Streaming SSR · Unhead

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- Core Concepts
  - [Titles & Title Templates](https://unhead.unjs.io/docs/svelte/head/guides/core-concepts/titles)
  - [Tag Sorting & Placement](https://unhead.unjs.io/docs/svelte/head/guides/core-concepts/positions)
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  - [Tag Deduplication](https://unhead.unjs.io/docs/svelte/head/guides/core-concepts/handling-duplicates)
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  - [Script Loading](https://unhead.unjs.io/docs/svelte/head/guides/core-concepts/loading-scripts)
  - [Reactivity](https://unhead.unjs.io/docs/svelte/head/guides/core-concepts/reactivity)
  - [StreamingNew](https://unhead.unjs.io/docs/svelte/head/guides/core-concepts/streaming)
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Core Concepts

# Streaming SSR

[Copy for LLMs](https://raw.githubusercontent.com/unjs/unhead/refs/heads/main/docs/0.svelte/head/guides/1.core-concepts/5.streaming.md)

Last updated Apr 10, 2026 by [Harlan Wilton](https://github.com/harlan-zw) in [doc: v3 beta -> stable](https://github.com/unjs/unhead/commit/17c123eb9e8974bf72973246efdf26a199fd11bd).

On this page

- [How It Works](#how-it-works)
- [Setup](#setup)
- [Usage](#usage)
- [When to Skip](#when-to-skip)

Standard SSR waits for everything to render before sending HTML. Streaming sends the document shell immediately, then streams content as async components resolve.

The problem: async components using `useHead()` set head tags _after_ the initial render. Without streaming support, those tags never reach the client's `<head>`.

Unhead's streaming integration solves this by injecting `<script>` patches into the stream as content resolves, updating the `<head>` in real-time.

## [How It Works](#how-it-works)

1. **Shell renders** - Initial `<head>` tags render with the document shell
2. **Async content resolves** - Components call `useHead()`
3. **Patches stream** - Unhead injects DOM updates as inline scripts
4. **Client hydrates** - The client head instance picks up the final state

## [Setup](#setup)

### [Vite Plugin](#vite-plugin)

The plugin transforms your components to enable streaming head updates:

```
// vite.config.ts
import { unheadSveltePlugin } from '@unhead/svelte/stream/vite'
import { svelte } from '@sveltejs/vite-plugin-svelte'
import { defineConfig } from 'vite'

export default defineConfig({
  plugins: [
    svelte(),
    unheadSveltePlugin(),
  ],
})
```

### [Server Entry](#server-entry)

```
// entry-server.ts
import { render as _render } from 'svelte/server'
import { createStreamableHead, UnheadContextKey } from '@unhead/svelte/stream/server'
import App from './App.svelte'

export async function render(url: string, template: string) {
  const { head, wrapStream } = createStreamableHead()
  const context = new Map()
  context.set(UnheadContextKey, head)

  const rendered = await _render(App, {
    props: { url },
    context,
  })

  const svelteStream = new ReadableStream({
    start(controller) {
      controller.enqueue(new TextEncoder().encode(rendered.body))
      controller.close()
    },
  })

  return wrapStream(svelteStream, template)
}
```

### [Client Entry](#client-entry)

```
// entry-client.ts
import { mount } from 'svelte'
import { createStreamableHead, UnheadContextKey } from '@unhead/svelte/stream/client'
import App from './App.svelte'

const head = createStreamableHead()
const context = new Map()
context.set(UnheadContextKey, head)

mount(App, {
  target: document.getElementById('app')!,
  context,
})
```

## [Usage](#usage)

Use `useHead()` normally in your components. Tags stream automatically as async content resolves:

```
<script lang="ts">
import { useHead } from '@unhead/svelte'

const { data } = $props()

useHead({
  title: data.title,
  meta: [
    { name: 'description', content: data.description }
  ]
})
</script>

<div>{data.content}</div>
```

## [When to Skip](#when-to-skip)

If your Svelte SSR is fully synchronous, stick with standard SSR. The streaming setup adds complexity for no benefit when all head tags are available at initial render.

[Edit this page](https://github.com/unjs/unhead/edit/main/docs/0.svelte/head/guides/1.core-concepts/5.streaming.md)

[Markdown For LLMs](https://raw.githubusercontent.com/unjs/unhead/refs/heads/main/docs/0.svelte/head/guides/1.core-concepts/5.streaming.md)

Did this page help you?

[Reactivity Use Svelte 5 $state() and $effect() with useHead() for reactive head tags. Use patch() to update entries.](https://unhead.unjs.io/docs/svelte/head/guides/core-concepts/reactivity) [Installation Add Schema.org to Svelte apps with @unhead/schema-org. Setup defineWebSite(), defineWebPage() for Google Rich Results.](https://unhead.unjs.io/docs/svelte/schema-org/guides/get-started/installation)

On this page

- [How It Works](#how-it-works)
- [Setup](#setup)
- [Usage](#usage)
- [When to Skip](#when-to-skip)

[GitHub](https://github.com/unjs/unhead) [ Discord](https://discord.com/invite/275MBUBvgP)

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[Part of the UnJS ecosystem](https://unjs.io/)

### Head Management

- [Getting Started](https://unhead.unjs.io/docs/svelte/head/guides/get-started/overview)
- [useHead](https://unhead.unjs.io/docs/svelte/head/api/composables/use-head)
- [useSeoMeta](https://unhead.unjs.io/docs/svelte/head/api/composables/use-seo-meta)
- [useHeadSafe](https://unhead.unjs.io/docs/svelte/head/api/composables/use-head-safe)
- [useScript](https://unhead.unjs.io/docs/svelte/head/api/composables/use-script)

### Schema.org

- [Getting Started](https://unhead.unjs.io/docs/svelte/schema-org/guides/get-started/overview)
- [useSchemaOrg](https://unhead.unjs.io/docs/svelte/schema-org/api/composables/use-schema-org)
- [Nodes](https://unhead.unjs.io/docs/svelte/schema-org/guides/core-concepts/nodes)
- [Recipes](https://unhead.unjs.io/docs/svelte/schema-org/guides/recipes/identity)

### Guides

- [Titles](https://unhead.unjs.io/docs/svelte/head/guides/core-concepts/titles)
- [Streaming SSR](https://unhead.unjs.io/docs/svelte/head/guides/core-concepts/streaming)
- [DOM Events](https://unhead.unjs.io/docs/svelte/head/guides/core-concepts/dom-event-handling)
- [Plugins](https://unhead.unjs.io/docs/svelte/head/guides/plugins/template-params)

### Tools

- [Meta Tag Generator](https://unhead.unjs.io/tools/meta-tag-generator)
- [OG Image Generator](https://unhead.unjs.io/tools/og-image-generator)
- [Schema.org Generator](https://unhead.unjs.io/tools/schema-generator)
- [Capo.js Analyzer](https://unhead.unjs.io/tools/capo-analyzer)

### Articles

- [What is Capo.js?](https://unhead.unjs.io/learn/guides/what-is-capo)

### Research

- [State of <head> in 2026](https://unhead.unjs.io/learn/research/state-of-head-2026)
- [Streaming Head Performance](https://unhead.unjs.io/learn/research/streaming-head-performance)
- [Capo.js Performance Research](https://unhead.unjs.io/learn/research/capo-performance-research)

Copyright © 2025-2026 Harlan Wilton - [MIT License](https://github.com/unjs/unhead/blob/main/license)