Svelte Streamdown

A Svelte port of Streamdown by Vercel - an all in one markdown renderer, designed specifically for AI-powered streaming applications.

📦 Installation

npm install svelte-streamdown# orpnpm add svelte-streamdown# oryarn add svelte-streamdown

🚀 Overview

Perfect for AI-powered applications that need to stream and render markdown content safely and beautifully, with support for incomplete markdown blocks, security hardening, and rich features like code highlighting, math expressions, and interactive diagrams.

✨ Main Features

🔄 Streaming-Optimized

• Incomplete Markdown Parsing: Handles unterminated blocks gracefully
• Progressive Rendering: Perfect for streaming AI responses
• Real-time Updates: Optimized for dynamic content
• Smooth Animations: Animate tokens and blocks as they are streamed.
• An incomplete signal [blocked] on the block still being streamed, so expensive renderers can wait and loading states need no JavaScript
• Capped, self-scrolling blocks: codeBlockMaxHeight / tableMaxHeight keep a long snippet or table pinned to its newest line while it streams

🔒 Security Hardening

• Image Origin Control: Whitelist allowed image sources
• Link Safety: Control link destinations

🎯 Fully Customizable Components & Theming

• Every component customizable with Svelte snippets
• Granular theming system - customize every part of every component
• Override default styling and behavior for any markdown element
• Full control over rendering with type-safe props
• Seamless integration with your design system

🎨 Built-in Typography Styles

Beautiful, responsive typography with built-in Tailwind CSS classes for headings, lists, code blocks, and more. Comes with a complete default theme that works out of the box.

📝 Extensive Markdown Features

Full support for

• Basic text marks: bold, italic, code, Strikethrough
• _Subscript and ^Superscript
• Links
• Headings (H1–H6)
• Blockquotes
• Github alert
• Ordered & unordered lists (including roman, alpha, nested)
• Task lists ([ ] and [x])
• Code blocks
• Mermaid diagrams
• Math , in $…$ / $$…$$ or the LaTeX \(…\) / \[…\] delimiters
• Escaping currency symbols ($140)
• Complex tables
• Footnotes 1
• Inline citations [ref] [ref2]
• MDX components (embed custom Svelte components)

🏷️ Raw HTML

renderHtml decides what happens to HTML in the markdown: off (the default) shows the source as literal text, true renders it, and a function lets you sanitize it yourself and return the string.

Pretty-printed HTML has one markdown trap: four leading spaces after a blank line is an indented code block, tag or not, so a nested <div> shows up in a code box instead of rendering. normalizeHtmlIndentation dedents tag lines before parsing and leaves <pre> and <code> bodies byte-for-byte:

<Streamdown {content} renderHtml normalizeHtmlIndentation />

The trap only bites where blocks are not trimmed: static, parseIncompleteMarkdown={false}, or calling the exported function on raw text. The default streaming path trims each block, which strips the leading four spaces, so it never sees the code box in the first place.

It is off by default because dedenting is lossy, and it is exported as a plain function (import { normalizeHtmlIndentation } from 'svelte-streamdown') if you would rather pre-process the string yourself.

💻 Interactive Code Blocks

• Syntax highlighting powered by @tanstack/highlight (synchronous, SSR-friendly, ~31KB min / ~11KB gzip for every language)
• Copy-to-clipboard functionality
• Download the snippet with the extension of its language
• Support any @tanstack/highlight theme, or your own
• Optional line numbers (lineNumbers, off by default)

Line numbers

lineNumbers numbers every code block. It is a CSS counter on the line that is already rendered — no extra element per line, and the numbers are pseudo-content, so copy, download and text selection only ever give you the code.

<Streamdown {content} lineNumbers />

Three words in the fence's info string, after the language, override the prop for that block: lineNumbers numbers it even when the prop is off, noLineNumbers leaves it unnumbered even when the prop is on, and startLine=N starts the count at N instead of 1 (anything non-numeric is ignored).

```ts startLine=10const a = 1;```​```ts noLineNumbersconst b = 2;```

The gutter's width and colour are theme data (theme.code.lineNumber), so they follow your theme like every other class.

🔢 Mathematical Expressions

LaTeX math support through KaTeX. Both delimiter styles are supported — dollars and the LaTeX delimiters LLMs usually emit:

• Inline math: $E = mc^2$ or \(E = mc^2\) renders inline as
• Block (display) math: $$ … $$ or \[ … \]

\( and \[ are always read as math delimiters — in static rendering as well as while streaming. Prose that uses them as literal-bracket escapes renders as math: \[optional\] is a display-math token, and an unterminated Use \[ to open a bracket is auto-closed into one by the completer (parseIncompleteMarkdown={false} stops that half). Escape the backslash — \\[ — to keep the sequence literal. See the 4.1.0 behaviour changes [blocked].)

KaTeX is an opt-in heavy component, so you must import the Math component and pass it via the components prop. Without it, math is rendered as raw text:

<script>	import { Streamdown } from 'svelte-streamdown';	import Math from 'svelte-streamdown/math'; // KaTeX math rendering</script>​<Streamdown {content} components={{ math: Math }} />

Pass KaTeX options through the katexConfig [blocked] prop (e.g. to set throwOnError or macros). See Bundle Optimization [blocked] for details on enabling heavy components.

🧜‍♀️ Mermaid Diagrams

• Render Mermaid diagrams from code blocks, drawn by pele — Mermaid syntax, no mermaid.js
• Live rendering while streaming: each finished line appears as it arrives
• Layout adapts to the column width (a wide flowchart turns vertical on a phone)
• Themed with CSS variables, so light/dark switches without a re-render
• Pan and Zoom
• Full screen mode
• Download as PNG, SVG or .mmd source

Example:

Complex table support

Tables copy and download as Markdown, HTML, CSV or TSV — see Controls [blocked] for the separator and filename options, and for the exported table utilities.

Fullscreen

Wide tables get an expand toggle next to copy and download: it lifts the table out of the flow (position: fixed, the whole viewport) so all of its columns are reachable, and the toolbar is repositioned to the top right so copy and download stay usable. Escape or the same button — now a close icon — collapses it and puts focus back on the toggle. While expanded the wrapper is a role="dialog" container labelled with translations.controls.table, and it carries data-expanded="true" plus the theme.table.expanded classes. It is deliberately not aria-modal: the toolbar is a sibling of the wrapper, not a child, so marking the wrapper modal would hide those buttons from assistive tech — that comes with a focus trap in a later release. Turn it off with controls={{ table: { fullscreen: false } }}.

Colspan

Rowspan

Footer

Column alignment

Multiple headers and very complex layout

Complex list support

decimal

1. First item
2. Second item
3. Third item

lower-alpha

1. First item
2. Second item
3. Third item

upper-alpha

1. First item
2. Second item
3. Third item

lower-roman

1. First item
2. Second item
3. Third item

upper-roman

1. First item
2. Second item
3. Third item

Nested Lists

1. First level (numeric) a. Second level (lowercase alpha) i. Third level (lowercase roman) - Fourth level (bullet) I. Fifth level (uppercase roman) A. Sixth level (uppercase alpha)

2. Back to the first level

Task List

• Uncompleted task
• Completed task
• Another uncompleted task
  • Nested uncompleted subtask
  • Nested completed subtask

Alert Support

Description List

Topic 1

Description 1

Topic 2

Description 2

Topic 3

Description 3

Topic 3

Description 3

Citation Support

Streamdown supports inline citations that allow you to reference external sources and display them in interactive popovers. Citations work out-of-the-box with a simple object structure and support nested references like this [cloudflare.website, vercel] will render into [cloudflare.website, vercel]

To enable inline citations, pass a sources object as a prop to the Streamdown component.

Basic Usage

<script>	import { Streamdown } from 'svelte-streamdown';​	let content = `According to [smith2023], AI is advancing rapidly. See also [nested.subsection] for related work.`;​	let sources = {		smith2023: {			title: 'AI Research Paper',			url: 'https://example.com/paper',			content: 'Detailed content of the citation...'		},		nested: {			subsection: {				title: 'Nested Citation',				url: 'https://example.com/nested'			}		}	};</script>​<Streamdown {content} {sources} />

Default Citation Structure

Citations work with objects containing these properties:

• title (or name or author): Display title for the citation
• url (or href, url, link or source): Link to the source
• content (or text, summary or excerpt): Rich content to display in carousel mode

Display Modes

Streamdown offers two ways to display citations:

• List View: Shows all citations in a compact list format
• Carousel View (default): Step-through navigation for multiple citations with full content display

You can control the display mode using the inlineCitationsMode prop:

<!-- List view --><Streamdown {content} {sources} inlineCitationsMode="list" />​<!-- Carousel view (default) --><Streamdown {content} {sources} inlineCitationsMode="carousel" />

Citation Popovers

Citations appear as clickable buttons that open popovers when clicked. The popover shows:

• Source title and URL (when available)
• Favicon from the source domain
• Rich content (in carousel mode)
• Navigation controls (in carousel mode for multiple citations)

Custom Citation Rendering

If your citation data structure doesn't match the default format, you can customize how citations are rendered using inlineCitationPreview, inlineCitationContent or inlineCitationPopover snippets:

<Streamdown {content} {sources}>	{#snippet inlineCitationPreview({ token })}		<!-- Customize the clickable citation button -->		{token.keys[0]}	{/snippet}​	{#snippet inlineCitationContent({ source, key, token })}		<!-- Customize content displayed in popover -->		<div class="custom-content">			<h4>{source.customTitle || key}</h4>			<p>{source.customDescription}</p>		</div>	{/snippet}</Streamdown>

These snippets allow you to:

• inlineCitationPreview: Customize the content of the clickable button that appears in the text
• inlineCitationContent: Customize how individual citation content is displayed within popovers
• inlineCitationPopover: Completely customize the list of citations

🔄 Differences from Original React Version

This Svelte port maintains feature parity with the original Streamdown while adapting to Svelte's patterns:

Tailwind CSS Setup

@import 'tailwindcss';/* Add Streamdown styles to your Tailwind build */@source "../node_modules/svelte-streamdown/**/*";

@import 'tailwindcss';/* Add Streamdown styles to your Tailwind build (stylesheet inside src/routes) */@source "../../node_modules/svelte-streamdown/**/*";

⚡ Streaming Performance & Memoization

Like the original Streamdown, svelte-streamdown avoids re-parsing the whole document on every streaming update — but it achieves this through Svelte 5's fine-grained reactivity rather than an explicit parse cache.

Here is how it works on each content update:

1. Block splitting: the incoming content is split into top-level markdown blocks with parseBlocks. This is a lightweight lexer pass that only computes each block's raw string.
2. Keyed rendering: blocks are rendered with a keyed {#each}, so existing block components are preserved across updates instead of being torn down and recreated.
3. Per-block memoized lexing: each block component derives its tokens from its own raw string (const tokens = $derived(lex(...))). A Svelte $derived only recomputes when its inputs change, so a block is only re-lexed (the expensive inline tokenization step) when its own raw string changes.

During streaming, newly received text almost always only changes the last block (and occasionally starts a new one). Every earlier block keeps an identical raw string, so Svelte skips its lex() call entirely — this is the equivalent of the memoized Block component in the React version. The block-splitting pass itself runs on every update, but it is the cheap pass; the costly inline parsing is what gets reused.

Code highlighting is incremental as well: a code block is only re-highlighted when its text changes. Highlighting itself is synchronous and runs during SSR, so there is no grammar to load, no loading state and no hydration flash.

The incremental block cache contract

Step 1 above keeps a small per-instance cache so that block splitting costs O(new text) rather than O(document) on each update. It seals every block except the last two and re-splits only the live tail.

Deciding whether an update is an append has to be cheap, so the sealed prefix is sampled, not rescanned: the first character of every sealed block, plus a fixed number of evenly spaced characters. Anything that fails a sample falls back to a full parse.

What that means in practice:

• Append-only updates are exact. This is what an LLM stream does, and what the component does with its own content prop.
• Replacing, shortening or restructuring the content is detected — a different length, a moved block boundary, or a changed block start all fail the checks and trigger a full reparse.
• A same-length edit in the middle of a sealed block can be missed, and that block will keep rendering its old text. If you bind content to an editor, or regenerate a block in the middle of a finished document, either pass static (which skips the streaming path) or force a fresh parse by re-keying the component:

{#key documentVersion}	<Streamdown content={editorValue} />{/key}

The incomplete signal

While a fence is still streaming, the block it produces is a guess: the closing ``` has not arrived, so the language, the last line and even whether it is a diagram at all can still change. Streamdown now says so out loud.

• The code and mermaid snippets receive an extra incomplete: boolean prop, as do custom components.code / components.mermaid components.
• The rendered container carries data-incomplete="true" while the fence is open, so a loading style needs no JavaScript: [data-streamdown-code][data-incomplete] { opacity: 0.7 }.
• Only the last block of a streaming document can be incomplete, and static never marks anything.

{#snippet code({ token, children, incomplete })}	<pre class:animate-pulse={incomplete}>{@render children()}</pre>{/snippet}

The built-in Mermaid component already acts on it. pele draws in well under a millisecond, so the diagram is redrawn on every chunk while the fence is open; a half-typed last line is dropped before parsing, and if the source still does not parse the last good diagram stays on screen. Parse errors are only shown once the fence is closed.

🎭 Animation System

Streamdown includes an animation system designed specifically for streaming AI content, providing smooth and engaging visual feedback as text appears on screen.

How It Works

The animation system works by:

1. Tokenization: Text is broken down into tokens (words or characters) based on your configuration
2. Sequential Animation: Each token animates as it is received
3. Block-level Animation: Entire blocks (paragraphs, headings, code blocks) animate as units

Animation Types

Choose from 4 distinct animation styles:

fade

A clean fade-in effect where text smoothly appears from transparent to opaque.

blur

Text starts slightly blurred and comes into focus while fading in, creating a smooth reveal effect.

slideUp

Text slides up from below while fading in, creating a dynamic upward motion.

slideDown

Text slides down from above while fading in, creating a dynamic downward motion.

🚀 Quick Start

Basic Usage

<script>	import { Streamdown } from 'svelte-streamdown';​	let content = `# Hello World​This is a **bold** text and this is *italic*.​\`\`\`javascriptconsole.log('Hello from Streamdown!');\`\`\``;</script>​<Streamdown {content} />

Advanced Usage with Custom Components

<script>	import { Streamdown } from 'svelte-streamdown';​	let content = `# Custom Components Example​This heading will use a custom component!`;</script>​<Streamdown {content}>	{#snippet heading({ children })}		<h1 class="mb-4 text-4xl font-bold text-blue-600">			{@render children()}		</h1>	{/snippet}</Streamdown>

Security Configuration

<script>	import { Streamdown } from 'svelte-streamdown';​	let markdown = `![Safe Image](https://trusted-domain.com/image.jpg)[Safe Link](https://trusted-domain.com/page)`;</script>​<Streamdown	{content}	allowedImagePrefixes={['https://trusted-domain.com']}	allowedLinkPrefixes={['https://trusted-domain.com']}/>

Prefixes can also be protocol-only, which allows any URL using that protocol. For example, 'https://' allows every HTTPS link while still blocking insecure http:// links, and 'mailto:' / 'tel:' allow email and phone links:

<Streamdown	{content}	allowedLinkPrefixes={['https://', 'mailto:']}	allowedImagePrefixes={['https://']}/>

📦 Bundle Optimization

Streamdown is optimized for minimal bundle size by making heavy components opt-in. By default, Code blocks, Mermaid diagrams, and Math expressions render as lightweight fallbacks (plain text). To enable full functionality, import and pass the components you need:

Enabling Heavy Components

<script>	import { Streamdown } from 'svelte-streamdown';	// Import only the components you need	import Code from 'svelte-streamdown/code'; // syntax highlighting	import Mermaid from 'svelte-streamdown/mermaid'; // Mermaid diagrams	import Math from 'svelte-streamdown/math'; // KaTeX math rendering</script>​<Streamdown {content} components={{ code: Code, mermaid: Mermaid, math: Math }} />

Component Dependencies

Fallback Behavior

When a heavy component is not provided:

• Code blocks: Render as plain <pre><code> without syntax highlighting
• Mermaid: Renders the mermaid source as a code block
• Math: Renders the raw LaTeX/math text

Highlight themes

The Code component bundles two themes out of the box: github-dark and github-light. By default highlightTheme follows the active color scheme (github-dark in dark mode, github-light otherwise), so basic light/dark theming works with no extra configuration.

To use any other theme (aurora-x, dracula, gruvbox-dark, gruvbox-light, monokai, nord, one-dark-pro, solarized-dark, solarized-light), import it from @tanstack/highlight/themes/<name> and register it via the highlightThemes prop. The key you register it under is the value you pass to highlightTheme:

<script lang="ts">	import { Streamdown } from 'svelte-streamdown';	import Code from 'svelte-streamdown/code'; // enables highlighting	import dracula from '@tanstack/highlight/themes/dracula';​	let { content } = $props();</script>​<Streamdown	{content}	components={{ code: Code }}	highlightThemes={{ dracula }}	highlightTheme="dracula"/>

A custom theme is just an object of the HighlightTheme shape (exported from svelte-streamdown) — a name, type, background, foreground and a color per token class:

<script lang="ts">	import { Streamdown, type HighlightTheme } from 'svelte-streamdown';	import Code from 'svelte-streamdown/code';	import githubDark from '@tanstack/highlight/themes/github-dark';​	const myTheme: HighlightTheme = {		...githubDark,		name: 'my-theme',		tokens: { ...githubDark.tokens, keyword: '#ff0088', comment: '#5a5a5a' }	};</script>​<Streamdown	{content}	components={{ code: Code }}	highlightThemes={{ 'my-theme': myTheme }}	highlightTheme="my-theme"/>

Dynamic (light/dark) theme switching

Register every theme you intend to switch between in highlightThemes, then drive highlightTheme from your color-scheme store. Switching is fully reactive:

<script lang="ts">	import { Streamdown } from 'svelte-streamdown';	import Code from 'svelte-streamdown/code';	import { mode } from 'mode-watcher';	import oneDarkPro from '@tanstack/highlight/themes/one-dark-pro';	import solarizedLight from '@tanstack/highlight/themes/solarized-light';​	let { content } = $props();​	const highlightTheme = $derived(mode.current === 'dark' ? 'one-dark-pro' : 'solarized-light');</script>​<Streamdown	{content}	components={{ code: Code }}	baseTheme="shadcn"	highlightThemes={{		'one-dark-pro': oneDarkPro,		'solarized-light': solarizedLight	}}	{highlightTheme}/>

Styling tokens with CSS

Every token span carries th-token plus a th-<class> class (th-keyword, th-string, th-comment, th-function, th-number, th-operator, th-tag, th-attr, …) alongside its inline color from the active theme, so you can tweak or override token styles from CSS:

[data-streamdown-code] .th-comment {	font-style: italic;}

Supported languages

All 30 scanners ship in the same ~31KB bundle, so there is nothing to lazy-load:

apache, cmake, cpp, css, diff, dockerfile, ejs, env, go, html, http, js, json, jsx, markdown, mermaid, nginx, php, plaintext, python, scheme, shell, sql, svelte, toml, ts, tsrx, tsx, vue, yaml.

Common aliases are normalized (javascript → js, typescript → ts, bash/sh/zsh → shell, py → python, jsonc/json5 → json, xml/htm → html, md → markdown, yml → yaml, docker → dockerfile).

You can add your own language with defineLanguage and the highlightLanguages prop. A language is a tokenizer returning { className, start, end } ranges:

<script lang="ts">	import { Streamdown, defineLanguage } from 'svelte-streamdown';	import Code from 'svelte-streamdown/code';​	const brainfuck = defineLanguage({		name: 'brainfuck',		tokenize: (code) =>			[...code.matchAll(/[+\-<>]+/g)].map((match) => ({				className: 'operator' as const,				start: match.index,				end: match.index + match[0].length			}))	});</script>​<Streamdown {content} components={{ code: Code }} highlightLanguages={[brainfuck]} />

Migrating from v3 (shiki)

v4 replaces shiki with @tanstack/highlight. Highlighting is now synchronous, renders during SSR and no longer has a skeleton/loading state. Remove shiki / @shikijs/* from your dependencies and rename the props:

- shikiTheme="nord"- shikiThemes={{ nord }}- shikiLanguages={[{ id: 'haskell', name: 'Haskell', import: () => import('@shikijs/langs/haskell') }]}+ highlightTheme="nord"+ highlightThemes={{ nord }}+ highlightLanguages={[myLanguage]}

• Theme objects come from @tanstack/highlight/themes/<name> instead of @shikijs/themes/<name>, and follow the HighlightTheme shape (not shiki's ThemeRegistration).
• highlightLanguages takes LanguageDefinitions built with defineLanguage, not lazy shiki grammar loaders.
• bundledLanguagesInfo, createLanguageSet and LanguageInfo are gone; defineLanguage, LanguageDefinition and HighlightTheme are exported instead.
• The code.skeleton theme key is gone (there is no loading state anymore).
• Languages shiki covered but @tanstack/highlight@0.1.0 does not (java, rust, c, c#, swift, kotlin, ruby, graphql, …) now render as plaintext.

📋 Props API

All Available Customizable Elements:

Text Elements: heading, p, strong, em, del

Links & Media: a, img

Lists: ul, ol, li

Code: code, codespan — code and mermaid also receive incomplete [blocked]

Tables: table, thead, tbody, tr, th, td, tfoot

Special Content: blockquote, hr, alert, mermaid, math, footnoteRef, inlineCitation

MDX Components: Handled via a single mdx snippet that receives token, props, and children. Use token.tagName to differentiate between components.

Note: The above elements are supported by Streamdown and should be customized using individual props or the theme system. MDX components require the mdx snippet.

🌍 Translations

Every string the components render themselves — alert titles, button labels, download menu entries, the copy announcements screen readers hear, the blocked-URL tooltips — comes from one nested translations object. Pass only the keys you want to change; the rest fall back to defaultTranslations, which is exported so you can read the shipped English values (or diff against them when a new key appears).

<script>	import { Streamdown, defaultTranslations } from 'svelte-streamdown';</script>​<Streamdown	{content}	translations={{		alert: { note: 'remarque', warning: 'attention' },		controls: {			copyCode: 'Copier le code',			copiedCode: 'Code copié',			downloadCode: 'Télécharger le code'		}	}}/>

The alert defaults are lowercase because the theme capitalizes them with CSS; if you drop that class, capitalize them here instead.

Control labels are used as both the title and the aria-label of the matching icon-only button, so translating them translates the accessible name too. copiedCode / copiedTable are announced through a visually hidden aria-live region after a copy.

🎛️ Controls

controls turns the code / table / mermaid toolbars on and off and configures what they do. controls={false} turns every control off, controls={true} (the default) turns them all on, and each section takes a boolean or an object:

type Controls =	| boolean	| {			code?:				| boolean				| {						enabled?: boolean;						copy?: boolean;						download?: boolean | { filename?: string | ((token: CodeToken) => string) };				  };			table?:				| boolean				| {						enabled?: boolean;						copy?: boolean;						download?: boolean | { filename?: string | ((token: TableToken) => string) };						fullscreen?: boolean;						csvSeparator?: ',' | ';' | '\t' | 'auto';				  };			mermaid?:				| boolean				| {						enabled?: boolean;						download?: boolean | { filename?: string | ((token: CodeToken) => string) };						mouseWheelZoom?: boolean;				  };	  };

<Streamdown	{content}	controls={{		code: { download: { filename: (token) => `snippet-${token.lang}` } },		table: { copy: false, csvSeparator: 'auto' },		mermaid: { mouseWheelZoom: false }	}}/>

• filename is the base name; the extension still comes from the content — languageExtensionMap for code (.ts, .py, … .txt), .csv / .tsv / .md / .html for tables, .svg / .png / .mmd for diagrams. The defaults are file, table and diagram.
• csvSeparator: 'auto' picks ; when the browser locale writes decimals with a comma (Excel reads , as a decimal point there), otherwise ,. CSV downloads carry a UTF-8 BOM so accented and CJK text opens correctly in Excel.
• mouseWheelZoom is a gesture rather than a button: it stays on unless you set it to false or turn every control off with controls={false}.
• table.fullscreen is the expand toggle in the table toolbar (on by default). It is a third action, so the toolbar survives copy: false, download: false; set all three to false to get rid of it.

Table export utilities

The table toolbar's DOM walk is exported, so a custom table snippet can build its own copy or download menu:

import {	extractTableData,	tableDataToCSV,	tableDataToTSV,	tableDataToMarkdown,	tableDataToHTML,	type TableData} from 'svelte-streamdown';​const data = extractTableData(document.querySelector('[data-streamdown-table="..."]')!);// { headers: string[], rows: string[][] } — <br> becomes \n, colspan/rowspan become empty cellstableDataToCSV(data, ';');tableDataToTSV(data);

tableDataToMarkdown and tableDataToHTML are there for callers that only hold a DOM table; the built-in menu copies Markdown from token.raw instead, which keeps the author's original inline formatting.

🎨 Theming System

Built-in Themes

Streamdown comes with two built-in themes:

• Default Theme: The standard theme with gray-based colors
• Shadcn Theme: A theme that uses shadcn/ui design tokens for seamless integration with shadcn-based projects

Beyond custom snippets, Streamdown provides a granular theming system that lets you customize every part of every component without writing custom snippets. You can use the built-in themes (default and shadcn) or create completely custom themes using the mergeTheme utility.

Theme Structure

Every component has multiple themeable parts. For example, the code component has:

code: {  base: 'bg-gray-100 rounded p-2 font-mono text-sm',           // Main code block  container: 'my-4 w-full overflow-hidden rounded-xl border',   // Wrapper container  header: 'flex items-center justify-between bg-gray-100/80',  // Header with language  button: 'cursor-pointer p-1 text-gray-600 transition-all',   // Copy button  language: 'ml-1 font-mono lowercase',                        // Language label  pre: 'overflow-x-auto font-mono p-0 bg-gray-100/40'        // Pre element}

Using Custom Themes

<script>	import { Streamdown } from 'svelte-streamdown';​	let content = `# Custom Theme Example​\`\`\`javascriptconsole.log('Beautiful code blocks!');\`\`\`​> This blockquote is also themed​| Header 1 | Header 2 ||----------|----------|| Cell 1   | Cell 2   |`;​	// Custom theme overrides	let customTheme = {		code: {			container: 'my-6 rounded-2xl border-2 border-purple-200 shadow-lg',			header: 'bg-purple-50 text-purple-700 font-medium',			button: 'text-purple-600 hover:text-purple-800 hover:bg-purple-100'		},		blockquote: {			base: 'border-l-8 border-purple-400 bg-purple-50 p-4 italic text-purple-800'		},		table: {			base: 'border-purple-200 shadow-md',			container: 'my-6 rounded-lg overflow-hidden'		},		th: {			base: 'bg-purple-100 px-6 py-3 text-purple-900 font-bold'		},		td: {			base: 'px-6 py-3 border-purple-100'		}	};</script>​<Streamdown {content} theme={customTheme} />

All Themeable Components

Each component supports multiple themeable parts:

Headings (h1-h6): base

Text Elements (p, strong, em, del): base

Lists (ul, ol, li): base

Links (a): base, blocked (for blocked/unsafe links)

Code (code): base, container, header, buttons, language, line, lineNumber, pre

Inline Code (inlineCode): base

Images (img): container, base, downloadButton

Tables (table, thead, tbody, tr, th, td): base, plus table and expanded (table only)

Blockquotes (blockquote): base

Alerts (alert): base, title, icon, plus type-specific styles (note, tip, warning, caution, important)

Mermaid (mermaid): base, buttons, icon, error

Mermaid theming

Diagrams are drawn by pele, which reads its colours from CSS variables instead of a Mermaid theme. Set them on any ancestor, or on .pele:

.my-diagrams {	--pele-bg: #fffcf0;	--pele-fg: #100f0f;	--pele-line: #6f6e69;	--pele-surface: #f2f0e5;	--pele-border: #cecdc3;	--pele-accent: #205ea6;}

The shadcn base theme already points them at --card, --muted, --border, --primary…, so diagrams follow .dark with no extra work. The tailwind theme keeps pele's light defaults to match its white card.

Math (math, inlineMath): base

Other (hr, sup, sub): base

Theme Merging

Themes are intelligently merged using Tailwind's class merging utility, so you only need to override the specific parts you want to customize while keeping the default styling for everything else.

🧩 MDX Component Support

Streamdown supports MDX-style JSX components, allowing you to embed custom Svelte components directly in your markdown content.

Basic Usage

<script>	import { Streamdown } from 'svelte-streamdown';​	let content = `# Using MDX Components​<Card title="Hello" count={42}>This is **markdown content** inside a component!</Card>​<Button label="Click me" active={true} />`;</script>​<Streamdown {content}>	{#snippet mdx({ token, props, children })}		{#if token.tagName === 'Card'}			<div class="rounded-lg border border-gray-200 p-4 shadow-sm">				<h3 class="text-xl font-bold">{props.title}</h3>				<p class="text-gray-600">Count: {props.count}</p>				<div class="mt-2">					{@render children()}				</div>			</div>		{:else if token.tagName === 'Button'}			<button class="rounded px-4 py-2 {props.active ? 'bg-blue-500 text-white' : 'bg-gray-200'}">				{props.label}			</button>		{:else}			{@render children()}		{/if}	{/snippet}</Streamdown>

Alternative: Using Svelte Components Directly

Instead of using the mdx snippet with conditional logic, you can pass Svelte components directly using the mdxComponents prop:

<script>	import { Streamdown } from 'svelte-streamdown';	import Card from './Card.svelte';	import Button from './Button.svelte';​	let content = `# Using MDX Components​<Card title="Hello" count={42}>This is **markdown content** inside a component!</Card>​<Button label="Click me" active={true} />`;</script>​<Streamdown {content} mdxComponents={{ Card, Button }} />

Your Svelte components (Card.svelte, Button.svelte) should accept props and a children snippet:

<!-- Card.svelte --><script>	let { title, count, children } = $props();</script>​<div class="rounded-lg border border-gray-200 p-4 shadow-sm">	<h3 class="text-xl font-bold">{title}</h3>	<p class="text-gray-600">Count: {count}</p>	<div class="mt-2">		{@render children()}	</div></div>

<!-- Button.svelte --><script>	let { label, active } = $props();</script>​<button class="rounded px-4 py-2 {active ? 'bg-blue-500 text-white' : 'bg-gray-200'}">	{label}</button>

This approach is cleaner when you have standalone component files, while the mdx snippet approach is better for inline component definitions or when you need shared logic across components.

Supported Syntax

Self-closing components:

<Component attr="value" count={42} enabled={true} />

Components with markdown children:

<Component title="Hello"># This is a headingThis **markdown** content will be parsed!</Component>

Attribute Types

MDX components support three attribute value types:

• Strings: attr="hello" → "hello"
• Numbers: count={42} or value={3.14} → 42, 3.14
• Booleans: active={true} or disabled={false} → true, false
• Expressions: value={variableName} → "variableName" (stored as string)

Component Naming

• PascalCase names are always components: <Card />, <MyComponent />, <Component123 />
• Lowercase and hyphenated names are opt-in. <card />, <ai-thinking> and <mention> are plain HTML unless you list them, so nothing you write today changes meaning.
• A name is listed by putting it in customTags, or simply by registering it in mdxComponents — its keys are allowlisted for you.

<Streamdown	content={`<ai-thinking>\nLet me **check** that.\n</ai-thinking>`}	customTags={['ai-thinking']}>	{#snippet mdx({ token, children })}		{#if token.tagName === 'ai-thinking'}			<aside class="text-sm text-gray-500">{@render children()}</aside>		{/if}	{/snippet}</Streamdown>

The allowlist is compiled once and shared by the tokenizer and the streaming completer, so a half-typed <ai-think is hidden while it streams rather than flashing as text.

Literal Tag Content

Some tags carry data, not markdown. List them in literalTagContent and their children become a single text token — underscores, asterisks and backticks survive untouched:

<!-- Markdown: <mention user_id="1">@_john_doe_</mention> --><Streamdown {content} literalTagContent={['mention']} /><!-- renders @_john_doe_, not @john_doe with an italic run -->

Attribute names may contain hyphens (<mention data-id="7">), and values use the same attr="string" / attr={expression} forms as PascalCase components.

Streaming Safety

MDX components are streaming-safe. Incomplete components are automatically handled during AI streaming:

• Incomplete tags like <Component attr not rendered to prevent runtime errors
• Unclosed components like <Card>content are auto-closed with </Card>
• Malformed attributes are escaped to prevent rendering errors
• Half-typed HTML tags are hidden too: Hello <div cla renders as Hello until the > arrives. This only ever applies to the very end of the block still streaming, and only to a name that could still grow into a common HTML element, a PascalCase component or a customTags entry, followed by nothing but attributes. A paragraph reading if a <b then c keeps its text wherever it sits, and so do Use the <div element to wrap it. and 3 < 5.

This ensures your UI remains stable even when receiving partial markdown from streaming AI responses.

Component Props

The mdx snippet receives three parameters:

• token: The full MdxToken with tagName, attributes, selfClosing, etc.
• props: Object containing all parsed attributes (e.g., props.title, props.count)
• children: Snippet containing parsed markdown content

Use token.tagName to determine which component is being rendered: <Card title="Hello" count={5}>Content</Card>

<!-- Markdown: <Card title="Hello" count={5}>Content</Card> --><Streamdown {content}>	{#snippet mdx({ token, props, children })}		{#if token.tagName === 'Card'}			<div>				<h3>{props.title}</h3>				<span>Count: {props.count}</span>				{@render children()}			</div>		{:else if token.tagName === 'Alert'}			<div class="alert alert-{props.type}">				{@render children()}			</div>		{:else}			<!-- Fallback for unknown components -->			{@render children()}		{/if}	{/snippet}</Streamdown>

💉 Extensibility

Streamdown is extensible through the use of custom extensions.

An extension is an object that has a name, a level and a tokenizer function.

• name: The name of the extension
• level: The level of the extension, can be block or inline
• tokenizer: The tokenizer function, see marked for more information

To render the extension custom tokens, you can then simply use the children snippet.

Example

<script lang="ts">	import { Streamdown, type Extension } from 'svelte-streamdown';	const markedCollapsible: Extension = {		name: 'collapsible',		level: 'block',		tokenizer(this, src) {			// Match [detail]...[detail] blocks (case insensitive)			const detailMatch = src.match(/^\[detail\](.*?)\[detail\]/is);​			if (detailMatch) {				const content = detailMatch[1] || '';				const tokens = this.lexer.blockTokens(content);​				return {					type: 'detail',					raw: detailMatch[0], // The entire matched string including tags					tokens				};			}​			return undefined;		}	};</script>​<Streamdown	extensions={[markedCollapsible]}	content={`[detail]	This is a collapsible **section**[detail]`}>	{#snippet children({ token, streamdown, children })}		{#if token.type === 'detail'}			<details>				<summary> Detail </summary>				<div>					{@render children()}				</div>			</details>		{/if}	{/snippet}</Streamdown>

🛠️ Development

Setup

# Clone the repositorygit clone <repository-url>cd svelte-streamdown​# Install dependenciespnpm install​# Start development serverpnpm dev​# Run testspnpm test​# Run the browser (component) tests — needs a Chromium binary:# pnpm exec playwright install chromiumpnpm test:browser​# Build for productionpnpm build

Building

# Build the librarypnpm build​# Preview the showcase apppnpm preview

🤝 Contributing

Contributions are welcome! This is a port of the original Streamdown project, so please:

1. Check the original Streamdown repository for upstream changes
2. Ensure compatibility with the original API
3. Maintain feature parity where possible
4. Add tests for new features if you want

📄 License

MIT

🙏 Acknowledgments

• Original Streamdown: Vercel for creating the original React component
• Svelte Community: For the amazing framework that made this port possible
• All Contributors: For helping improve and maintain this project

Made with ❤️ and 🤖