beautiful-mermaid

Render Mermaid diagrams as beautiful SVGs or ASCII art

Ultra-fast, fully themeable, zero DOM dependencies. Built for the AI era.

Live Demo & Samples

→ Use it live in Craft Agents

Why We Built This

Diagrams are essential for AI-assisted programming. When you're working with an AI coding assistant, being able to visualize data flows, state machines, and system architecture—directly in your terminal or chat interface—makes complex concepts instantly graspable.

Mermaid is the de facto standard for text-based diagrams. It's brilliant. But the default renderer has problems:

• Aesthetics — Might be personal preference, but wished they looked more professional
• Complex theming — Customizing colors requires wrestling with CSS classes
• No terminal output — Can't render to ASCII for CLI tools
• Heavy dependencies — Pulls in a lot of code for simple diagrams

We built beautiful-mermaid at Craft to power diagrams in Craft Agents. It's fast, beautiful, and works everywhere—from rich UIs to plain terminals.

The ASCII rendering engine is based on mermaid-ascii by Alexander Grooff. We ported it from Go to TypeScript and extended it Thank you Alexander for the excellent foundation! (And inspiration that this was possible.)

Features

• 5 diagram types — Flowcharts, State, Sequence, Class, and ER diagrams
• Dual output — SVG for rich UIs, ASCII/Unicode for terminals
• 15 built-in themes — And dead simple to add your own
• Full Shiki compatibility — Use any VS Code theme directly
• Live theme switching — CSS custom properties, no re-render needed
• Mono mode — Beautiful diagrams from just 2 colors
• Zero DOM dependencies — Pure TypeScript, works everywhere
• Ultra-fast — Renders 100+ diagrams in under 500ms

Installation

npm install beautiful-mermaid
# or
bun add beautiful-mermaid
# or
pnpm add beautiful-mermaid

Quick Start

SVG Output

import { renderMermaid } from 'beautiful-mermaid'

const svg = await renderMermaid(`
  graph TD
    A[Start] --> B{Decision}
    B -->|Yes| C[Action]
    B -->|No| D[End]
`)

ASCII Output

import { renderMermaidAscii } from 'beautiful-mermaid'

const ascii = renderMermaidAscii(`graph LR; A --> B --> C`)

┌───┐     ┌───┐     ┌───┐
│   │     │   │     │   │
│ A │────►│ B │────►│ C │
│   │     │   │     │   │
└───┘     └───┘     └───┘

Browser (Script Tag)

For non-bundled environments, include via CDN:

<script src="https://unpkg.com/beautiful-mermaid/dist/beautiful-mermaid.browser.global.js"></script>
<script>
  const { renderMermaid, THEMES } = beautifulMermaid;
  renderMermaid('graph TD; A-->B').then(svg => { ... });
</script>

Also available via jsDelivr. The bundle exposes renderMermaid, renderMermaidAscii, THEMES, DEFAULTS, and fromShikiTheme on the global beautifulMermaid object.

Theming

The theming system is the heart of beautiful-mermaid. It's designed to be both powerful and dead simple.

The Two-Color Foundation

Every diagram needs just two colors: background (bg) and foreground (fg). That's it. From these two colors, the entire diagram is derived using color-mix():

const svg = await renderMermaid(diagram, {
  bg: '#1a1b26',  // Background
  fg: '#a9b1d6',  // Foreground
})

This is Mono Mode—a coherent, beautiful diagram from just two colors. The system automatically derives:

| Element | Derivation | |---------|------------| | Text | --fg at 100% | | Secondary text | --fg at 60% into --bg | | Edge labels | --fg at 40% into --bg | | Connectors | --fg at 30% into --bg | | Arrow heads | --fg at 50% into --bg | | Node fill | --fg at 3% into --bg | | Node stroke | --fg at 20% into --bg |

Enriched Mode

For richer themes, you can provide optional "enrichment" colors that override specific derivations:

const svg = await renderMermaid(diagram, {
  bg: '#1a1b26',
  fg: '#a9b1d6',
  // Optional enrichment:
  line: '#3d59a1',    // Edge/connector color
  accent: '#7aa2f7',  // Arrow heads, highlights
  muted: '#565f89',   // Secondary text, labels
  surface: '#292e42', // Node fill tint
  border: '#3d59a1',  // Node stroke
})

If an enrichment color isn't provided, it falls back to the color-mix() derivation. This means you can provide just the colors you care about.

CSS Custom Properties = Live Switching

All colors are CSS custom properties on the <svg> element. This means you can switch themes instantly without re-rendering:

// Switch theme by updating CSS variables
svg.style.setProperty('--bg', '#282a36')
svg.style.setProperty('--fg', '#f8f8f2')
// The entire diagram updates immediately

Built-in Themes

15 carefully curated themes ship out of the box:

| Theme | Type | Background | Accent | |-------|------|------------|--------| | zinc-light | Light | #FFFFFF | Derived | | zinc-dark | Dark | #18181B | Derived | | tokyo-night | Dark | #1a1b26 | #7aa2f7 | | tokyo-night-storm | Dark | #24283b | #7aa2f7 | | tokyo-night-light | Light | #d5d6db | #34548a | | catppuccin-mocha | Dark | #1e1e2e | #cba6f7 | | catppuccin-latte | Light | #eff1f5 | #8839ef | | nord | Dark | #2e3440 | #88c0d0 | | nord-light | Light | #eceff4 | #5e81ac | | dracula | Dark | #282a36 | #bd93f9 | | github-light | Light | #ffffff | #0969da | | github-dark | Dark | #0d1117 | #4493f8 | | solarized-light | Light | #fdf6e3 | #268bd2 | | solarized-dark | Dark | #002b36 | #268bd2 | | one-dark | Dark | #282c34 | #c678dd |

import { renderMermaid, THEMES } from 'beautiful-mermaid'

const svg = await renderMermaid(diagram, THEMES['tokyo-night'])

Adding Your Own Theme

Creating a theme is trivial. At minimum, just provide bg and fg:

const myTheme = {
  bg: '#0f0f0f',
  fg: '#e0e0e0',
}

const svg = await renderMermaid(diagram, myTheme)

Want richer colors? Add any of the optional enrichments:

const myRichTheme = {
  bg: '#0f0f0f',
  fg: '#e0e0e0',
  accent: '#ff6b6b',  // Pop of color for arrows
  muted: '#666666',   // Subdued labels
}

Full Shiki Compatibility

Use any VS Code theme directly via Shiki integration. This gives you access to hundreds of community themes:

import { getSingletonHighlighter } from 'shiki'
import { renderMermaid, fromShikiTheme } from 'beautiful-mermaid'

// Load any theme from Shiki's registry
const highlighter = await getSingletonHighlighter({
  themes: ['vitesse-dark', 'rose-pine', 'material-theme-darker']
})

// Extract diagram colors from the theme
const colors = fromShikiTheme(highlighter.getTheme('vitesse-dark'))

const svg = await renderMermaid(diagram, colors)

The fromShikiTheme() function intelligently maps VS Code editor colors to diagram roles:

| Editor Color | Diagram Role | |--------------|--------------| | editor.background | bg | | editor.foreground | fg | | editorLineNumber.foreground | line | | focusBorder / keyword token | accent | | comment token | muted | | editor.selectionBackground | surface | | editorWidget.border | border |

Supported Diagrams

Flowcharts

graph TD
  A[Start] --> B{Decision}
  B -->|Yes| C[Process]
  B -->|No| D[End]
  C --> D

All directions supported: TD (top-down), LR (left-right), BT (bottom-top), RL (right-left).

State Diagrams

stateDiagram-v2
  [*] --> Idle
  Idle --> Processing: start
  Processing --> Complete: done
  Complete --> [*]

Sequence Diagrams

sequenceDiagram
  Alice->>Bob: Hello Bob!
  Bob-->>Alice: Hi Alice!
  Alice->>Bob: How are you?
  Bob-->>Alice: Great, thanks!

Class Diagrams

classDiagram
  Animal <|-- Duck
  Animal <|-- Fish
  Animal: +int age
  Animal: +String gender
  Animal: +isMammal() bool
  Duck: +String beakColor
  Duck: +swim()
  Duck: +quack()

ER Diagrams

erDiagram
  CUSTOMER ||--o{ ORDER : places
  ORDER ||--|{ LINE_ITEM : contains
  PRODUCT ||--o{ LINE_ITEM : "is in"

ASCII Output

For terminal environments, CLI tools, or anywhere you need plain text, render to ASCII or Unicode box-drawing characters:

import { renderMermaidAscii } from 'beautiful-mermaid'

// Unicode mode (default) — prettier box drawing
const unicode = renderMermaidAscii(`graph LR; A --> B`)

// Pure ASCII mode — maximum compatibility
const ascii = renderMermaidAscii(`graph LR; A --> B`, { useAscii: true })

Unicode output:

┌───┐     ┌───┐
│   │     │   │
│ A │────►│ B │
│   │     │   │
└───┘     └───┘

ASCII output:

+---+     +---+
|   |     |   |
| A |---->| B |
|   |     |   |
+---+     +---+

ASCII Options

renderMermaidAscii(diagram, {
  useAscii: false,      // true = ASCII, false = Unicode (default)
  paddingX: 5,          // Horizontal spacing between nodes
  paddingY: 5,          // Vertical spacing between nodes
  boxBorderPadding: 1,  // Padding inside node boxes
})

API Reference

renderMermaid(text, options?): Promise<string>

Render a Mermaid diagram to SVG. Auto-detects diagram type.

Parameters:

• text — Mermaid source code
• options — Optional RenderOptions object

RenderOptions:

| Option | Type | Default | Description | |--------|------|---------|-------------| | bg | string | #FFFFFF | Background color | | fg | string | #27272A | Foreground color | | line | string? | — | Edge/connector color | | accent | string? | — | Arrow heads, highlights | | muted | string? | — | Secondary text, labels | | surface | string? | — | Node fill tint | | border | string? | — | Node stroke color | | font | string | Inter | Font family | | transparent | boolean | false | Render with transparent background |

renderMermaidAscii(text, options?): string

Render a Mermaid diagram to ASCII/Unicode text. Synchronous.

AsciiRenderOptions:

| Option | Type | Default | Description | |--------|------|---------|-------------| | useAscii | boolean | false | Use ASCII instead of Unicode | | paddingX | number | 5 | Horizontal node spacing | | paddingY | number | 5 | Vertical node spacing | | boxBorderPadding | number | 1 | Inner box padding |

fromShikiTheme(theme): DiagramColors

Extract diagram colors from a Shiki theme object.

THEMES: Record<string, DiagramColors>

Object containing all 15 built-in themes.

DEFAULTS: { bg: string, fg: string }

Default colors (#FFFFFF / #27272A).

Attribution

The ASCII rendering engine is based on mermaid-ascii by Alexander Grooff. We ported it from Go to TypeScript and extended it with:

• Sequence diagram support
• Class diagram support
• ER diagram support
• Unicode box-drawing characters
• Configurable spacing and padding

Thank you Alexander for the excellent foundation!

License

MIT — see LICENSE for details.

Built with care by the team at Craft