---
name: mermaid-diagrams
description: Create software diagrams using Mermaid (.mmd) syntax — class, sequence, flowchart, ERD, C4, state, git, gantt, pie. WHEN: "mermaid diagram", "create .mmd", "mermaid flowchart", "render mermaid", "visualize with mermaid". DO NOT USE FOR: PlantUML (.puml) diagrams — use plantuml skill.
license: MIT
metadata:
  author: KANINI
  version: 1.0.0
---

# Mermaid Diagramming

Create professional software diagrams using Mermaid's text-based syntax. Mermaid renders diagrams from simple text definitions, making diagrams version-controllable, easy to update, and maintainable alongside code.

## Quick Reference

| | |
|---|---|
| **Best for** | Creating Mermaid (.mmd) diagrams — class, sequence, flowchart, ERD, C4, state |
| **Primary capabilities** | 9 diagram types · text-based syntax · native GitHub/GitLab/Notion rendering |
| **References** | `references/class-diagrams.md` · `references/sequence-diagrams.md` · `references/flowcharts.md` · `references/erd-diagrams.md` · `references/c4-diagrams.md` · `references/advanced-features.md` |

## Core Syntax Structure

All Mermaid diagrams follow this pattern:

```mermaid
diagramType
  definition content
```

**Key principles:**
- First line declares diagram type (e.g., `classDiagram`, `sequenceDiagram`, `flowchart`)
- Use `%%` for comments
- Line breaks and indentation improve readability but aren't required
- Unknown words break diagrams; parameters fail silently

## Diagram Type Selection Guide

**Choose the right diagram type:**

1. **Class Diagrams** - Domain modeling, OOP design, entity relationships
   - Domain-driven design documentation
   - Object-oriented class structures
   - Entity relationships and dependencies

2. **Sequence Diagrams** - Temporal interactions, message flows
   - API request/response flows
   - User authentication flows
   - System component interactions
   - Method call sequences

3. **Flowcharts** - Processes, algorithms, decision trees
   - User journeys and workflows
   - Business processes
   - Algorithm logic
   - Deployment pipelines

4. **Entity Relationship Diagrams (ERD)** - Database schemas
   - Table relationships
   - Data modeling
   - Schema design

5. **C4 Diagrams** - Software architecture at multiple levels
   - System Context (systems and users)
   - Container (applications, databases, services)
   - Component (internal structure)
   - Code (class/interface level)

6. **State Diagrams** - State machines, lifecycle states
7. **Git Graphs** - Version control branching strategies
8. **Gantt Charts** - Project timelines, scheduling
9. **Pie/Bar Charts** - Data visualization

## Quick Start Examples

### Class Diagram (Domain Model)
```mermaid
classDiagram
    Title -- Genre
    Title *-- Season
    Title *-- Review
    User --> Review : creates
    
    class Title {
        +string name
        +int releaseYear
        +play()
    }
    
    class Genre {
        +string name
        +getTopTitles()
    }
```

### Sequence Diagram (API Flow)
```mermaid
sequenceDiagram
    participant User
    participant API
    participant Database
    
    User->>API: POST /login
    API->>Database: Query credentials
    Database-->>API: Return user data
    alt Valid credentials
        API-->>User: 200 OK + JWT token
    else Invalid credentials
        API-->>User: 401 Unauthorized
    end
```

### Flowchart (User Journey)
```mermaid
flowchart TD
    Start([User visits site]) --> Auth{Authenticated?}
    Auth -->|No| Login[Show login page]
    Auth -->|Yes| Dashboard[Show dashboard]
    Login --> Creds[Enter credentials]
    Creds --> Validate{Valid?}
    Validate -->|Yes| Dashboard
    Validate -->|No| Error[Show error]
    Error --> Login
```

### ERD (Database Schema)
```mermaid
erDiagram
    USER ||--o{ ORDER : places
    ORDER ||--|{ LINE_ITEM : contains
    PRODUCT ||--o{ LINE_ITEM : includes
    
    USER {
        int id PK
        string email UK
        string name
        datetime created_at
    }
    
    ORDER {
        int id PK
        int user_id FK
        decimal total
        datetime created_at
    }
```

## Detailed References

For in-depth guidance on specific diagram types, see:

- **[references/class-diagrams.md](references/class-diagrams.md)** - Domain modeling, relationships (association, composition, aggregation, inheritance), multiplicity, methods/properties
- **[references/sequence-diagrams.md](references/sequence-diagrams.md)** - Actors, participants, messages (sync/async), activations, loops, alt/opt/par blocks, notes
- **[references/flowcharts.md](references/flowcharts.md)** - Node shapes, connections, decision logic, subgraphs, styling
- **[references/erd-diagrams.md](references/erd-diagrams.md)** - Entities, relationships, cardinality, keys, attributes
- **[references/c4-diagrams.md](references/c4-diagrams.md)** - System context, container, component diagrams, boundaries
- **[references/advanced-features.md](references/advanced-features.md)** - Themes, styling, configuration, layout options

## Best Practices

1. **Start Simple** - Begin with core entities/components, add details incrementally
2. **Use Meaningful Names** - Clear labels make diagrams self-documenting
3. **Comment Extensively** - Use `%%` comments to explain complex relationships
4. **Keep Focused** - One diagram per concept; split large diagrams into multiple focused views
5. **Version Control** - Store `.mmd` files alongside code for easy updates
6. **Add Context** - Include titles and notes to explain diagram purpose
7. **Iterate** - Refine diagrams as understanding evolves

## Configuration and Theming

Configure diagrams using frontmatter:

```mermaid
---
config:
  theme: base
  themeVariables:
    primaryColor: "#ff6b6b"
---
flowchart LR
    A --> B
```

**Available themes:** default, forest, dark, neutral, base

**Layout options:**
- `layout: dagre` (default) - Classic balanced layout
- `layout: elk` - Advanced layout for complex diagrams (requires integration)

**Look options:**
- `look: classic` - Traditional Mermaid style
- `look: handDrawn` - Sketch-like appearance

## Exporting and Rendering

**Native support in:**
- GitHub/GitLab - Automatically renders in Markdown
- VS Code - With Markdown Mermaid extension
- Notion, Obsidian, Confluence - Built-in support

**Export options:**
- [Mermaid Live Editor](https://mermaid.live) - Online editor with PNG/SVG export
- Mermaid CLI - `npm install -g @mermaid-js/mermaid-cli` then `mmdc -i input.mmd -o output.png`
- Docker - `docker run --rm -v $(pwd):/data minlag/mermaid-cli -i /data/input.mmd -o /data/output.png`

## Common Pitfalls

- **Breaking characters** - Avoid `{}` in comments, use proper escape sequences for special characters
- **Syntax errors** - Misspellings break diagrams; validate syntax in Mermaid Live
- **Overcomplexity** - Split complex diagrams into multiple focused views
- **Missing relationships** - Document all important connections between entities

## When to Create Diagrams

**Always diagram when:**
- Starting new projects or features
- Documenting complex systems
- Explaining architecture decisions
- Designing database schemas
- Planning refactoring efforts
- Onboarding new team members

**Use diagrams to:**
- Align stakeholders on technical decisions
- Document domain models collaboratively
- Visualize data flows and system interactions
- Plan before coding
- Create living documentation that evolves with code

## Error Handling

| Error | Message | Remediation |
|---|---|---|
| Syntax error breaks diagram | (Mermaid parse error) | Validate at mermaid.live; fix identifier typos, check `{}` escapes |
| Unsupported diagram type | — | See "Diagram Type Selection Guide"; pick nearest fit |
| Complex layout fails | "Dagre overflow" | Switch to `layout: elk` or split into multiple diagrams |
| Special characters break output | — | Use proper escape sequences; avoid `{}` in comments |
