Nextjs Development

Next.js Development

Optimized for Next.js 16+, React 19+, TypeScript 5.5+, Turbopack, and App Router-first architectures.

Comprehensive reference for Next.js (latest: 16.2.4) with the App Router, TypeScript, and modern patterns. Covers project structure, Server/Client Components, data fetching, caching with the use cache directive, Server Actions, MCP devtools integration, and performance optimization.

• Leverage native parallel subagent dispatch and 200k+ context windows where available.

Component Review Rubric Reference

Apply the shared Component Review Rubric before approving Next.js components, then run the Next.js-specific checks below.

Anti-Patterns

• Mixing server and client responsibilities: Bundle size, caching, and auth decisions become harder to reason about.
• Using legacy synchronous request APIs: Modern Next.js expects async request surfaces such as params, headers(), and cookies().
• Skipping route-level loading and error states: Streaming apps feel broken when only the happy path is implemented.

Verification Protocol

Before claiming "skill applied successfully":

1. Pass/fail: The Nextjs Development guidance is tied to a concrete route, component, screen, or design artifact.
2. Pass/fail: Component states cover loading, empty, error, success, and responsive breakpoints where applicable.
3. Pass/fail: Accessibility, visual hierarchy, and interaction behavior are reviewed against the shared component rubric.
4. Pressure-test scenario: Review the component on a narrow mobile viewport, keyboard-only path, and slow-loading state.
5. Success metric: Zero generic UI approval; every approval cites rendered behavior or source evidence.

Before and After Example

// Before
export default function ProductPage({ params }: { params: { id: string } }) {
  const [product, setProduct] = useState<Product | null>(null);
  useEffect(() => {
    fetch(`/api/products/${params.id}`).then((r) => r.json()).then(setProduct);
  }, [params.id]);
  return product ? <ProductView product={product} /> : <Spinner />;
}

// After
export default async function ProductPage({
  params,
}: {
  params: Promise<{ id: string }>
}) {
  const { id } = await params;
  const product = await getProduct(id);
  return <ProductView product={product} />;
}

Moves data fetching into the server component and follows the async request API used in current Next.js releases.

Activation Conditions

Use symptom -> action triggers: when one matches, apply this skill and verify with the protocol below.

App Router & Routing

• Creating or modifying page.tsx, layout.tsx, loading.tsx, error.tsx, not-found.tsx
• Working with dynamic routes [slug], catch-all [...slug], optional catch-all [[...slug]]
• Parallel routes @slot, intercepting routes, route groups (group)
• New v15/v16 file conventions: forbidden.tsx, proxy.ts, template.tsx, unauthorized.tsx

Server & Client Components

• Deciding when to use "use client" or "use server" directives
• Component boundary questions, RSC + RCC composition patterns
• Passing Server Components as children/props to Client Components
• taint API for data security

Data Fetching & Caching

• Using use cache directive (replaces cache: 'force-cache')
• cacheTag(), cacheLife(), revalidateTag(), revalidatePath()
• Async Request APIs: await cookies(), await headers(), await params, await searchParams
• after() for post-response work, connection() for dynamic rendering

Server Actions & Forms

• "use server" in functions or module scope
• <Form> component with client-side navigation
• Form validation, optimistic updates, error handling
• after() for side-effects after action completes

Performance & Turbopack

• next dev with Turbopack (default in v15+, stable)
• Image optimization with next/image
• Font subsetting with next/font
• Lazy loading, bundle optimization, serverComponentsHmrCache

Next.js MCP Dev Tools

• Querying live errors, logs, routes from the running dev server
• Using next-devtools-mcp with coding agents (requires Next.js 16+)
• Upgrading to Next.js 16 with codemods
• Enabling Cache Components feature

Part 1: Project Setup & Config

Creating a New Project

npx create-next-app@latest my-app --typescript --tailwind --eslint --app

TypeScript Config (next.config.ts)

import type { NextConfig } from 'next'

const nextConfig: NextConfig = {
  reactCompiler: true,          // stable in v16
  reactStrictMode: true,
  serverExternalPackages: ['sharp'],  // renamed from serverComponentsExternalPackages in v15
  experimental: {
    turbopackFileSystemCache: true,   // persist Turbopack cache across restarts
    serverComponentsHmrCache: true,   // cache fetch responses during HMR
  },
  cacheLife: {                        // custom cache profiles
    frequent: { stale: 60, revalidate: 60, expire: 3600 },
    daily: { stale: 3600, revalidate: 3600, expire: 86400 },
  },
}

export default nextConfig

Project Structure

my-app/
├── app/
│   ├── layout.tsx          # Root layout (required)
│   ├── page.tsx            # Home page
│   ├── loading.tsx         # Streaming skeleton
│   ├── error.tsx           # Error boundary
│   ├── not-found.tsx       # 404 page
│   ├── forbidden.tsx       # 403 page (v16)
│   ├── unauthorized.tsx    # 401 page (v16)
│   ├── (marketing)/        # Route group (no URL segment)
│   │   └── about/page.tsx
│   ├── blog/
│   │   └── [slug]/page.tsx # Dynamic route
│   └── api/
│       └── route.ts        # Route Handler
├── components/             # Shared RSC/RCC components
├── lib/                    # Server utilities
├── public/                 # Static assets
├── next.config.ts          # TypeScript config (v15+)
├── .mcp.json               # MCP server config (v16)
└── instrumentation.ts      # Server lifecycle hooks (stable v15)

Part 2: App Router Routing

File Conventions

Dynamic Routes

// app/blog/[slug]/page.tsx
// IMPORTANT: params is now async in Next.js 15+
export default async function BlogPost({
  params,
}: {
  params: Promise<{ slug: string }>
}) {
  const { slug } = await params  // must await in v15+
  return <h1>{slug}</h1>
}

// Generate static paths
export async function generateStaticParams() {
  const posts = await fetchPosts()
  return posts.map((post) => ({ slug: post.slug }))
}

Route Groups & Parallel Routes

app/
├── (auth)/                 # Route group: no URL impact
│   ├── login/page.tsx      # /login
│   └── register/page.tsx   # /register
├── @modal/                 # Parallel route (slot)
│   └── photo/[id]/page.tsx
├── layout.tsx              # Receives { children, modal } props
└── page.tsx

Intercepting Routes

app/
├── photos/[id]/page.tsx    # Full page: /photos/123
└── @modal/
    └── (.)photos/[id]/     # Intercept same-level route
        └── page.tsx        # Renders as modal without full navigation

searchParams (async in v15+)

// app/search/page.tsx
export default async function SearchPage({
  searchParams,
}: {
  searchParams: Promise<{ q: string; page: string }>
}) {
  const { q, page } = await searchParams  // must await in v15+
  return <Results query={q} page={Number(page)} />
}

Part 3: Server & Client Components

Decision Tree

Does the component need:
  - onClick, onChange, event handlers?    → "use client"
  - useState, useEffect, useReducer?      → "use client"
  - Browser-only APIs (window, localStorage)?  → "use client"
  - useRouter, useParams, useSearchParams?     → "use client"
  
Otherwise:
  - Direct DB/API access without extra fetch?  → Server Component (default)
  - Large dependencies (marked-js, date-fns)?  → Server Component (not in JS bundle)
  - Access cookies(), headers(), auth tokens?  → Server Component

Component Composition Pattern

// ✅ Pass Server Components as children to Client Components
// app/page.tsx (Server Component)
import { ClientWrapper } from '@/components/ClientWrapper'
import { ServerData } from '@/components/ServerData'

export default function Page() {
  return (
    <ClientWrapper>
      <ServerData /> {/* Server Component as child — no "use client" boundary issue */}
    </ClientWrapper>
  )
}

// components/ClientWrapper.tsx
"use client"
import { useState } from 'react'

export function ClientWrapper({ children }: { children: React.ReactNode }) {
  const [open, setOpen] = useState(false)
  return <div => setOpen(!open)}>{children}</div>
}

Directives Reference

Data Security with taint

// next.config.ts
const nextConfig: NextConfig = {
  experimental: { taint: true }
}

// lib/user.ts (Server)
import { experimental_taintUniqueValue } from 'react'

export async function getUser(id: string) {
  const user = await db.user.findUnique({ where: { id } })
  // Prevent accidental serialization of sensitive fields
  experimental_taintUniqueValue(
    'Do not pass user.passwordHash to Client',
    user,
    user.passwordHash
  )
  return user
}

Part 4: Data Fetching & Caching

The use cache Directive (Next.js 15+)

use cache replaces the old cache: 'force-cache' approach and works at the file, component, or function level.

// Cache an entire async function
async function getProducts() {
  'use cache'
  const data = await fetch('https://api.example.com/products')
  return data.json()
}

// Cache a Server Component
async function ProductList() {
  'use cache'
  cacheLife('daily')          // use named profile from next.config.ts
  cacheTag('products')        // tag for targeted revalidation

  const products = await getProducts()
  return <ul>{products.map(p => <li key={p.id}>{p.name}</li>)}</ul>
}

cacheLife Profiles

// Built-in profiles
cacheLife('seconds')   // stale: 0,  revalidate: 1,    expire: 60
cacheLife('minutes')   // stale: 60, revalidate: 60,   expire: 3600
cacheLife('hours')     // stale: 3600, revalidate: 3600, expire: 86400
cacheLife('days')      // stale: 86400, revalidate: 86400, expire: 604800
cacheLife('weeks')     // stale: 604800, revalidate: 604800, expire: 2592000
cacheLife('max')       // stale: 2592000, revalidate: 2592000, expire: Infinity

// Custom profile (defined in next.config.ts)
cacheLife('frequent')  // stale: 60, revalidate: 60, expire: 3600

Targeted Revalidation with cacheTag

// app/actions.ts
'use server'
import { revalidateTag } from 'next/cache'

export async function updateProduct(id: string, data: FormData) {
  await db.products.update({ where: { id }, data: Object.fromEntries(data) })
  revalidateTag('products')      // invalidates all cached items with this tag
  revalidateTag(`product-${id}`) // fine-grained invalidation
}

// app/products/[id]/page.tsx
async function ProductPage({ params }: { params: Promise<{ id: string }> }) {
  'use cache'
  const { id } = await params
  cacheTag('products', `product-${id}`)
  const product = await db.products.findUnique({ where: { id } })
  return <Product data={product} />
}

fetch Cache Behavior (v15+ defaults changed)

// GET route handlers are NO LONGER cached by default in v15+
// Explicitly opt-in to caching:
const res = await fetch('https://api.example.com/data', {
  next: { revalidate: 3600, tags: ['products'] }
})

// Force dynamic (never cache):
const res = await fetch('https://api.example.com/data', {
  cache: 'no-store'
})

// ISR — revalidate every N seconds:
export const revalidate = 3600  // segment-level option

Async Request APIs (v15 Breaking Change)

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

// BEFORE (v14): synchronous
const cookieStore = cookies()

// AFTER (v15+): must await
const cookieStore = await cookies()
const headersList = await headers()

// params and searchParams also async in page/layout props
const { slug } = await params
const { q } = await searchParams

after() — Post-Response Side Effects

import { after } from 'next/server'

export async function GET(request: Request) {
  const data = await fetchData()

  // Fires AFTER response is sent to client
  after(async () => {
    await logAnalyticsEvent('data-fetched', { timestamp: Date.now() })
  })

  return Response.json(data)
}

connection() — Force Dynamic Rendering

import { connection } from 'next/server'

export default async function Page() {
  // Signals this component requires a live request (opts out of static rendering)
  await connection()
  const realTimeData = await fetchLiveData()
  return <Dashboard data={realTimeData} />
}

Part 5: Server Actions & Forms

Server Actions

// app/actions.ts
'use server'
import { revalidatePath } from 'next/cache'
import { redirect } from 'next/navigation'
import { z } from 'zod'

const CreatePostSchema = z.object({
  title: z.string().min(1).max(200),
  content: z.string().min(10),
})

export async function createPost(formData: FormData) {
  const parsed = CreatePostSchema.safeParse({
    title: formData.get('title'),
    content: formData.get('content'),
  })

  if (!parsed.success) {
    return { error: parsed.error.flatten().fieldErrors }
  }

  const post = await db.posts.create({ data: parsed.data })
  revalidatePath('/blog')
  redirect(`/blog/${post.id}`)
}

<Form> Component (Next.js 15+)

import Form from 'next/form'

export default function SearchForm() {
  // <Form> replaces <form> for client-side navigation + prefetching
  return (
    <Form action="/search">
      <input name="q" placeholder="Search..." />
      <button type="submit">Search</button>
    </Form>
  )
}

Optimistic Updates with useOptimistic

'use client'
import { useOptimistic, useTransition } from 'react'
import { toggleLike } from '@/app/actions'

export function LikeButton({ postId, initialLikes }: Props) {
  const [optimisticLikes, addOptimisticLike] = useOptimistic(
    initialLikes,
    (state, delta: number) => state + delta
  )
  const [isPending, startTransition] = useTransition()

  return (
    <button
      => startTransition(async () => {
        addOptimisticLike(1)
        await toggleLike(postId)
      })}
      disabled={isPending}
    >
      {optimisticLikes} Likes
    </button>
  )
}

Part 6: Next.js MCP Dev Tools

The next-devtools-mcp package enables coding agents to connect to the live Next.js development server. Requires Next.js 16+.

Setup

// .mcp.json (project root)
{
  "mcpServers": {
    "next-devtools": {
      "command": "npx",
      "args": ["-y", "next-devtools-mcp@latest"]
    }
  }
}

Next.js 16+ includes a built-in MCP endpoint at /_next/mcp in the development server. next-devtools-mcp automatically discovers and connects to running instances — even across multiple ports.

Available MCP Tools

Usage Patterns

# Ask the agent about runtime state
"What errors are currently in my application?"
→ Agent calls get_errors → analyzes build/type/runtime errors → suggests fixes

# Debug a specific route
"Why is /dashboard rendering statically instead of dynamically?"
→ Agent calls get_page_metadata with route=/dashboard → shows rendering config

# Navigate the codebase
"What Server Actions exist in this app?"
→ Agent calls get_project_metadata → then get_server_action_by_id for each action

# Upgrade workflow
"Help me upgrade to Next.js 16"
→ Agent calls upgrade_nextjs_16 → runs codemods → handles breaking changes

# Enable new features
"Set up Cache Components for this project"
→ Agent calls enable_cache_components → configures next.config.ts + shows patterns

Part 7: Performance & Turbopack

Turbopack (Default in v15+)

# Turbopack is now the default — no flag needed
npm run dev           # Uses Turbopack automatically

# Opt back to webpack if needed
npm run dev -- --webpack

# Enable Turbopack filesystem cache (persist across restarts)
# next.config.ts
experimental: { turbopackFileSystemCache: true }

Benchmark vs webpack: 76.7% faster cold starts, 96.3% faster HMR.

Image Optimization

import Image from 'next/image'

export function Hero() {
  return (
    <Image
      src="/hero.jpg"
      alt="Hero image"
      width={1200}
      height={600}
      priority          // LCP image: preloads synchronously
      sizes="(max-width: 768px) 100vw, 1200px"
      placeholder="blur"
      blurDataURL="data:image/jpeg;base64,..."
    />
  )
}

Font Optimization

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

const inter = Inter({
  subsets: ['latin'],
  variable: '--font-inter',    // CSS variable for Tailwind
  display: 'swap',
})

// app/layout.tsx
export default function RootLayout({ children }: { children: React.ReactNode }) {
  return (
    <html lang="en" className={inter.variable}>
      <body>{children}</body>
    </html>
  )
}

React Compiler

// next.config.ts — stable in v16
const nextConfig: NextConfig = {
  reactCompiler: true,  // eliminates manual useMemo/useCallback
}

Bundle Optimization

// next.config.ts
const nextConfig: NextConfig = {
  // Avoid importing entire icon libraries
  // next/font handles subsetting automatically
  bundlePagesRouterDependencies: true,  // renamed from bundlePagesExternals in v15
  experimental: {
    optimizePackageImports: ['lucide-react', '@heroicons/react'],
  },
}

Part 8: Metadata & SEO

Static Metadata

// app/layout.tsx or app/page.tsx
import type { Metadata } from 'next'

export const metadata: Metadata = {
  title: {
    template: '%s | My App',
    default: 'My App',
  },
  description: 'App description for SEO',
  openGraph: {
    type: 'website',
    url: 'https://example.com',
    images: [{ url: '/og-image.jpg', width: 1200, height: 630 }],
  },
  robots: { index: true, follow: true },
  metadataBase: new URL('https://example.com'),
}

Dynamic Metadata

// app/blog/[slug]/page.tsx
export async function generateMetadata({
  params,
}: {
  params: Promise<{ slug: string }>
}): Promise<Metadata> {
  const { slug } = await params  // async in v15+
  const post = await getPost(slug)

  return {
    title: post.title,
    description: post.excerpt,
    openGraph: {
      images: [{ url: post.cover, width: 1200, height: 630 }],
    },
  }
}

Part 9: Route Handlers & Middleware

Route Handlers (Uncached by Default in v15+)

// app/api/products/route.ts
import { NextRequest, NextResponse } from 'next/server'
import { after } from 'next/server'

// GET is NO LONGER cached by default in v15+
export async function GET(request: NextRequest) {
  const { searchParams } = new URL(request.url)
  const category = searchParams.get('category')

  const products = await db.products.findMany({
    where: category ? { category } : undefined,
  })

  // Side effect after response
  after(() => logRequest(request.url))

  return NextResponse.json(products)
}

// Opt-in to caching for a route segment
export const revalidate = 3600  // revalidate every hour
export const dynamic = 'force-static'  // always static

Middleware

// middleware.ts (project root)
import { NextResponse } from 'next/server'
import type { NextRequest } from 'next/server'

export function middleware(request: NextRequest) {
  const token = request.cookies.get('auth-token')?.value

  if (!token && request.nextUrl.pathname.startsWith('/dashboard')) {
    return NextResponse.redirect(new URL('/login', request.url))
  }

  return NextResponse.next()
}

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

Part 10: Instrumentation

Server-Side Lifecycle (instrumentation.ts)

// instrumentation.ts (stable in v15)
export async function register() {
  // Runs once when the server starts (Node.js + Edge)
  if (process.env.NEXT_RUNTIME === 'nodejs') {
    // Initialize OpenTelemetry, Sentry, etc.
    const { initTracing } = await import('./lib/tracing')
    await initTracing()
  }
}

export async function onRequestError(
  error: Error,
  request: { path: string; method: string },
  context: { routeType: string }
) {
  // Centralized error reporting
  await reportError(error, { path: request.path })
}

Client-Side Instrumentation (instrumentation-client.ts)

// instrumentation-client.ts (v16)
export function onRouteChange({ path }: { path: string }) {
  // Track route changes for analytics
  analytics.track('page_view', { path })
}

export function onCaughtError(error: Error) {
  // Capture client-side errors
  Sentry.captureException(error)
}

Part 11: Auth Interrupts (v16)

// middleware.ts or Server Component
import { forbidden, unauthorized } from 'next/navigation'

export default async function AdminPage() {
  const session = await getSession()

  if (!session) {
    unauthorized()  // renders unauthorized.tsx
  }

  if (!session.user.isAdmin) {
    forbidden()     // renders forbidden.tsx
  }

  return <AdminDashboard />
}

Part 12: Upgrading to v15/v16

Automated Codemods

# Upgrade to v15 (handles async Request APIs automatically)
npx @next/codemod@latest upgrade

# Or upgrade to v16 specifically
npx @next/codemod@latest upgrade next@16

# Available codemods
npx @next/codemod@latest next-async-request-api .
npx @next/codemod@latest next-og-import .

Key v15 Breaking Changes

Modern Component and Testing Examples

Server Components

export default async function DashboardPage() {
  const metrics = await getDashboardMetrics();
  return <Dashboard metrics={metrics} />;
}

Error Boundaries

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

export default function Error({ reset }: { reset: () => void }) {
  return <button dashboard</button>;
}

Accessibility Testing Tools

import AxeBuilder from '@axe-core/playwright';

test('dashboard has no critical accessibility issues', async ({ page }) => {
  await page.goto('/dashboard');
  const results = await new AxeBuilder({ page }).analyze();
  expect(results.violations).toEqual([]);
});

Common Pitfalls

• Mixing server and client responsibilities: It bloats bundles and makes caching or auth decisions harder to reason about.
• Using old synchronous request APIs: Current Next.js releases expect async params, searchParams, cookies(), and headers().
• Skipping error and loading states: Streaming routes feel broken when only the happy path is modeled.

Cross-Client Portability

This skill is written to stay usable across GitHub Copilot, Claude Code, and Codex.

• GitHub Copilot: keep the folder in a Copilot-visible skill path or wrap the workflow in project instructions when folder discovery is unavailable.
• Claude Code: keep the folder in a local skills directory or a compatible plugin source.
• Codex: install or sync the folder into $CODEX_HOME/skills/nextjs-development and restart Codex after major changes.

MCP Availability And Fallback

Preferred MCP Server: Next.js MCP

• Fallback prompt: "Use the Next.js Development skill without MCP. Rely on the local SKILL.md, bundled references or scripts, and manual verification. Show the exact commands, evidence, and final checks you used before concluding."
• Use next dev, next build, next lint, browser console output, and local server logs when live MCP diagnostics are unavailable.
• Verify routing, rendering mode, and data-fetching behavior with the bundled examples and a running dev server.

Related Skills

• react-development: Use it when the workflow also needs React component architecture and client or server boundaries.
• javascript-development: Use it when the workflow also needs modern JavaScript and TypeScript application code.
• web-testing: Use it when the workflow also needs browser and end-to-end testing evidence.
• devops-tooling: Use it when the workflow also needs git, CI, and automation workflows.