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) — 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.

Option B: Kroki API (no install)

curl --version  # Just need curl

Workflow

1. Check deps — mmdc --version and confirm a headless Chrome is installed (a bare --version pass does NOT mean export works); fall back to Kroki if either is missing
2. Pick diagram type — choose from table below
3. Generate — write .mmd file to disk
4. Validate — run 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.

# Validate with mmdc (local)
mmdc -i diagram.mmd -o /tmp/test.png 2>&1

# Validate with Kroki (if mmdc unavailable)
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"

# 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. Install the browser (see Prerequisites) or validate via Kroki instead of "fixing" correct syntax.

Self-Check (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, suggest the user fine-tune at mermaid.live.

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 architecture
Mind Map	mindmap	topic breakdowns
User Journey	journey	user-experience flows

Syntax Reference

Flowchart: See reference/FLOWCHART.md Sequence: See reference/SEQUENCE.md Class & ER: See reference/CLASS-ER.md Other types: See reference/OTHER-TYPES.md

Examples

Example 1: API Authentication Flow

User prompt:

Create a sequence diagram for JWT authentication

Generated .mmd:

sequenceDiagram
  participant C as Client
  participant G as API Gateway
  participant A as Auth Service
  participant D as Database

  C->>G: POST /login {email, password}
  G->>A: validate(credentials)
  A->>D: SELECT user WHERE email=?
  D-->>A: user record
  A-->>A: verify password hash
  A-->>G: 200 OK + JWT token
  G-->>C: {token: "eyJhbG..."}

Output files: auth-flow.mmd + auth-flow.png

---

Example 2: Microservices Architecture

User prompt:

Draw an e-commerce microservices architecture

Generated .mmd:

flowchart TD
  subgraph Clients
    M[Mobile App]
    W[Web App]
  end

  GW[API Gateway]

  subgraph Services
    US[User Service]
    OS[Order Service]
    PS[Product Service]
    PAY[Payment Service]
  end

  subgraph Data
    UDB[(User DB)]
    ODB[(Order DB)]
    PDB[(Product DB)]
    REDIS[(Redis Cache)]
  end

  M & W --> GW
  GW --> US & OS & PS & PAY
  US --> UDB
  OS --> ODB
  PS --> PDB
  PAY --> REDIS

Output files: ecommerce-arch.mmd + ecommerce-arch.png

---

Example 3: Order State Machine

User prompt:

Show order lifecycle states

Generated .mmd:

stateDiagram-v2
  [*] --> Pending : order created
  Pending --> Confirmed : payment success
  Pending --> Cancelled : timeout/cancel
  Confirmed --> Shipped : dispatched
  Shipped --> Delivered : received
  Delivered --> [*]
  Cancelled --> [*]

Output files: order-states.mmd + order-states.png

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"