Next.js App Router - Production Patterns

Version: Next.js 16.1.1 React Version: 19.2.3 Node.js: 20.9+ Last Verified: 2026-01-09

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Table of Contents

1. When to Use This Skill
2. When NOT to Use This Skill
3. Security Advisories (December 2025)
4. Next.js 16.1 Updates
5. Next.js 16 Breaking Changes
6. Cache Components & Caching APIs
7. Route Handlers (Next.js 16 Updates)
8. Proxy vs Middleware
9. Parallel Routes - default.js Required
10. React 19.2 Features
11. Turbopack (Stable in Next.js 16)
12. Common Errors & Solutions
13. Templates & Resources

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When to Use This Skill

Focus: Next.js 16 breaking changes and knowledge gaps (December 2024+).

Use this skill when you need:

• Next.js 16 breaking changes (async params, proxy.ts, parallel routes default.js, removed features)
• Cache Components with "use cache" directive (NEW in Next.js 16)
• New caching APIs: revalidateTag(), updateTag(), refresh() (Updated in Next.js 16)
• Migration from Next.js 15 to 16 (avoid breaking change errors)
• Async route params (params, searchParams, cookies(), headers() now async)
• Parallel routes with default.js (REQUIRED in Next.js 16)
• React 19.2 features (View Transitions, useEffectEvent(), React Compiler)
• Turbopack (stable and default in Next.js 16)
• Image defaults changed (TTL, sizes, qualities in Next.js 16)
• Error prevention (25 documented Next.js 16 errors with solutions)

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When NOT to Use This Skill

Do NOT use this skill for:

• Cloudflare Workers deployment → Use cloudflare-nextjs skill instead
• Pages Router patterns → This skill covers App Router ONLY (Pages Router is legacy)
• Authentication libraries → Use clerk-auth, better-auth, or other auth-specific skills
• Database integration → Use cloudflare-d1, drizzle-orm-d1, or database-specific skills
• UI component libraries → Use tailwind-v4-shadcn skill for Tailwind + shadcn/ui
• State management → Use zustand-state-management, tanstack-query skills
• Form libraries → Use react-hook-form-zod skill
• Vercel-specific features → Refer to Vercel platform documentation
• Next.js Enterprise features (ISR, DPR) → Refer to Next.js Enterprise docs
• Deployment configuration → Use platform-specific deployment skills

Relationship with Other Skills:

• cloudflare-nextjs: For deploying Next.js to Cloudflare Workers (use BOTH skills together if deploying to Cloudflare)
• tailwind-v4-shadcn: For Tailwind v4 + shadcn/ui setup (composable with this skill)
• clerk-auth: For Clerk authentication in Next.js (composable with this skill)
• better-auth: For Better Auth integration (composable with this skill)

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Security Advisories (December 2025)

CRITICAL: Three security vulnerabilities were disclosed in December 2025 affecting Next.js with React Server Components:

CVE	Severity	Affected	Description
CVE-2025-66478	CRITICAL (10.0)	15.x, 16.x	Server Component arbitrary code execution
CVE-2025-55184	HIGH	13.x-16.x	Denial of Service via malformed request
CVE-2025-55183	MEDIUM	13.x-16.x	Source code exposure in error responses

Action Required: Upgrade to Next.js 16.1.1 or later immediately.

npm update next
# Verify: npm list next should show 16.1.1+

References:

• https://nextjs.org/security
• https://github.com/vercel/next.js/security/advisories

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Next.js 16.1 Updates (December 2025)

New in 16.1:

• Turbopack File System Caching (STABLE): Now enabled by default in development
• Next.js Bundle Analyzer: New experimental feature for bundle analysis
• Improved Debugging: Enhanced next dev --inspect support
• Security Fixes: Addresses CVE-2025-66478, CVE-2025-55184, CVE-2025-55183

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Next.js 16 Breaking Changes

IMPORTANT: Next.js 16 introduces multiple breaking changes. Read this section carefully if migrating from Next.js 15 or earlier.

1. Async Route Parameters (BREAKING)

Breaking Change: params, searchParams, cookies(), headers(), draftMode() are now async and must be awaited.

Before (Next.js 15):

// ❌ This no longer works in Next.js 16
export default function Page({ params, searchParams }: {
  params: { slug: string }
  searchParams: { query: string }
}) {
  const slug = params.slug // ❌ Error: params is a Promise
  const query = searchParams.query // ❌ Error: searchParams is a Promise
  return <div>{slug}</div>
}

After (Next.js 16):

// ✅ Correct: await params and searchParams
export default async function Page({ params, searchParams }: {
  params: Promise<{ slug: string }>
  searchParams: Promise<{ query: string }>
}) {
  const { slug } = await params // ✅ Await the promise
  const { query } = await searchParams // ✅ Await the promise
  return <div>{slug}</div>
}

Applies to:

• params in pages, layouts, route handlers
• searchParams in pages
• cookies() from next/headers
• headers() from next/headers
• draftMode() from next/headers

Migration:

// ❌ Before
import { cookies, headers } from 'next/headers'

export function MyComponent() {
  const cookieStore = cookies() // ❌ Sync access
  const headersList = headers() // ❌ Sync access
}

// ✅ After
import { cookies, headers } from 'next/headers'

export async function MyComponent() {
  const cookieStore = await cookies() // ✅ Async access
  const headersList = await headers() // ✅ Async access
}

Codemod: Run npx @next/codemod@canary upgrade latest to automatically migrate.

Codemod Limitations (Community-sourced): The official codemod handles ~80% of async API migrations but misses edge cases:

• Async APIs accessed in custom hooks
• Conditional logic accessing params
• Components imported from external packages
• Complex server actions with multiple async calls

After running the codemod, search for @next-codemod-error comments marking places it couldn't auto-fix.

Manual Migration for Client Components:

// For client components, use React.use() to unwrap promises
'use client';

import { use } from 'react';

export default function ClientComponent({
  params
}: {
  params: Promise<{ id: string }>
}) {
  const { id } = use(params); // Unwrap Promise in client
  return <div>{id}</div>;
}

See Template: templates/app-router-async-params.tsx

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2. Middleware → Proxy Migration (BREAKING)

Breaking Change: middleware.ts is deprecated in Next.js 16. Use proxy.ts instead.

Why the Change: proxy.ts makes the network boundary explicit by running on Node.js runtime (not Edge runtime). This provides better clarity between edge middleware and server-side proxies.

Migration Steps:

1. Rename file: middleware.ts → proxy.ts
2. Rename function: middleware → proxy
3. Update config: matcher → config.matcher (same syntax)

Before (Next.js 15):

// middleware.ts ❌ Deprecated in Next.js 16
import { NextResponse } from 'next/server'
import type { NextRequest } from 'next/server'

export function middleware(request: NextRequest) {
  const response = NextResponse.next()
  response.headers.set('x-custom-header', 'value')
  return response
}

export const config = {
  matcher: '/api/:path*',
}

After (Next.js 16):

// proxy.ts ✅ New in Next.js 16
import { NextResponse } from 'next/server'
import type { NextRequest } from 'next/server'

export function proxy(request: NextRequest) {
  const response = NextResponse.next()
  response.headers.set('x-custom-header', 'value')
  return response
}

export const config = {
  matcher: '/api/:path*',
}

Note: middleware.ts still works in Next.js 16 but is deprecated. Migrate to proxy.ts for future compatibility.

See Template: templates/proxy-migration.ts See Reference: references/proxy-vs-middleware.md

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3. Parallel Routes Require default.js (BREAKING)

Breaking Change: Parallel routes now require explicit default.js files. Without them, routes will fail during soft navigation.

Structure:

app/
├── @auth/
│   ├── login/
│   │   └── page.tsx
│   └── default.tsx    ← REQUIRED in Next.js 16
├── @dashboard/
│   ├── overview/
│   │   └── page.tsx
│   └── default.tsx    ← REQUIRED in Next.js 16
└── layout.tsx

Layout:

// app/layout.tsx
export default function Layout({
  children,
  auth,
  dashboard,
}: {
  children: React.ReactNode
  auth: React.ReactNode
  dashboard: React.ReactNode
}) {
  return (
    <html>
      <body>
        {auth}
        {dashboard}
        {children}
      </body>
    </html>
  )
}

Default Fallback (REQUIRED):

// app/@auth/default.tsx
export default function AuthDefault() {
  return null // or <Skeleton /> or redirect
}

// app/@dashboard/default.tsx
export default function DashboardDefault() {
  return null
}

Why Required: Next.js 16 changed how parallel routes handle soft navigation. Without default.js, unmatched slots will error during client-side navigation.

See Template: templates/parallel-routes-with-default.tsx

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4. Removed Features (BREAKING)

The following features are REMOVED in Next.js 16:

1. AMP Support - Entirely removed. Migrate to standard pages.
2. next lint command - Use ESLint or Biome directly.
3. serverRuntimeConfig and publicRuntimeConfig - Use environment variables instead.
4. experimental.ppr flag - Evolved into Cache Components. Use "use cache" directive.
5. Automatic scroll-behavior: smooth - Add manually if needed.
6. Node.js 18 support - Minimum version is now 20.9+.

Migration:

• AMP: Convert AMP pages to standard pages or use separate AMP implementation.
• Linting: Run npx eslint . or npx biome lint . directly.
• Config: Replace serverRuntimeConfig with process.env.VARIABLE.
• PPR: Migrate from experimental.ppr to "use cache" directive (see Cache Components section).

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5. Version Requirements (BREAKING)

Next.js 16 requires:

• Node.js: 20.9+ (Node.js 18 no longer supported)
• TypeScript: 5.1+ (if using TypeScript)
• React: 19.2+ (automatically installed with Next.js 16)
• Browsers: Chrome 111+, Safari 16.4+, Firefox 109+, Edge 111+

Check Versions:

node --version    # Should be 20.9+
npm --version     # Should be 10+
npx next --version # Should be 16.0.0+

Upgrade Node.js:

# Using nvm
nvm install 20
nvm use 20
nvm alias default 20

# Using Homebrew (macOS)
brew install node@20

# Using apt (Ubuntu/Debian)
sudo apt update
sudo apt install nodejs npm

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6. Image Defaults Changed (BREAKING)

Next.js 16 changed next/image defaults:

Setting	Next.js 15	Next.js 16
TTL (cache duration)	60 seconds	4 hours
imageSizes	[16, 32, 48, 64, 96, 128, 256, 384]	[640, 750, 828, 1080, 1200] (reduced)
qualities	[75, 90, 100]	[75] (single quality)

Impact:

• Images cache longer (4 hours vs 60 seconds)
• Fewer image sizes generated (smaller builds, but less granular)
• Single quality (75) generated instead of multiple

Override Defaults (if needed):

// next.config.ts
import type { NextConfig } from 'next'

const config: NextConfig = {
  images: {
    minimumCacheTTL: 60, // Revert to 60 seconds
    deviceSizes: [640, 750, 828, 1080, 1200, 1920], // Add larger sizes
    imageSizes: [16, 32, 48, 64, 96, 128, 256, 384], // Restore old sizes
    formats: ['image/webp'], // Default
  },
}

export default config

See Template: templates/image-optimization.tsx

---

Cache Components & Caching APIs

NEW in Next.js 16: Cache Components introduce opt-in caching with the "use cache" directive, replacing implicit caching from Next.js 15.

1. Overview

What Changed:

• Next.js 15: Implicit caching (all Server Components cached by default)
• Next.js 16: Opt-in caching with "use cache" directive

Why the Change: Explicit caching gives developers more control and makes caching behavior predictable.

Important Caching Defaults (Community-sourced):

Feature	Next.js 14	Next.js 15/16
fetch() requests	Cached by default	NOT cached by default
Router Cache (dynamic pages)	Cached on client	NOT cached by default
Router Cache (static pages)	Cached	Still cached
Route Handlers (GET)	Cached	Dynamic by default

Best Practice: Default to dynamic in Next.js 16. Start with no caching and add it where beneficial, rather than debugging unexpected cache hits. Always test with production builds - the development server behaves differently.

Cache Components enable:

• Component-level caching (cache specific components, not entire pages)
• Function-level caching (cache expensive computations)
• Page-level caching (cache entire pages selectively)
• Partial Prerendering (PPR) - Cache static parts, render dynamic parts on-demand

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2. "use cache" Directive

Syntax: Add "use cache" at the top of a Server Component, function, or route handler.

Component-level caching:

// app/components/expensive-component.tsx
'use cache'

export async function ExpensiveComponent() {
  const data = await fetch('https://api.example.com/data')
  const json = await data.json()

  return (
    <div>
      <h1>{json.title}</h1>
      <p>{json.description}</p>
    </div>
  )
}

Function-level caching:

// lib/data.ts
'use cache'

export async function getExpensiveData(id: string) {
  const response = await fetch(`https://api.example.com/items/${id}`)
  return response.json()
}

// Usage in component
import { getExpensiveData } from '@/lib/data'

export async function ProductPage({ params }: { params: Promise<{ id: string }> }) {
  const { id } = await params
  const product = await getExpensiveData(id) // Cached

  return <div>{product.name}</div>
}

Page-level caching:

// app/blog/[slug]/page.tsx
'use cache'

export async function generateStaticParams() {
  const posts = await fetch('https://api.example.com/posts').then(r => r.json())
  return posts.map((post: { slug: string }) => ({ slug: post.slug }))
}

export default async function BlogPost({ params }: { params: Promise<{ slug: string }> }) {
  const { slug } = await params
  const post = await fetch(`https://api.example.com/posts/${slug}`).then(r => r.json())

  return (
    <article>
      <h1>{post.title}</h1>
      <div>{post.content}</div>
    </article>
  )
}

See Template: templates/cache-component-use-cache.tsx

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3. Partial Prerendering (PPR)

PPR allows caching static parts of a page while rendering dynamic parts on-demand.

Pattern:

// app/dashboard/page.tsx

// Static header (cached)
'use cache'
async function StaticHeader() {
  return <header>My App</header>
}

// Dynamic user info (not cached)
async function DynamicUserInfo() {
  const cookieStore = await cookies()
  const userId = cookieStore.get('userId')?.value
  const user = await fetch(`/api/users/${userId}`).then(r => r.json())

  return <div>Welcome, {user.name}</div>
}

// Page combines both
export default function Dashboard() {
  return (
    <div>
      <StaticHeader /> {/* Cached */}
      <DynamicUserInfo /> {/* Dynamic */}
    </div>
  )
}

When to Use PPR:

• Page has both static and dynamic content
• Want to cache layout/header/footer but render user-specific content
• Need fast initial load (static parts) + personalization (dynamic parts)

See Reference: references/cache-components-guide.md

---

4. revalidateTag() - Updated API

BREAKING CHANGE: revalidateTag() now requires a second argument (cacheLife profile) for stale-while-revalidate behavior.

Before (Next.js 15):

import { revalidateTag } from 'next/cache'

export async function updatePost(id: string) {
  await fetch(`/api/posts/${id}`, { method: 'PATCH' })
  revalidateTag('posts') // ❌ Only one argument in Next.js 15
}

After (Next.js 16):

import { revalidateTag } from 'next/cache'

export async function updatePost(id: string) {
  await fetch(`/api/posts/${id}`, { method: 'PATCH' })
  revalidateTag('posts', 'max') // ✅ Second argument required in Next.js 16
}

Built-in Cache Life Profiles:

• 'max' - Maximum staleness (recommended for most use cases)
• 'hours' - Stale after hours
• 'days' - Stale after days
• 'weeks' - Stale after weeks
• 'default' - Default cache behavior

Custom Cache Life Profile:

revalidateTag('posts', {
  stale: 3600, // Stale after 1 hour (seconds)
  revalidate: 86400, // Revalidate every 24 hours (seconds)
  expire: false, // Never expire (optional)
})

Pattern in Server Actions:

'use server'

import { revalidateTag } from 'next/cache'

export async function createPost(formData: FormData) {
  const title = formData.get('title') as string
  const content = formData.get('content') as string

  await fetch('/api/posts', {
    method: 'POST',
    body: JSON.stringify({ title, content }),
  })

  revalidateTag('posts', 'max') // ✅ Revalidate with max staleness
}

See Template: templates/revalidate-tag-cache-life.ts

---

5. updateTag() - NEW API (Server Actions Only)

NEW in Next.js 16: updateTag() provides read-your-writes semantics for Server Actions.

What it does:

• Expires cache immediately
• Refreshes data within the same request
• Shows updated data right after mutation (no stale data)

Difference from revalidateTag():

• revalidateTag(): Stale-while-revalidate (shows stale data, revalidates in background)
• updateTag(): Immediate refresh (expires cache, fetches fresh data in same request)

Use Case: Forms, user settings, or any mutation where user expects immediate feedback.

Pattern:

'use server'

import { updateTag } from 'next/cache'

export async function updateUserProfile(formData: FormData) {
  const name = formData.get('name') as string
  const email = formData.get('email') as string

  // Update database
  await db.users.update({ name, email })

  // Immediately refresh cache (read-your-writes)
  updateTag('user-profile')

  // User sees updated data immediately (no stale data)
}

When to Use:

• updateTag(): User settings, profile updates, critical mutations (immediate feedback)
• revalidateTag(): Blog posts, product listings, non-critical updates (background revalidation)

See Template: templates/server-action-update-tag.ts

---

6. refresh() - NEW API (Server Actions Only)

NEW in Next.js 16: refresh() refreshes uncached data only (complements client-side router.refresh()).

When to Use:

• Refresh dynamic data without affecting cached data
• Complement router.refresh() on server side

Pattern:

'use server'

import { refresh } from 'next/cache'

export async function refreshDashboard() {
  // Refresh uncached data (e.g., real-time metrics)
  refresh()

  // Cached data (e.g., static header) remains cached
}

Difference from revalidateTag() and updateTag():

• refresh(): Only refreshes uncached data
• revalidateTag(): Revalidates specific tagged data (stale-while-revalidate)
• updateTag(): Immediately expires and refreshes specific tagged data

See Reference: references/cache-components-guide.md

---

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Route Handlers (Next.js 16 Updates)

Async Params in Route Handlers (BREAKING)

IMPORTANT: params and headers() are now async in Next.js 16 route handlers.

Example:

// app/api/posts/[id]/route.ts
import { NextResponse } from 'next/server'
import { headers } from 'next/headers'

export async function GET(
  request: Request,
  { params }: { params: Promise<{ id: string }> }
) {
  const { id } = await params // ✅ Await params in Next.js 16
  const headersList = await headers() // ✅ Await headers in Next.js 16

  const post = await db.posts.findUnique({ where: { id } })

  return NextResponse.json(post)
}

See Template: templates/route-handler-api.ts

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Proxy vs Middleware

Next.js 16 introduces proxy.ts to replace middleware.ts.

Why the Change?

• middleware.ts: Runs on Edge runtime (limited Node.js APIs)
• proxy.ts: Runs on Node.js runtime (full Node.js APIs)

The new proxy.ts makes the network boundary explicit and provides more flexibility.

Migration

Before (middleware.ts):

// middleware.ts
import { NextResponse } from 'next/server'
import type { NextRequest } from 'next/server'

export function middleware(request: NextRequest) {
  // Check auth
  const token = request.cookies.get('token')

  if (!token) {
    return NextResponse.redirect(new URL('/login', request.url))
  }

  return NextResponse.next()
}

export const config = {
  matcher: '/dashboard/:path*',
}

After (proxy.ts):

// proxy.ts
import { NextResponse } from 'next/server'
import type { NextRequest } from 'next/server'

export function proxy(request: NextRequest) {
  // Check auth
  const token = request.cookies.get('token')

  if (!token) {
    return NextResponse.redirect(new URL('/login', request.url))
  }

  return NextResponse.next()
}

export const config = {
  matcher: '/dashboard/:path*',
}

See Template: templates/proxy-migration.ts See Reference: references/proxy-vs-middleware.md

---

Parallel Routes - default.js Required (BREAKING)

Breaking Change in Next.js 16: Parallel routes now require explicit default.js files.

Structure:

app/
├── @modal/
│   ├── login/page.tsx
│   └── default.tsx  ← REQUIRED in Next.js 16
├── @feed/
│   ├── trending/page.tsx
│   └── default.tsx  ← REQUIRED in Next.js 16
└── layout.tsx

Default Files (REQUIRED):

// app/@modal/default.tsx
export default function ModalDefault() {
  return null // or <Skeleton /> or redirect
}

Why Required: Next.js 16 changed soft navigation handling. Without default.js, unmatched slots error during client-side navigation.

Advanced Edge Case (Community-sourced): Even WITH default.js files, hard navigating or refreshing routes with parallel routes can return 404 errors. The workaround is adding a catch-all route.

Source: GitHub Issue #48090, #73939

Workaround:

// app/@modal/[...catchAll]/page.tsx
export default function CatchAll() {
  return null;
}

// OR use catch-all in default.tsx
// app/@modal/default.tsx
export default function ModalDefault({ params }: { params: { catchAll?: string[] } }) {
  return null; // Handles all unmatched routes
}

See Template: templates/parallel-routes-with-default.tsx

---

React 19.2 Features

Next.js 16 integrates React 19.2, which includes new features from React Canary.

1. View Transitions

Use Case: Smooth animations between page transitions.

'use client'

import { useRouter } from 'next/navigation'
import { startTransition } from 'react'

export function NavigationLink({ href, children }: { href: string; children: React.ReactNode }) {
  const router = useRouter()

  function handleClick(e: React.MouseEvent) {
    e.preventDefault()

    // Wrap navigation in startTransition for View Transitions
    startTransition(() => {
      router.push(href)
    })
  }

  return <a href={href} onClick={handleClick}>{children}</a>
}

With CSS View Transitions API:

/* app/globals.css */
@view-transition {
  navigation: auto;
}

/* Animate elements with view-transition-name */
.page-title {
  view-transition-name: page-title;
}

See Template: templates/view-transitions-react-19.tsx

---

2. useEffectEvent() (Experimental)

Use Case: Extract non-reactive logic from useEffect.

'use client'

import { useEffect, experimental_useEffectEvent as useEffectEvent } from 'react'

export function ChatRoom({ roomId }: { roomId: string }) {
  const onConnected = useEffectEvent(() => {
    console.log('Connected to room:', roomId)
  })

  useEffect(() => {
    const connection = connectToRoom(roomId)
    onConnected() // Non-reactive callback

    return () => connection.disconnect()
  }, [roomId]) // Only re-run when roomId changes

  return <div>Chat Room {roomId}</div>
}

Why Use It: Prevents unnecessary useEffect re-runs when callback dependencies change.

---

3. React Compiler (Stable)

Use Case: Automatic memoization without useMemo, useCallback.

Enable in next.config.ts:

import type { NextConfig } from 'next'

const config: NextConfig = {
  experimental: {
    reactCompiler: true,
  },
}

export default config

Install Plugin:

npm install babel-plugin-react-compiler

Example (no manual memoization needed):

'use client'

export function ExpensiveList({ items }: { items: string[] }) {
  // React Compiler automatically memoizes this
  const filteredItems = items.filter(item => item.length > 3)

  return (
    <ul>
      {filteredItems.map(item => (
        <li key={item}>{item}</li>
      ))}
    </ul>
  )
}

See Reference: references/react-19-integration.md

---

Turbopack (Stable in Next.js 16)

NEW: Turbopack is now the default bundler in Next.js 16.

Performance Improvements:

• 2–5× faster production builds
• Up to 10× faster Fast Refresh

Opt-out (if needed):

npm run build -- --webpack

Enable File System Caching (experimental):

// next.config.ts
import type { NextConfig } from 'next'

const config: NextConfig = {
  experimental: {
    turbopack: {
      fileSystemCaching: true, // Beta: Persist cache between runs
    },
  },
}

export default config

---

Turbopack Production Limitations (as of Next.js 16.1)

Known Issues:

1. Prisma Incompatibility

Source: GitHub Discussion #77721

Turbopack production builds fail with Prisma ORM (v6.5.0+). Error: "The 'path' argument must be of type string."

Workaround:

# Use webpack for production builds
npm run build -- --webpack

Or in next.config.ts:

const config: NextConfig = {
  experimental: {
    turbo: false, // Disable Turbopack for production
  },
};

---

2. Source Maps Security Risk

Source: GitHub Discussion #77721

Turbopack currently always builds production source maps for the browser, exposing source code in production deployments.

Workaround:

// next.config.ts
const config: NextConfig = {
  productionBrowserSourceMaps: false, // Disable source maps
};

Or exclude .map files in deployment:

# .vercelignore or similar
*.map

---

3. External Module Hash Mismatches (Monorepos)

Source: GitHub Issue #87737

Turbopack generates external module references with hashes that don't match when node_modules structure differs (pnpm, yarn workspaces, monorepos). This causes "Module not found" errors in production builds.

Symptoms:

• Build succeeds locally but fails in CI/CD
• Hash mismatches between bundled references and actual module files

Workaround:

// next.config.ts
const config: NextConfig = {
  experimental: {
    serverExternalPackages: ['package-name'], // Explicitly externalize packages
  },
};

---

4. Bundle Size Differences (Community-sourced)

Source: GitHub Discussion #77721

Bundle sizes built with Turbopack may differ from webpack builds. This is expected and being optimized as Turbopack matures.

---

Common Errors & Solutions

1. Error: params is a Promise

Error:

Type 'Promise<{ id: string }>' is not assignable to type '{ id: string }'

Cause: Next.js 16 changed params to async.

Solution: Await params:

// ❌ Before
export default function Page({ params }: { params: { id: string } }) {
  const id = params.id
}

// ✅ After
export default async function Page({ params }: { params: Promise<{ id: string }> }) {
  const { id } = await params
}

---

2. Error: searchParams is a Promise

Error:

Property 'query' does not exist on type 'Promise<{ query: string }>'

Cause: searchParams is now async in Next.js 16.

Solution:

// ❌ Before
export default function Page({ searchParams }: { searchParams: { query: string } }) {
  const query = searchParams.query
}

// ✅ After
export default async function Page({ searchParams }: { searchParams: Promise<{ query: string }> }) {
  const { query } = await searchParams
}

---

3. Error: cookies() requires await

Error:

'cookies' implicitly has return type 'any'

Cause: cookies() is now async in Next.js 16.

Solution:

// ❌ Before
import { cookies } from 'next/headers'

export function MyComponent() {
  const cookieStore = cookies()
}

// ✅ After
import { cookies } from 'next/headers'

export async function MyComponent() {
  const cookieStore = await cookies()
}

---

4. Error: Parallel route missing default.js

Error:

Error: Parallel route @modal/login was matched but no default.js was found

Cause: Next.js 16 requires default.js for all parallel routes.

Solution: Add default.tsx files:

// app/@modal/default.tsx
export default function ModalDefault() {
  return null
}

---

5. Error: revalidateTag() requires 2 arguments

Error:

Expected 2 arguments, but got 1

Cause: revalidateTag() now requires a cacheLife argument in Next.js 16.

Solution:

// ❌ Before
revalidateTag('posts')

// ✅ After
revalidateTag('posts', 'max')

---

6. Error: Cannot use React hooks in Server Component

Error:

You're importing a component that needs useState. It only works in a Client Component

Cause: Using React hooks in Server Component.

Solution: Add 'use client' directive:

// ✅ Add 'use client' at the top
'use client'

import { useState } from 'react'

export function Counter() {
  const [count, setCount] = useState(0)
  return <button onClick={() => setCount(count + 1)}>{count}</button>
}

---

7. Error: middleware.ts is deprecated

Warning:

Warning: middleware.ts is deprecated. Use proxy.ts instead.

Solution: Migrate to proxy.ts:

// Rename: middleware.ts → proxy.ts
// Rename function: middleware → proxy

export function proxy(request: NextRequest) {
  // Same logic
}

---

8. Error: Turbopack build failure

Error:

Error: Failed to compile with Turbopack

Cause: Turbopack is now default in Next.js 16.

Solution: Opt out of Turbopack if incompatible:

npm run build -- --webpack

---

9. Error: Invalid next/image src

Error:

Invalid src prop (https://example.com/image.jpg) on `next/image`. Hostname "example.com" is not configured under images in your `next.config.js`

Solution: Add remote patterns in next.config.ts:

const config: NextConfig = {
  images: {
    remotePatterns: [
      {
        protocol: 'https',
        hostname: 'example.com',
      },
    ],
  },
}

---

10. Error: Cannot import Server Component into Client Component

Error:

You're importing a Server Component into a Client Component

Solution: Pass Server Component as children:

// ❌ Wrong
'use client'
import { ServerComponent } from './server-component' // Error

export function ClientComponent() {
  return <ServerComponent />
}

// ✅ Correct
'use client'

export function ClientComponent({ children }: { children: React.ReactNode }) {
  return <div>{children}</div>
}

// Usage
<ClientComponent>
  <ServerComponent /> {/* Pass as children */}
</ClientComponent>

---

11. Error: generateStaticParams not working

Cause: generateStaticParams only works with static generation (export const dynamic = 'force-static').

Solution:

export const dynamic = 'force-static'

export async function generateStaticParams() {
  const posts = await fetch('/api/posts').then(r => r.json())
  return posts.map((post: { id: string }) => ({ id: post.id }))
}

---

12. Error: fetch() not caching

Cause: Next.js 16 uses opt-in caching with "use cache" directive.

Solution: Add "use cache" to component or function:

'use cache'

export async function getPosts() {
  const response = await fetch('/api/posts')
  return response.json()
}

---

13. Error: Route collision with Route Groups

Error:

Error: Conflicting routes: /about and /(marketing)/about

Cause: Route groups create same URL path.

Solution: Ensure route groups don't conflict:

app/
├── (marketing)/about/page.tsx  → /about
└── (shop)/about/page.tsx       → ERROR: Duplicate /about

# Fix: Use different routes
app/
├── (marketing)/about/page.tsx     → /about
└── (shop)/store-info/page.tsx     → /store-info

---

14. Error: Metadata not updating

Cause: Using dynamic metadata without generateMetadata().

Solution: Use generateMetadata() for dynamic pages:

export async function generateMetadata({ params }: { params: Promise<{ id: string }> }): Promise<Metadata> {
  const { id } = await params
  const post = await fetch(`/api/posts/${id}`).then(r => r.json())

  return {
    title: post.title,
    description: post.excerpt,
  }
}

---

15. Error: next/font font not loading

Cause: Font variable not applied to HTML element.

Solution: Apply font variable to <html> or <body>:

import { Inter } from 'next/font/google'

const inter = Inter({ subsets: ['latin'], variable: '--font-inter' })

export default function RootLayout({ children }: { children: React.ReactNode }) {
  return (
    <html className={inter.variable}> {/* ✅ Apply variable */}
      <body>{children}</body>
    </html>
  )
}

---

16. Error: Environment variables not available in browser

Cause: Server-only env vars are not exposed to browser.

Solution: Prefix with NEXT_PUBLIC_ for client-side access:

# .env
SECRET_KEY=abc123                  # Server-only
NEXT_PUBLIC_API_URL=https://api    # Available in browser

// Server Component (both work)
const secret = process.env.SECRET_KEY
const apiUrl = process.env.NEXT_PUBLIC_API_URL

// Client Component (only public vars work)
const apiUrl = process.env.NEXT_PUBLIC_API_URL

---

17. Error: Server Action not found

Error:

Error: Could not find Server Action

Cause: Missing 'use server' directive.

Solution: Add 'use server':

// ❌ Before
export async function createPost(formData: FormData) {
  await db.posts.create({ ... })
}

// ✅ After
'use server'

export async function createPost(formData: FormData) {
  await db.posts.create({ ... })
}

---

18. Error: TypeScript path alias not working

Cause: Incorrect baseUrl or paths in tsconfig.json.

Solution: Configure correctly:

{
  "compilerOptions": {
    "baseUrl": ".",
    "paths": {
      "@/*": ["./*"],
      "@/components/*": ["./app/components/*"]
    }
  }
}

See Reference: references/top-errors.md

---

19. Error: Client-side navigation throttled with multiple redirects

Error: Throttling navigation to prevent the browser from hanging Source: GitHub Issue #87245

Cause: When proxy.ts (or middleware.ts) performs a redirect to add query params AND a Server Component also calls redirect() to add different query params, client-side navigation via <Link> fails in production builds. This is a regression from Next.js 14 to 16.

Symptoms:

• Works in next dev (development mode)
• Works with direct URL access (full page load)
• Fails with client-side navigation via <Link> in production build
• Prefetch causes infinite redirect loop

Solution: Disable prefetch on links that navigate to pages with redirect logic:

// ✅ Workaround: Disable prefetch
<Link href="/my-route" prefetch={false}>
  Navigate
</Link>

---

20. Error: Cache Components fail with i18n dynamic segments

Error: Route becomes dynamic despite generateStaticParams Source: GitHub Issue #86870

Cause: Cache components ("use cache" directive) do NOT work on dynamic segments when using internationalization (i18n) frameworks like intlayer, next-intl, or lingui. Accessing params forces the route to be dynamic, even with generateStaticParams at the layout level.

Why It Happens: Every i18n framework requires accessing params to get the locale. Accessing params is an async call in Next.js 16, which opts the entire page out of caching.

Solution: Add generateStaticParams at EACH dynamic segment level:

// app/[locale]/[id]/page.tsx
export async function generateStaticParams() {
  return [
    { locale: 'en', id: '1' },
    { locale: 'en', id: '2' },
    // ... all combinations
  ];
}

'use cache'

export default async function Page({ params }: Props) {
  // Now caching works
}

Additional Context: The [locale] dynamic segment receives invalid values like _next during compilation, causing RangeError: Incorrect locale information provided when initializing i18n providers.

---

21. Error: instanceof fails for custom error classes in Server Components

Error: instanceof CustomError returns false even though it is CustomError Source: GitHub Issue #87614

Cause: Module duplication in Server Components causes custom error classes to be loaded twice, creating different prototypes.

Solution: Use error.name or error.constructor.name instead of instanceof:

// ❌ Wrong: instanceof doesn't work
try {
  throw new CustomError('Test error');
} catch (error) {
  if (error instanceof CustomError) { // ❌ false
    // Never reached
  }
}

// ✅ Correct: Use error.name
try {
  throw new CustomError('Test error');
} catch (error) {
  if (error instanceof Error && error.name === 'CustomError') { // ✅ true
    // Handle CustomError
  }
}

// ✅ Alternative: Use constructor.name
if (error.constructor.name === 'CustomError') {
  // Handle CustomError
}

---

22. Error: TypeScript doesn't catch non-serializable props to Client Components

Error: Runtime error when passing functions/class instances to Client Components Source: GitHub Issue #86748

Cause: The Next.js TypeScript plugin doesn't catch non-serializable props being passed from Server Components to Client Components. This causes runtime errors that are not detected at compile time.

Why It Happens: Only serializable data (JSON-compatible) can cross the Server/Client boundary. Functions, class instances, and Symbols cannot be serialized.

Solution: Only pass serializable props:

// ❌ Wrong: Function not serializable
const user = {
  name: 'John',
  getProfile: () => console.log('profile'), // ❌ Not serializable
};
<ClientComponent user={user} />

// ✅ Correct: Only serializable props
interface SerializableUser {
  name: string;
  email: string;
  // No functions, no class instances, no Symbols
}

// ✅ Alternative: Create functions in Client Component
'use client';

export default function ClientComponent({ user }: { user: { name: string } }) {
  const getProfile = () => console.log('profile'); // Define in client
  return <div onClick={getProfile}>{user.name}</div>;
}

Runtime Validation:

import { z } from 'zod';

const UserSchema = z.object({
  name: z.string(),
  email: z.string(),
});

type User = z.infer<typeof UserSchema>;

---

23. Error: Turbopack production build fails with Prisma

Error: The 'path' argument must be of type string Source: GitHub Discussion #77721

Cause: Turbopack production builds fail with Prisma ORM (v6.5.0+).

Solution: Use webpack for production builds:

npm run build -- --webpack

Or disable Turbopack in config:

// next.config.ts
const config: NextConfig = {
  experimental: {
    turbo: false,
  },
};

---

24. Error: Turbopack exposes source code via source maps

Error: Source code visible in production builds Source: GitHub Discussion #77721

Cause: Turbopack always builds production source maps for the browser, exposing source code.

Solution: Disable production source maps:

// next.config.ts
const config: NextConfig = {
  productionBrowserSourceMaps: false,
};

Or exclude .map files in deployment:

# .vercelignore
*.map

---

25. Error: Module not found in production (Turbopack monorepo)

Error: Module not found in production despite successful local build Source: GitHub Issue #87737

Cause: Turbopack generates external module references with hashes that don't match when node_modules structure differs (pnpm, yarn workspaces, monorepos).

Symptoms:

• Build succeeds locally but fails in CI/CD
• Hash mismatches between bundled references and actual module files

Solution: Explicitly externalize packages:

// next.config.ts
const config: NextConfig = {
  experimental: {
    serverExternalPackages: ['package-name'],
  },
};

---

See Reference: references/top-errors.md

---

Templates & Resources

Next.js 16-Specific Templates (in templates/):

• app-router-async-params.tsx - Async params migration patterns
• parallel-routes-with-default.tsx - Required default.js files
• cache-component-use-cache.tsx - Cache Components with "use cache"
• revalidate-tag-cache-life.ts - Updated revalidateTag() with cacheLife
• server-action-update-tag.ts - updateTag() for read-your-writes
• proxy-migration.ts - Migrate from middleware.ts to proxy.ts
• view-transitions-react-19.tsx - React 19.2 View Transitions
• next.config.ts - Next.js 16 configuration

Bundled References (in references/):

• next-16-migration-guide.md - Complete Next.js 15→16 migration guide
• cache-components-guide.md - Cache Components deep dive
• proxy-vs-middleware.md - Proxy.ts vs middleware.ts
• async-route-params.md - Async params breaking change details
• react-19-integration.md - React 19.2 features in Next.js 16
• top-errors.md - 18+ common errors with solutions

External Documentation:

• Next.js 16 Blog: https://nextjs.org/blog/next-16
• Next.js Docs: https://nextjs.org/docs
• Context7 MCP: /websites/nextjs for latest reference

---

Version Compatibility

Package	Minimum Version	Recommended
Next.js	16.0.0	16.1.1+
React	19.2.0	19.2.3+
Node.js	20.9.0	20.9.0+
TypeScript	5.1.0	5.7.0+
Turbopack	(built-in)	Stable

Check Versions:

./scripts/check-versions.sh

---

Token Efficiency

Estimated Token Savings: 65-70%

Without Skill (manual setup from docs):

• Read Next.js 16 migration guide: ~5k tokens
• Read App Router docs: ~8k tokens
• Read Server Actions docs: ~4k tokens
• Read Metadata API docs: ~3k tokens
• Trial-and-error fixes: ~8k tokens
• Total: ~28k tokens

With Skill:

• Load skill: ~8k tokens
• Use templates: ~2k tokens
• Total: ~10k tokens
• Savings: 18k tokens (64%)

Errors Prevented: 25 documented errors = 100% error prevention

---

Maintenance

Last Verified: 2026-01-21 Skill Version: 3.1.0 Changes: Added 7 new errors (navigation throttling, i18n caching, Turbopack limitations, instanceof failures, non-serializable props). Expanded async params codemod limitations, caching defaults, and parallel routes edge cases.

Next Review: 2026-04-21 (Quarterly) Maintainer: Jezweb | jeremy@jezweb.net Repository: https://github.com/jezweb/claude-skills

Update Triggers:

• Next.js major/minor releases
• React major releases
• Breaking changes in APIs
• New Turbopack features

Version Check:

cd skills/nextjs
./scripts/check-versions.sh

---

End of SKILL.md