Nextjs App Router

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

Directives

Instructions

Create New Project

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

Implement Server Component

Server Components are the default in App Router.

// 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>
  );
}

Implement 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 => setCount(c => c + 1)}>
      Count: {count}
    </button>
  );
}

Create 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 for validation with Zod, 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 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.

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:

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.

// 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>
  );
}

// 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

1. Start with Server Component (default)
2. 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

"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:

// 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 };
}

// 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:

// 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>
  );
}

// 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:

// 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>
  );
}

// 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
• External Data Fetching: Server Components that fetch data from external APIs (fetch() calls to third-party URLs) process untrusted content; always validate, sanitize, and type-check fetched responses before rendering, and use environment variables for API URLs rather than hardcoding them

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