Mermaid.js Guide for Claude Code

This is an advanced guide to building mermaid diagrams

Testing output with mermaid-cli

You should test any mermaid diagrams you draw: 1. extract just the mermaid diagram from the markdown file, 2. call mermaid-cli to convert it to a png file 3. check for any syntax errors returned 4. read the png file to see if it looks like what was wanted 5. delete the png file if all is OK

There are two ways to call mermaid-cli - (1) specify - as the filename and pipe the contents in:

cat << EOF  | npx -p @mermaid-js/mermaid-cli mmdc -s 3 -i - -o tmp.png
    graph TD
    A[Client] --> B[Load Balancer]
EOF

or (2) write the output to a temporary file, then pass the name as a parameter:

npx -p @mermaid-js/mermaid-cli mmdc -s 3 -i tmp.md -o tmp.png

Input can be:

• a mermaid file .mmd with just mermaid content
• a markdown file .md with embedded mermaid diagrams - note if you specify output as tmp.png it will make a file per diagram, tmp-1.png, tmp-2.png and so on

You can get more help for mermaid-cli with

npx -p @mermaid-js/mermaid-cli mmdc -h

My preferences

• I like light backgrounds, so any text displayed on the background should be black or dark
• For text on shapes, you should make sure that either you use light text on a dark shape, or dark text on a light shape.

Quick Reference

• Test diagrams: npx -p @mermaid-js/mermaid-cli mmdc -i tmp.mmd -o tmp.png then read the PNG
• Hand-drawn look: Add config: look: handDrawn in frontmatter
• Invisible subgraphs: Use classDef invisible fill:#0000,stroke:#0000;
• New shape syntax: NodeName@{shape: diamond, label: "Decision"}
• Common shapes: rect, rounded, diamond (decisions), cyl (database), doc (document), hex (process), trap-b (trapezoid), lean-r (I/O)

Frontmatter configuration

If you need to configure a mermaid diagram - standalone or embedded in a markdown file - you can do it with YAML frontmatter:

---
config:
  look: handDrawn
  theme: forest
---
graph LR
 A --> B

Customizing using themes

You can set a theme in the frontmatter as above - unless instructed otherwise you should probably leave this as the default, or use base which then allows you to customize things like the main colour scheme:

---
config:
  theme: base
  themeVariables:
    primaryColor: 'red'
    primaryTextColor: 'white'
    fontSize: 32px
    lineColor: 'green'
---
graph TD
 A --> B

Theme variables if you need them are at https://mermaid.js.org/config/theming.html#theme-variables

Flowchart Features

Shape Syntax

Use NodeName@{shape: shapeName, label: "Label"} for custom shapes:

flowchart TB
    decision@{shape: diamond, label: "Decision"}
    database@{shape: cyl, label: "Database"}
    document@{shape: doc, label: "Document"}
    input@{shape: lean-r, label: "Input/Output"}

Invisible Subgraphs for Layout Control

A powerful technique for grouping related nodes without visual borders:

flowchart LR
    %% Define invisible style
    classDef invisible fill:#0000, stroke:#0000;
    
    %% Group related components invisibly
    subgraph api_layer[" "]
        api1[API Gateway]
        api2[Auth Service]
        api3[Rate Limiter]
    end
    
    subgraph services[" "]
        svc1[User Service]
        svc2[Order Service]
        svc3[Payment Service]
    end
    
    %% Apply invisible style
    class api_layer,services invisible;
    
    %% Connections
    api1 --> svc1
    api1 --> svc2
    api2 --> svc1
    api3 --> api1

This technique:

• Groups nodes logically for better layout
• Maintains visual separation
• Helps Mermaid’s layout engine position related items together
• Uses empty label [" "] to hide subgraph titles

Using classDef

Define reusable styles:

flowchart TB
    classDef primary fill:#1e40af,stroke:#1e3a8a,color:#fff;
    classDef danger fill:#dc2626,stroke:#b91c1c,color:#fff;
    
    start[Start]:::primary
    error[Error]:::danger
    
    start --> error

Sequence Diagrams

sequenceDiagram
    actor User
    participant API
    participant DB as Database
    
    %% Grouping with boxes
    box rgba(100,100,255,0.1) Frontend
        participant UI
        participant Cache
    end
    
    %% Parallel execution
    par Process A
        API->>DB: Query users
    and Process B
        API->>Cache: Check cache
    end
    
    %% Conditional flow
    alt Cache hit
        Cache-->>API: Return cached data
    else Cache miss
        DB-->>API: Return fresh data
    end

Advanced Techniques

Styling Edges

Use linkStyle to style specific edges (numbered in order of definition):

flowchart LR
    A[Start] --> B[Process]
    B --> C[End]
    
    linkStyle 0 stroke:#ff3,stroke-width:4px
    linkStyle 1 stroke:#f9f,stroke-width:2px,stroke-dasharray: 5 5

General Tips

• Group related nodes - Use subgraphs (visible or invisible) to help the layout engine
• Define styles once - Use classDef at the top rather than inline styles
• Limit diagram complexity - Break large diagrams into multiple smaller ones
• Use appropriate diagram types - Don’t force complex relationships into flowcharts when other diagrams would be clearer

Best Practices

1. Diagram selection: Flowchart for processes, Sequence for interactions, ER for databases
2. Style consistently: Use classDef with semantic names (primary, error)
3. Layout control: Use invisible subgraphs, test orientations (TB, LR, RL, BT)
4. Keep it simple: Break complex diagrams into smaller ones