name: nextjs description: | Build Next.js 16 apps with App Router, Server Components/Actions, Cache Components ("use cache"), and async route params. Includes proxy.ts (replaces middleware.ts) and React 19.2.

Use when: building Next.js 16 projects, or troubleshooting async params (Promise types), "use cache" directives, parallel route 404s (missing default.js), or proxy.ts CORS. allowed-tools: ["Read", "Write", "Edit", "Bash", "Glob", "Grep"]

Next.js App Router - Production Patterns

Version: Next.js 16.0.0 React Version: 19.2.0 Node.js: 20.9+ Last Verified: 2025-10-24

When to Use This Skill
When NOT to Use This Skill
Next.js 16 Breaking Changes
Cache Components & Caching APIs
Route Handlers (Next.js 16 Updates)
Proxy vs Middleware
Parallel Routes - default.js Required
React 19.2 Features
Turbopack (Stable in Next.js 16)
Common Errors & Solutions
Templates & Resources

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 (18+ documented Next.js 16 errors with solutions)

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)

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.

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

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:

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

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

4. Removed Features (BREAKING)

The following features are REMOVED in Next.js 16:

AMP Support - Entirely removed. Migrate to standard pages.
next lint command - Use ESLint or Biome directly.
serverRuntimeConfig and publicRuntimeConfig - Use environment variables instead.
experimental.ppr flag - Evolved into Cache Components. Use "use cache" directive.
Automatic scroll-behavior: smooth - Add manually if needed.
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).

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

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.

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

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

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

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

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.

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

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

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	16.0.0+
React	19.2.0	19.2.0+
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: 18+ common mistakes = 100% error prevention

Maintenance

Last Verified: 2025-10-24 Next Review: 2026-01-24 (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

nextjs

Use when: building Next.js 16 projects, or troubleshooting async params (Promise types), "use cache" directives, parallel route 404s (missing default.js), or proxy.ts CORS. allowed-tools: ["Read", "Write", "Edit", "Bash", "Glob", "Grep"]

Next.js App Router - Production Patterns

Table of Contents

When to Use This Skill

When NOT to Use This Skill

Next.js 16 Breaking Changes

1. Async Route Parameters (BREAKING)

2. Middleware → Proxy Migration (BREAKING)

3. Parallel Routes Require default.js (BREAKING)

4. Removed Features (BREAKING)

5. Version Requirements (BREAKING)

6. Image Defaults Changed (BREAKING)

Cache Components & Caching APIs

1. Overview

2. "use cache" Directive

3. Partial Prerendering (PPR)

4. revalidateTag() - Updated API

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

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

Route Handlers (Next.js 16 Updates)

Async Params in Route Handlers (BREAKING)

Proxy vs Middleware

Why the Change?

Migration

Parallel Routes - default.js Required (BREAKING)

React 19.2 Features

1. View Transitions

2. useEffectEvent() (Experimental)

3. React Compiler (Stable)

Turbopack (Stable in Next.js 16)

Common Errors & Solutions

1. Error: params is a Promise

2. Error: searchParams is a Promise

3. Error: cookies() requires await

4. Error: Parallel route missing default.js

5. Error: revalidateTag() requires 2 arguments

6. Error: Cannot use React hooks in Server Component

7. Error: middleware.ts is deprecated

8. Error: Turbopack build failure

9. Error: Invalid next/image src

10. Error: Cannot import Server Component into Client Component

11. Error: generateStaticParams not working

12. Error: fetch() not caching

13. Error: Route collision with Route Groups

14. Error: Metadata not updating

15. Error: next/font font not loading

16. Error: Environment variables not available in browser

17. Error: Server Action not found

18. Error: TypeScript path alias not working

Templates & Resources

Version Compatibility

Token Efficiency

Maintenance

Related Skills

3. Parallel Routes Require `default.js` (BREAKING)

2. `"use cache"` Directive

4. `revalidateTag()` - Updated API

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

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

2. `useEffectEvent()` (Experimental)

1. Error: `params` is a Promise

2. Error: `searchParams` is a Promise

3. Error: `cookies()` requires await

4. Error: Parallel route missing `default.js`

5. Error: `revalidateTag()` requires 2 arguments

7. Error: `middleware.ts` is deprecated

9. Error: Invalid `next/image` src

11. Error: `generateStaticParams` not working

12. Error: `fetch()` not caching

15. Error: `next/font` font not loading