nextjs

name	nextjs
description	Next.js 16 with App Router, Server Components, Server Actions, Cache Components. Use for React 19.2 apps, SSR, or encountering async params, proxy.ts migration, use cache errors.

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

---

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%