Mermaid Diagrams

Mermaid Diagrams

Portable Mermaid.js diagramming for architectural documentation. Not tied to any blog platform, theme, or rendering engine. Diagrams can be rendered via CLI, embedded in markdown, or served as HTML snippets.

When to Use

Load this skill when:

• Rendering C4 flowchart approximations after the C4 level and model have been chosen; use c4-diagramming for C4 modeling decisions
• Producing sequence diagrams for interaction flows
• Designing flowcharts for process documentation
• Building state/class/ER diagrams for specification
• Generating any diagram that needs to render in both agent-facing and human-facing contexts
• Reviewing whether a structurally valid Mermaid diagram is the right communication artifact for its audience and output surface

Do NOT load when:

• A plain text outline communicates the relationship more clearly than a diagram.
• The target renderer cannot execute Mermaid and no pre-rendering path is available.

PDF Output — Pre-render Required

Mermaid code blocks (mermaid) do NOT render in the Pandoc → HTML → Puppeteer PDF pipeline. The pipeline generates static HTML with no JavaScript execution.

For any diagram destined for PDF output:

1. Create the diagram as a standalone .mmd file
2. Pre-render to SVG: npx @mermaid-js/mermaid-cli@11.16.0 -i diagram.mmd -o diagram.svg --width 800
3. Choose one embedding method: use raw inline SVG by default, or base64 data URIs when the renderer corrupts raw SVG.
4. Strip hardcoded max-width pixel values from the SVG tags
5. Use flowchart TD (portrait) not flowchart LR (landscape) — see references/portrait-layout.md
6. Add page-break divs before/after each full-page diagram

Do NOT leave ```mermaid code blocks in markdown that will go through Pandoc. They render as raw monospace text.

See references/pdf-rendering-pipeline.md for the full pipeline with Puppeteer setup, SVG styling fixes, and QA checklist.

Communication Review

Before authoring, state the audience, reader job, target surface, and intended read order. Choose the smallest diagram type and scope that supports that job. Keep abstraction, node shapes, edge semantics, labels, boundary treatment, and visual hierarchy consistent. Syntax validation and a successful render are necessary but not sufficient: a diagram can be valid Mermaid and still be the wrong communication artifact because it is too dense, hides the primary path, mixes representational conventions, or leaves the reader to infer uncertainty.

Load references/diagram-communication.md when creating, revising, or reviewing a diagram for people rather than only testing grammar. Use its review record for narrative, signal-to-noise, labels and legends, color-independent meaning, accessibility fallback, uncertainty, and rendered review evidence. C4 level/model ownership stays with c4-diagramming; architecture decisions stay with software-architecture; full accessibility conformance stays with web-accessibility.

Supported Diagram Types

C4 Model Guidance

Mermaid has experimental native C4 syntax (C4Context, C4Container, C4Component) but it is unsupported on GitHub and most markdown renderers. GitHub's built-in mermaid renderer does not bundle the C4 plugin — C4-syntax blocks render as raw code rather than diagrams. Use one of these approaches instead:

1. Flowchart workarounds (GitHub-compatible) — Convert C4 diagrams to standard flowchart syntax using subgraphs for boundaries, styled node boxes for Person/System/Container/Db, and labelled edges for Rel. See references/c4-to-flowchart.md for the full conversion pattern.
2. Structurizr DSL — use for real C4 diagrams. Render via Structurizr CLI or export to Mermaid SVG. Best for formal architecture documentation that doesn't live in GitHub markdown.
3. Hybrid approach — maintain a full C4 model in Structurizr DSL and include a flowchart-based approximation in Markdown for inline readability.

C4 → Flowchart Conversion Pattern

See references/c4-to-flowchart.md for worked examples of all three C4 levels.

GitHub Compatibility Reference

Rendering

CLI (mmdc) — for PDF/SVG/PNG output

npx @mermaid-js/mermaid-cli@11.16.0 -i diagram.mmd -o diagram.svg
npx @mermaid-js/mermaid-cli@11.16.0 -i diagram.mmd -o diagram.png
npx @mermaid-js/mermaid-cli@11.16.0 -i diagram.mmd -o diagram.pdf

Requires Puppeteer + Chromium (~1.7GB). Use the Docker image for isolated rendering:

docker run --rm -v $(pwd):/data ghcr.io/mermaid-js/mermaid-cli mermaid-cli -i /data/diagram.mmd -o /data/diagram.svg

CDN (HTML) — for inline web rendering

<script src="https://cdn.jsdelivr.net/npm/mermaid@11/dist/mermaid.min.js"></script>
<script>mermaid.initialize({startOnLoad:true});</script>
<div class="mermaid">
flowchart LR
  A-->B
</div>

Validation

// Node.js validation
import { parse } from 'mermaid';
try {
  parse('flowchart LR\n  A-->B');
  console.log('Valid');
} catch (e) {
  console.error('Invalid:', e.message);
}

Scripts

Theming

Mermaid uses a base theme with customizable theme variables. Set them with an init directive at the top of the diagram. Consult the Mermaid documentation for the complete version-specific variable set.

Key theme variables:

• primaryColor, primaryTextColor, primaryBorderColor
• secondaryColor, tertiaryColor
• lineColor, fontFamily, fontSize
• background (outer background), mainBkg (element background)

Anti-Patterns

Portability

This skill is intentionally host-neutral. Use your agent's normal mechanisms to load the references, templates, and scripts listed here. Do not assume a particular profile system, task orchestrator, memory service, or response-handoff format.

When not to use

• Use c4-diagramming when the task is to choose or maintain C4 system-context, container, component, or code-level models.
• Use software-architecture for architecture drivers, tradeoffs, decisions, and fitness evidence rather than diagram syntax or rendering.
• Use web-accessibility for complete WCAG/ARIA conformance and assistive-technology evaluation; this skill supplies diagram-specific checks and fallbacks only.