Mermaid Diagrams

Generate .mmd text files and export to PNG/SVG/PDF using mmdc (local) or Kroki API (no install).

Key advantage: Text-based syntax with fully automatic layout — no x/y coordinates needed.

When to use / when NOT to use

Use this skill for: diagrams-as-code with automatic layout (flowchart, sequence, class, state, ER, gantt, mindmap, architecture) — text source that lives in git and embeds in Markdown.

Do NOT use it — route elsewhere — for:

• Pixel-precise placement, custom layout, branded icons, or heavy styling → drawio.
• A hand-drawn / sketchy aesthetic → excalidraw or tldraw.
• A freeform whiteboard or freehand strokes → tldraw.
• Strict, conventional UML notation → plantuml.

Prerequisites

Option A: Local (mmdc) — also needs a headless Chrome (mmdc renders via Puppeteer)

npm install -g @mermaid-js/mermaid-cli
npx puppeteer browsers install chrome-headless-shell   # required — mmdc has no bundled browser
mmdc --version

mmdc --version succeeds even with no Chrome installed, but every export then fails with Could not find Chrome. Install the browser above (or set PUPPETEER_EXECUTABLE_PATH to a system Chrome). If you can't, use Kroki (Option B) — it needs no browser.

CI / Docker: mmdc crashes with Running as root without --no-sandbox in containers. Pass the bundled puppeteer config (see scripts/puppeteer-config.json):

mmdc -p scripts/puppeteer-config.json -i diagram.mmd -o diagram.png

Option B: Kroki API (no install)

curl --version  # Just need curl

Workflow

1. Check deps — validate via Kroki (needs only curl); check mmdc --version and a headless Chrome only when local export (PNG quality / PDF) is wanted
2. Pick diagram type — choose from table below
3. Generate — write .mmd file to disk
4. Validate — Kroki-first (see Validation; REQUIRED before export)
5. Export — use mmdc or Kroki API to produce PNG/SVG/PDF
6. Self-check (vision) — read the exported PNG and fix readability/layout defects that automatic layout can't prevent (clipped labels, cramped density, wrong orientation), then re-validate + re-export. Max 2 rounds; skip if no vision. See Self-Check (vision) below.
7. Review loop — show the image to the user, apply the minimal .mmd edit per request, re-export until approved (5-round safety valve). See Review Loop below.
8. Report — tell user the output file paths

Validation (Required)

NEVER export a diagram without validating first.

Prefer Kroki for validation — it needs no browser and sidesteps the Could not find Chrome trap entirely. Use mmdc for validation only when offline.

# Validate with Kroki (preferred — no browser needed)
curl -s -X POST -H "Content-Type: text/plain" --data-binary @diagram.mmd https://kroki.io/mermaid/svg -o /tmp/test.svg && echo "Valid" || echo "Invalid"

# Validate with mmdc (offline fallback — requires headless Chrome)
mmdc -i diagram.mmd -o /tmp/test.svg 2>&1

# If error, fix the .mmd file and validate again
# Only proceed to export after validation passes

Common validation errors:

• Missing quotes around labels with special characters
• Wrong arrow syntax (use ->> for sequence, --> for flowchart)
• Undeclared participants in sequence diagrams

A Could not find Chrome (or puppeteer) error from mmdc is a setup problem, not a diagram error — the .mmd may be perfectly valid. Validate via Kroki instead of "fixing" correct syntax.

Self-Check (vision)

Zero-cost pre-check: before spending a vision call, read the PNG dimensions (sips -g pixelWidth -g pixelHeight diagram.png on macOS, file diagram.png elsewhere). An extreme aspect ratio (longer side > 8x the shorter) is a "wrong orientation" defect — fix it without vision.

Validation (above) only proves the syntax is legal — it says nothing about whether the rendered diagram is readable. After exporting, use the agent's vision capability to read the PNG and catch what automatic layout can't prevent. Mermaid positions everything itself, so the failures here are about content and readability, not overlaps:

Check	What to look for	Fix
Label truncation	Node / edge text clipped or cut off	Shorten the label, or wrap it with <br/>
Cramped, unreadable density	Too many nodes crammed together; tangled lines	Flip direction (TD↔LR), split into subgraphs, or reduce nodes
Wrong orientation / aspect	Diagram far too wide or too tall to read	Change flowchart TD↔LR (or set direction in class/state)
Edge spaghetti	Many edges crossing, hard to follow	Reorder node declarations so connected nodes sit adjacent; group with subgraph
Wrong diagram type	Type doesn't suit the content (e.g. flowchart for a timeline)	Switch type (gantt, sequenceDiagram, stateDiagram-v2, …)
Low contrast	Text blends into the node fill	Adjust classDef / theme so text contrasts the fill

• Max 2 self-check rounds — if issues remain after 2 fixes, show the user anyway.
• Re-validate (syntax) and re-export after every fix.
• If vision is unavailable, skip self-check and show the PNG directly.

Review Loop

After self-check, show the exported image and collect feedback. Apply the minimal .mmd edit for each request, then re-validate and re-export:

User request	Edit action
Change a label	Edit the node / edge text in the .mmd
Add / remove a node or edge	Add or delete the matching line
Change a color	Add / adjust a classDef and class <node> <className>
Change layout direction	Swap TD↔LR (flowchart) or set direction (class / state)
Restructure / group	Wrap related nodes in a subgraph, or regenerate

• Overwrite the same diagram.mmd / diagram.png each round — don't create v1, v2, …
• Safety valve: after 5 rounds, generate a mermaid.live handoff link (see next section) so the user fine-tunes the current diagram in the browser.

Mermaid.live Handoff

When the review loop ends (approved or safety-valve), generate a one-click link that opens the current .mmd in the mermaid.live editor for interactive fine-tuning:

python3 scripts/mermaid_live_link.py diagram.mmd
# -> https://mermaid.live/edit#pako:...

Pure zlib + base64 (raw-deflate + URL-safe base64, same encoding mermaid.live uses), works offline. The script self-verifies with a round-trip decode before printing.

Batch Mode (Repo Survey)

When the user asks to diagram a whole repo ("survey this codebase", "map the architecture"):

1. Scan — locate schemas (SQL / ORM models), API routes, and state machines / lifecycle enums in the codebase
2. Propose — one diagram per finding: ER for the schema, sequence for a main API flow, state for a lifecycle, flowchart for overall service layout; confirm the set with the user before generating
3. Generate + export — run the standard workflow per diagram into one output folder
4. Overview page — inline every exported SVG into a single self-contained index.html (title + one section per diagram, no CDN), so the user opens one file

Same validation and self-check rules apply to every diagram in the batch.

Diagram Types

Type	Keyword	Use for
Flowchart	flowchart TD/LR	processes, pipelines, decisions
Sequence	sequenceDiagram	API calls, message passing
Class	classDiagram	OOP models, data structures
ER	erDiagram	database schemas
State	stateDiagram-v2	state machines, lifecycle
Gantt	gantt	project timelines
Pie	pie	proportions
Git Graph	gitGraph	branch strategies
C4 Context	C4Context	high-level system context
Architecture	architecture-beta	cloud / CI/CD service layouts
Mind Map	mindmap	topic breakdowns
User Journey	journey	user-experience flows
Use Case	usecase-beta	actor–system interactions (UML)
Cynefin	cynefin-beta	sense-making / complexity domains
Event Modeling	eventmodeling	event-driven system timelines
Tree View	treeView-beta	file / directory hierarchies
Wardley Maps	wardley-beta	business strategy / value chains

Syntax Reference

Flowchart: See reference/FLOWCHART.md Sequence: See reference/SEQUENCE.md Class & ER: See reference/CLASS-ER.md Architecture: See reference/ARCHITECTURE.md Use Case: See reference/USECASE.md Other types: See reference/OTHER-TYPES.md Themes & styling: See reference/THEMES.md

Examples

See reference/EXAMPLES.md for four worked examples (JWT auth sequence, microservices architecture, order state machine, cloud architecture) — each with the user prompt, the generated .mmd, and the output files.

Export Commands

Option 1: Local Export (mmdc)

Requires mmdc installed locally. Best for offline use.

# PNG (recommended: 2048px wide, white background)
mmdc -i diagram.mmd -o diagram.png -w 2048 --backgroundColor white

# PNG with theme — valid -t values: default | dark | neutral | forest
# (`base` is NOT a valid -t value; it only works inside a %%{init: {'theme':'base'}}%% directive)
mmdc -i diagram.mmd -o diagram.png -w 2048 --backgroundColor white --theme neutral

# SVG
mmdc -i diagram.mmd -o diagram.svg

# PDF
mmdc -i diagram.mmd -o diagram.pdf

Option 2: Kroki API (No Install Required)

Use Kroki when mmdc is not available. No local dependencies needed.

# SVG via Kroki
curl -X POST -H "Content-Type: text/plain" --data-binary @diagram.mmd https://kroki.io/mermaid/svg -o diagram.svg

# PNG via Kroki
curl -X POST -H "Content-Type: text/plain" --data-binary @diagram.mmd https://kroki.io/mermaid/png -o diagram.png

# PDF is NOT supported by Kroki for Mermaid — POSTing to /mermaid/pdf returns
# HTTP 400 ("Unsupported output format: pdf for mermaid. Must be one of png or svg").
# For PDF, use the local mmdc path instead:  mmdc -i diagram.mmd -o diagram.pdf

Kroki advantages:

• No local installation required
• Works on any system with curl
• Supports 20+ diagram types (PlantUML, GraphViz, D2, etc.)

When to use Kroki:

• mmdc installation fails
• Quick one-off diagrams
• CI/CD pipelines without Node.js

Common Mistakes

Mistake	Fix
mmdc not found	npm install -g @mermaid-js/mermaid-cli
mmdc error Could not find Chrome	Install the headless browser: npx puppeteer browsers install chrome-headless-shell (or use Kroki)
Kroki PDF fails with HTTP 400	Kroki does PNG/SVG only for Mermaid; use local mmdc for PDF
Valid diagram reported "invalid" by mmdc	The error is a Chrome/puppeteer setup failure, not a syntax error — don't rewrite correct .mmd; fix the browser or validate via Kroki
Wrong arrow in sequence	Use ->> for request, -->> for response
Special chars in label	Wrap in quotes: A["Label: value"]
Blank/small output	Add -w 2048 flag
Participant order wrong	Declare participant explicitly at top
Subgraph name with spaces	Wrap in quotes: subgraph "My Layer"