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, and 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 | Purpose | Directive | Purpose | |------|---------|-----------|---------| | page.tsx | Route page | "use server" | Server Action function | | layout.tsx | Shared layout | "use client" | Client Component boundary | | loading.tsx | Suspense loading | "use cache" | Explicit caching (Next.js 16) | | error.tsx | Error boundary | | | | not-found.tsx | 404 page | | | | route.ts | API Route Handler | | | | proxy.ts | Routing boundary | | |

Instructions

Create New Project

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

Server Component

Server Components are the default in App Router. They run on the server and can use async/await.

tsx
// app/users/page.tsx
async function getUsers() {
  const apiUrl = process.env.API_URL;
  const res = await fetch(`${apiUrl}/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>;
}

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>;
}

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 Zod validation, 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 patterns.

Route Handler

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

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

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

Dynamic segments use [param]:

ts
// app/api/users/[id]/route.ts
export async function GET(request: NextRequest, { params }: { params: Promise<{ id: string }> }) {
  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);
}

Next.js 16 Async APIs

All Next.js APIs are async in version 16.

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

export default async function Page() {
  const cookieStore = await cookies();
  const headersList = await headers();
  const session = cookieStore.get("session")?.value;
  const userAgent = headersList.get("user-agent");
  return <div>...</div>;
}

Params and searchParams are also Promise-based:

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.

Parallel Routes

Use @folder convention for parallel route slots.

tsx
// app/dashboard/layout.tsx
export default function DashboardLayout({ children, team, analytics }: Record<string, 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), browser APIs (window, document), event handlers (onClick, onSubmit), or 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

Performance Checklist:

• Use loading.tsx for Suspense boundaries
• Use next/image for optimized images
• Use next/font for font optimization
• Add error.tsx and not-found.tsx for error handling

Examples

Example 1: Blog Post Form with Server Action

Input: Create a form to submit blog posts with Zod 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: Cached Product Page

Input: Create a cached product page with on-demand 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></article>;
}

ts
// 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 stats areas

Output:

tsx
// app/dashboard/layout.tsx
export default function DashboardLayout({ children, sidebar, stats }: Record<string, 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></>;
}

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:

• Using await in a Client Component causes a build error
• Accessing window or document in Server Components throws an error
• Forgetting to await cookies() or headers() in Next.js 16 returns a Promise instead of the value
• Server Actions without proper validation can expose the database to unauthorized access
• External Data Fetching: Server Components that fetch() third-party URLs process untrusted content — always validate, sanitize, and type-check responses; use environment variables for API URLs rather than hardcoding them

References

• references/app-router-fundamentals.md — Server/Client Components, file conventions, navigation
• references/routing-patterns.md — Parallel routes, intercepting routes, route groups
• 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