Next.js App Router (Next.js 16+)

Build modern React applications using Next.js 16+ with App Router architecture.

Overview

This skill provides patterns for:

Server Components (default) and Client Components ("use client")
Server Actions for mutations and form handling
Route Handlers for API endpoints
Explicit caching with "use cache" directive
Parallel and intercepting routes
Next.js 16 async APIs and proxy.ts

When to Use

Activate when user requests involve:

"Create a Next.js 16 project", "Set up App Router"
"Server Component", "Client Component", "use client"
"Server Action", "form submission", "mutation"
"Route Handler", "API endpoint", "route.ts"
"use cache", "cacheLife", "cacheTag", "revalidation"
"parallel routes", "@slot", "intercepting routes"
"proxy.ts", "migrate from middleware.ts"
"layout.tsx", "page.tsx", "loading.tsx", "error.tsx", "not-found.tsx"
"generateMetadata", "next/image", "next/font"

Quick Reference

File Conventions

File	Purpose
`page.tsx`	Route page component
`layout.tsx`	Shared layout wrapper
`loading.tsx`	Suspense loading UI
`error.tsx`	Error boundary
`not-found.tsx`	404 page
`template.tsx`	Re-mounting layout
`route.ts`	API Route Handler
`default.tsx`	Parallel route fallback
`proxy.ts`	Routing boundary (Next.js 16)

Directives

Directive	Purpose
`"use server"`	Mark Server Action functions
`"use client"`	Mark Client Component boundary
`"use cache"`	Enable explicit caching (Next.js 16)

Instructions

Create New Project

bash

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

Implement Server Component

Server Components are the default in App Router.

tsx

// app/users/page.tsx
async function getUsers() {
  const res = await fetch('https://api.example.com/users');
  return res.json();
}

export default async function UsersPage() {
  const users = await getUsers();
  return (
    <main>
      {users.map(user => <UserCard key={user.id} user={user} />)}
    </main>
  );
}

Implement Client Component

Add

"use client"

when using hooks, browser APIs, or event handlers.

tsx

"use client";

import { useState } from "react";

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

Create Server Action

Define actions in separate files with "use server" directive.

tsx

// app/actions.ts
"use server";

import { revalidatePath } from "next/cache";

export async function createUser(formData: FormData) {
  const name = formData.get("name") as string;
  const email = formData.get("email") as string;

  await db.user.create({ data: { name, email } });
  revalidatePath("/users");
}

Use with forms in Client Components:

tsx

"use client";

import { useActionState } from "react";
import { createUser } from "./actions";

export default function UserForm() {
  const [state, formAction, pending] = useActionState(createUser, {});

  return (
    <form action={formAction}>
      <input name="name" />
      <input name="email" type="email" />
      <button type="submit" disabled={pending}>
        {pending ? "Creating..." : "Create"}
      </button>
    </form>
  );
}

See references/server-actions.md for validation with Zod, optimistic updates, and advanced patterns.

Configure Caching

Use "use cache" directive for explicit caching (Next.js 16+).

tsx

"use cache";

import { cacheLife, cacheTag } from "next/cache";

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

  cacheTag(`product-${id}`);
  cacheLife("hours");

  const product = await fetchProduct(id);
  return <ProductDetail product={product} />;
}

See references/caching-strategies.md for cache profiles, on-demand revalidation, and advanced caching patterns.

Create Route Handler

Implement API endpoints using Route Handlers.

// app/api/users/route.ts
import { NextRequest, NextResponse } from "next/server";

export async function GET(request: NextRequest) {
  const users = await db.user.findMany();
  return NextResponse.json(users);
}

export async function POST(request: NextRequest) {
  const body = await request.json();
  const user = await db.user.create({ data: body });
  return NextResponse.json(user, { status: 201 });
}

Dynamic segments use

[param]

// app/api/users/[id]/route.ts
interface RouteParams {
  params: Promise<{ id: string }>;
}

export async function GET(request: NextRequest, { params }: RouteParams) {
  const { id } = await params;
  const user = await db.user.findUnique({ where: { id } });

  if (!user) {
    return NextResponse.json({ error: "Not found" }, { status: 404 });
  }

  return NextResponse.json(user);
}

Handle Next.js 16 Async APIs

All Next.js APIs are async in version 16.

tsx

import { cookies, headers, draftMode } from "next/headers";

export default async function Page() {
  const cookieStore = await cookies();
  const headersList = await headers();
  const { isEnabled } = await draftMode();

  const session = cookieStore.get("session")?.value;
  const userAgent = headersList.get("user-agent");

  return <div>...</div>;
}

Params and searchParams are also async:

tsx

export default async function Page({
  params,
  searchParams,
}: {
  params: Promise<{ slug: string }>;
  searchParams: Promise<{ sort?: string }>;
}) {
  const { slug } = await params;
  const { sort } = await searchParams;
  // ...
}

See references/nextjs16-migration.md for migration guide and proxy.ts configuration.

Implement Parallel Routes

Use

@folder

convention for parallel route slots.

tsx

// app/dashboard/layout.tsx
export default function DashboardLayout({
  children,
  team,
  analytics,
}: {
  children: React.ReactNode;
  team: React.ReactNode;
  analytics: React.ReactNode;
}) {
  return (
    <div>
      {children}
      <div className="grid grid-cols-2">
        {team}
        {analytics}
      </div>
    </div>
  );
}

tsx

// app/dashboard/@team/page.tsx
export default function TeamPage() {
  return <div>Team Section</div>;
}

// app/dashboard/@analytics/page.tsx
export default function AnalyticsPage() {
  return <div>Analytics Section</div>;
}

See references/routing-patterns.md for intercepting routes, route groups, and dynamic routes.

Best Practices

Server vs Client Decision

Start with Server Component (default)
Use Client Component only for:
- React hooks (useState, useEffect, useContext)
- Browser APIs (window, document, localStorage)
- Event handlers (onClick, onSubmit)
- Client-only libraries

Data Fetching

Fetch in Server Components when possible
Use React's
```
cache()
```
for deduplication
Parallelize independent fetches
Add Suspense boundaries with
```
loading.tsx
```

Caching Strategy

tsx

"use cache";
import { cacheLife, cacheTag } from "next/cache";

// Set cache duration
cacheLife("hours");

// Tag for revalidation
cacheTag("resource-name");

Performance Checklist

Use
```
loading.tsx
```
for Suspense boundaries
Use
```
next/image
```
for optimized images
Use
```
next/font
```
for font optimization
Enable React Compiler in
```
next.config.ts
```
Add
```
error.tsx
```
for error handling
Add
```
not-found.tsx
```
for 404 handling

Examples

Example 1: Create Blog Post with Server Action

Input: Create a form to submit blog posts with validation

Output:

tsx

// app/blog/actions.ts
"use server";

import { z } from "zod";
import { revalidatePath } from "next/cache";

const schema = z.object({
  title: z.string().min(5),
  content: z.string().min(10),
});

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

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

  await db.post.create({ data: parsed.data });
  revalidatePath("/blog");
  return { success: true };
}

tsx

// app/blog/new/page.tsx
"use client";

import { useActionState } from "react";
import { createPost } from "../actions";

export default function NewPostPage() {
  const [state, formAction, pending] = useActionState(createPost, {});

  return (
    <form action={formAction}>
      <input name="title" placeholder="Title" />
      {state.errors?.title && <span>{state.errors.title[0]}</span>}

      <textarea name="content" placeholder="Content" />
      {state.errors?.content && <span>{state.errors.content[0]}</span>}

      <button type="submit" disabled={pending}>
        {pending ? "Publishing..." : "Publish"}
      </button>
    </form>
  );
}

Example 2: Product Page with Caching

Input: Create a cached product page with revalidation

Output:

tsx

// app/products/[id]/page.tsx
"use cache";

import { cacheLife, cacheTag } from "next/cache";

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

  cacheTag(`product-${id}`, "products");
  cacheLife("hours");

  const product = await db.product.findUnique({ where: { id } });

  if (!product) {
    notFound();
  }

  return (
    <article>
      <h1>{product.name}</h1>
      <p>{product.description}</p>
      <p>${product.price}</p>
    </article>
  );
}

tsx

// app/api/revalidate/route.ts
import { revalidateTag } from "next/cache";
import { NextResponse } from "next/server";

export async function POST(request: Request) {
  const { tag } = await request.json();
  revalidateTag(tag);
  return NextResponse.json({ revalidated: true });
}

Example 3: Dashboard with Parallel Routes

Input: Create a dashboard with sidebar and main content areas

Output:

tsx

// app/dashboard/layout.tsx
export default function DashboardLayout({
  children,
  sidebar,
  stats,
}: {
  children: React.ReactNode;
  sidebar: React.ReactNode;
  stats: React.ReactNode;
}) {
  return (
    <div className="flex">
      <aside className="w-64">{sidebar}</aside>
      <main className="flex-1">
        <div className="grid grid-cols-3">{stats}</div>
        {children}
      </main>
    </div>
  );
}

tsx

// app/dashboard/@sidebar/page.tsx
export default function Sidebar() {
  return <nav>{/* Navigation links */}</nav>;
}

// app/dashboard/@stats/page.tsx
export default async function Stats() {
  const stats = await fetchStats();
  return (
    <>
      <div>Users: {stats.users}</div>
      <div>Orders: {stats.orders}</div>
      <div>Revenue: {stats.revenue}</div>
    </>
  );
}

Constraints and Warnings

Constraints

Server Components cannot use browser APIs or React hooks
Client Components cannot be async (no direct data fetching)
```
cookies()
```
,
```
headers()
```
,
```
draftMode()
```
are async in Next.js 16
```
params
```
and
```
searchParams
```
are Promise-based in Next.js 16
Server Actions must be defined with "use server" directive

Warnings

Attempting to use
```
await
```
in a Client Component will cause a build error
Accessing
```
window
```
or
```
document
```
in Server Components will throw an error
Forgetting to await
```
cookies()
```
or
```
headers()
```
in Next.js 16 will result in a Promise instead of the actual values
Server Actions without proper validation can expose your database to unauthorized access

References

Consult these files for detailed patterns:

references/app-router-fundamentals.md - Server/Client Components, file conventions, navigation, next/image, next/font
references/routing-patterns.md - Parallel routes, intercepting routes, route groups, dynamic routes
references/caching-strategies.md - "use cache", cacheLife, cacheTag, revalidation
references/server-actions.md - Server Actions, useActionState, validation, optimistic updates
references/nextjs16-migration.md - Async APIs, proxy.ts, Turbopack, config
references/data-fetching.md - Data patterns, Suspense, streaming
references/metadata-api.md - generateMetadata, OpenGraph, sitemap

nextjs-app-router

NPX Install

Tags

SKILL.md Content

Next.js App Router (Next.js 16+)

Overview

When to Use

Quick Reference

File Conventions

Directives

Instructions

Create New Project

Implement Server Component

Implement Client Component

Create Server Action

Configure Caching

Create Route Handler

Handle Next.js 16 Async APIs

Implement Parallel Routes

Best Practices

Server vs Client Decision

Data Fetching

Caching Strategy

Performance Checklist

Examples

Example 1: Create Blog Post with Server Action

Example 2: Product Page with Caching

Example 3: Dashboard with Parallel Routes

Constraints and Warnings

Constraints

Warnings

References