creating-mermaid-diagrams

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):

Option B: Kroki API (no install)

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.

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:

• 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:

• 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:

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

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.

Option 2: Kroki API (No Install Required)

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

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