pretty-mermaid

Pretty Mermaid

Create or render Mermaid diagrams with the bundled Node.js CLI. Use SVG for scalable documentation, PNG for sharing or raster-only consumers, and ASCII or Unicode for terminals and plain text.

Working directory

Treat the directory containing this file as <skill-root>. Run bundled scripts from that directory, or invoke them with absolute paths. Keep user source and rendered output in the user's requested location; do not copy the renderer into their project.

Workflow

1. Determine whether the user supplied Mermaid source or needs a diagram authored from prose.
2. Choose the diagram type and output format from the tables below.
3. Read only the relevant reference file when syntax, theme selection, or API behavior needs more detail.
4. Save new source as a .mmd file, preserving user terminology and relationships.
5. Render with a named theme or explicit colors.
6. Inspect the result. Fix syntax, clipping, crowded layout, or unclear labels and render again.
7. Return the source and output paths, plus the selected format and theme.

Do not overwrite an existing source or output file unless the user asked for replacement.

Choose a diagram type

Need	Diagram type	Starter
Process, decision tree, architecture	Flowchart	flowchart LR
API calls, messages, interactions	Sequence	sequenceDiagram
Lifecycle or finite-state machine	State	stateDiagram-v2
Classes, modules, relationships	Class	classDiagram
Database entities and cardinality	ER	erDiagram
Bars, lines, trends, comparisons	XY chart	xychart-beta

Read references/DIAGRAM_TYPES.md when authoring non-trivial Mermaid syntax.

Choose an output

Output	Best for	Notes
SVG	READMEs, docs, slides, websites	Scalable, themed, supports transparency
PNG	Chats, previews, raster-only tools	Set --format png; no external converter required
Unicode	Modern terminals and readable text previews	Default ASCII renderer output
Plain ASCII	Logs and restricted terminals	Add --use-ascii
ANSI-colored text	Interactive terminals	Set --color-mode

Core commands

Run these from <skill-root>.

List themes

node scripts/themes.mjs

Render SVG

node scripts/render.mjs \
  --input diagram.mmd \
  --output diagram.svg \
  --theme tokyo-night

Render terminal text

node scripts/render.mjs \
  --input diagram.mmd \
  --output diagram.txt \
  --format ascii \
  --color-mode none

Add --use-ascii when Unicode box-drawing characters are not acceptable.

Render PNG

node scripts/render.mjs \
  --input diagram.mmd \
  --output diagram.png \
  --format png \
  --width 1200 \
  --theme tokyo-night

Batch render a directory

node scripts/batch.mjs \
  --input-dir ./diagrams \
  --output-dir ./rendered \
  --format svg \
  --theme github-dark \
  --workers 4

Use batch rendering for three or more diagrams or when consistent options must be applied to a directory.

Theme selection

• General dark documentation: tokyo-night
• GitHub dark or light surfaces: github-dark, github-light
• Print and presentations: zinc-light
• High-contrast color: dracula
• Cool, restrained palette: nord, nord-light

Read references/THEMES.md or open docs/THEME_GALLERY.md when visual theme choice matters. A named theme can be refined with explicit color flags.

Useful options

Shared styling

Option	Purpose
--theme <name>	Apply one of the 15 built-in themes
--bg, --fg	Set required base colors
--line, --accent, --muted	Refine connectors, highlights, and secondary text
--surface, --border	Refine node fill and stroke
--font <name>	Set the SVG font family

SVG

Option	Purpose
--transparent	Remove the SVG background
--padding <n>	Set canvas padding
--node-spacing <n>	Set horizontal node spacing
--layer-spacing <n>	Set vertical layer spacing
--component-spacing <n>	Separate disconnected components
--interactive	Enable XY chart hover tooltips

PNG

Option	Purpose
--width <n>	Set output width from 100 to 10000 pixels while preserving aspect ratio
--transparent	Preserve a transparent background

Terminal output

Option	Purpose
--use-ascii	Replace Unicode box drawing with plain ASCII
--padding-x, --padding-y	Tune diagram spacing
--box-border-padding	Tune padding inside node boxes
--color-mode <mode>	none, auto, ansi16, ansi256, truecolor, or html

Run node scripts/render.mjs --help or node scripts/batch.mjs --help for the authoritative CLI list.

Authoring guidance

• Prefer short, concrete labels; preserve domain-specific terms from the user.
• Use explicit edge labels when a branch or message is ambiguous.
• Keep large diagrams readable by splitting unrelated concerns instead of shrinking text.
• Use LR for wide flows and TB for narrow documents.
• Avoid communicating meaning through color alone.
• Use a light theme for print and confirm contrast against the final background.
• For unfamiliar syntax, start from assets/example_diagrams/ and consult the diagram reference.

Validation

After rendering:

1. Confirm the command exits successfully and the output file is non-empty.
2. Confirm SVG output begins with <svg; confirm PNG output opens as a valid image; confirm text output contains visible diagram content.
3. Inspect visual output when layout matters, especially long labels, CJK text, disconnected components, and XY charts.
4. Confirm arrows, cardinalities, states, and labels match the source request.
5. Report any renderer limitation instead of silently dropping unsupported syntax.

Run both npm test and npm run validate when changing this skill, its scripts, templates, or references.

Troubleshooting

• Missing dependency: run npm install in <skill-root>; the CLI also attempts a first-run install.
• Unknown theme: run node scripts/themes.mjs and use an exact listed name.
• Parse error: consult references/DIAGRAM_TYPES.md, reduce to the failing statement, then restore the diagram incrementally.
• Crowded SVG: increase --node-spacing, --layer-spacing, or --component-spacing.
• PNG color error: use concrete hex values for custom colors; unresolved external CSS variables cannot be rasterized.
• Terminal color escape codes in redirected output: use --color-mode none.

Reference routing

Resource	Read or use when
references/DIAGRAM_TYPES.md	Authoring or debugging Mermaid syntax
references/THEMES.md	Comparing themes or defining custom colors
references/api_reference.md	Extending scripts or calling beautiful-mermaid directly
docs/THEME_GALLERY.md	Choosing a theme visually
assets/example_diagrams/	Starting from a supported diagram template
scripts/render.mjs	Rendering one diagram
scripts/batch.mjs	Rendering a directory in parallel
scripts/themes.mjs	Listing installed themes