markdown-mermaid

name	markdown-mermaid
description	Author Mermaid diagrams that render correctly in GitHub, VS Code, and Mermaid 10+ consumers. Applies a config-driven init directive + linkStyle + semantic classDef vocabulary from `.github/config/brand-palette.json` (default: 6-role semantic palette + typography). Covers renderer pitfalls, mode fragility, and diagram-tool selection. Use when embedding Mermaid in markdown, debugging silent render failures, or choosing between Mermaid / Excalidraw / D2 / PlantUML.
lastReviewed	2026-08-01T00:00:00.000Z

Markdown & Mermaid

Author Mermaid that renders cleanly on first attempt and doesn't drift from the project's visual identity. LLMs generate correct Mermaid syntax natively; this skill adds the two things they don't have:

1. The project's palette + linkStyle + classDef vocabulary (from .github/config/brand-palette.json, shared with the illustrator plugin's svg-banner + illustrator agent + flint-chart)
2. Renderer-specific pitfalls (colons breaking timeline/gitGraph/gantt parsers, unicode escapes, reserved-word handling; the footguns native gen trips on)

Unified 2026-07-30: palette values moved from the retired mermaid-init.json into the shared brand-palette.json. One file, all visual layers.

Init directive (always required)

Every diagram starts with an init directive derived from .github/config/brand-palette.json. Values come from the semantic section (classDef fills/strokes/text), the brand.muted value (edge / link color), and the typography.linkStroke / linkStrokeWidth values. Default (Alex ACT 6-role semantic palette):

```mermaid
%%{init: {'theme': 'base', 'themeVariables': {'lineColor': '#57606a', 'primaryColor': '#ddf4ff', 'primaryBorderColor': '#0969da', 'primaryTextColor': '#1f2328', 'edgeLabelBackground': '#ffffff'}}}%%
flowchart LR
    A[Input]:::blue --> B[Process]:::purple --> C[Output]:::green

    classDef blue fill:#ddf4ff,color:#0550ae,stroke:#80ccff
    classDef green fill:#d3f5db,color:#1a7f37,stroke:#6fdd8b
    classDef purple fill:#d8b9ff,color:#6639ba,stroke:#bf8aff
    classDef gold fill:#fff8c5,color:#9a6700,stroke:#d4a72c
    classDef red fill:#ffebe9,color:#cf222e,stroke:#f5a3a3
    classDef neutral fill:#eaeef2,color:#24292f,stroke:#d0d7de

    linkStyle default stroke:#57606a,stroke-width:1.5px
```

Three components, one source:

• %%{init: ...}%% themeVariables derived from brand-palette.json (edgeLabelBackground = white constant; other theme colors track the palette)
• classDef <name> fill:... color:... stroke:... for each entry in semantic (blue / green / purple / gold / red / neutral)
• linkStyle default <spec> from typography.linkStroke + linkStrokeWidth

To override the palette for a project, edit .github/config/brand-palette.json (rebrands mermaid + the illustrator plugin's svg-banner + illustrator + flint at once). To use a different palette for one diagram, override the init directive inline (rare; usually you want fleet-wide consistency).

Semantic classDef vocabulary

Apply :::<class> to every node. Semantic roles ship with the default config:

Class	Role	Example
:::blue	Input, source, start	A[Audio file]:::blue
:::green	Output, result, success	C[Transcript]:::green
:::purple	Processing, model, transformation	B[WhisperX]:::purple
:::gold	Decision, condition, gate	D{Valid?}:::gold
:::red	Error, warning, failure	E[Retry]:::red
:::neutral	Context, optional, out-of-scope	F[Cache]:::neutral

Consistency across the fleet matters more than clever per-diagram creativity — a reader who has seen one Alex-brand diagram should recognize the second immediately.

Mode fragility (renderer footguns)

Several Mermaid modes fail silently on colons and special characters. Default to flowchart for arbitrary text.

Mode	Status	Constraint
flowchart	Safe	None — handles any content
sequenceDiagram	Safe	Standard message format
classDiagram	Safe	Standard notation
erDiagram	Safe	Standard notation
stateDiagram	Caution	Colons in state names break parsing
journey	Caution	Score format is sensitive
timeline	Fragile	Colons in events (: is separator)
gitGraph	Fragile	Long chains with quoted colon-tags break
gantt	Fragile	dateFormat HH:mm mis-parses task lines

Rule: If your labels contain colons, times (HH:MM), or complex text — use flowchart with subgraphs.

Debug silent failure: check browser console; simplify content; test incrementally; try flowchart — if it works there, the mode was the problem.

Full pitfall catalog (P1–P9, unicode/emoji failures, layout patterns, classDiagram + architecture-beta gotchas, reserved-word handling, cross-diagram compatibility matrix) in references/pitfalls.md — 844 lines of renderer-specific gotchas that LLM native generation reliably trips on.

Multi-line labels

Use <br/> for line breaks, NOT \n. \n renders as literal text. This is the single most common defect in LLM-generated Mermaid.

%% Correct
A[Line one<br/>Line two]:::blue

%% Wrong — renders "Line one\nLine two" as one string
A[Line one\nLine two]:::blue

Size the graph and its frame together

Author the source so the diagram has a readable natural shape before relying on a renderer:

• Prefer TD for sequential flows with several steps or multi-line labels.
• Split diagrams wider than 4:1 unless horizontal comparison is the point.
• Keep labels short enough that a node communicates one idea.
• Do not combine width: 100% with a fixed or capped SVG height. That creates a page-wide viewport with a small graph centered inside it.

The Alex docs-shell applies one deterministic fit after Mermaid renders:

1. Crop once to the root graph bounds plus 16–32px of padding.
2. Derive the ideal SVG width from the cropped viewBox and source font size, targeting 16px labels.
3. Shrink-wrap compact diagrams instead of stretching every SVG to page width.
4. Use contained horizontal scrolling only when the graph cannot preserve a 13px desktop or 11px mobile label floor inside the available width.

Runtime fitting removes wasted viewport space; it does not repair an over-dense source diagram. Refactor the Mermaid source first when the graph still needs scrolling or extreme height after fitting.

Diagram-tool selection

Showing	Best tool	Why
Process, workflow, decision tree	Mermaid flowchart	Native GitHub, wide renderer support
System architecture (technical)	Mermaid flowchart + subgraphs	Same
System architecture (executive)	D2 (external)	Cleaner, less busy
Sequence / API interactions	Mermaid sequenceDiagram	Native
Class relationships	Mermaid classDiagram	Native
Timeline / roadmap	Mermaid gantt (but see fragility above)	Native but fragile
Free-form / whiteboard	Excalidraw (external)	LLM cannot generate; user draws
Data / metrics	flint-chart plugin	Real chart rendering, not diagram
Brand / hero banner	Alex_ACT_Illustrator_Plugin svg-banner	Not a diagram; different domain

For Mermaid alternatives (D2, PlantUML, Graphviz, WaveDrom) with syntax examples and VS Code extension setup, see references/tool-ecosystem.md.

VS Code 1.109+ native chat rendering

The renderMermaidDiagram chat tool (deferred; tool_search for "mermaid" to load) renders diagrams interactively in Copilot Chat — pan, zoom, fullscreen, copy source. Use it for iterative design in chat; use fenced code blocks for docs that live in .md files.

References

Bulk content lives in references/ — loaded on demand:

• references/pitfalls.md — parser pitfalls P1–P9, unicode/emoji, reserved words, cross-diagram compatibility
• references/tool-ecosystem.md — Mermaid / D2 / PlantUML / Excalidraw comparison + VS Code setup
• references/diagram-reference.md — diagram types, node shapes, edge styles, per-diagram theming
• references/markdown-best-practices.md — document structure, figure/table conventions, Shields.io

Related

• lint-clean-markdown — writing markdown that passes markdownlint on the first attempt. Reach for it when the concern is lint compliance rather than diagram rendering.

Falsifiability

Revisit by 2026-10-30 (90 days) or sooner if any of the following fires:

• Diagrams authored per this skill fail to render in GitHub or VS Code preview ≥3 times in a quarter (skill guidance out of date)
• The mode-fragility warnings stop applying because Mermaid.js fixed the underlying parser bugs (check mermaid.js changelog)
• A heir configures a brand override in .github/config/brand-palette.json and reports the config schema is too tight (missing a field they need) ≥2 times in a quarter
• The default palette + built-in fallback (if we add one to a future renderer helper) drift out of sync
• The trim itself was wrong: diagrams start rendering without the palette because the skill body was doing more work than it looked (the always-loaded palette section carried the discipline; the config-pointer doesn't)