Next.js App Router - Production Patterns

Version: Next.js 16.0.3 React Version: 19.2.0 Node.js: 20.9+ Last Verified: 2025-11-21

Table of Contents

1. When to Use This Skill
2. When NOT to Use This Skill
3. Next.js 16 Breaking Changes
4. Cache Components & Caching APIs
5. Server Components
6. Server Actions
7. Route Handlers
8. React 19.2 Features
9. Metadata API
10. Image & Font Optimization
11. Top 5 Critical Errors
12. Performance Patterns
13. TypeScript Configuration

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When to Use This Skill

Use this skill when you need:

• Next.js 16 App Router patterns (layouts, loading, error boundaries, routing)
• Server Components best practices (data fetching, composition, streaming)
• Server Actions patterns (forms, mutations, revalidation, error handling)
• 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 (async params, proxy.ts, parallel routes)
• Route Handlers (API endpoints, webhooks, streaming responses)
• Proxy patterns (proxy.ts replaces middleware.ts in Next.js 16)
• Async route params (params, searchParams, cookies(), headers() now async)
• Parallel routes with default.js (breaking change in Next.js 16)
• React 19.2 features (View Transitions, useEffectEvent(), React Compiler)
• Metadata API (SEO, Open Graph, Twitter Cards, sitemaps)
• Image optimization (next/image with updated defaults in Next.js 16)
• Performance optimization (lazy loading, code splitting, PPR, ISR)

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, auth-js, 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

---

Next.js 16 Breaking Changes

CRITICAL: Next.js 16 has multiple breaking changes. For detailed migration steps, see references/next-16-migration-guide.md.

Breaking Change	Before	After
Async params	params.slug	const { slug } = await params
Async headers	cookies() sync	await cookies()
Middleware	middleware.ts	proxy.ts (renamed)
Parallel routes	default.js optional	default.js required
Caching	Auto-cached fetch	Opt-in with "use cache"
revalidateTag()	1 argument	2 arguments (tag + cacheLife)
Node.js	18.x+	20.9+ required
React	18.x	19.2+ required

Quick Fix for Async Params:

// ✅ Next.js 16 pattern
export default async function Page({ params }: { params: Promise<{ id: string }> }) {
  const { id } = await params
  return <div>{id}</div>
}

Codemod: bunx @next/codemod@canary upgrade latest

See: references/next-16-migration-guide.md for complete migration guide with examples.

---

Cache Components & Caching APIs

"use cache" Directive (NEW in Next.js 16)

'use cache'

export async function BlogPosts() {
  const posts = await db.posts.findMany()
  return posts.map(post => <article key={post.id}>{post.title}</article>)
}

Caching APIs Summary

API	Purpose	Example
"use cache"	Opt-in component/function caching	'use cache' at top
revalidateTag()	Invalidate by tag	revalidateTag('posts', 'max')
updateTag()	Update cache without revalidation	updateTag('posts', newData)
refresh()	Refresh current page	refresh()
revalidatePath()	Invalidate by path	revalidatePath('/posts')

PPR (Partial Prerendering)

// next.config.ts
const config = { experimental: { ppr: true } }

// page.tsx
export const experimental_ppr = true

export default function Page() {
  return (
    <>
      <StaticHeader />
      <Suspense fallback={<Skeleton />}>
        <DynamicContent />
      </Suspense>
    </>
  )
}

See: references/caching-apis.md for complete caching API reference with ISR, tag-based revalidation, and advanced patterns.

---

Server Components

Server Components are the default in App Router. They run on the server and can fetch data, access databases, and keep logic server-side.

// app/posts/page.tsx (Server Component by default)
export default async function PostsPage() {
  const posts = await db.posts.findMany()
  return <div>{posts.map(p => <article key={p.id}>{p.title}</article>)}</div>
}

Streaming with Suspense

import { Suspense } from 'react'

export default function Page() {
  return (
    <div>
      <Header />
      <Suspense fallback={<Skeleton />}>
        <Posts />
      </Suspense>
    </div>
  )
}

Server vs Client Components

Server Components	Client Components
Data fetching, DB access	Interactivity (onClick)
Sensitive logic	React hooks (useState)
Large dependencies	Browser APIs
Static content	Real-time updates

Client Component (requires 'use client'):

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

---

Server Actions

Server Actions are async functions that run on the server, callable from Client or Server Components.

Basic Server Action

// app/actions.ts
'use server'

import { revalidatePath } from 'next/cache'

export async function createPost(formData: FormData) {
  const title = formData.get('title') as string
  await db.posts.create({ data: { title } })
  revalidatePath('/posts')
}

Form Handling

// Server Component Form (simplest)
import { createPost } from './actions'

export default function NewPostPage() {
  return (
    <form action={createPost}>
      <input name="title" required />
      <button type="submit">Create</button>
    </form>
  )
}

Available Patterns

Pattern	Use Case	Reference
Client Form with Loading	useFormState + useFormStatus	templates/server-action-form.tsx
Error Handling	Return { error } or { success }	references/server-actions-patterns.md
Optimistic Updates	useOptimistic hook	references/server-actions-patterns.md
File Upload	FormData + blob storage	references/server-actions-patterns.md
Redirect After Action	redirect() function	references/server-actions-patterns.md

See: references/server-actions-patterns.md for error handling, optimistic updates, file uploads, and advanced patterns.

---

Route Handlers

Route Handlers are the App Router equivalent of API Routes.

// app/api/posts/route.ts
export async function GET(request: Request) {
  const posts = await db.posts.findMany()
  return Response.json({ posts })
}

export async function POST(request: Request) {
  const body = await request.json()
  const post = await db.posts.create({ data: body })
  return Response.json({ post }, { status: 201 })
}

Dynamic Routes (with async params):

// app/api/posts/[id]/route.ts
export async function GET(
  request: Request,
  { params }: { params: Promise<{ id: string }> }
) {
  const { id } = await params // Await in Next.js 16
  const post = await db.posts.findUnique({ where: { id } })
  return post ? Response.json({ post }) : Response.json({ error: 'Not found' }, { status: 404 })
}

See: templates/route-handler-api.ts for search params, webhooks, and streaming patterns.

---

React 19.2 Features

Feature	Usage
React Compiler	experimental: { reactCompiler: true } - Auto-memoization
View Transitions	useTransition() + router.push()
useEffectEvent	Stable event handlers without deps

---

Metadata API

// Static metadata
export const metadata: Metadata = {
  title: 'My Blog',
  description: 'A blog about Next.js',
  openGraph: { title: 'My Blog', images: ['/og-image.jpg'] },
}

// Dynamic metadata (await params in Next.js 16)
export async function generateMetadata({ params }: { params: Promise<{ id: string }> }) {
  const { id } = await params
  const post = await db.posts.findUnique({ where: { id } })
  return { title: post.title, description: post.excerpt }
}

---

Image & Font Optimization

// next/image
import Image from 'next/image'
<Image src="/profile.jpg" alt="Profile" width={500} height={500} priority />

// next/font
import { Inter } from 'next/font/google'
const inter = Inter({ subsets: ['latin'], variable: '--font-inter' })
<html className={inter.variable}>

Remote images: Configure images.remotePatterns in next.config.ts.

---

Top 5 Critical Errors

Error 1: params is a Promise

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

Solution: Await params in Next.js 16:

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

---

Error 2: middleware.ts is deprecated

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

Solution: Rename file and function:

// Rename: middleware.ts → proxy.ts
// Rename function: middleware → proxy
export function proxy(request: NextRequest) {
  // Same logic
}

---

Error 3: Parallel route missing default.js

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

Solution: Add default.tsx:

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

---

Error 4: Cannot use React hooks in Server Component

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

Solution: Add 'use client':

'use client'

import { useState } from 'react'

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

---

Error 5: fetch() not caching

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

Solution: Add "use cache" directive:

'use cache'

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

---

See All 18 Errors: references/error-catalog.md

---

Performance Patterns

Pattern	Usage
Lazy Loading	const HeavyComp = dynamic(() => import('./Heavy'), { ssr: false })
Code Splitting	Automatic per route - each page.tsx gets own bundle
Turbopack	Default in Next.js 16, opt out with --webpack flag
PPR	experimental: { ppr: true } + <Suspense> boundaries

---

TypeScript Configuration

{
  "compilerOptions": {
    "strict": true,
    "baseUrl": ".",
    "paths": { "@/*": ["./*"] }
  }
}

---

When to Load References

Reference	Load When...
next-16-migration-guide.md	Migrating from Next.js 15, async params errors, proxy.ts setup
server-actions-patterns.md	Error handling, optimistic updates, file uploads, advanced forms
caching-apis.md	ISR, tag-based revalidation, updateTag(), refresh(), PPR details
error-catalog.md	Debugging any Next.js error, comprehensive error solutions
top-errors.md	Quick fixes for the 5 most common Next.js errors

---

Bundled Resources

Type	Files
References	error-catalog.md, top-errors.md, next-16-migration-guide.md, server-actions-patterns.md, caching-apis.md
Templates	async-params-page.tsx, server-action-form.tsx, route-handler-api.ts, cache-component-use-cache.tsx, parallel-routes-with-default.tsx, proxy-migration.ts

---

Related Skills

Skill	Purpose
cloudflare-nextjs	Deploy to Cloudflare Workers
tailwind-v4-shadcn	Styling
clerk-auth	Authentication
drizzle-orm-d1	Database
react-hook-form-zod	Forms
zustand-state-management	Client state

Official Docs: https://nextjs.org/docs | App Router: https://nextjs.org/docs/app

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Version: Next.js 16.0.0 | React 19.2.0 | Node.js 20.9+ | TypeScript 5.3+ Production Tested: E-commerce, SaaS, content sites | Token Savings: 65-70%