/nextjs-app-router

name: nextjs-app-router
description: Provides patterns and code examples for building Next.js 16+ applications with App Router architecture. Use when creating projects with App Router, implementing Server Components and Client Components ("use client"), creating Server Actions for forms, building Route Handlers (route.ts), configuring caching with "use cache" directive (cacheLife, cacheTag), setting up parallel routes (`@slot`) or intercepting routes, migrating to proxy.ts, or working with App Router file conventions (layout.tsx, page.tsx, loading.tsx, error.tsx).
allowed-tools: Read, Write, Edit, Bash

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

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.

// 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.

"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.

// 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:

"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](references/server-actions.md) for Zod validation, optimistic updates, and advanced patterns.

Configure Caching

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

"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](references/caching-strategies.md) for cache profiles, on-demand revalidation, and advanced patterns.

Route Handler

// 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]`:

// 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.

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:

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

Se