Next.js (v16+) — App Router

You are an expert in Next.js 16 with the App Router. Always prefer the App Router over the legacy Pages Router unless the user's project explicitly uses Pages Router.

Critical Pattern: Lazy Initialization for Build-Safe Modules

Never initialize database clients (Neon, Drizzle), Redis (Upstash), or service SDKs (Resend, Slack) at module scope.

During next build, static generation can evaluate modules before runtime env vars are present, which causes startup crashes. Always initialize these clients lazily inside getter functions.

import { drizzle } from 'drizzle-orm/neon-http'
import { neon } from '@neondatabase/serverless'

let _db: ReturnType<typeof drizzle> | null = null

export function getDb() {
  if (!_db) _db = drizzle(neon(process.env.DATABASE_URL!))
  return _db
}

Apply the same lazy singleton pattern to Redis and SDK clients (getRedis(), getResend(), getSlackClient()) instead of creating them at import time.

Scaffolding

When running create-next-app, always pass --yes to skip interactive prompts (React Compiler, import alias) that hang in non-interactive shells:

npx create-next-app@latest my-app --yes --typescript --tailwind --eslint --app --src-dir --import-alias "@/*" --turbopack --use-npm

If the target directory contains ANY non-Next.js files (.claude/, CLAUDE.md, .git/, config files, etc.), you MUST add --force. Without it, create-next-app will fail with "The directory contains files that could conflict" and block scaffolding. Check the directory first — if it has anything in it, use --force:

npx create-next-app@latest . --yes --force --typescript --tailwind --eslint --app --src-dir --import-alias "@/*" --turbopack --use-npm

Geist Font Fix (Tailwind v4 + shadcn)

shadcn init rewrites globals.css with --font-sans: var(--font-sans) in @theme inline — a circular reference that falls back to Times/serif. Even var(--font-geist-sans) doesn't work because Tailwind v4's @theme inline resolves at CSS parse time, not runtime.

Two fixes required after create-next-app + shadcn init:

Use literal font names in globals.css (not CSS variable references):

@theme inline {
  /* CORRECT — literal names that @theme can resolve at parse time */
  --font-sans: "Geist", "Geist Fallback", ui-sans-serif, system-ui, sans-serif;
  --font-mono: "Geist Mono", "Geist Mono Fallback", ui-monospace, monospace;
}

Move font variable classNames from <body> to <html> in layout.tsx:

// app/layout.tsx — CORRECT
<html lang="en" className={`${geistSans.variable} ${geistMono.variable}`}>
  <body className="antialiased">

// app/layout.tsx — WRONG (default scaffold output)
<html lang="en">
  <body className={`${geistSans.variable} ${geistMono.variable} antialiased`}>

Always apply both fixes after running create-next-app + shadcn init with Tailwind v4.

UI Defaults for App Router Pages

When building pages, layouts, and route-level UI in this stack, default to shadcn/ui + Geist instead of raw Tailwind scaffolding.

Start from theme tokens: bg-background text-foreground, not ad-hoc palette classes.
Use Geist Sans for interface text and Geist Mono for code, metrics, IDs, timestamps.
Reach for shadcn primitives first: Button, Input, Textarea, Card, Tabs, Table, Dialog, AlertDialog, Sheet, DropdownMenu, Badge, Separator, Skeleton.
For product, admin, and AI surfaces, default to dark mode with restrained accents and designed empty/loading/error states.
Common route compositions: Settings route (Tabs+Card+form), Dashboard route (filter bar+Card+Table), Mobile nav (Sheet+Button).

Key Architecture

Next.js 16 uses React 19.2 features and the App Router (file-system routing under app/). Ensure React 19.2.4+ for security patches (see CVE section below).

File Conventions

layout.tsx — Persistent wrapper, preserves state across navigations
page.tsx — Unique UI for a route, makes route publicly accessible
loading.tsx — Suspense fallback shown while segment loads
error.tsx — Error boundary for a segment
not-found.tsx — 404 UI for a segment
route.ts — API endpoint (Route Handler)
template.tsx — Like layout but re-mounts on navigation
default.tsx — Fallback for parallel routes

Routing

Dynamic segments: [id], catch-all: [...slug], optional catch-all: [[...slug]]
Route groups: (group) — organize without affecting URL
Parallel routes: @slot — render multiple pages in same layout
Intercepting routes: (.), (..), (...), (..)(..) — modal patterns

Server Components (Default)

All components in the App Router are Server Components by default. They:

Run on the server only, ship zero JavaScript to the client
Can directly await data (fetch, DB queries, file system)
Cannot use useState, useEffect, or browser APIs
Cannot use event handlers (onClick, onChange)

// app/users/page.tsx — Server Component (default)
export default async function UsersPage() {
  const users = await db.query('SELECT * FROM users')
  return <UserList users={users} />
}

Client Components

Add 'use client' at the top of the file when you need interactivity or browser APIs.

'use client'
import { useState } from 'react'

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

Rule: Push 'use client' as far down the component tree as possible. Keep data fetching in Server Components and pass data down as props.

Server Actions / Server Functions

Async functions marked with 'use server' that run on the server. Use for mutations.

// app/actions.ts
'use server'

export async function createUser(formData: FormData) {
  const name = formData.get('name') as string
  await db.insert('users', { name })
  revalidatePath('/users')
}

Use Server Actions for:

Form submissions and data mutations
In-app mutations with revalidatePath / revalidateTag

Use Route Handlers (route.ts) for:

Public APIs consumed by external clients
Webhooks
Large file uploads
Streaming responses

Cache Components (Next.js 16)

The 'use cache' directive enables component and function-level caching.

'use cache'

export async function CachedUserList() {
  cacheLife('hours') // Configure cache duration
  cacheTag('users')  // Tag for on-demand invalidation
  const users = await db.query('SELECT * FROM users')
  return <UserList users={users} />
}

Cache Scope Variants

'use cache' supports scope modifiers that control where cached data is stored:

// Default — cached in the deployment's local data cache
'use cache'

// Remote cache — shared across all deployments and regions (Vercel only)
'use cache: remote'

// Private cache — per-request cache, never shared between users
'use cache: private'

Variant	Shared across deployments?	Shared across users?	Use case
`'use cache'`	No (per-deployment)	Yes	Default, most use cases
`'use cache: remote'`	Yes	Yes	Expensive computations shared globally
`'use cache: private'`	No	No	User-specific cached data (e.g., profile)

Cache Handler Configuration

Next.js 16 uses cacheHandlers (plural) to configure separate handlers for different cache types:

// next.config.ts
const nextConfig = {
  cacheHandlers: {
    default: require.resolve('./cache-handler-default.mjs'),
    remote: require.resolve('./cache-handler-remote.mjs'),
    fetch: require.resolve('./cache-handler-fetch.mjs'),
  },
}

Important: cacheHandlers (plural, Next.js 16+) replaces cacheHandler (singular, Next.js 15). The singular form configured one handler for all cache types. The plural form allows per-type handlers (default, remote, fetch). Using the old singular cacheHandler in Next.js 16 triggers a deprecation warning.

Cache Invalidation

Invalidate with updateTag('users') from a Server Action (immediate expiration, Server Actions only) or revalidateTag('users', 'max') for stale-while-revalidate from Server Actions or Route Handlers.

Important: The single-argument revalidateTag(tag) is deprecated in Next.js 16. Always pass a cacheLife profile as the second argument (e.g., 'max', 'hours', 'days').

Function	Context	Behavior
`updateTag(tag)`	Server Actions only	Immediate expiration, read-your-own-writes
`revalidateTag(tag, 'max')`	Server Actions + Route Handlers	Stale-while-revalidate (recommended)
`revalidateTag(tag, { expire: 0 })`	Route Handlers (webhooks)	Immediate expiration from external triggers

Proxy (formerly Middleware)

In Next.js 16, middleware.ts is renamed to proxy.ts. It runs exclusively on the Node.js runtime — the Edge runtime is not supported in proxy and cannot be configured.

File location: Place proxy.ts at the same level as your app/ directory:

Standard project: proxy.ts at project root
With --src-dir (or srcDir: true): src/proxy.ts (inside src/, alongside app/)

If you place proxy.ts in the wrong location, Next.js will silently ignore it and no request interception will occur.

Constraints:

Proxy can only rewrite, redirect, or modify headers — it cannot return full response bodies. Use Route Handlers for that.
Config flags are renamed: skipMiddlewareUrlNormalize → skipProxyUrlNormalize.
Keep it light: use for high-level traffic control (e.g., redirecting users without a session cookie). Detailed auth should live in Server Components or Server Actions.
OpenNext note: OpenNext doesn't support proxy.ts yet — keep using middleware.ts if self-hosting with OpenNext.

// proxy.ts (or src/proxy.ts if using src directory)
import type { NextRequest } from 'next/server'

export function proxy(request: NextRequest) {
  // Rewrite, redirect, set headers, etc.
}

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

Upgrading

Use the built-in upgrade command (available since 16.1.0):

pnpm next upgrade        # or npm/yarn/bun equivalent

For versions before 16.1.0: npx @next/codemod@canary upgrade latest

If your AI coding assistant supports MCP, the Next.js DevTools MCP can automate upgrade and migration tasks.

What's New in Next.js 16.1

Next.js 16.1 (December 2025, latest stable patch: 16.1.6) builds on 16.0 with developer experience improvements:

Turbopack File System Caching (Stable) — Compiler artifacts are now cached on disk between next dev restarts, delivering up to 14× faster startup on large projects. Enabled by default, no config needed. File system caching for next build is planned next.
Bundle Analyzer (Experimental) — New built-in bundle analyzer works with Turbopack. Offers route-specific filtering, import tracing, and RSC boundary analysis to identify bloated dependencies in both server and client bundles. Enable with experimental.bundleAnalyzer: true in next.config.
next dev --inspect — Debug your dev server without global NODE_OPTIONS=--inspect. Applies the inspector only to the relevant process.
next upgrade command — New CLI command to simplify version upgrades: npx @next/codemod@canary upgrade latest.
Transitive External Dependencies — Turbopack correctly resolves and externalizes transitive deps in serverExternalPackages without extra config.
20 MB smaller installs — Streamlined Turbopack caching layer reduces node_modules footprint.

React 19.2 Features

Next.js 16+ uses React 19.2. These features are available in App Router applications:

`useEffectEvent` Hook

Creates a stable function that always accesses the latest props and state without triggering effect re-runs. Use when your effect needs to read a value but shouldn't re-run when that value changes:

'use client'
import { useEffect, useEffectEvent } from 'react'

function ChatRoom({ roomId, theme }: { roomId: string; theme: string }) {
  const onConnected = useEffectEvent(() => {
    showNotification(`Connected to ${roomId}`, theme) // reads latest theme
  })

  useEffect(() => {
    const connection = createConnection(roomId)
    connection.on('connected', onConnected)
    connection.connect()
    return () => connection.disconnect()
  }, [roomId]) // theme is NOT a dependency — onConnected reads it via useEffectEvent
}

Common use cases: logging with current state, notifications using current theme, callbacks that need fresh values but aren't the trigger for the effect.

`<Activity>` Component

Preserves component state when hiding and showing UI, without unmounting. Solves the classic tradeoff between unmounting (loses state) and CSS display:none (effects keep running):

'use client'
import { Activity, useState } from 'react'

function TabContainer() {
  const [activeTab, setActiveTab] = useState('inbox')

  return (
    <div>
      <nav>
        <button onClick={() => setActiveTab('inbox')}>Inbox</button>
        <button onClick={() => setActiveTab('drafts')}>Drafts</button>
      </nav>
      <Activity mode={activeTab === 'inbox' ? 'visible' : 'hidden'}>
        <InboxPanel />
      </Activity>
      <Activity mode={activeTab === 'drafts' ? 'visible' : 'hidden'}>
        <DraftsPanel />
      </Activity>
    </div>
  )
}

Use for: tabbed interfaces, modals, sidebars, background tasks — anywhere you need to maintain component state without keeping everything actively rendered. When mode="hidden", effects are suspended (not running in the background).

View Transitions API

React 19.2 supports the browser View Transitions API for animating elements across navigations. Next.js 16 has built-in support — elements can animate between route changes without manual transition logic.

Key change: useId now generates IDs with _r_ prefix (instead of :r:) to be valid for view-transition-name and XML 1.0 names.

Layout Deduplication During Prefetching

Next.js 16 deduplicates shared layouts during prefetching. When multiple <Link> components point to routes with a shared layout, the layout is downloaded once instead of separately for each link.

Impact: A page with 50 product links that share a layout downloads ~198KB instead of ~2.4MB — a 92% reduction in prefetch network transfer.

Combined with incremental prefetching, Next.js only fetches route segments not already in cache, cancels prefetch requests when links leave the viewport, re-prefetches on hover or viewport re-entry, and re-prefetches when data is invalidated.

Bundle Analyzer (`next experimental-analyze`)

Built-in bundle analyzer that works with Turbopack (available since Next.js 16.1):

# Analyze and serve results in browser
next experimental-analyze --serve

# Analyze with custom port
next experimental-analyze --serve --port 4001

# Write analysis to .next/diagnostics/analyze (no server)
next experimental-analyze

Features:

Route-specific filtering between client and server bundles
Full import chain tracing — see exactly why a module is included
Traces imports across RSC boundaries and dynamic imports
No application build required — analyzes module graph directly

Save output for comparison: cp -r .next/diagnostics/analyze ./analyze-before-refactor

Legacy: For projects not using Turbopack, use @next/bundle-analyzer with ANALYZE=true npm run build.

Next.js 16.2 (Canary)

Next.js 16.2 is currently in canary (latest: 16.2.0-canary.84, March 2026). Key areas in development:

Turbopack File System Caching for next build — Extending the stable next dev FS cache to production builds for faster CI.
Proxy refinements — Continued iteration on proxy.ts (the Node.js-runtime replacement for middleware.ts introduced in 16.0). The proxy API is stabilizing with improved request context and streaming support.
React Compiler optimizations — Further automatic memoization improvements building on the stable React Compiler in 16.0.

Note: Canary releases are not recommended for production. Track progress at the Next.js Changelog or GitHub Releases.

DevTools MCP

Next.js 16 includes Next.js DevTools MCP, a Model Context Protocol integration for AI-assisted debugging. It enables AI agents to diagnose issues, explain behavior, and suggest fixes within your development workflow. If your AI coding assistant supports MCP, DevTools MCP can also automate upgrade and migration tasks.

Breaking Changes in Next.js 16

Async Request APIs: cookies(), headers(), params, searchParams are all async — must await them
Proxy replaces Middleware: Rename middleware.ts → proxy.ts, runs on Node.js only (Edge not supported)
Turbopack is top-level config: Move from experimental.turbopack to turbopack in next.config
View Transitions: Built-in support for animating elements across navigations
Node.js 20.9+ required: Dropped support for Node 18
TypeScript 5.1+ required

React 19 Gotchas

`useRef()` Requires an Initial Value

React 19 strict mode enforces that useRef() must be called with an explicit initial value. Omitting it causes a type error or runtime warning:

// WRONG — React 19 strict mode complains
const ref = useRef()       // ❌ missing initial value
const ref = useRef<HTMLDivElement>()  // ❌ still missing

// CORRECT
const ref = useRef<HTMLDivElement>(null)  // ✅
const ref = useRef(0)                     // ✅

This affects all useRef calls in client components. The fix is always to pass an explicit initial value (usually null for DOM refs).

Security: Critical CVEs

Multiple vulnerabilities affect all Next.js App Router applications (13.4+, 14.x, 15.x, 16.x). Upgrade immediately.

CVE-2025-66478 / CVE-2025-55182 — Remote Code Execution (CVSS 10.0, Critical)

A deserialization vulnerability in the React Server Components (RSC) "Flight" protocol allows unauthenticated remote code execution via crafted HTTP requests. Near-100% exploit reliability against default configurations. Actively exploited in the wild. No workaround — upgrade is required. Rotate all application secrets after patching.

Affects: Next.js App Router applications (15.x, 16.x, 14.3.0-canary.77+)
Does NOT affect: Pages Router apps, Edge Runtime, Next.js 13.x, stable Next.js 14.x

CVE-2025-55184 — Denial of Service (CVSS 7.5, High)

Specially crafted HTTP requests cause the server process to hang indefinitely, consuming CPU and blocking legitimate users. No authentication required, low attack complexity.

CVE-2025-55183 — Source Code Exposure (CVSS 5.3, Medium)

Malformed HTTP requests can trick Server Actions into returning their compiled source code instead of the expected response. Environment variables are not exposed, but any hardcoded secrets in Server Action code can leak.

CVE-2026-23864 — Denial of Service via Memory Exhaustion (CVSS 7.5, High)

Disclosed January 26, 2026. The original DoS fix for CVE-2025-55184 was incomplete — additional vectors allow specially crafted HTTP requests to Server Function endpoints to crash the server via out-of-memory exceptions or excessive CPU usage. No authentication required.

CVE-2025-29927 — Middleware Authorization Bypass (CVSS 9.1, Critical)

An attacker can bypass middleware-based authorization by sending a crafted x-middleware-subrequest header, skipping all middleware logic. This affects any Next.js application that relies on middleware.ts (or proxy.ts in 16+) as the sole authorization gate. Patched in Next.js 14.2.25, 15.2.3, and all 16.x releases.

Mitigation: Never rely on middleware/proxy as the only auth layer. Always re-validate authorization in Server Components, Server Actions, or Route Handlers. If you cannot patch immediately, block x-middleware-subrequest at your reverse proxy or WAF.

Patched Versions

Release Line	Minimum Safe Version (all CVEs)
14.x	`next@14.2.35`
15.0.x	`next@15.0.5`
15.1.x	`next@15.1.9`
15.2.x	`next@15.2.6`
15.3.x	`next@15.3.6`
15.4.x	`next@15.4.8`
15.5.x	`next@15.5.7`
16.0.x	`next@16.0.11`
16.1.x	`next@16.1.5`

Upgrade React to at least 19.0.1 / 19.1.2 / 19.2.1 for the RCE fix (CVE-2025-55182), and 19.2.4+ to fully address all DoS vectors (CVE-2025-55184, CVE-2025-67779, CVE-2026-23864).

# Upgrade to latest patched versions
npm install next@latest react@latest react-dom@latest

Vercel deployed WAF rules automatically for hosted projects, but WAF is defense-in-depth, not a substitute for patching.

Rendering Strategy Decision

Strategy	When to Use
SSG (`generateStaticParams`)	Content rarely changes, maximum performance
ISR (`revalidate: N`)	Content changes periodically, acceptable staleness
SSR (Server Components)	Per-request fresh data, personalized content
Cache Components (`'use cache'`)	Mix static shell with dynamic parts
Client Components	Interactive UI, browser APIs needed
Streaming (Suspense)	Show content progressively as data loads

Rendering Strategy Guidance

Choosing a rendering strategy?
├─ Content changes less than once per day?
│  ├─ Same for all users? → SSG (`generateStaticParams`)
│  └─ Personalized? → SSG shell + client fetch for personalized parts
│
├─ Content changes frequently but can be slightly stale?
│  ├─ Revalidate on schedule? → ISR with `revalidate: N` seconds
│  └─ Revalidate on demand? → `revalidateTag()` or `revalidatePath()`
│
├─ Content must be fresh on every request?
│  ├─ Cacheable per-request? → Cache Components (`'use cache'` + `cacheLife`)
│  ├─ Personalized per-user? → SSR with Streaming (Suspense boundaries)
│  └─ Real-time? → Client-side with SWR/React Query + SSR for initial load
│
└─ Mostly static with one dynamic section?
   └─ Partial Prerendering: static shell + Suspense for dynamic island

Caching Strategy Matrix

Data Type	Strategy	Implementation
Static assets (JS, CSS, images)	Immutable cache	Automatic with Vercel (hashed filenames)
API responses (shared)	Cache Components	`'use cache'` + `cacheLife('hours')`
API responses (per-user)	No cache or short TTL	`cacheLife({ revalidate: 60 })` with user-scoped key
Configuration data	Edge Config	`@vercel/edge-config` (< 5ms reads)
Database queries	ISR + on-demand	`revalidateTag('products')` on write
Full pages	SSG / ISR	`generateStaticParams` + `revalidate`
Search results	Client-side + SWR	`useSWR` with stale-while-revalidate

Image Optimization Pattern

// BEFORE: Unoptimized, causes LCP & CLS issues
<img src="/hero.jpg" />

// AFTER: Optimized with next/image
import Image from 'next/image';
<Image src="/hero.jpg" width={1200} height={600} priority alt="Hero" />

Font Loading Pattern

// BEFORE: External font causes CLS
<link href="https://fonts.googleapis.com/css2?family=Inter" rel="stylesheet" />

// AFTER: Zero-CLS with next/font
import { Inter } from 'next/font/google';
const inter = Inter({ subsets: ['latin'] });

Cache Components Pattern

// BEFORE: Re-fetches on every request
async function ProductList() {
  const products = await db.query('SELECT * FROM products');
  return <ul>{products.map(p => <li key={p.id}>{p.name}</li>)}</ul>;
}

// AFTER: Cached with automatic revalidation
'use cache';
import { cacheLife } from 'next/cache';

async function ProductList() {
  cacheLife('hours');
  const products = await db.query('SELECT * FROM products');
  return <ul>{products.map(p => <li key={p.id}>{p.name}</li>)}</ul>;
}

Optimistic UI Pattern

// Instant feedback while Server Action processes
'use client';
import { useOptimistic } from 'react';

function LikeButton({ count, onLike }) {
  const [optimisticCount, addOptimistic] = useOptimistic(count);
  return (
    <button onClick={() => { addOptimistic(count + 1); onLike(); }}>
      {optimisticCount} likes
    </button>
  );
}

OG Image Generation

Next.js supports file-based OG image generation via opengraph-image.tsx and twitter-image.tsx special files. These use @vercel/og (built on Satori) to render JSX to images at the Edge runtime.

File Convention

Place an opengraph-image.tsx (or twitter-image.tsx) in any route segment to auto-generate social images for that route:

// app/blog/[slug]/opengraph-image.tsx
import { ImageResponse } from 'next/og'

export const runtime = 'edge'
export const alt = 'Blog post'
export const size = { width: 1200, height: 630 }
export const contentType = 'image/png'

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

  return new ImageResponse(
    (
      <div
        style={{
          fontSize: 48,
          background: 'linear-gradient(to bottom, #000, #111)',
          color: 'white',
          width: '100%',
          height: '100%',
          display: 'flex',
          alignItems: 'center',
          justifyContent: 'center',
          padding: 48,
        }}
      >
        {post.title}
      </div>
    ),
    { ...size }
  )
}

Key Points

ImageResponse — Import from next/og (re-exports @vercel/og). Renders JSX to PNG/SVG images.
Edge runtime — OG image routes run on the Edge runtime by default. Export runtime = 'edge' explicitly for clarity.
Exports — alt, size, and contentType configure the generated <meta> tags automatically.
Static or dynamic — Without params, the image is generated at build time. With dynamic segments, it generates per-request.
Supported CSS — Satori supports a Flexbox subset. Use inline style objects (no Tailwind). display: 'flex' is required on containers.
Fonts — Load custom fonts via fetch and pass to ImageResponse options: { fonts: [{ name, data, style, weight }] }.
Twitter fallback — If no twitter-image.tsx exists, opengraph-image.tsx is used for Twitter cards too.

When to Use

Approach	When
`opengraph-image.tsx` file	Dynamic per-route OG images with data fetching
Static `opengraph-image.png` file	Same image for every page in a segment
`generateMetadata` with `openGraph.images`	Point to an external image URL

Deployment on Vercel

Zero-config: Vercel auto-detects Next.js and optimizes
vercel dev for local development with Vercel features
Server Components → Serverless/Edge Functions automatically
Image optimization via next/image (automatic on Vercel)
Font optimization via next/font (automatic on Vercel)

Common Patterns

Data Fetching in Server Components

// Parallel data fetching
const [users, posts] = await Promise.all([
  getUsers(),
  getPosts(),
])

Streaming with Suspense

import { Suspense } from 'react'

export default function Page() {
  return (
    <div>
      <h1>Dashboard</h1>
      <Suspense fallback={<Skeleton />}>
        <SlowDataComponent />
      </Suspense>
    </div>
  )
}

Error Handling

// app/dashboard/error.tsx
'use client'

export default function Error({ error, reset }: {
  error: Error & { digest?: string }
  reset: () => void
}) {
  return (
    <div>
      <h2>Something went wrong</h2>
      <button onClick={() => reset()}>Try again</button>
    </div>
  )
}