Mermaid Studio

Mermaid Studio

Expert-level Mermaid diagram creation, validation, and multi-format rendering. Creates diagrams from descriptions or code analysis, validates syntax, and renders to SVG, PNG, or ASCII with professional theming.

Golden Rules — Elegant Diagrams by Default

Every diagram MUST follow these principles. They are not optional — they define the difference between a mediocre diagram and a gold-standard one.

Rule 1: Always Use an Init Directive for Professional Styling

NEVER create a diagram without an %%{init} directive or frontmatter config. The default Mermaid theme produces harsh black lines and generic colors. Always apply a curated palette.

For general diagrams (flowchart, sequence, state, class, ERD):

%%{init: {'theme': 'base', 'themeVariables': {
  'primaryColor': '#4f46e5', 'primaryTextColor': '#ffffff',
  'primaryBorderColor': '#3730a3', 'lineColor': '#94a3b8',
  'secondaryColor': '#10b981', 'tertiaryColor': '#f59e0b',
  'background': '#ffffff', 'mainBkg': '#f8fafc',
  'nodeBorder': '#cbd5e1', 'clusterBkg': '#f1f5f9',
  'clusterBorder': '#e2e8f0', 'titleColor': '#1e293b',
  'edgeLabelBackground': '#ffffff', 'textColor': '#334155'
}}}%%

⚠️ Font Warning: Do NOT set fontFamily in theme variables. The Mermaid default font (trebuchet ms, verdana, arial, sans-serif) works everywhere. Setting system-ui, Segoe UI, or -apple-system will render as Times New Roman in headless Chromium (used by mmdc).

For C4 diagrams — see the dedicated C4 styling section below.

For architecture-beta diagrams — see the dedicated AWS/Architecture section below.

Rule 2: Soft Lines, Never Harsh Black

The single biggest visual improvement is using lineColor: '#94a3b8' (slate-400) instead of the default black. This creates a modern, breathable diagram. For dark themes, use lineColor: '#64748b' (slate-500).

Rule 3: Limit Density — Breathe

• Maximum 15 nodes per diagram (not 20 — fewer is more elegant)
• Use subgraph or boundaries to create whitespace and visual grouping
• Prefer LR (left-right) for process flows — it reads more naturally
• Use invisible links (A ~~~ B) to add spacing when the layout is cramped

Rule 4: Meaningful Labels and Consistent Style

• Node IDs: camelCase (orderService, not s1 or os)
• Labels: short, clear natural language ([Order Service])
• Arrows: action verbs with protocol info ("Sends order via gRPC")
• Descriptions: one-line, role-focused ("Handles order lifecycle")

Rule 5: Color Harmony Over Color Variety

Use max 3-4 colors per diagram. Map colors to meaning:

• Blue tones (#4f46e5, #3b82f6) → primary systems, internal services
• Green tones (#10b981, #059669) → success states, data stores
• Amber tones (#f59e0b, #d97706) → external systems, warnings
• Slate tones (#64748b, #94a3b8) → lines, borders, secondary elements
• Red tones (#ef4444) → errors ONLY, never as decoration

Modes of Operation

This skill operates in three modes based on user intent:

Default to Full mode when intent is unclear.

Step 1: Understand the Request

Before writing any Mermaid code, determine:

1. What to diagram — system, flow, schema, architecture, code structure?
2. Which diagram type — use the Decision Matrix below
3. Output format — code only, SVG, PNG, or ASCII?
4. Theme preference — ask only if rendering; default to base theme with curated palette

Diagram Type Decision Matrix

If the user's description doesn't clearly map to one type, suggest 2-3 options with a brief rationale for each, then let them choose.

When to Load References

Load reference files ONLY when needed for the specific diagram type:

• C4 diagrams → Read references/c4-architecture.md BEFORE writing code
• AWS/Cloud architecture → Read references/aws-architecture.md BEFORE writing code
• Code-to-diagram → Read references/code-to-diagram.md BEFORE analyzing
• Theming/styling → Read references/themes.md when user requests custom themes
• Syntax errors → Read references/troubleshooting.md when validation fails
• Any diagram type details → Read references/diagram-types.md for comprehensive syntax

Step 2: Create the Diagram

2.1 — Write Mermaid Code

Follow these principles in order of priority:

1. Elegance first — every diagram must look professional with init directives and curated colors
2. Correctness — valid syntax that renders without errors
3. Clarity — meaningful labels, logical flow direction, clear relationships
4. Simplicity — under 15 nodes per diagram; split complex systems into multiple
5. Consistency — uniform naming (camelCase for IDs, descriptive labels in brackets)

2.2 — Structure Rules

%% Diagram: [Purpose] | Author: [auto] | Date: [auto]
%%{init: {'theme': 'base', 'themeVariables': { ... }}}%%
[diagramType]
    [content]

CRITICAL: The %%{init} directive MUST go on the very first non-comment line, BEFORE the diagram type declaration. Alternatively, use YAML frontmatter at the absolute start of the file.

Naming conventions:

• Node IDs: camelCase, descriptive (orderService, not s1)
• Labels: natural language in brackets ([Order Service])
• Relationships: action verbs ("Sends order to", "Reads from")

Layout best practices:

• TD (top-down) for hierarchical flows and processes
• LR (left-right) for timelines, pipelines, and sequential processes
• RL for right-to-left reading contexts
• Use subgraph to group related nodes; name subgraphs meaningfully
• Add direction inside subgraphs when needed for different flow

2.3 — Quick Reference Examples

Flowchart (with professional styling):

%%{init: {'theme': 'base', 'themeVariables': {
  'primaryColor': '#4f46e5', 'primaryTextColor': '#fff',
  'primaryBorderColor': '#3730a3', 'lineColor': '#94a3b8',
  'secondaryColor': '#10b981', 'tertiaryColor': '#f59e0b',
  'background': '#ffffff', 'mainBkg': '#f8fafc',
  'nodeBorder': '#cbd5e1', 'clusterBkg': '#f1f5f9',
  'clusterBorder': '#e2e8f0', 'titleColor': '#1e293b',
  'edgeLabelBackground': '#ffffff'
}}}%%
flowchart TD
    Start([Start]) --> Input[/User Input/]
    Input --> Validate{Valid?}
    Validate -->|Yes| Process[Process Data]
    Validate -->|No| Error[Show Error]
    Error --> Input
    Process --> Save[(Save to DB)]
    Save --> End([End])

For sequence diagram and ERD styling examples, read references/themes.md.

C4 Context (with MANDATORY elegant styling):

C4Context
    title System Context — E-Commerce Platform

    Person(customer, "Customer", "Places orders online")
    System(platform, "E-Commerce Platform", "Handles orders and payments")
    System_Ext(payment, "Payment Gateway", "Processes transactions")
    System_Ext(email, "Email Service", "Sends notifications")

    Rel(customer, platform, "Uses", "HTTPS")
    Rel(platform, payment, "Processes payments", "API")
    Rel(platform, email, "Sends emails", "SMTP")

    UpdateRelStyle(customer, platform, $textColor="#475569", $lineColor="#94a3b8")
    UpdateRelStyle(platform, payment, $textColor="#475569", $lineColor="#94a3b8")
    UpdateRelStyle(platform, email, $textColor="#475569", $lineColor="#94a3b8")

    UpdateLayoutConfig($c4ShapeInRow="3", $c4BoundaryInRow="1")

Architecture (AWS with Iconify icons):

architecture-beta
    group vpc(logos:aws-vpc)[VPC]

    service api(logos:aws-api-gateway)[API Gateway] in vpc
    service lambda(logos:aws-lambda)[Lambda] in vpc
    service db(logos:aws-dynamodb)[DynamoDB] in vpc
    service s3(logos:aws-s3)[S3 Bucket]

    api:R --> L:lambda
    lambda:R --> L:db
    lambda:B --> T:s3

IMPORTANT: Architecture-beta diagrams with logos:* icons require icon pack registration. When rendering with the render script, use the --icons logos flag. If rendering in a markdown viewer that doesn't support icon packs, use the built-in icons (cloud, database, disk, server, internet) as fallback. Read references/aws-architecture.md for the complete icon catalog and rendering instructions.

For comprehensive syntax of ALL diagram types, read references/diagram-types.md.

C4 Diagrams — Mandatory Styling Guide

C4 diagrams have fixed element styling (blue boxes for systems, gray for persons, etc.), but their relationship lines default to harsh black which creates visual noise. You MUST apply these styling rules:

The C4 Styling Pattern

Every C4 diagram MUST include these directives at the end:

    %% === MANDATORY STYLING ===
    %% Apply soft line colors to ALL relationships
    UpdateRelStyle(fromAlias, toAlias, $textColor="#475569", $lineColor="#94a3b8")
    %% Repeat for each Rel() in the diagram

    %% Optimize layout spacing
    UpdateLayoutConfig($c4ShapeInRow="3", $c4BoundaryInRow="1")

C4 Color Values Reference

C4 Layout Tips

CRITICAL — Maximum 6 Rel() per diagram. More than 6 relationships causes Dagre to route arrows through nodes, creating unreadable spaghetti. If your system needs more connections, split it into multiple focused diagrams.

• Use $c4ShapeInRow="3" for most diagrams (prevents horizontal crowding)
• Use $c4ShapeInRow="2" for diagrams with long labels
• Use $c4BoundaryInRow="1" always (stacks boundaries vertically for clarity)
• Apply $offsetY="-10" to UpdateRelStyle when labels overlap with elements
• Prefer tree-shaped topologies (1 in, 1-2 out per node) over mesh topologies
• Declare elements in flow order — the order of Container() declarations affects layout
• Use directional Rel_D, Rel_R, etc. only as a last resort when auto-layout creates overlapping

For comprehensive C4 syntax, examples, and patterns, read references/c4-architecture.md.

Step 3: Validate

Before rendering, ALWAYS validate the Mermaid syntax:

node ~/.agents/skills/mermaid-studio/scripts/validate.mjs <file.mmd>

If validation fails:

1. Read the error message carefully
2. Consult references/troubleshooting.md for common fixes
3. Fix the syntax and re-validate
4. Maximum 3 fix attempts before asking the user for clarification

Step 4: Render

4.1 — Setup (First Run Only)

bash ~/.agents/skills/mermaid-studio/scripts/setup.sh

Installs mermaid-cli, beautiful-mermaid, mermaid, puppeteer, and Chromium into ${XDG_CACHE_HOME:-~/.cache}/mermaid-studio (override with MERMAID_STUDIO_DIR). Run once per environment.

4.2 — Single Diagram Rendering

# SVG (default — best quality)
node ~/.agents/skills/mermaid-studio/scripts/render.mjs -i diagram.mmd -o diagram.svg

# PNG (for documents/presentations)
node ~/.agents/skills/mermaid-studio/scripts/render.mjs -i diagram.mmd -o diagram.png --width 1200

# ASCII (for terminals/READMEs)
node ~/.agents/skills/mermaid-studio/scripts/render-ascii.mjs -i diagram.mmd

# With icon packs (architecture-beta with AWS/tech icons)
node ~/.agents/skills/mermaid-studio/scripts/render.mjs -i diagram.mmd -o diagram.svg --icons logos,fa

The --icons flag registers Iconify packs before rendering. Packs: logos (AWS/tech), fa (Font Awesome). Use logos for AWS.

4.3 — Batch Rendering

For multiple diagrams at once:

node ~/.agents/skills/mermaid-studio/scripts/batch.mjs \
  --input-dir ./diagrams \
  --output-dir ./rendered \
  --format svg \
  --theme default \
  --workers 4

4.4 — Available Themes

beautiful-mermaid (15): tokyo-night | tokyo-night-storm | tokyo-night-light | dracula | nord | nord-light | catppuccin-mocha | catppuccin-latte | github-dark | github-light | solarized-dark | solarized-light | one-dark | zinc-dark | zinc-light

mermaid-cli native (5): default | forest | dark | neutral | base

Custom theme: --theme base --config '{"theme":"base","themeVariables":{"primaryColor":"#4f46e5","lineColor":"#94a3b8"}}'

For the full theme catalog, read references/themes.md. The render script auto-selects the best engine (mmdc primary, beautiful-mermaid fallback, Puppeteer for icon packs).

Step 5: Code-to-Diagram (When Requested)

When the user asks to visualize existing code or architecture:

1. Read references/code-to-diagram.md for the analysis methodology
2. Analyze the codebase to identify the right diagram type:
   • Module dependencies → Flowchart or Class diagram
   • API routes and handlers → Sequence diagram
   • Database models/schemas → ERD
   • Service architecture → C4 Container or Architecture diagram
   • State machines in code → State diagram
3. Generate the .mmd file with proper init directives (Golden Rule 1)
4. Validate and render as usual

Troubleshooting Quick Reference

For comprehensive troubleshooting, read references/troubleshooting.md.

Output Conventions

• Save .mmd source files alongside rendered outputs
• Naming: {purpose}-{type}.mmd (e.g., auth-flow-sequence.mmd)
• For batch: maintain input filename, change extension
• Always provide both the .mmd source and rendered file to the user