serialized config files in production
+  // During prod server, we can load from the serialized config.
   if (phase === PHASE_PRODUCTION_SERVER) {
-    // Helper to try loading serialized config
     const tryLoadSerializedConfig = (
       configPath: string
     ): NextConfigComplete | null => {

From 5c3b362cb2f295b867dfde2e1bebb922737a180b Mon Sep 17 00:00:00 2001
From: devjiwonchoi <devjiwonchoi@gmail.com>
Date: Fri, 19 Sep 2025 11:00:34 +0200
Subject: [PATCH 05/28] errors.json

---
 packages/next/errors.json | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/packages/next/errors.json b/packages/next/errors.json
index 542ec27051b0..e93fb5776ae3 100644
--- a/packages/next/errors.json
+++ b/packages/next/errors.json
@@ -808,5 +808,6 @@
   "807": "Expected a %s response header.",
   "808": "Invalid binary HMR message: insufficient data (expected %s bytes, got %s)",
   "809": "Invalid binary HMR message of type %s",
-  "810": "React debug channel stream error"
+  "810": "React debug channel stream error",
+  "811": "Failed to load the serialized config file \"%s\" in \"%s\"."
 }

From 330b4f6a1d122973f261d75d4533d4e555af9d97 Mon Sep 17 00:00:00 2001
From: devjiwonchoi <devjiwonchoi@gmail.com>
Date: Fri, 19 Sep 2025 16:17:15 +0200
Subject: [PATCH 06/28] test: update to ensure it loads from serialized

---
 .../app-dir/next-config-serialized/app/page.tsx     |  2 +-
 .../next-config-serialized.test.ts                  | 13 +++++++------
 .../app-dir/next-config-serialized/next.config.js   |  3 +++
 3 files changed, 11 insertions(+), 7 deletions(-)

diff --git a/test/production/app-dir/next-config-serialized/app/page.tsx b/test/production/app-dir/next-config-serialized/app/page.tsx
index ff7159d9149f..71e8787438c0 100644
--- a/test/production/app-dir/next-config-serialized/app/page.tsx
+++ b/test/production/app-dir/next-config-serialized/app/page.tsx
@@ -1,3 +1,3 @@
 export default function Page() {
-  return <p>hello world</p>
+  return <p>hello world {process.env.foo}</p>
 }
diff --git a/test/production/app-dir/next-config-serialized/next-config-serialized.test.ts b/test/production/app-dir/next-config-serialized/next-config-serialized.test.ts
index db9d58b5e9ec..cecaada1dbda 100644
--- a/test/production/app-dir/next-config-serialized/next-config-serialized.test.ts
+++ b/test/production/app-dir/next-config-serialized/next-config-serialized.test.ts
@@ -16,16 +16,16 @@ describe('next-config-serialized', () => {
     await next.build()
     expect(await next.hasFile('.next/required-server-files.json')).toBe(true)
 
-    // Rename the file so it can't be loaded.
+    // Rename the config file so it can't be loaded.
     await next.renameFile('next.config.js', 'next.config.noop.js')
     await retry(async () => {
       expect(await next.hasFile('next.config.noop.js')).toBe(true)
     })
 
-    await next.start()
+    await next.start({ skipBuild: true })
 
     const browser = await next.browser('/')
-    expect(await browser.elementByCss('p').text()).toBe('hello world')
+    expect(await browser.elementByCss('p').text()).toBe('hello world foo')
 
     // Restore the file for the next case.
     await next.renameFile('next.config.noop.js', 'next.config.js')
@@ -39,18 +39,19 @@ describe('next-config-serialized', () => {
 
     await next.build()
     expect(await next.hasFile('out/required-server-files.json')).toBe(true)
+    // next-config-serialized.json created next to next.config.js
     expect(await next.hasFile('next-config-serialized.json')).toBe(true)
 
-    // Rename the file so it can't be loaded.
+    // Rename the config file so it can't be loaded.
     await next.renameFile('next.config.js', 'next.config.noop.js')
     await retry(async () => {
       expect(await next.hasFile('next.config.noop.js')).toBe(true)
     })
 
-    await next.start()
+    await next.start({ skipBuild: true })
 
     const browser = await next.browser('/')
-    expect(await browser.elementByCss('p').text()).toBe('hello world')
+    expect(await browser.elementByCss('p').text()).toBe('hello world foo')
 
     // Restore the file for the next case.
     await next.renameFile('next.config.noop.js', 'next.config.js')
diff --git a/test/production/app-dir/next-config-serialized/next.config.js b/test/production/app-dir/next-config-serialized/next.config.js
index f5c3fc144eba..f43f757aadae 100644
--- a/test/production/app-dir/next-config-serialized/next.config.js
+++ b/test/production/app-dir/next-config-serialized/next.config.js
@@ -3,6 +3,9 @@
  */
 const nextConfig = {
   // distDir: 'out',
+  env: {
+    foo: 'foo',
+  },
 }
 
 module.exports = nextConfig

From 1899b7275610088d18126f507c45926a72b1daec Mon Sep 17 00:00:00 2001
From: devjiwonchoi <devjiwonchoi@gmail.com>
Date: Fri, 19 Sep 2025 16:19:01 +0200
Subject: [PATCH 07/28] fall back to loading the config

---
 packages/next/src/server/config.ts | 8 +++-----
 1 file changed, 3 insertions(+), 5 deletions(-)

diff --git a/packages/next/src/server/config.ts b/packages/next/src/server/config.ts
index 516f5f5239ed..be237849f1ac 100644
--- a/packages/next/src/server/config.ts
+++ b/packages/next/src/server/config.ts
@@ -1357,9 +1357,9 @@ export default async function loadConfig(
 
   // During prod server, we can load from the serialized config.
   if (phase === PHASE_PRODUCTION_SERVER) {
-    const tryLoadSerializedConfig = (
+    function tryLoadSerializedConfig(
       configPath: string
-    ): NextConfigComplete | null => {
+    ): NextConfigComplete | null {
       try {
         if (!existsSync(configPath)) {
           return null
@@ -1403,9 +1403,7 @@ export default async function loadConfig(
       return fromSerialized
     }
 
-    throw new Error(
-      `Failed to load the serialized config file "${SERIALIZED_CONFIG_FILE}" in "${targetDir}".`
-    )
+    // Fall back to loading the config.
   }
 
   const curLog = silent

From 3a40f5678f064ce41384b83f32d5107fb8a39a19 Mon Sep 17 00:00:00 2001
From: devjiwonchoi <devjiwonchoi@gmail.com>
Date: Fri, 19 Sep 2025 18:46:53 +0200
Subject: [PATCH 08/28] test: update tests

---
 test/e2e/config-schema-check/index.test.ts    |  4 +-
 test/e2e/next-phase/index.test.ts             |  5 +-
 .../test/index.test.js                        | 47 ++++++++++---------
 .../custom-server/test/index.test.js          |  5 +-
 4 files changed, 32 insertions(+), 29 deletions(-)

diff --git a/test/e2e/config-schema-check/index.test.ts b/test/e2e/config-schema-check/index.test.ts
index 9e483ae0dcb3..72c1f2c60de6 100644
--- a/test/e2e/config-schema-check/index.test.ts
+++ b/test/e2e/config-schema-check/index.test.ts
@@ -58,8 +58,8 @@ describe('next.config.js schema validating - invalid config', () => {
 
       expect(output).toContain('Invalid next.config.js options detected')
       expect(output).toContain('badKey')
-      // for next start and next build we both display the warnings
-      expect(warningTimes).toBe(isNextStart ? 2 : 1)
+      // With serialized config, warnings only appear during build, not during start
+      expect(warningTimes).toBe(1)
 
       return 'success'
     }, 'success')
diff --git a/test/e2e/next-phase/index.test.ts b/test/e2e/next-phase/index.test.ts
index 3ff81f18156f..d02d3bf7a9ae 100644
--- a/test/e2e/next-phase/index.test.ts
+++ b/test/e2e/next-phase/index.test.ts
@@ -26,7 +26,8 @@ describe('next-phase', () => {
     const phases = {
       dev: 'phase-development-server',
       build: 'phase-production-build',
-      start: 'phase-production-server',
+      // Serialized next config will use the config from build.
+      start: 'phase-production-build',
     }
     const currentPhase = isNextDev ? phases.dev : phases.build
     const nonExistedPhase = isNextDev ? phases.build : phases.dev
@@ -39,8 +40,6 @@ describe('next-phase', () => {
 
     if (isNextDev) {
       expect(next.cliOutput).not.toContain(phases.start)
-    } else {
-      expect(next.cliOutput).toContain(phases.start)
     }
   })
 })
diff --git a/test/integration/config-experimental-warning/test/index.test.js b/test/integration/config-experimental-warning/test/index.test.js
index fb7e3f2ee7e6..1f16961067a3 100644
--- a/test/integration/config-experimental-warning/test/index.test.js
+++ b/test/integration/config-experimental-warning/test/index.test.js
@@ -219,8 +219,10 @@ describe('Config Experimental Warning', () => {
         expect(stdout).toMatch(' ✓ parallelServerCompiles')
       })
 
-      it('should show unrecognized experimental features in warning but not in start log experiments section', async () => {
-        configFile.write(`
+      // In prod, it will load a serialized config, so the warning will not appear during start.
+      if (global.isNextDev) {
+        it('should show unrecognized experimental features in warning but not in start log experiments section', async () => {
+          configFile.write(`
         module.exports = {
           experimental: {
             appDir: true
@@ -228,28 +230,29 @@ describe('Config Experimental Warning', () => {
         }
       `)
 
-        await collectStdoutFromBuild(appDir)
-        const port = await findPort()
-        let stdout = ''
-        let stderr = ''
-        app = await nextStart(appDir, port, {
-          onStdout(msg) {
-            stdout += msg
-          },
-          onStderr(msg) {
-            stderr += msg
-          },
-        })
+          await collectStdoutFromBuild(appDir)
+          const port = await findPort()
+          let stdout = ''
+          let stderr = ''
+          app = await nextStart(appDir, port, {
+            onStdout(msg) {
+              stdout += msg
+            },
+            onStderr(msg) {
+              stderr += msg
+            },
+          })
 
-        await check(() => {
-          const cliOutput = stripAnsi(stdout)
-          const cliOutputErr = stripAnsi(stderr)
-          expect(cliOutput).not.toContain(experimentalHeader)
-          expect(cliOutputErr).toContain(
-            `Unrecognized key(s) in object: 'appDir' at "experimental"`
-          )
+          await check(() => {
+            const cliOutput = stripAnsi(stdout)
+            const cliOutputErr = stripAnsi(stderr)
+            expect(cliOutput).not.toContain(experimentalHeader)
+            expect(cliOutputErr).toContain(
+              `Unrecognized key(s) in object: 'appDir' at "experimental"`
+            )
+          })
         })
-      })
+      }
     }
   )
 })
diff --git a/test/integration/custom-server/test/index.test.js b/test/integration/custom-server/test/index.test.js
index 3e67277192a8..42684dc7c55f 100644
--- a/test/integration/custom-server/test/index.test.js
+++ b/test/integration/custom-server/test/index.test.js
@@ -145,8 +145,9 @@ describe.each([
       'production mode',
       () => {
         beforeAll(async () => {
-          await nextBuild(appDir)
-          await startServer({ GENERATE_ETAGS: 'true', NODE_ENV: 'production' })
+          // Set env during build for serialized next config.
+          await nextBuild(appDir, [], { env: { GENERATE_ETAGS: 'true' } })
+          await startServer({ NODE_ENV: 'production' })
         })
         afterAll(() => killApp(server))
 

From 5431ec69ee137286f7884d3d2624b532ad0023b2 Mon Sep 17 00:00:00 2001
From: devjiwonchoi <devjiwonchoi@gmail.com>
Date: Fri, 19 Sep 2025 19:12:54 +0200
Subject: [PATCH 09/28] test: update
 test/integration/config-experimental-warning/test/index.test.js

---
 .../test/index.test.js                        | 83 ++++++++++---------
 1 file changed, 45 insertions(+), 38 deletions(-)

diff --git a/test/integration/config-experimental-warning/test/index.test.js b/test/integration/config-experimental-warning/test/index.test.js
index 1f16961067a3..72e0619829c2 100644
--- a/test/integration/config-experimental-warning/test/index.test.js
+++ b/test/integration/config-experimental-warning/test/index.test.js
@@ -21,20 +21,25 @@ const experimentalHeader = ' - Experiments (use with caution):'
 let app
 async function collectStdoutFromDev(appDir) {
   let stdout = ''
+  let stderr = ''
   const port = await findPort()
   app = await launchApp(appDir, port, {
     onStdout(msg) {
       stdout += msg
     },
+    onStderr(msg) {
+      stderr += msg
+    },
   })
-  return stdout
+  return { stdout, stderr }
 }
 
 async function collectStdoutFromBuild(appDir) {
-  const { stdout } = await nextBuild(appDir, [], {
+  const { stdout, stderr } = await nextBuild(appDir, [], {
     stdout: true,
+    stderr: true,
   })
-  return stdout
+  return { stdout, stderr }
 }
 
 describe('Config Experimental Warning', () => {
@@ -58,7 +63,7 @@ describe('Config Experimental Warning', () => {
       }
     `)
 
-    const stdout = await collectStdoutFromDev(appDir)
+    const { stdout } = await collectStdoutFromDev(appDir)
     expect(stdout).not.toMatch(experimentalHeader)
   })
 
@@ -69,7 +74,7 @@ describe('Config Experimental Warning', () => {
       }
     `)
 
-    const stdout = await collectStdoutFromDev(appDir)
+    const { stdout } = await collectStdoutFromDev(appDir)
     expect(stdout).not.toMatch(experimentalHeader)
   })
 
@@ -82,7 +87,7 @@ describe('Config Experimental Warning', () => {
       }
     `)
 
-    const stdout = await collectStdoutFromDev(appDir)
+    const { stdout } = await collectStdoutFromDev(appDir)
     expect(stdout).toMatch(experimentalHeader)
     expect(stdout).toMatch(' ✓ workerThreads')
   })
@@ -96,7 +101,7 @@ describe('Config Experimental Warning', () => {
       })
     `)
 
-    const stdout = await collectStdoutFromDev(appDir)
+    const { stdout } = await collectStdoutFromDev(appDir)
     expect(stdout).toMatch(experimentalHeader)
     expect(stdout).toMatch(' ✓ workerThreads')
   })
@@ -110,7 +115,7 @@ describe('Config Experimental Warning', () => {
       })
     `)
 
-    const stdout = await collectStdoutFromDev(appDir)
+    const { stdout } = await collectStdoutFromDev(appDir)
     expect(stdout).not.toContain(experimentalHeader)
     expect(stdout).not.toContain('workerThreads')
   })
@@ -124,7 +129,7 @@ describe('Config Experimental Warning', () => {
       }
     `)
 
-    const stdout = await collectStdoutFromDev(appDir)
+    const { stdout } = await collectStdoutFromDev(appDir)
     expect(stdout).toMatch(experimentalHeader)
     expect(stdout).toMatch(' ⨯ prerenderEarlyExit')
   })
@@ -138,7 +143,7 @@ describe('Config Experimental Warning', () => {
       }
     `)
 
-    const stdout = await collectStdoutFromDev(appDir)
+    const { stdout } = await collectStdoutFromDev(appDir)
     expect(stdout).toMatch(experimentalHeader)
     expect(stdout).toMatch(' · cpus: 2')
   })
@@ -152,7 +157,7 @@ describe('Config Experimental Warning', () => {
       }
     `)
 
-    const stdout = await collectStdoutFromDev(appDir)
+    const { stdout } = await collectStdoutFromDev(appDir)
     expect(stdout).toMatch(experimentalHeader)
     expect(stdout).toMatch(' · ppr: "incremental"')
   })
@@ -167,11 +172,29 @@ describe('Config Experimental Warning', () => {
       }
     `)
 
-    const stdout = await collectStdoutFromDev(appDir)
+    const { stdout } = await collectStdoutFromDev(appDir)
     expect(stdout).toContain(experimentalHeader)
     expect(stdout).toContain(' ✓ workerThreads')
     expect(stdout).toContain(' ✓ scrollRestoration')
   })
+
+  it('should show unrecognized experimental features in warning but not in start log experiments section', async () => {
+    configFile.write(`
+        module.exports = {
+          experimental: {
+            appDir: true
+          }
+        }
+      `)
+
+    const { stderr } = await collectStdoutFromDev(appDir)
+    await check(() => {
+      const cliOutput = stripAnsi(stderr)
+      expect(stderr).toContain(
+        `Unrecognized key(s) in object: 'appDir' at "experimental"`
+      )
+    })
+  })
   ;(process.env.TURBOPACK_DEV ? describe.skip : describe)(
     'production mode',
     () => {
@@ -210,7 +233,7 @@ describe('Config Experimental Warning', () => {
           }
         }
       `)
-        const stdout = await collectStdoutFromBuild(appDir)
+        const { stdout } = await collectStdoutFromBuild(appDir)
         expect(stdout).toMatch(experimentalHeader)
         expect(stdout).toMatch(' · cpus: 2')
         expect(stdout).toMatch(' ✓ workerThreads')
@@ -220,9 +243,8 @@ describe('Config Experimental Warning', () => {
       })
 
       // In prod, it will load a serialized config, so the warning will not appear during start.
-      if (global.isNextDev) {
-        it('should show unrecognized experimental features in warning but not in start log experiments section', async () => {
-          configFile.write(`
+      it('should show unrecognized experimental features in warning but not in start log experiments section', async () => {
+        configFile.write(`
         module.exports = {
           experimental: {
             appDir: true
@@ -230,29 +252,14 @@ describe('Config Experimental Warning', () => {
         }
       `)
 
-          await collectStdoutFromBuild(appDir)
-          const port = await findPort()
-          let stdout = ''
-          let stderr = ''
-          app = await nextStart(appDir, port, {
-            onStdout(msg) {
-              stdout += msg
-            },
-            onStderr(msg) {
-              stderr += msg
-            },
-          })
-
-          await check(() => {
-            const cliOutput = stripAnsi(stdout)
-            const cliOutputErr = stripAnsi(stderr)
-            expect(cliOutput).not.toContain(experimentalHeader)
-            expect(cliOutputErr).toContain(
-              `Unrecognized key(s) in object: 'appDir' at "experimental"`
-            )
-          })
+        const { stderr } = await collectStdoutFromBuild(appDir)
+        await check(() => {
+          const cliOutput = stripAnsi(stderr)
+          expect(stderr).toContain(
+            `Unrecognized key(s) in object: 'appDir' at "experimental"`
+          )
         })
-      }
+      })
     }
   )
 })

From c6e5c0309b095317f4536eb81ae92a8817c773dd Mon Sep 17 00:00:00 2001
From: devjiwonchoi <devjiwonchoi@gmail.com>
Date: Fri, 19 Sep 2025 19:19:31 +0200
Subject: [PATCH 10/28] test: fix lint

---
 .../config-experimental-warning/test/index.test.js    | 11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)

diff --git a/test/integration/config-experimental-warning/test/index.test.js b/test/integration/config-experimental-warning/test/index.test.js
index 72e0619829c2..da26e1f0f740 100644
--- a/test/integration/config-experimental-warning/test/index.test.js
+++ b/test/integration/config-experimental-warning/test/index.test.js
@@ -178,7 +178,7 @@ describe('Config Experimental Warning', () => {
     expect(stdout).toContain(' ✓ scrollRestoration')
   })
 
-  it('should show unrecognized experimental features in warning but not in start log experiments section', async () => {
+  it('should show unrecognized experimental features in warning but not in start log experiments section during dev', async () => {
     configFile.write(`
         module.exports = {
           experimental: {
@@ -190,12 +190,11 @@ describe('Config Experimental Warning', () => {
     const { stderr } = await collectStdoutFromDev(appDir)
     await check(() => {
       const cliOutput = stripAnsi(stderr)
-      expect(stderr).toContain(
+      expect(cliOutput).toContain(
         `Unrecognized key(s) in object: 'appDir' at "experimental"`
       )
     })
-  })
-  ;(process.env.TURBOPACK_DEV ? describe.skip : describe)(
+  })(process.env.TURBOPACK_DEV ? describe.skip : describe)(
     'production mode',
     () => {
       it('should not show next app info in next start', async () => {
@@ -243,7 +242,7 @@ describe('Config Experimental Warning', () => {
       })
 
       // In prod, it will load a serialized config, so the warning will not appear during start.
-      it('should show unrecognized experimental features in warning but not in start log experiments section', async () => {
+      it('should show unrecognized experimental features in warning but not in start log experiments section during build', async () => {
         configFile.write(`
         module.exports = {
           experimental: {
@@ -255,7 +254,7 @@ describe('Config Experimental Warning', () => {
         const { stderr } = await collectStdoutFromBuild(appDir)
         await check(() => {
           const cliOutput = stripAnsi(stderr)
-          expect(stderr).toContain(
+          expect(cliOutput).toContain(
             `Unrecognized key(s) in object: 'appDir' at "experimental"`
           )
         })

From e632b94b01ffdd1cc2b3d8a640db5b7e4a796a53 Mon Sep 17 00:00:00 2001
From: devjiwonchoi <devjiwonchoi@gmail.com>
Date: Fri, 19 Sep 2025 19:31:15 +0200
Subject: [PATCH 11/28] test: fix lint

---
 test/e2e/config-schema-check/index.test.ts | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/test/e2e/config-schema-check/index.test.ts b/test/e2e/config-schema-check/index.test.ts
index 72c1f2c60de6..16a0c27737c8 100644
--- a/test/e2e/config-schema-check/index.test.ts
+++ b/test/e2e/config-schema-check/index.test.ts
@@ -31,7 +31,7 @@ describe('next.config.js schema validating - defaultConfig', () => {
 })
 
 describe('next.config.js schema validating - invalid config', () => {
-  const { next, isNextStart, skipped } = nextTestSetup({
+  const { next, skipped } = nextTestSetup({
     files: {
       'pages/index.js': `
     export default function Page() {

From ac81ea217740187f65f7d914d51553854df078ba Mon Sep 17 00:00:00 2001
From: devjiwonchoi <devjiwonchoi@gmail.com>
Date: Fri, 19 Sep 2025 19:35:25 +0200
Subject: [PATCH 12/28] test: i hate semi false

---
 .../integration/config-experimental-warning/test/index.test.js | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/test/integration/config-experimental-warning/test/index.test.js b/test/integration/config-experimental-warning/test/index.test.js
index da26e1f0f740..a931d933a6ca 100644
--- a/test/integration/config-experimental-warning/test/index.test.js
+++ b/test/integration/config-experimental-warning/test/index.test.js
@@ -194,7 +194,8 @@ describe('Config Experimental Warning', () => {
         `Unrecognized key(s) in object: 'appDir' at "experimental"`
       )
     })
-  })(process.env.TURBOPACK_DEV ? describe.skip : describe)(
+  })
+  ;(process.env.TURBOPACK_DEV ? describe.skip : describe)(
     'production mode',
     () => {
       it('should not show next app info in next start', async () => {

From 6a6ff0d375da60341238f3a640663d1e967fba6e Mon Sep 17 00:00:00 2001
From: devjiwonchoi <devjiwonchoi@gmail.com>
Date: Fri, 19 Sep 2025 22:58:20 +0200
Subject: [PATCH 13/28] add serializeNextConfigForProduction flag

---
 packages/next/src/server/config-shared.ts | 11 +++++++++++
 packages/next/src/server/config.ts        |  9 ++++++++-
 2 files changed, 19 insertions(+), 1 deletion(-)

diff --git a/packages/next/src/server/config-shared.ts b/packages/next/src/server/config-shared.ts
index d95a27c8a2ac..4491443eacb3 100644
--- a/packages/next/src/server/config-shared.ts
+++ b/packages/next/src/server/config-shared.ts
@@ -818,6 +818,17 @@ export interface ExperimentalConfig {
    * instead of `{distDir}`.
    */
   isolatedDevBuild?: boolean
+
+  /**
+   * When enabled, the produciton server will use the serialized config file
+   * instead of the original config file. This can save the time of loading the
+   * config file, especially when you are using `next.config.ts`. When the `distDir`
+   * is set, the serialized config file will be written to the same directory as the
+   * original config file.
+   *
+   * @default true
+   */
+  serializeNextConfigForProduction?: boolean
 }
 
 export type ExportPathMap = {
diff --git a/packages/next/src/server/config.ts b/packages/next/src/server/config.ts
index be237849f1ac..6f764f9e238e 100644
--- a/packages/next/src/server/config.ts
+++ b/packages/next/src/server/config.ts
@@ -1387,7 +1387,11 @@ export default async function loadConfig(
     const fromManifest = tryLoadSerializedConfig(
       join(dir, '.next', SERVER_FILES_MANIFEST)
     )
-    if (fromManifest) {
+    if (
+      fromManifest &&
+      // Don't return here and will eventually fall back to loading the config.
+      fromManifest.experimental?.serializeNextConfigForProduction
+    ) {
       return fromManifest
     }
 
@@ -1396,6 +1400,9 @@ export default async function loadConfig(
     const configPath = await findUp(CONFIG_FILES, { cwd: dir })
     const targetDir = configPath ? dirname(configPath) : dir
 
+    // Even though serializeNextConfigForProduction is enabled, we still need to check
+    // the existSync because there's no way to know if serializeNextConfigForProduction
+    // is enabled until we load the config.
     const fromSerialized = tryLoadSerializedConfig(
       join(targetDir, SERIALIZED_CONFIG_FILE)
     )

From 15b5f8eab239ff9142cbc09574e765e2b17e5739 Mon Sep 17 00:00:00 2001
From: devjiwonchoi <devjiwonchoi@gmail.com>
Date: Fri, 19 Sep 2025 23:00:11 +0200
Subject: [PATCH 14/28] test: disable serializeNextConfigForProduction for nx
 test

---
 .../app-dir/nx-handling/apps/next-nx-test/next.config.js   | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/test/e2e/app-dir/nx-handling/apps/next-nx-test/next.config.js b/test/e2e/app-dir/nx-handling/apps/next-nx-test/next.config.js
index 70cc8d82b4b5..5ced4df9e2c4 100644
--- a/test/e2e/app-dir/nx-handling/apps/next-nx-test/next.config.js
+++ b/test/e2e/app-dir/nx-handling/apps/next-nx-test/next.config.js
@@ -10,6 +10,13 @@ const nextConfig = {
   // Use this to set Nx-specific options
   // See: https://nx.dev/recipes/next/next-config-setup
   nx: {},
+  experimental: {
+    // Disable because nx tries to copy the config to the dist dir
+    // and expect to load the config inside the dist dir again.
+    // In this case, the serialized config file will be relative
+    // to the original config, not the one inside the dist dir.
+    serializeNextConfigForProduction: false,
+  },
 }
 
 const plugins = [

From a782d26609e59660d5735d715d812d1e262f39ce Mon Sep 17 00:00:00 2001
From: devjiwonchoi <devjiwonchoi@gmail.com>
Date: Fri, 19 Sep 2025 23:02:58 +0200
Subject: [PATCH 15/28] update comment

---
 packages/next/src/server/config-shared.ts | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/packages/next/src/server/config-shared.ts b/packages/next/src/server/config-shared.ts
index 4491443eacb3..4493bbbafd16 100644
--- a/packages/next/src/server/config-shared.ts
+++ b/packages/next/src/server/config-shared.ts
@@ -823,8 +823,9 @@ export interface ExperimentalConfig {
    * When enabled, the produciton server will use the serialized config file
    * instead of the original config file. This can save the time of loading the
    * config file, especially when you are using `next.config.ts`. When the `distDir`
-   * is set, the serialized config file will be written to the same directory as the
-   * original config file.
+   * is set, the serialized config `next-config-serialized.json` will be written to
+   * the same directory as the original config file. This is because Next.js doesn't
+   * know the `distDir` until loading the config.
    *
    * @default true
    */

From 9b98fb5e8807da3db6b3ff0ffa5e04bc82ca3b0d Mon Sep 17 00:00:00 2001
From: devjiwonchoi <devjiwonchoi@gmail.com>
Date: Fri, 19 Sep 2025 23:05:48 +0200
Subject: [PATCH 16/28] test: update comment

---
 test/e2e/app-dir/nx-handling/apps/next-nx-test/next.config.js | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/test/e2e/app-dir/nx-handling/apps/next-nx-test/next.config.js b/test/e2e/app-dir/nx-handling/apps/next-nx-test/next.config.js
index 5ced4df9e2c4..2223f3829b48 100644
--- a/test/e2e/app-dir/nx-handling/apps/next-nx-test/next.config.js
+++ b/test/e2e/app-dir/nx-handling/apps/next-nx-test/next.config.js
@@ -13,8 +13,8 @@ const nextConfig = {
   experimental: {
     // Disable because nx tries to copy the config to the dist dir
     // and expect to load the config inside the dist dir again.
-    // In this case, the serialized config file will be relative
-    // to the original config, not the one inside the dist dir.
+    // In this case, the `distDir` will be relative to the original config,
+    // not the config inside the dist dir, and cause distDir path mismatch.
     serializeNextConfigForProduction: false,
   },
 }

From 21d753b1f052ddc269cd88b5bd5f4d974ade27aa Mon Sep 17 00:00:00 2001
From: Jiwon Choi <devjiwonchoi@gmail.com>
Date: Fri, 19 Sep 2025 23:39:22 +0200
Subject: [PATCH 17/28] config.experimental?.serializeNextConfigForProduction
 when write next-config-ser.json

Co-authored-by: vercel[bot] <35613825+vercel[bot]@users.noreply.github.com>
---
 packages/next/src/build/index.ts | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/packages/next/src/build/index.ts b/packages/next/src/build/index.ts
index 973fa8c8b665..b375410947b3 100644
--- a/packages/next/src/build/index.ts
+++ b/packages/next/src/build/index.ts
@@ -2729,7 +2729,7 @@ export default async function build(
       // the prod server will not know the distDir until loading the config.
       // Therefore we write the serialized config to the same directory as the
       // original config file.
-      if (config.output !== 'standalone' && config.distDir !== '.next') {
+      if (config.output !== 'standalone' && config.distDir !== '.next' && config.experimental?.serializeNextConfigForProduction) {
         const serializedConfigPath = path.join(
           // Write to the same directory as the original config file.
           config.configFile ? path.dirname(config.configFile) : dir,

From b20019d798fa578e1b0c7c72de3c08b91db9841f Mon Sep 17 00:00:00 2001
From: devjiwonchoi <devjiwonchoi@gmail.com>
Date: Fri, 19 Sep 2025 23:40:41 +0200
Subject: [PATCH 18/28] set serializeNextConfigForProduction to true on default
 config

---
 packages/next/src/server/config-shared.ts | 1 +
 1 file changed, 1 insertion(+)

diff --git a/packages/next/src/server/config-shared.ts b/packages/next/src/server/config-shared.ts
index 4493bbbafd16..1d86f6b50820 100644
--- a/packages/next/src/server/config-shared.ts
+++ b/packages/next/src/server/config-shared.ts
@@ -1507,6 +1507,7 @@ export const defaultConfig = Object.freeze({
     browserDebugInfoInTerminal: false,
     optimizeRouterScrolling: false,
     isolatedDevBuild: false,
+    serializeNextConfigForProduction: true,
   },
   htmlLimitedBots: undefined,
   bundlePagesRouterDependencies: false,

From 7a63bfb8fd9fc99c8c72c3fbcabafe42c783a542 Mon Sep 17 00:00:00 2001
From: devjiwonchoi <devjiwonchoi@gmail.com>
Date: Fri, 19 Sep 2025 23:45:53 +0200
Subject: [PATCH 19/28] fix lint

---
 packages/next/src/build/index.ts | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/packages/next/src/build/index.ts b/packages/next/src/build/index.ts
index b375410947b3..153cb18a6bf3 100644
--- a/packages/next/src/build/index.ts
+++ b/packages/next/src/build/index.ts
@@ -2729,7 +2729,11 @@ export default async function build(
       // the prod server will not know the distDir until loading the config.
       // Therefore we write the serialized config to the same directory as the
       // original config file.
-      if (config.output !== 'standalone' && config.distDir !== '.next' && config.experimental?.serializeNextConfigForProduction) {
+      if (
+        config.output !== 'standalone' &&
+        config.distDir !== '.next' &&
+        config.experimental?.serializeNextConfigForProduction
+      ) {
         const serializedConfigPath = path.join(
           // Write to the same directory as the original config file.
           config.configFile ? path.dirname(config.configFile) : dir,

From bd9356aace285e630e117c3d5b77349c5ecc9e67 Mon Sep 17 00:00:00 2001
From: devjiwonchoi <devjiwonchoi@gmail.com>
Date: Sat, 20 Sep 2025 00:08:31 +0200
Subject: [PATCH 20/28] add to schema

---
 packages/next/src/server/config-schema.ts | 1 +
 1 file changed, 1 insertion(+)

diff --git a/packages/next/src/server/config-schema.ts b/packages/next/src/server/config-schema.ts
index e7769094cd1c..a897c5517782 100644
--- a/packages/next/src/server/config-schema.ts
+++ b/packages/next/src/server/config-schema.ts
@@ -526,6 +526,7 @@ export const configSchema: zod.ZodType<NextConfig> = z.lazy(() =>
           ])
           .optional(),
         optimizeRouterScrolling: z.boolean().optional(),
+        serializeNextConfigForProduction: z.boolean().optional(),
       })
       .optional(),
     exportPathMap: z

From ca0b9bd20935ee017a1cd4f4fb6752875175d109 Mon Sep 17 00:00:00 2001
From: devjiwonchoi <devjiwonchoi@gmail.com>
Date: Sat, 20 Sep 2025 21:11:41 +0200
Subject: [PATCH 21/28] add
 __NEXT_EXPERIMENTAL_SERIALIZE_NEXT_CONFIG_FOR_PRODUCTION flag

---
 packages/next/src/build/index.ts          | 6 +++++-
 packages/next/src/server/config-shared.ts | 7 +++++--
 packages/next/src/server/config.ts        | 5 ++++-
 3 files changed, 14 insertions(+), 4 deletions(-)

diff --git a/packages/next/src/build/index.ts b/packages/next/src/build/index.ts
index 153cb18a6bf3..3883953e1dd3 100644
--- a/packages/next/src/build/index.ts
+++ b/packages/next/src/build/index.ts
@@ -2732,7 +2732,11 @@ export default async function build(
       if (
         config.output !== 'standalone' &&
         config.distDir !== '.next' &&
-        config.experimental?.serializeNextConfigForProduction
+        (config.experimental?.serializeNextConfigForProduction ||
+          // This flag is used to be enabled on the tests.
+          process.env
+            .__NEXT_EXPERIMENTAL_SERIALIZE_NEXT_CONFIG_FOR_PRODUCTION ===
+            'true')
       ) {
         const serializedConfigPath = path.join(
           // Write to the same directory as the original config file.
diff --git a/packages/next/src/server/config-shared.ts b/packages/next/src/server/config-shared.ts
index 1d86f6b50820..ad7f0cc4711b 100644
--- a/packages/next/src/server/config-shared.ts
+++ b/packages/next/src/server/config-shared.ts
@@ -820,7 +820,7 @@ export interface ExperimentalConfig {
   isolatedDevBuild?: boolean
 
   /**
-   * When enabled, the produciton server will use the serialized config file
+   * When enabled, the production server will use the serialized config file
    * instead of the original config file. This can save the time of loading the
    * config file, especially when you are using `next.config.ts`. When the `distDir`
    * is set, the serialized config `next-config-serialized.json` will be written to
@@ -1507,7 +1507,10 @@ export const defaultConfig = Object.freeze({
     browserDebugInfoInTerminal: false,
     optimizeRouterScrolling: false,
     isolatedDevBuild: false,
-    serializeNextConfigForProduction: true,
+    serializeNextConfigForProduction:
+      // This flag is used to be enabled on the tests.
+      process.env.__NEXT_EXPERIMENTAL_SERIALIZE_NEXT_CONFIG_FOR_PRODUCTION ===
+      'true',
   },
   htmlLimitedBots: undefined,
   bundlePagesRouterDependencies: false,
diff --git a/packages/next/src/server/config.ts b/packages/next/src/server/config.ts
index 6f764f9e238e..b5825073b9f8 100644
--- a/packages/next/src/server/config.ts
+++ b/packages/next/src/server/config.ts
@@ -1390,7 +1390,10 @@ export default async function loadConfig(
     if (
       fromManifest &&
       // Don't return here and will eventually fall back to loading the config.
-      fromManifest.experimental?.serializeNextConfigForProduction
+      (fromManifest.experimental?.serializeNextConfigForProduction ||
+        // This flag is used to be enabled on the tests.
+        process.env.__NEXT_EXPERIMENTAL_SERIALIZE_NEXT_CONFIG_FOR_PRODUCTION ===
+          'true')
     ) {
       return fromManifest
     }

From a27a1517daa81216d94029fa9317a56ddd31741c Mon Sep 17 00:00:00 2001
From: devjiwonchoi <devjiwonchoi@gmail.com>
Date: Sat, 20 Sep 2025 21:13:26 +0200
Subject: [PATCH 22/28] ci: enable
 __NEXT_EXPERIMENTAL_SERIALIZE_NEXT_CONFIG_FOR_PRODUCTION flag

---
 .github/workflows/build_and_test.yml | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/.github/workflows/build_and_test.yml b/.github/workflows/build_and_test.yml
index a741e858daaa..14a095351aa4 100644
--- a/.github/workflows/build_and_test.yml
+++ b/.github/workflows/build_and_test.yml
@@ -956,6 +956,7 @@ jobs:
       afterBuild: |
         export __NEXT_EXPERIMENTAL_PPR=true # for compatibility with the existing tests
         export __NEXT_EXPERIMENTAL_CACHE_COMPONENTS=true
+        export __NEXT_EXPERIMENTAL_SERIALIZE_NEXT_CONFIG_FOR_PRODUCTION=true
         export NEXT_EXTERNAL_TESTS_FILTERS="test/experimental-tests-manifest.json"
 
         node run-tests.js \
@@ -978,6 +979,7 @@ jobs:
       afterBuild: |
         export __NEXT_EXPERIMENTAL_PPR=true # for compatibility with the existing tests
         export __NEXT_EXPERIMENTAL_CACHE_COMPONENTS=true
+        export __NEXT_EXPERIMENTAL_SERIALIZE_NEXT_CONFIG_FOR_PRODUCTION=true
         export NEXT_EXTERNAL_TESTS_FILTERS="test/experimental-tests-manifest.json"
         export NEXT_TEST_MODE=dev
 
@@ -1002,6 +1004,7 @@ jobs:
       afterBuild: |
         export __NEXT_EXPERIMENTAL_PPR=true # for compatibility with the existing tests
         export __NEXT_EXPERIMENTAL_CACHE_COMPONENTS=true
+        export __NEXT_EXPERIMENTAL_SERIALIZE_NEXT_CONFIG_FOR_PRODUCTION=true
         export NEXT_EXTERNAL_TESTS_FILTERS="test/experimental-tests-manifest.json"
         export NEXT_TEST_MODE=start
 

From 0a25ac88bea40fe89c95ddeed867321db356dc99 Mon Sep 17 00:00:00 2001
From: devjiwonchoi <devjiwonchoi@gmail.com>
Date: Sat, 20 Sep 2025 21:34:33 +0200
Subject: [PATCH 23/28] flatten logic

---
 packages/next/src/server/config.ts | 118 +++++++++++++++++------------
 1 file changed, 71 insertions(+), 47 deletions(-)

diff --git a/packages/next/src/server/config.ts b/packages/next/src/server/config.ts
index b5825073b9f8..c422fb4f051e 100644
--- a/packages/next/src/server/config.ts
+++ b/packages/next/src/server/config.ts
@@ -1355,62 +1355,86 @@ export default async function loadConfig(
     return standaloneConfig
   }
 
+  let path: string | undefined
   // During prod server, we can load from the serialized config.
   if (phase === PHASE_PRODUCTION_SERVER) {
-    function tryLoadSerializedConfig(
-      configPath: string
-    ): NextConfigComplete | null {
-      try {
-        if (!existsSync(configPath)) {
-          return null
-        }
-
-        const parsed = JSON.parse(readFileSync(configPath, 'utf8'))
-        const config = parsed.config
-
-        // Cache the config
-        configCache.set(cacheKey, {
-          config,
-          rawConfig: config,
-          configuredExperimentalFeatures: [],
-        })
-
-        return config
-      } catch {
-        // Continue to next option
-      }
-      return null
-    }
-
     // Try load from ".next" dir since we don't know the
     // distDir until loading the config.
-    const fromManifest = tryLoadSerializedConfig(
-      join(dir, '.next', SERVER_FILES_MANIFEST)
-    )
-    if (
-      fromManifest &&
-      // Don't return here and will eventually fall back to loading the config.
-      (fromManifest.experimental?.serializeNextConfigForProduction ||
-        // This flag is used to be enabled on the tests.
-        process.env.__NEXT_EXPERIMENTAL_SERIALIZE_NEXT_CONFIG_FOR_PRODUCTION ===
-          'true')
-    ) {
-      return fromManifest
+    try {
+      const possiblyServerFilesManifestPath = join(
+        dir,
+        '.next',
+        SERVER_FILES_MANIFEST
+      )
+      if (existsSync(possiblyServerFilesManifestPath)) {
+        const parsed = JSON.parse(
+          readFileSync(possiblyServerFilesManifestPath, 'utf8')
+        )
+        const config: NextConfigComplete = parsed.config
+
+        if (
+          // Don't return here and will eventually fall back to loading the config.
+          config?.experimental?.serializeNextConfigForProduction ||
+          // This flag is used to be enabled on the tests.
+          process.env
+            .__NEXT_EXPERIMENTAL_SERIALIZE_NEXT_CONFIG_FOR_PRODUCTION === 'true'
+        ) {
+          // Cache the config
+          configCache.set(cacheKey, {
+            config,
+            rawConfig: config,
+            configuredExperimentalFeatures: [],
+          })
+
+          return config
+        }
+      }
+    } catch {
+      // Continue to next option
     }
 
     // If the custom distDir is set, we write the serialized config
     // to the same directory as the original config file.
-    const configPath = await findUp(CONFIG_FILES, { cwd: dir })
-    const targetDir = configPath ? dirname(configPath) : dir
+    path = await findUp(CONFIG_FILES, { cwd: dir })
+    const targetDir = path ? dirname(path) : dir
 
-    // Even though serializeNextConfigForProduction is enabled, we still need to check
+    // Even though serializeNextConfigForProduction might be disabled, we still need to check
     // the existSync because there's no way to know if serializeNextConfigForProduction
-    // is enabled until we load the config.
-    const fromSerialized = tryLoadSerializedConfig(
-      join(targetDir, SERIALIZED_CONFIG_FILE)
-    )
-    if (fromSerialized) {
-      return fromSerialized
+    // is disabled until we load the config.
+    const serializedConfigPath = join(targetDir, SERIALIZED_CONFIG_FILE)
+    try {
+      if (existsSync(serializedConfigPath)) {
+        const parsed = JSON.parse(readFileSync(serializedConfigPath, 'utf8'))
+        const config: NextConfigComplete = parsed.config
+
+        if (
+          // Don't return here and will eventually fall back to loading the config.
+          config?.experimental?.serializeNextConfigForProduction ||
+          // This flag is used to be enabled on the tests.
+          process.env
+            .__NEXT_EXPERIMENTAL_SERIALIZE_NEXT_CONFIG_FOR_PRODUCTION === 'true'
+        ) {
+          // Cache the config
+          configCache.set(cacheKey, {
+            config,
+            rawConfig: config,
+            configuredExperimentalFeatures: [],
+          })
+
+          return config
+        }
+      }
+    } catch (cause) {
+      if (
+        // This flag is used to be enabled on the tests.
+        process.env.__NEXT_EXPERIMENTAL_SERIALIZE_NEXT_CONFIG_FOR_PRODUCTION ===
+        'true'
+      ) {
+        throw new Error(
+          `Failed to load serialized config "${serializedConfigPath}".`,
+          { cause }
+        )
+      }
     }
 
     // Fall back to loading the config.
@@ -1461,7 +1485,7 @@ export default async function loadConfig(
     return config
   }
 
-  const path = await findUp(CONFIG_FILES, { cwd: dir })
+  path ??= await findUp(CONFIG_FILES, { cwd: dir })
 
   // If config file was found
   if (path?.length) {

From 7a94269b9e03bc436f1c8de91e23a887ba6c180b Mon Sep 17 00:00:00 2001
From: devjiwonchoi <devjiwonchoi@gmail.com>
Date: Sat, 20 Sep 2025 21:34:49 +0200
Subject: [PATCH 24/28] test: enable serializeNextConfigForProduction

---
 packages/next/src/server/config-shared.ts                     | 2 --
 test/production/app-dir/next-config-serialized/next.config.js | 3 +++
 2 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/packages/next/src/server/config-shared.ts b/packages/next/src/server/config-shared.ts
index ad7f0cc4711b..0f85b81ce588 100644
--- a/packages/next/src/server/config-shared.ts
+++ b/packages/next/src/server/config-shared.ts
@@ -826,8 +826,6 @@ export interface ExperimentalConfig {
    * is set, the serialized config `next-config-serialized.json` will be written to
    * the same directory as the original config file. This is because Next.js doesn't
    * know the `distDir` until loading the config.
-   *
-   * @default true
    */
   serializeNextConfigForProduction?: boolean
 }
diff --git a/test/production/app-dir/next-config-serialized/next.config.js b/test/production/app-dir/next-config-serialized/next.config.js
index f43f757aadae..81a6ab13bae1 100644
--- a/test/production/app-dir/next-config-serialized/next.config.js
+++ b/test/production/app-dir/next-config-serialized/next.config.js
@@ -6,6 +6,9 @@ const nextConfig = {
   env: {
     foo: 'foo',
   },
+  experimental: {
+    serializeNextConfigForProduction: true,
+  },
 }
 
 module.exports = nextConfig

From 89edac042f1ccc0c47446a708835ee69154898c2 Mon Sep 17 00:00:00 2001
From: devjiwonchoi <devjiwonchoi@gmail.com>
Date: Sat, 20 Sep 2025 21:40:18 +0200
Subject: [PATCH 25/28] use nullish coalescing

---
 packages/next/src/build/index.ts   | 3 ++-
 packages/next/src/server/config.ts | 6 ++++--
 2 files changed, 6 insertions(+), 3 deletions(-)

diff --git a/packages/next/src/build/index.ts b/packages/next/src/build/index.ts
index 3883953e1dd3..1c987144ed14 100644
--- a/packages/next/src/build/index.ts
+++ b/packages/next/src/build/index.ts
@@ -2732,7 +2732,8 @@ export default async function build(
       if (
         config.output !== 'standalone' &&
         config.distDir !== '.next' &&
-        (config.experimental?.serializeNextConfigForProduction ||
+        // Use nullish coalescing (??) since we don't want to return when it's false.
+        (config.experimental?.serializeNextConfigForProduction ??
           // This flag is used to be enabled on the tests.
           process.env
             .__NEXT_EXPERIMENTAL_SERIALIZE_NEXT_CONFIG_FOR_PRODUCTION ===
diff --git a/packages/next/src/server/config.ts b/packages/next/src/server/config.ts
index c422fb4f051e..d7575f6cc304 100644
--- a/packages/next/src/server/config.ts
+++ b/packages/next/src/server/config.ts
@@ -1374,7 +1374,8 @@ export default async function loadConfig(
 
         if (
           // Don't return here and will eventually fall back to loading the config.
-          config?.experimental?.serializeNextConfigForProduction ||
+          // Use nullish coalescing (??) since we don't want to return when it's false.
+          config?.experimental?.serializeNextConfigForProduction ??
           // This flag is used to be enabled on the tests.
           process.env
             .__NEXT_EXPERIMENTAL_SERIALIZE_NEXT_CONFIG_FOR_PRODUCTION === 'true'
@@ -1409,7 +1410,8 @@ export default async function loadConfig(
 
         if (
           // Don't return here and will eventually fall back to loading the config.
-          config?.experimental?.serializeNextConfigForProduction ||
+          // Use nullish coalescing (??) since we don't want to return when it's false.
+          config?.experimental?.serializeNextConfigForProduction ??
           // This flag is used to be enabled on the tests.
           process.env
             .__NEXT_EXPERIMENTAL_SERIALIZE_NEXT_CONFIG_FOR_PRODUCTION === 'true'

From 3830f4974556d91b7dc93cebdb162b4dd24e5302 Mon Sep 17 00:00:00 2001
From: devjiwonchoi <devjiwonchoi@gmail.com>
Date: Sat, 20 Sep 2025 21:40:25 +0200
Subject: [PATCH 26/28] update error json

---
 packages/next/errors.json | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/packages/next/errors.json b/packages/next/errors.json
index e93fb5776ae3..228f1b85c5ec 100644
--- a/packages/next/errors.json
+++ b/packages/next/errors.json
@@ -809,5 +809,5 @@
   "808": "Invalid binary HMR message: insufficient data (expected %s bytes, got %s)",
   "809": "Invalid binary HMR message of type %s",
   "810": "React debug channel stream error",
-  "811": "Failed to load the serialized config file \"%s\" in \"%s\"."
+  "811": "Failed to load serialized config \"%s\"."
 }

From d54197a156c3a6edf2d5a46afca9e2803ff63621 Mon Sep 17 00:00:00 2001
From: devjiwonchoi <devjiwonchoi@gmail.com>
Date: Sat, 20 Sep 2025 22:53:30 +0200
Subject: [PATCH 27/28] test: update tests

---
 test/e2e/config-schema-check/index.test.ts    | 14 +++++--
 test/e2e/next-phase/index.test.ts             | 13 +++++-
 .../test/index.test.js                        | 41 ++++++++++++++++---
 .../custom-server/test/index.test.js          | 18 ++++++--
 4 files changed, 72 insertions(+), 14 deletions(-)

diff --git a/test/e2e/config-schema-check/index.test.ts b/test/e2e/config-schema-check/index.test.ts
index 16a0c27737c8..cf9904d5c797 100644
--- a/test/e2e/config-schema-check/index.test.ts
+++ b/test/e2e/config-schema-check/index.test.ts
@@ -31,7 +31,7 @@ describe('next.config.js schema validating - defaultConfig', () => {
 })
 
 describe('next.config.js schema validating - invalid config', () => {
-  const { next, skipped } = nextTestSetup({
+  const { next, isNextStart, skipped } = nextTestSetup({
     files: {
       'pages/index.js': `
     export default function Page() {
@@ -58,8 +58,16 @@ describe('next.config.js schema validating - invalid config', () => {
 
       expect(output).toContain('Invalid next.config.js options detected')
       expect(output).toContain('badKey')
-      // With serialized config, warnings only appear during build, not during start
-      expect(warningTimes).toBe(1)
+      if (
+        process.env.__NEXT_EXPERIMENTAL_SERIALIZE_NEXT_CONFIG_FOR_PRODUCTION ===
+        'true'
+      ) {
+        // With serialized config, warnings only appear during build, not during start
+        expect(warningTimes).toBe(1)
+      } else {
+        // for next start and next build we both display the warnings
+        expect(warningTimes).toBe(isNextStart ? 2 : 1)
+      }
 
       return 'success'
     }, 'success')
diff --git a/test/e2e/next-phase/index.test.ts b/test/e2e/next-phase/index.test.ts
index d02d3bf7a9ae..95c0612b9123 100644
--- a/test/e2e/next-phase/index.test.ts
+++ b/test/e2e/next-phase/index.test.ts
@@ -23,11 +23,16 @@ describe('next-phase', () => {
   if (skipped) return
 
   it('should render page with next phase correctly', async () => {
+    const isSerializedNextConfigForProduction =
+      process.env.__NEXT_EXPERIMENTAL_SERIALIZE_NEXT_CONFIG_FOR_PRODUCTION ===
+      'true'
     const phases = {
       dev: 'phase-development-server',
       build: 'phase-production-build',
-      // Serialized next config will use the config from build.
-      start: 'phase-production-build',
+      start: isSerializedNextConfigForProduction
+        ? // Serialized next config will use the config from build.
+          'phase-production-build'
+        : 'phase-production-server',
     }
     const currentPhase = isNextDev ? phases.dev : phases.build
     const nonExistedPhase = isNextDev ? phases.build : phases.dev
@@ -40,6 +45,10 @@ describe('next-phase', () => {
 
     if (isNextDev) {
       expect(next.cliOutput).not.toContain(phases.start)
+    } else {
+      if (isSerializedNextConfigForProduction) {
+        expect(next.cliOutput).toContain(phases.start)
+      }
     }
   })
 })
diff --git a/test/integration/config-experimental-warning/test/index.test.js b/test/integration/config-experimental-warning/test/index.test.js
index a931d933a6ca..1e458891cfe4 100644
--- a/test/integration/config-experimental-warning/test/index.test.js
+++ b/test/integration/config-experimental-warning/test/index.test.js
@@ -187,10 +187,10 @@ describe('Config Experimental Warning', () => {
         }
       `)
 
-    const { stderr } = await collectStdoutFromDev(appDir)
+    const { stdout, stderr } = await collectStdoutFromDev(appDir)
     await check(() => {
-      const cliOutput = stripAnsi(stderr)
-      expect(cliOutput).toContain(
+      expect(stripAnsi(stdout)).not.toContain(experimentalHeader)
+      expect(stripAnsi(stderr)).toContain(
         `Unrecognized key(s) in object: 'appDir' at "experimental"`
       )
     })
@@ -252,13 +252,42 @@ describe('Config Experimental Warning', () => {
         }
       `)
 
-        const { stderr } = await collectStdoutFromBuild(appDir)
+        const { stderr: buildStderr } = await collectStdoutFromBuild(appDir)
+
+        const port = await findPort()
+        let stdout = ''
+        let stderr = ''
+        app = await nextStart(appDir, port, {
+          onStdout(msg) {
+            stdout += msg
+          },
+          onStderr(msg) {
+            stderr += msg
+          },
+        })
+
         await check(() => {
-          const cliOutput = stripAnsi(stderr)
-          expect(cliOutput).toContain(
+          // TODO: experimentalHeader shows on build, likely not validating properly.
+          // expect(stripAnsi(buildStdout)).not.toContain(experimentalHeader)
+          expect(stripAnsi(buildStderr)).toContain(
             `Unrecognized key(s) in object: 'appDir' at "experimental"`
           )
         })
+
+        // For serialized config, the warning will not appear during start.
+        if (
+          process.env
+            .__NEXT_EXPERIMENTAL_SERIALIZE_NEXT_CONFIG_FOR_PRODUCTION !== 'true'
+        ) {
+          await check(() => {
+            const cliOutput = stripAnsi(stdout)
+            const cliOutputErr = stripAnsi(stderr)
+            expect(cliOutput).not.toContain(experimentalHeader)
+            expect(cliOutputErr).toContain(
+              `Unrecognized key(s) in object: 'appDir' at "experimental"`
+            )
+          })
+        }
       })
     }
   )
diff --git a/test/integration/custom-server/test/index.test.js b/test/integration/custom-server/test/index.test.js
index 42684dc7c55f..eb54720cd25c 100644
--- a/test/integration/custom-server/test/index.test.js
+++ b/test/integration/custom-server/test/index.test.js
@@ -145,9 +145,21 @@ describe.each([
       'production mode',
       () => {
         beforeAll(async () => {
-          // Set env during build for serialized next config.
-          await nextBuild(appDir, [], { env: { GENERATE_ETAGS: 'true' } })
-          await startServer({ NODE_ENV: 'production' })
+          if (
+            process.env
+              .__NEXT_EXPERIMENTAL_SERIALIZE_NEXT_CONFIG_FOR_PRODUCTION ===
+            'true'
+          ) {
+            // Set env during build for serialized next config.
+            await nextBuild(appDir, [], { env: { GENERATE_ETAGS: 'true' } })
+            await startServer({ NODE_ENV: 'production' })
+          } else {
+            await nextBuild(appDir)
+            await startServer({
+              GENERATE_ETAGS: 'true',
+              NODE_ENV: 'production',
+            })
+          }
         })
         afterAll(() => killApp(server))
 

From e999ffbbcecaa38ba2582a0f2f58d7649910934b Mon Sep 17 00:00:00 2001
From: devjiwonchoi <devjiwonchoi@gmail.com>
Date: Sat, 20 Sep 2025 23:12:17 +0200
Subject: [PATCH 28/28] test: remove unnecessary addition

---
 .../test/index.test.js                         | 18 ------------------
 1 file changed, 18 deletions(-)

diff --git a/test/integration/config-experimental-warning/test/index.test.js b/test/integration/config-experimental-warning/test/index.test.js
index 1e458891cfe4..40d8e4fe8ba3 100644
--- a/test/integration/config-experimental-warning/test/index.test.js
+++ b/test/integration/config-experimental-warning/test/index.test.js
@@ -177,24 +177,6 @@ describe('Config Experimental Warning', () => {
     expect(stdout).toContain(' ✓ workerThreads')
     expect(stdout).toContain(' ✓ scrollRestoration')
   })
-
-  it('should show unrecognized experimental features in warning but not in start log experiments section during dev', async () => {
-    configFile.write(`
-        module.exports = {
-          experimental: {
-            appDir: true
-          }
-        }
-      `)
-
-    const { stdout, stderr } = await collectStdoutFromDev(appDir)
-    await check(() => {
-      expect(stripAnsi(stdout)).not.toContain(experimentalHeader)
-      expect(stripAnsi(stderr)).toContain(
-        `Unrecognized key(s) in object: 'appDir' at "experimental"`
-      )
-    })
-  })
   ;(process.env.TURBOPACK_DEV ? describe.skip : describe)(
     'production mode',
     () => {