Mermaid Skill

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.

Option B: Kroki API (no install)

curl --version  # Just need curl

Workflow

• 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
• Pick diagram type — choose from table below
• Generate — write .mmd file to disk
• Validate — run validation (REQUIRED before export)
• Export — use mmdc or Kroki API to produce PNG/SVG/PDF
• 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.
• 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.
• 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 system context |

| Architecture | architecture-beta | cloud / CI/CD service layouts |

| 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

Architecture: See reference/ARCHITECTURE.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

Example 4: Cloud Architecture

User prompt:

> Draw a simple service architecture for an API

Generated .mmd:

architecture-beta
  group api(cloud)[API]

  service gateway(internet)[Gateway] in api
  service db(database)[Database] in api
  service cache(disk)[Cache] in api

  gateway:R --> L:db
  gateway:B --> T:cache

Output files: api-architecture.mmd + api-architecture.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" |