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This skill ensures AI agents write correct, modern Nuxt 4 code and avoid common mistakes from outdated Nuxt 3 patterns. Always read this before writing any Nuxt code.

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name	nuxt-4-skill
description	This skill ensures AI agents write correct, modern Nuxt 4 code and avoid common mistakes from outdated Nuxt 3 patterns. Always read this before writing any Nuxt code.

Nuxt 4 Development Skill

Purpose: This skill ensures AI agents write correct, modern Nuxt 4 code and avoid common mistakes from outdated Nuxt 3 patterns. Always read this before writing any Nuxt code. Last verified: February 2025 | Nuxt 4.x stable (released July 2025)

1. Critical Context: Nuxt 4 vs Nuxt 3

Nuxt 4 was officially released in July 2025. It is a stability-focused major release that builds on Nuxt 3 with breaking changes in project structure, data fetching, TypeScript, and tooling. Nuxt 3 reaches end-of-maintenance in January 2026.

If you have training data from Nuxt 3, many patterns are now outdated. Always prefer the patterns documented in this skill.

Key Differences at a Glance

Area	Nuxt 3 (Old)	Nuxt 4 (Current)
Directory structure	Flat root (`pages/`, `components/` at root)	`app/` directory (source code lives in `app/`)
Data fetching reactivity	Deep reactive refs by default	`shallowRef` by default for better performance
Data fetching keys	Independent per call	Singleton: shared refs for same key
Route middleware	`defineNuxtRouteMiddleware` (some guides)	`defineNuxtRouteMiddleware` with async support
`<head>` management	Unhead v1	Unhead v2 (removed `vmid`, `hid`, `children`)
Component names	Vue default naming	Normalized to match Nuxt auto-import pattern
Module loading in layers	Project modules loaded first (incorrect)	Layer modules first, project modules last (correct)
Route metadata	Available on both `route.name` and `route.meta.name`	Only on `route.name` (deduplicated)
Page meta scanning	Before `pages:extend` hook	After `pages:extend` hook
Node.js requirement	16+	18.20+ (LTS 20+ recommended)
Generate config	`generate.exclude`, `generate.routes`	Removed. Use `nitro.prerender` instead
Inline styles	All CSS inlined	Only Vue component CSS inlined; global CSS as separate file

2. Project Structure (CRITICAL CHANGE)

Nuxt 4 Default Structure

my-nuxt-app/
├── app/                    # ← ALL application source code goes here
│   ├── assets/
│   ├── components/
│   ├── composables/
│   ├── layouts/
│   ├── middleware/          # Route middleware (client-side)
│   ├── pages/
│   ├── plugins/
│   ├── utils/
│   ├── app.vue
│   ├── app.config.ts
│   ├── error.vue
│   ├── router.options.ts
│   └── spa-loading-template.html
├── content/                # Nuxt Content (at root, NOT inside app/)
├── layers/                 # Nuxt Layers (at root)
├── modules/                # Local modules (at root)
├── public/                 # Static assets (at root)
├── shared/                 # Shared code between app and server
├── server/                 # Server code (at root, NOT inside app/)
│   ├── api/
│   ├── middleware/          # Server middleware (Nitro/h3)
│   ├── plugins/
│   ├── routes/
│   └── utils/
├── nuxt.config.ts
├── package.json
└── tsconfig.json

Rules

The ~ alias now points to app/ by default (your srcDir).
~/components resolves to app/components/, ~/pages to app/pages/, etc.
server/, content/, layers/, modules/, and public/ stay at the project root, NOT inside app/.
If the project still uses the flat (Nuxt 3) structure, Nuxt auto-detects it — but always prefer the new structure for new projects.

Why This Matters

Performance: Prevents .git/ and node_modules/ from being scanned by file watchers.
IDE Type Safety: Separates app context from server context for accurate auto-completion.

Reverting to Nuxt 3 Structure (if needed)

// nuxt.config.ts
export default defineNuxtConfig({
  srcDir: '.',
  dir: {
    app: 'app',
  },
})

3. Data Fetching (SIGNIFICANT CHANGES)

The Three Methods

Method	Use Case
`useFetch`	Simple data fetching in components (wrapper around `useAsyncData` + `$fetch`)
`useAsyncData`	Complex async operations, custom fetch logic
`$fetch`	Direct HTTP calls (event handlers, non-component code, client-only actions)

Critical Nuxt 4 Changes

3.1 Shallow Reactivity by Default

// Nuxt 4: data is a shallowRef (NOT deeply reactive)
const { data } = await useFetch('/api/users')

// If you need deep reactivity (rare, impacts performance):
const { data } = await useFetch('/api/users', { deep: true })

Do NOT set deep: true globally unless absolutely necessary. ShallowRef provides massive performance improvements on nested data structures.

3.2 Singleton Data Fetching (Shared Refs)

All calls to useAsyncData/useFetch with the same key now share the same data, error, and status refs.

// ✅ CORRECT: Same key = shared data across components
// Component A
const { data: product } = await useFetch(`/api/products/${id}`, {
  key: `product-${id}`
})

// Component B (different file, same key)
const { data: product } = await useFetch(`/api/products/${id}`, {
  key: `product-${id}`
})
// → No duplicate API call! Uses the same shared ref.

Important Rules:

All calls with the same explicit key must have identical deep, transform, pick, getCachedData, and default options.
Extract shared data fetching into composables to avoid inconsistencies:

// app/composables/useProduct.ts
export function useProduct(productId: string) {
  return useAsyncData(
    `product-${productId}`,
    () => $fetch(`/api/products/${productId}`),
    {
      deep: true,
      transform: (product) => ({
        ...product,
        formattedPrice: `${product.price.toFixed(2)} kr`,
      }),
    }
  )
}

3.3 `getCachedData` Now Receives Context

// Nuxt 4: getCachedData has a context parameter
useAsyncData('key', fetchFunction, {
  getCachedData: (key, nuxtApp, ctx) => {
    // ctx.cause can be: 'initial' | 'refresh:hook' | 'refresh:manual' | 'watch'
    
    // Example: Always refetch on manual refresh
    if (ctx.cause === 'refresh:manual') return undefined
    
    return nuxtApp.payload.data[key] || nuxtApp.static.data[key]
  }
})

3.4 Reactive Keys

// Nuxt 4: Keys can be reactive (computed, ref, getter)
const id = computed(() => route.params.id)

const { data: post } = await useFetch(() => `/api/posts/${id.value}`)
// → Automatically refetches when id changes

3.5 Automatic Data Cleanup

When the last component using a useAsyncData key unmounts, Nuxt automatically removes the cached data to prevent memory leaks.

3.6 Common Data Fetching Mistakes to Avoid

// ❌ WRONG: Using $fetch in component setup (causes double fetch on SSR + hydration)
const data = ref(null)
data.value = await $fetch('/api/users')

// ✅ CORRECT: Use useFetch or useAsyncData in components
const { data } = await useFetch('/api/users')

// ❌ WRONG: Conflicting options with same key
const { data: a } = useAsyncData('users', () => $fetch('/api/users'), { deep: false })
const { data: b } = useAsyncData('users', () => $fetch('/api/users'), { deep: true })
// → Nuxt 4 will warn about this!

// ✅ CORRECT: Use $fetch in server routes and event handlers
// server/api/combined.get.ts
export default defineEventHandler(async () => {
  const [users, posts] = await Promise.all([
    $fetch('/api/users'),
    $fetch('/api/posts'),
  ])
  return { users, posts }
})

4. Server API Routes

Server routes live in server/ at the project root (NOT inside app/).

File-Based Routing

server/
├── api/
│   ├── hello.ts            → GET/POST/etc. /api/hello
│   ├── hello.get.ts        → GET /api/hello
│   ├── hello.post.ts       → POST /api/hello
│   ├── users/
│   │   ├── index.get.ts    → GET /api/users
│   │   ├── index.post.ts   → POST /api/users
│   │   └── [id].get.ts     → GET /api/users/:id
├── routes/
│   └── health.ts           → /health (no /api prefix)
├── middleware/
│   └── log.ts              → Runs on every request
├── plugins/
│   └── startup.ts          → Nitro server plugins
└── utils/
    └── auth.ts             → Auto-imported server utilities

Writing Event Handlers

// server/api/users.get.ts
export default defineEventHandler(async (event) => {
  // Read query parameters
  const query = getQuery(event)
  
  // Return data (auto-serialized as JSON)
  return { users: ['Alice', 'Bob'] }
})

// server/api/users.post.ts
export default defineEventHandler(async (event) => {
  // Read and validate body
  const body = await readValidatedBody(event, z.object({
    name: z.string().min(1),
    email: z.string().email(),
  }).parse)
  
  // Return created resource
  return { user: body }
})

// server/api/users/[id].get.ts
export default defineEventHandler(async (event) => {
  const id = getRouterParam(event, 'id')
  // Or with validation:
  // const { id } = await getValidatedRouterParams(event, z.object({ id: z.string() }).parse)
  return { id, name: 'Alice' }
})

Error Handling in Server Routes

// ✅ CORRECT: Use createError to throw proper HTTP errors
export default defineEventHandler(async (event) => {
  const user = await findUser(event)
  if (!user) {
    throw createError({
      statusCode: 404,
      message: 'User not found',
    })
  }
  return user
})

// ❌ WRONG: Returning error objects (sends 200 with error body)
export default defineEventHandler(async (event) => {
  const user = await findUser(event)
  if (!user) {
    return { code: 404, message: 'User not found' } // DON'T DO THIS
  }
  return user
})

Server Middleware vs Server Utils (Important Pattern)

Prefer server utils over global server middleware for route-specific logic:

// server/utils/auth.ts (Auto-imported)
export async function requireAuth(event: H3Event) {
  const session = await getUserSession(event)
  if (!session) {
    throw createError({ statusCode: 401, message: 'Unauthorized' })
  }
  return session
}

// server/api/protected-data.get.ts
export default defineEventHandler(async (event) => {
  const session = await requireAuth(event) // Explicit, clear
  return { secret: 'data' }
})

Only use global server middleware for truly global concerns (logging, CORS headers):

// server/middleware/log.ts
export default defineEventHandler((event) => {
  console.log('Request:', getRequestURL(event))
  // Don't return anything — middleware should not respond
})

Event Handler with Guards Pattern (Nuxt 4 / h3)

// server/api/admin/dashboard.get.ts
export default defineEventHandler({
  onRequest: [
    (event) => requireAuth(event),
    (event) => requireRole(event, 'admin'),
  ],
  handler: async (event) => {
    return { dashboard: 'data' }
  },
})

5. Route Middleware (Client-Side)

Route middleware lives in app/middleware/ and runs in the Vue/browser context (not server).

Types of Middleware

Named: app/middleware/auth.ts → applied via definePageMeta
Global: app/middleware/tracking.global.ts → runs on every route change
Inline: Defined directly in definePageMeta

// app/middleware/auth.ts
export default defineNuxtRouteMiddleware((to, from) => {
  const { isLoggedIn } = useAuth()
  if (!isLoggedIn) {
    return navigateTo('/login')
  }
})

// Usage in a page:
// app/pages/dashboard.vue

definePageMeta({
  middleware: ['auth'], // or middleware: 'auth'
})

Nuxt 4: Async Middleware Support

// Nuxt 4 allows async middleware natively
export default defineNuxtRouteMiddleware(async (to, from) => {
  const { data } = await useFetch('/api/auth/check')
  if (!data.value?.authenticated) {
    return navigateTo('/login')
  }
})

Return Values

undefined or nothing → continue navigation
navigateTo('/path') → redirect (302 on server)
navigateTo('/path', { redirectCode: 301 }) → permanent redirect
abortNavigation() → cancel navigation
abortNavigation(error) → cancel with error

6. `nuxt.config.ts` — Nuxt 4 Patterns

// nuxt.config.ts
export default defineNuxtConfig({
  // ✅ Nuxt 4 defaults — no compatibilityVersion needed
  
  // Runtime config (with full IntelliSense in Nuxt 4)
  runtimeConfig: {
    apiSecret: '', // Server-only, set via NUXT_API_SECRET env var
    public: {
      apiBase: '', // Client + Server, set via NUXT_PUBLIC_API_BASE
    },
  },

  // Modules
  modules: [
    '@nuxtjs/supabase',
    '@nuxt/ui',
  ],

  // ❌ REMOVED in Nuxt 4 — don't use:
  // generate: { exclude: [...], routes: [...] }
  
  // ✅ Use nitro.prerender instead:
  nitro: {
    prerender: {
      ignore: ['/admin', '/private'],
      routes: ['/sitemap.xml'],
    },
  },

  // Route rules (SSR, caching, redirects)
  routeRules: {
    '/': { prerender: true },
    '/api/**': { cache: { maxAge: 60 * 60 } },
    '/old-page': { redirect: { to: '/new-page', statusCode: 302 } },
  },

  // Testing Nuxt 5 features early (optional)
  // future: {
  //   compatibilityVersion: 5,
  // },
})

7. `<head>` Management (Unhead v2)

Removed Props

// ❌ Nuxt 3 / Unhead v1 — REMOVED in Nuxt 4
useHead({
  meta: [{ name: 'description', content: '...', vmid: 'desc', hid: 'desc' }]
})

// ✅ Nuxt 4 / Unhead v2
useHead({
  meta: [{ name: 'description', content: '...' }]
})

Import from `#imports` Instead of `@unhead/vue`

// ✅ Preferred in Nuxt 4
import { useHead, useSeoMeta } from '#imports'

// ❌ Still works but not recommended
import { useHead } from '@unhead/vue'

`useSeoMeta` — Preferred for SEO

// app/pages/about.vue

useSeoMeta({
  title: 'About Us',
  description: 'Learn about our company.',
  ogTitle: 'About Us',
  ogDescription: 'Learn about our company.',
  ogImage: '/images/og-about.png',
})

8. Composables & Auto-Imports

Where to Place Composables

app/composables/ — auto-imported in app context
app/utils/ — auto-imported in app context
server/utils/ — auto-imported in server context
shared/ — shared between app and server (new in Nuxt 4)

Writing Composables

// app/composables/useCounter.ts
export function useCounter(initial = 0) {
  const count = useState('counter', () => initial)
  
  function increment() {
    count.value++
  }
  
  return { count, increment }
}

// Usage — auto-imported, no import needed:
// app/pages/index.vue

const { count, increment } = useCounter()

`useState` for Shared State

// ✅ Use useState for SSR-safe reactive state shared across components
const user = useState('user', () => null)

// ❌ DON'T use plain ref for shared state (causes hydration mismatches)
const user = ref(null)

9. TypeScript in Nuxt 4

Nuxt 4 generates a tsconfig.json with project references that separate:

app/ context (Vue, browser APIs, auto-imports)
server/ context (h3, Nitro, Node APIs)
shared/ context (code shared between both)
Configuration files

This means:

Server utils are NOT available in app/ code (and vice versa)
Use shared/ for types and functions needed in both contexts
IDE auto-complete is context-aware

Type-Safe Fetch

// Nuxt 4 infers API response types from server routes
const { data } = await useFetch('/api/users')
// data is typed based on the return type of server/api/users.ts

10. Common Mistakes & Anti-Patterns

❌ Mistake 1: Placing `server/` Inside `app/`

# WRONG
app/
  server/     ← Server code must be at project root!
  pages/

# CORRECT
app/
  pages/
server/       ← At root level

❌ Mistake 2: Using `generate` Config

// WRONG — removed in Nuxt 4
export default defineNuxtConfig({
  generate: {
    routes: ['/about', '/contact']
  }
})

// CORRECT
export default defineNuxtConfig({
  nitro: {
    prerender: {
      routes: ['/about', '/contact']
    }
  }
})

❌ Mistake 3: Using `$fetch` in Component Setup

// WRONG — causes double fetching (SSR + client hydration)

const users = ref([])
users.value = await $fetch('/api/users')


// CORRECT — SSR-safe, transfers payload to client

const { data: users } = await useFetch('/api/users')

❌ Mistake 4: Using `vmid` or `hid` in `useHead`

// WRONG — removed in Unhead v2
useHead({ meta: [{ name: 'desc', content: '...', hid: 'desc' }] })

// CORRECT
useHead({ meta: [{ name: 'description', content: '...' }] })

❌ Mistake 5: Returning Error Objects from Server Routes

// WRONG — returns 200 with error body
return { statusCode: 404, message: 'Not found' }

// CORRECT — throws proper HTTP error
throw createError({ statusCode: 404, message: 'Not found' })

❌ Mistake 6: Accessing Route Meta Duplicates

// WRONG (Nuxt 4) — metadata is no longer duplicated
const name = route.meta.name

// CORRECT
const name = route.name

❌ Mistake 7: Deep Reactivity Assumptions

// In Nuxt 4, data from useFetch is a shallowRef
const { data } = await useFetch('/api/nested-object')

// WRONG — mutating nested properties won't trigger reactivity
data.value.nested.property = 'new value'

// CORRECT — replace the entire value or use deep: true
data.value = { ...data.value, nested: { ...data.value.nested, property: 'new value' } }
// OR use deep: true (performance cost):
const { data } = await useFetch('/api/nested-object', { deep: true })

❌ Mistake 8: Using `pages:extend` to Override Page Meta

// WRONG in Nuxt 4 — metadata scanning happens after pages:extend
hooks: {
  'pages:extend'(pages) {
    pages[0].meta.layout = 'custom' // May not work
  }
}

// CORRECT — use pages:resolved
hooks: {
  'pages:resolved'(pages) {
    pages[0].meta.layout = 'custom'
  }
}

11. SSR-Safe Cookie and Session Handling

When calling protected API routes during SSR, the cookie is not automatically forwarded:

// ❌ WRONG — cookie not forwarded during SSR
const { data } = await useFetch('/api/protected')

// ✅ CORRECT — forward cookies during SSR
const { data } = await useFetch('/api/protected', {
  headers: useRequestHeaders(['cookie']),
})

For auth patterns, consider using nuxt-auth-utils which handles this automatically with useUserSession().

12. Environment Variables & Runtime Config

// nuxt.config.ts
export default defineNuxtConfig({
  runtimeConfig: {
    // Server-only (never exposed to client)
    supabaseServiceKey: '',       // → NUXT_SUPABASE_SERVICE_KEY
    
    // Client + server
    public: {
      supabaseUrl: '',            // → NUXT_PUBLIC_SUPABASE_URL
      supabaseAnonKey: '',        // → NUXT_PUBLIC_SUPABASE_ANON_KEY
    },
  },
})

// In app code (client or server)
const config = useRuntimeConfig()
config.public.supabaseUrl

// In server code only
const config = useRuntimeConfig()
config.supabaseServiceKey // Server-only key

13. Migration Checklist (Nuxt 3 → 4)

Update Node.js to 18.20+ (LTS 20+ recommended)
Install Nuxt 4: npm install nuxt@^4.0.0
Run codemods: npx codemod@0.18.7 nuxt/4/migration-recipe
Move source files into app/ directory
Keep server/, content/, public/ at project root
Replace generate config with nitro.prerender
Remove vmid/hid from useHead calls
Review getCachedData implementations (new context parameter)
Check useAsyncData keys for conflicting options
Update route.meta.name to route.name
Update pages:extend meta overrides to pages:resolved
Test thoroughly — especially data fetching, middleware, and SSR

14. Recommended Module Ecosystem

Module	Purpose
`@nuxt/ui`	UI component library (v4 for Nuxt 4)
`@nuxtjs/supabase`	Supabase integration
`nuxt-auth-utils`	Session/auth management
`@nuxt/content`	Content management (v3 for Nuxt 4)
`@nuxtjs/tailwindcss`	Tailwind CSS integration
`@nuxt/image`	Image optimization
`@nuxt/eslint`	ESLint configuration
`@pinia/nuxt`	State management

Always check module compatibility with Nuxt 4 before installing. Many popular modules have released Nuxt 4-compatible versions.

15. Official Documentation Links

Nuxt 4 Docs: https://nuxt.com/docs/4.x
Upgrade Guide: https://nuxt.com/docs/4.x/getting-started/upgrade
Data Fetching: https://nuxt.com/docs/4.x/getting-started/data-fetching
Server Directory: https://nuxt.com/docs/4.x/directory-structure/server
Middleware: https://nuxt.com/docs/4.x/directory-structure/app/middleware
Deployment: https://nuxt.com/docs/4.x/getting-started/deployment
API Reference: https://nuxt.com/docs/4.x/api
Nuxt 4 Announcement: https://nuxt.com/blog/v4
Roadmap: https://nuxt.com/blog/roadmap-v4

Agent Instructions

When writing Nuxt code:

Always use the app/ directory structure for new projects.
Never use $fetch in component <script setup> — use useFetch or useAsyncData.
Remember shallowRef is the default — don't assume deep reactivity on fetched data.
Use createError() for server route errors, never return error objects.
Forward cookies with useRequestHeaders(['cookie']) when calling protected APIs during SSR.
Prefer server utils over global server middleware for route-specific auth/validation.
Use #imports instead of importing from @unhead/vue or other Nuxt-provided packages directly.
Check the Nuxt 4 docs at https://nuxt.com/docs/4.x for anything not covered here.
Nuxt auto-imports are available — don't manually import defineEventHandler, useFetch, useHead, etc. unless needed for type clarity.
Use shared/ directory for code that needs to run in both app and server contexts.

name	nuxt-4-skill
description	This skill ensures AI agents write correct, modern Nuxt 4 code and avoid common mistakes from outdated Nuxt 3 patterns. Always read this before writing any Nuxt code.

Nuxt 4 Development Skill

Purpose: This skill ensures AI agents write correct, modern Nuxt 4 code and avoid common mistakes from outdated Nuxt 3 patterns. Always read this before writing any Nuxt code. Last verified: February 2025 | Nuxt 4.x stable (released July 2025)

1. Critical Context: Nuxt 4 vs Nuxt 3

If you have training data from Nuxt 3, many patterns are now outdated. Always prefer the patterns documented in this skill.

Key Differences at a Glance

Area	Nuxt 3 (Old)	Nuxt 4 (Current)
Directory structure	Flat root (`pages/`, `components/` at root)	`app/` directory (source code lives in `app/`)
Data fetching reactivity	Deep reactive refs by default	`shallowRef` by default for better performance
Data fetching keys	Independent per call	Singleton: shared refs for same key
Route middleware	`defineNuxtRouteMiddleware` (some guides)	`defineNuxtRouteMiddleware` with async support
`<head>` management	Unhead v1	Unhead v2 (removed `vmid`, `hid`, `children`)
Component names	Vue default naming	Normalized to match Nuxt auto-import pattern
Module loading in layers	Project modules loaded first (incorrect)	Layer modules first, project modules last (correct)
Route metadata	Available on both `route.name` and `route.meta.name`	Only on `route.name` (deduplicated)
Page meta scanning	Before `pages:extend` hook	After `pages:extend` hook
Node.js requirement	16+	18.20+ (LTS 20+ recommended)
Generate config	`generate.exclude`, `generate.routes`	Removed. Use `nitro.prerender` instead
Inline styles	All CSS inlined	Only Vue component CSS inlined; global CSS as separate file

2. Project Structure (CRITICAL CHANGE)

Nuxt 4 Default Structure

my-nuxt-app/
├── app/                    # ← ALL application source code goes here
│   ├── assets/
│   ├── components/
│   ├── composables/
│   ├── layouts/
│   ├── middleware/          # Route middleware (client-side)
│   ├── pages/
│   ├── plugins/
│   ├── utils/
│   ├── app.vue
│   ├── app.config.ts
│   ├── error.vue
│   ├── router.options.ts
│   └── spa-loading-template.html
├── content/                # Nuxt Content (at root, NOT inside app/)
├── layers/                 # Nuxt Layers (at root)
├── modules/                # Local modules (at root)
├── public/                 # Static assets (at root)
├── shared/                 # Shared code between app and server
├── server/                 # Server code (at root, NOT inside app/)
│   ├── api/
│   ├── middleware/          # Server middleware (Nitro/h3)
│   ├── plugins/
│   ├── routes/
│   └── utils/
├── nuxt.config.ts
├── package.json
└── tsconfig.json

Rules

The ~ alias now points to app/ by default (your srcDir).
~/components resolves to app/components/, ~/pages to app/pages/, etc.
server/, content/, layers/, modules/, and public/ stay at the project root, NOT inside app/.
If the project still uses the flat (Nuxt 3) structure, Nuxt auto-detects it — but always prefer the new structure for new projects.

Why This Matters

Performance: Prevents .git/ and node_modules/ from being scanned by file watchers.
IDE Type Safety: Separates app context from server context for accurate auto-completion.

Reverting to Nuxt 3 Structure (if needed)

// nuxt.config.ts
export default defineNuxtConfig({
  srcDir: '.',
  dir: {
    app: 'app',
  },
})

3. Data Fetching (SIGNIFICANT CHANGES)

The Three Methods

Method	Use Case
`useFetch`	Simple data fetching in components (wrapper around `useAsyncData` + `$fetch`)
`useAsyncData`	Complex async operations, custom fetch logic
`$fetch`	Direct HTTP calls (event handlers, non-component code, client-only actions)

Critical Nuxt 4 Changes

3.1 Shallow Reactivity by Default

// Nuxt 4: data is a shallowRef (NOT deeply reactive)
const { data } = await useFetch('/api/users')

// If you need deep reactivity (rare, impacts performance):
const { data } = await useFetch('/api/users', { deep: true })

Do NOT set deep: true globally unless absolutely necessary. ShallowRef provides massive performance improvements on nested data structures.

3.2 Singleton Data Fetching (Shared Refs)

All calls to useAsyncData/useFetch with the same key now share the same data, error, and status refs.

// ✅ CORRECT: Same key = shared data across components
// Component A
const { data: product } = await useFetch(`/api/products/${id}`, {
  key: `product-${id}`
})

// Component B (different file, same key)
const { data: product } = await useFetch(`/api/products/${id}`, {
  key: `product-${id}`
})
// → No duplicate API call! Uses the same shared ref.

Important Rules:

All calls with the same explicit key must have identical deep, transform, pick, getCachedData, and default options.
Extract shared data fetching into composables to avoid inconsistencies:

// app/composables/useProduct.ts
export function useProduct(productId: string) {
  return useAsyncData(
    `product-${productId}`,
    () => $fetch(`/api/products/${productId}`),
    {
      deep: true,
      transform: (product) => ({
        ...product,
        formattedPrice: `${product.price.toFixed(2)} kr`,
      }),
    }
  )
}

3.3 `getCachedData` Now Receives Context

// Nuxt 4: getCachedData has a context parameter
useAsyncData('key', fetchFunction, {
  getCachedData: (key, nuxtApp, ctx) => {
    // ctx.cause can be: 'initial' | 'refresh:hook' | 'refresh:manual' | 'watch'
    
    // Example: Always refetch on manual refresh
    if (ctx.cause === 'refresh:manual') return undefined
    
    return nuxtApp.payload.data[key] || nuxtApp.static.data[key]
  }
})

3.4 Reactive Keys

// Nuxt 4: Keys can be reactive (computed, ref, getter)
const id = computed(() => route.params.id)

const { data: post } = await useFetch(() => `/api/posts/${id.value}`)
// → Automatically refetches when id changes

3.5 Automatic Data Cleanup

When the last component using a useAsyncData key unmounts, Nuxt automatically removes the cached data to prevent memory leaks.

3.6 Common Data Fetching Mistakes to Avoid

// ❌ WRONG: Using $fetch in component setup (causes double fetch on SSR + hydration)
const data = ref(null)
data.value = await $fetch('/api/users')

// ✅ CORRECT: Use useFetch or useAsyncData in components
const { data } = await useFetch('/api/users')

// ❌ WRONG: Conflicting options with same key
const { data: a } = useAsyncData('users', () => $fetch('/api/users'), { deep: false })
const { data: b } = useAsyncData('users', () => $fetch('/api/users'), { deep: true })
// → Nuxt 4 will warn about this!

// ✅ CORRECT: Use $fetch in server routes and event handlers
// server/api/combined.get.ts
export default defineEventHandler(async () => {
  const [users, posts] = await Promise.all([
    $fetch('/api/users'),
    $fetch('/api/posts'),
  ])
  return { users, posts }
})

4. Server API Routes

Server routes live in server/ at the project root (NOT inside app/).

File-Based Routing

server/
├── api/
│   ├── hello.ts            → GET/POST/etc. /api/hello
│   ├── hello.get.ts        → GET /api/hello
│   ├── hello.post.ts       → POST /api/hello
│   ├── users/
│   │   ├── index.get.ts    → GET /api/users
│   │   ├── index.post.ts   → POST /api/users
│   │   └── [id].get.ts     → GET /api/users/:id
├── routes/
│   └── health.ts           → /health (no /api prefix)
├── middleware/
│   └── log.ts              → Runs on every request
├── plugins/
│   └── startup.ts          → Nitro server plugins
└── utils/
    └── auth.ts             → Auto-imported server utilities

Writing Event Handlers

// server/api/users.get.ts
export default defineEventHandler(async (event) => {
  // Read query parameters
  const query = getQuery(event)
  
  // Return data (auto-serialized as JSON)
  return { users: ['Alice', 'Bob'] }
})

// server/api/users.post.ts
export default defineEventHandler(async (event) => {
  // Read and validate body
  const body = await readValidatedBody(event, z.object({
    name: z.string().min(1),
    email: z.string().email(),
  }).parse)
  
  // Return created resource
  return { user: body }
})

// server/api/users/[id].get.ts
export default defineEventHandler(async (event) => {
  const id = getRouterParam(event, 'id')
  // Or with validation:
  // const { id } = await getValidatedRouterParams(event, z.object({ id: z.string() }).parse)
  return { id, name: 'Alice' }
})

Error Handling in Server Routes

// ✅ CORRECT: Use createError to throw proper HTTP errors
export default defineEventHandler(async (event) => {
  const user = await findUser(event)
  if (!user) {
    throw createError({
      statusCode: 404,
      message: 'User not found',
    })
  }
  return user
})

// ❌ WRONG: Returning error objects (sends 200 with error body)
export default defineEventHandler(async (event) => {
  const user = await findUser(event)
  if (!user) {
    return { code: 404, message: 'User not found' } // DON'T DO THIS
  }
  return user
})

Server Middleware vs Server Utils (Important Pattern)

Prefer server utils over global server middleware for route-specific logic:

// server/utils/auth.ts (Auto-imported)
export async function requireAuth(event: H3Event) {
  const session = await getUserSession(event)
  if (!session) {
    throw createError({ statusCode: 401, message: 'Unauthorized' })
  }
  return session
}

// server/api/protected-data.get.ts
export default defineEventHandler(async (event) => {
  const session = await requireAuth(event) // Explicit, clear
  return { secret: 'data' }
})

Only use global server middleware for truly global concerns (logging, CORS headers):

// server/middleware/log.ts
export default defineEventHandler((event) => {
  console.log('Request:', getRequestURL(event))
  // Don't return anything — middleware should not respond
})

Event Handler with Guards Pattern (Nuxt 4 / h3)

// server/api/admin/dashboard.get.ts
export default defineEventHandler({
  onRequest: [
    (event) => requireAuth(event),
    (event) => requireRole(event, 'admin'),
  ],
  handler: async (event) => {
    return { dashboard: 'data' }
  },
})

5. Route Middleware (Client-Side)

Route middleware lives in app/middleware/ and runs in the Vue/browser context (not server).

Types of Middleware

Named: app/middleware/auth.ts → applied via definePageMeta
Global: app/middleware/tracking.global.ts → runs on every route change
Inline: Defined directly in definePageMeta

// app/middleware/auth.ts
export default defineNuxtRouteMiddleware((to, from) => {
  const { isLoggedIn } = useAuth()
  if (!isLoggedIn) {
    return navigateTo('/login')
  }
})

// Usage in a page:
// app/pages/dashboard.vue

definePageMeta({
  middleware: ['auth'], // or middleware: 'auth'
})

Nuxt 4: Async Middleware Support

// Nuxt 4 allows async middleware natively
export default defineNuxtRouteMiddleware(async (to, from) => {
  const { data } = await useFetch('/api/auth/check')
  if (!data.value?.authenticated) {
    return navigateTo('/login')
  }
})

Return Values

undefined or nothing → continue navigation
navigateTo('/path') → redirect (302 on server)
navigateTo('/path', { redirectCode: 301 }) → permanent redirect
abortNavigation() → cancel navigation
abortNavigation(error) → cancel with error

6. `nuxt.config.ts` — Nuxt 4 Patterns

// nuxt.config.ts
export default defineNuxtConfig({
  // ✅ Nuxt 4 defaults — no compatibilityVersion needed
  
  // Runtime config (with full IntelliSense in Nuxt 4)
  runtimeConfig: {
    apiSecret: '', // Server-only, set via NUXT_API_SECRET env var
    public: {
      apiBase: '', // Client + Server, set via NUXT_PUBLIC_API_BASE
    },
  },

  // Modules
  modules: [
    '@nuxtjs/supabase',
    '@nuxt/ui',
  ],

  // ❌ REMOVED in Nuxt 4 — don't use:
  // generate: { exclude: [...], routes: [...] }
  
  // ✅ Use nitro.prerender instead:
  nitro: {
    prerender: {
      ignore: ['/admin', '/private'],
      routes: ['/sitemap.xml'],
    },
  },

  // Route rules (SSR, caching, redirects)
  routeRules: {
    '/': { prerender: true },
    '/api/**': { cache: { maxAge: 60 * 60 } },
    '/old-page': { redirect: { to: '/new-page', statusCode: 302 } },
  },

  // Testing Nuxt 5 features early (optional)
  // future: {
  //   compatibilityVersion: 5,
  // },
})

7. `<head>` Management (Unhead v2)

Removed Props

// ❌ Nuxt 3 / Unhead v1 — REMOVED in Nuxt 4
useHead({
  meta: [{ name: 'description', content: '...', vmid: 'desc', hid: 'desc' }]
})

// ✅ Nuxt 4 / Unhead v2
useHead({
  meta: [{ name: 'description', content: '...' }]
})

Import from `#imports` Instead of `@unhead/vue`

// ✅ Preferred in Nuxt 4
import { useHead, useSeoMeta } from '#imports'

// ❌ Still works but not recommended
import { useHead } from '@unhead/vue'

`useSeoMeta` — Preferred for SEO

// app/pages/about.vue

useSeoMeta({
  title: 'About Us',
  description: 'Learn about our company.',
  ogTitle: 'About Us',
  ogDescription: 'Learn about our company.',
  ogImage: '/images/og-about.png',
})

8. Composables & Auto-Imports

Where to Place Composables

app/composables/ — auto-imported in app context
app/utils/ — auto-imported in app context
server/utils/ — auto-imported in server context
shared/ — shared between app and server (new in Nuxt 4)

Writing Composables

// app/composables/useCounter.ts
export function useCounter(initial = 0) {
  const count = useState('counter', () => initial)
  
  function increment() {
    count.value++
  }
  
  return { count, increment }
}

// Usage — auto-imported, no import needed:
// app/pages/index.vue

const { count, increment } = useCounter()

`useState` for Shared State

// ✅ Use useState for SSR-safe reactive state shared across components
const user = useState('user', () => null)

// ❌ DON'T use plain ref for shared state (causes hydration mismatches)
const user = ref(null)

9. TypeScript in Nuxt 4

Nuxt 4 generates a tsconfig.json with project references that separate:

app/ context (Vue, browser APIs, auto-imports)
server/ context (h3, Nitro, Node APIs)
shared/ context (code shared between both)
Configuration files

This means:

Server utils are NOT available in app/ code (and vice versa)
Use shared/ for types and functions needed in both contexts
IDE auto-complete is context-aware

Type-Safe Fetch

// Nuxt 4 infers API response types from server routes
const { data } = await useFetch('/api/users')
// data is typed based on the return type of server/api/users.ts

10. Common Mistakes & Anti-Patterns

❌ Mistake 1: Placing `server/` Inside `app/`

# WRONG
app/
  server/     ← Server code must be at project root!
  pages/

# CORRECT
app/
  pages/
server/       ← At root level

❌ Mistake 2: Using `generate` Config

// WRONG — removed in Nuxt 4
export default defineNuxtConfig({
  generate: {
    routes: ['/about', '/contact']
  }
})

// CORRECT
export default defineNuxtConfig({
  nitro: {
    prerender: {
      routes: ['/about', '/contact']
    }
  }
})

❌ Mistake 3: Using `$fetch` in Component Setup

// WRONG — causes double fetching (SSR + client hydration)

const users = ref([])
users.value = await $fetch('/api/users')


// CORRECT — SSR-safe, transfers payload to client

const { data: users } = await useFetch('/api/users')

❌ Mistake 4: Using `vmid` or `hid` in `useHead`

// WRONG — removed in Unhead v2
useHead({ meta: [{ name: 'desc', content: '...', hid: 'desc' }] })

// CORRECT
useHead({ meta: [{ name: 'description', content: '...' }] })

❌ Mistake 5: Returning Error Objects from Server Routes

// WRONG — returns 200 with error body
return { statusCode: 404, message: 'Not found' }

// CORRECT — throws proper HTTP error
throw createError({ statusCode: 404, message: 'Not found' })

❌ Mistake 6: Accessing Route Meta Duplicates

// WRONG (Nuxt 4) — metadata is no longer duplicated
const name = route.meta.name

// CORRECT
const name = route.name

❌ Mistake 7: Deep Reactivity Assumptions

// In Nuxt 4, data from useFetch is a shallowRef
const { data } = await useFetch('/api/nested-object')

// WRONG — mutating nested properties won't trigger reactivity
data.value.nested.property = 'new value'

// CORRECT — replace the entire value or use deep: true
data.value = { ...data.value, nested: { ...data.value.nested, property: 'new value' } }
// OR use deep: true (performance cost):
const { data } = await useFetch('/api/nested-object', { deep: true })

❌ Mistake 8: Using `pages:extend` to Override Page Meta

// WRONG in Nuxt 4 — metadata scanning happens after pages:extend
hooks: {
  'pages:extend'(pages) {
    pages[0].meta.layout = 'custom' // May not work
  }
}

// CORRECT — use pages:resolved
hooks: {
  'pages:resolved'(pages) {
    pages[0].meta.layout = 'custom'
  }
}

11. SSR-Safe Cookie and Session Handling

When calling protected API routes during SSR, the cookie is not automatically forwarded:

// ❌ WRONG — cookie not forwarded during SSR
const { data } = await useFetch('/api/protected')

// ✅ CORRECT — forward cookies during SSR
const { data } = await useFetch('/api/protected', {
  headers: useRequestHeaders(['cookie']),
})

For auth patterns, consider using nuxt-auth-utils which handles this automatically with useUserSession().

12. Environment Variables & Runtime Config

// nuxt.config.ts
export default defineNuxtConfig({
  runtimeConfig: {
    // Server-only (never exposed to client)
    supabaseServiceKey: '',       // → NUXT_SUPABASE_SERVICE_KEY
    
    // Client + server
    public: {
      supabaseUrl: '',            // → NUXT_PUBLIC_SUPABASE_URL
      supabaseAnonKey: '',        // → NUXT_PUBLIC_SUPABASE_ANON_KEY
    },
  },
})

// In app code (client or server)
const config = useRuntimeConfig()
config.public.supabaseUrl

// In server code only
const config = useRuntimeConfig()
config.supabaseServiceKey // Server-only key

13. Migration Checklist (Nuxt 3 → 4)

Update Node.js to 18.20+ (LTS 20+ recommended)
Install Nuxt 4: npm install nuxt@^4.0.0
Run codemods: npx codemod@0.18.7 nuxt/4/migration-recipe
Move source files into app/ directory
Keep server/, content/, public/ at project root
Replace generate config with nitro.prerender
Remove vmid/hid from useHead calls
Review getCachedData implementations (new context parameter)
Check useAsyncData keys for conflicting options
Update route.meta.name to route.name
Update pages:extend meta overrides to pages:resolved
Test thoroughly — especially data fetching, middleware, and SSR

14. Recommended Module Ecosystem

Module	Purpose
`@nuxt/ui`	UI component library (v4 for Nuxt 4)
`@nuxtjs/supabase`	Supabase integration
`nuxt-auth-utils`	Session/auth management
`@nuxt/content`	Content management (v3 for Nuxt 4)
`@nuxtjs/tailwindcss`	Tailwind CSS integration
`@nuxt/image`	Image optimization
`@nuxt/eslint`	ESLint configuration
`@pinia/nuxt`	State management

Always check module compatibility with Nuxt 4 before installing. Many popular modules have released Nuxt 4-compatible versions.

15. Official Documentation Links

Nuxt 4 Docs: https://nuxt.com/docs/4.x
Upgrade Guide: https://nuxt.com/docs/4.x/getting-started/upgrade
Data Fetching: https://nuxt.com/docs/4.x/getting-started/data-fetching
Server Directory: https://nuxt.com/docs/4.x/directory-structure/server
Middleware: https://nuxt.com/docs/4.x/directory-structure/app/middleware
Deployment: https://nuxt.com/docs/4.x/getting-started/deployment
API Reference: https://nuxt.com/docs/4.x/api
Nuxt 4 Announcement: https://nuxt.com/blog/v4
Roadmap: https://nuxt.com/blog/roadmap-v4

Agent Instructions

When writing Nuxt code:

Always use the app/ directory structure for new projects.
Never use $fetch in component <script setup> — use useFetch or useAsyncData.
Remember shallowRef is the default — don't assume deep reactivity on fetched data.
Use createError() for server route errors, never return error objects.
Forward cookies with useRequestHeaders(['cookie']) when calling protected APIs during SSR.
Prefer server utils over global server middleware for route-specific auth/validation.
Use #imports instead of importing from @unhead/vue or other Nuxt-provided packages directly.
Check the Nuxt 4 docs at https://nuxt.com/docs/4.x for anything not covered here.
Nuxt auto-imports are available — don't manually import defineEventHandler, useFetch, useHead, etc. unless needed for type clarity.
Use shared/ directory for code that needs to run in both app and server contexts.

nuxt-4-skill

Nuxt 4 Development Skill

1. Critical Context: Nuxt 4 vs Nuxt 3

Key Differences at a Glance

2. Project Structure (CRITICAL CHANGE)

Nuxt 4 Default Structure

Rules

Why This Matters

Reverting to Nuxt 3 Structure (if needed)

3. Data Fetching (SIGNIFICANT CHANGES)

The Three Methods

Critical Nuxt 4 Changes

3.1 Shallow Reactivity by Default

3.2 Singleton Data Fetching (Shared Refs)

3.3 getCachedData Now Receives Context

3.4 Reactive Keys

3.5 Automatic Data Cleanup

3.6 Common Data Fetching Mistakes to Avoid

4. Server API Routes

File-Based Routing

Writing Event Handlers

Error Handling in Server Routes

Server Middleware vs Server Utils (Important Pattern)

Event Handler with Guards Pattern (Nuxt 4 / h3)

5. Route Middleware (Client-Side)

Types of Middleware

Nuxt 4: Async Middleware Support

Return Values

6. nuxt.config.ts — Nuxt 4 Patterns

7. <head> Management (Unhead v2)

Removed Props

Import from #imports Instead of @unhead/vue

useSeoMeta — Preferred for SEO

8. Composables & Auto-Imports

Where to Place Composables

Writing Composables

useState for Shared State

9. TypeScript in Nuxt 4

Type-Safe Fetch

10. Common Mistakes & Anti-Patterns

❌ Mistake 1: Placing server/ Inside app/

❌ Mistake 2: Using generate Config

❌ Mistake 3: Using $fetch in Component Setup

❌ Mistake 4: Using vmid or hid in useHead

❌ Mistake 5: Returning Error Objects from Server Routes

❌ Mistake 6: Accessing Route Meta Duplicates

❌ Mistake 7: Deep Reactivity Assumptions

❌ Mistake 8: Using pages:extend to Override Page Meta

11. SSR-Safe Cookie and Session Handling

12. Environment Variables & Runtime Config

13. Migration Checklist (Nuxt 3 → 4)

14. Recommended Module Ecosystem

15. Official Documentation Links

Agent Instructions

Nuxt 4 Development Skill

1. Critical Context: Nuxt 4 vs Nuxt 3

Key Differences at a Glance

2. Project Structure (CRITICAL CHANGE)

Nuxt 4 Default Structure

Rules

Why This Matters

Reverting to Nuxt 3 Structure (if needed)

3. Data Fetching (SIGNIFICANT CHANGES)

The Three Methods

Critical Nuxt 4 Changes

3.1 Shallow Reactivity by Default

3.2 Singleton Data Fetching (Shared Refs)

3.3 getCachedData Now Receives Context

3.4 Reactive Keys

3.5 Automatic Data Cleanup

3.6 Common Data Fetching Mistakes to Avoid

4. Server API Routes

File-Based Routing

Writing Event Handlers

Error Handling in Server Routes

Server Middleware vs Server Utils (Important Pattern)

Event Handler with Guards Pattern (Nuxt 4 / h3)

5. Route Middleware (Client-Side)

Types of Middleware

Nuxt 4: Async Middleware Support

3.3 `getCachedData` Now Receives Context

6. `nuxt.config.ts` — Nuxt 4 Patterns

7. `<head>` Management (Unhead v2)

Import from `#imports` Instead of `@unhead/vue`

`useSeoMeta` — Preferred for SEO

`useState` for Shared State

❌ Mistake 1: Placing `server/` Inside `app/`

❌ Mistake 2: Using `generate` Config

❌ Mistake 3: Using `$fetch` in Component Setup

❌ Mistake 4: Using `vmid` or `hid` in `useHead`

❌ Mistake 8: Using `pages:extend` to Override Page Meta

3.3 `getCachedData` Now Receives Context

6. `nuxt.config.ts` — Nuxt 4 Patterns

7. `<head>` Management (Unhead v2)

Import from `#imports` Instead of `@unhead/vue`

`useSeoMeta` — Preferred for SEO

`useState` for Shared State

❌ Mistake 1: Placing `server/` Inside `app/`

❌ Mistake 2: Using `generate` Config

❌ Mistake 3: Using `$fetch` in Component Setup

❌ Mistake 4: Using `vmid` or `hid` in `useHead`

❌ Mistake 8: Using `pages:extend` to Override Page Meta