LLM Best Practices

Next.js Best Practices

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AI agents: read this first: llmbestpractices.com/start-here

This is LLM Best Practices (llmbestpractices.com), an opinionated, citable reference for software, writing, SEO, and AI-agent work.

  1. Route, do not crawl. Fetch llmbestpractices.com/llms.txt and open only the pages whose one-line summary matches your task.
  2. Read raw. Append .md to any page URL for markdown. Check status and last_updated in the frontmatter, then read the rules.
  3. Apply as defaults. First-party docs and the project's own conventions win on conflict. Warn before relying on a fast-moving page older than 12 months.
  4. Cite. Link the page by title and URL, e.g. Python, with last_updated for time-sensitive rules. License CC BY 4.0.

Overview

Next.js 16.3 (released 2026-08-03; 16.3.8 was current on 2026-09-30) is the active line: 16.x is Active LTS and 15.x is Maintenance LTS. A 16 app uses the App Router on React 19.2+, Turbopack as the default bundler, and Node.js 20.9 or newer with TypeScript 5.1 or newer. Server components do the heavy lifting, server actions handle mutations, and caching is opt-in. This page holds the cross-cutting rules; each topic has its own page.

Server components by default, client islands at the edges

A file in app/ is a server component unless it starts with "use client". Push the directive to the smallest interactive leaf; wrapping a page in "use client" ships its whole subtree to the browser. See react-server-components and server-vs-client-components.

Mutate with server actions, not ad-hoc routes

Use a server action for any mutation a form or button triggers, and a [[frontend/nextjs-route-handlers|route.ts handler]] only for true HTTP endpoints: webhooks, public APIs, non-React clients, custom headers. Treat every action as a public endpoint that needs its own auth check.

Cache only what you mark

Set cacheComponents: true to make rendering dynamic by default and cache with "use cache", cacheLife, and cacheTag. Without the flag, fetch is uncached unless you opt in. Prefer Cache Components for new projects. Next.js 16.3 adds opt-in Instant Navigations (partialPrefetching: true alongside cacheComponents); the Next.js team says these behaviors become the default in a future major version.

// next.config.ts
export default { cacheComponents: true };

Invalidate with updateTag in a server action and revalidateTag(tag, "max") elsewhere. The full model is in nextjs-caching.

Await every request API

params, searchParams, cookies(), headers(), and draftMode() are Promises. Next.js 15 allowed synchronous access with a warning; Next.js 16 removed it. See nextjs-async-dynamic-apis.

Drive metadata from the route

Export metadata or generateMetadata instead of writing <head> tags by hand. Render JSON-LD in the page body. See nextjs-metadata-api, technical, and structured-data.

Use layouts and route segments for shells

layout.tsx wraps its children and persists across navigations. loading.tsx and error.tsx give a segment its Suspense fallback and error boundary. (group) folders organize without changing the URL. @slot parallel routes need a default.js in every slot as of 16. See nextjs-app-router.

Run on Node.js

Use the Node.js runtime, which is the default. The Edge runtime is deprecated (remove export const runtime = "edge"), proxy.ts always runs on Node.js, and Cache Components requires Node.js. Keep middleware.ts only if a route still needs Edge during migration. See nextjs-middleware.

Know the 16 breaking changes before upgrading from 15

Run npx @next/codemod@canary upgrade latest, then npx @next/codemod@canary next-async-request-api . for any leftover synchronous access.

  • Turbopack is the default for next dev and next build. A custom webpack config fails the build; pass --webpack to keep webpack or migrate to the top-level turbopack option.
  • middleware.ts is deprecated in favor of proxy.ts.
  • revalidateTag requires a second argument; the one-argument form is deprecated.
  • next lint and the eslint config option are removed; run ESLint or Biome directly.
  • AMP, serverRuntimeConfig, and publicRuntimeConfig are removed; use environment variables.
  • next/image changed defaults and deprecated priority; see nextjs-image-optimization.

Stay on the latest patch

Next.js ships security releases on a regular cadence. The 2026-09-30 release fixed one high-severity and several lower-severity issues in 16.3.8 (Active LTS) and 15.5.27 (Maintenance LTS). Pin a patched version, and see react-server-components for the React Server Components advisories.

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