Turn a stabilized design — a list of components and the connections between them — into a polished, self-contained HTML + SVG diagram. Components are boxes, arrows are communication/data flow, dashed boundaries are regions or trust zones. This is the diagramming engine for the whole plugin; the orchestrator and building blocks hand off here when it's time to draw.
Self-contained by design: one HTML file with inline SVG and CSS, a system monospace font (no web-font fetch), and no dependency to render — it draws fully offline. The only external calls are two pinned, SRI-protected CDN scripts that power the optional PNG/PDF export; remove them and the diagram still renders — only the export buttons are lost.
When to reach for this
At step 3 (high-level design) and step 6 (deep-dive) of the reasoning loop, and any time a design is clear enough to draw: the component list and their connections are known. Also for failure-path diagrams (step 5) — showing a degradation flow is part of the design.
When NOT to
Before the design has stabilized — don't draw boxes that aren't yet justified (that's failure mode #7). A quick ASCII sketch in the conversation is fine for early thinking; render with this skill once the components have earned their place.
How to build a diagram
- Gather the spec from the design: the list of components (each with a type),
the directed connections (with protocol/label), and any region/boundary
groupings. The
system-designdesign-doc already produces this. - Copy the template
assets/template.htmlto a working file. It carries the dark theme, grid background, fonts, the export toolbar, and the summary-card layout — keep those intact. - Map each component to a semantic type (color) using the table below.
- Place boxes and draw arrows following the layout/spacing rules in
references/design-system.md(draw arrows before boxes so they sit behind; mask arrows behind semi-transparent fills; keep ≥40px vertical gaps; legends go outside boundary boxes). - Label the flow — number arrows to show request order; use dashed arrows for async/replication/fallback; dashed boundaries for regions and security groups.
- Fill the summary cards with the key decisions/trade-offs (ties to the design doc), update the title/footer.
- Open it:
open <file>.htmlto view; the toolbar exports PNG/PDF.
Semantic colors (map components to types)
These align with how the building blocks think about a system.
| Component type | Fill (rgba) | Stroke | Used for |
|---|---|---|---|
| Frontend / client | rgba(8,51,68,0.4) |
#22d3ee |
web/mobile clients, edge |
| Backend / service | rgba(6,78,59,0.4) |
#34d399 |
app/API services, workers |
| Database / store | rgba(76,29,149,0.4) |
#a78bfa |
SQL/NoSQL, object store |
| Cache | rgba(8,51,68,0.4) |
#38bdf8 |
Redis/Memcached, CDN cache |
| Message bus / queue | rgba(251,146,60,0.3) |
#fb923c |
Kafka, SQS, queues, streams |
| Cloud / managed | rgba(120,53,15,0.3) |
#fbbf24 |
managed services, regions |
| Security / auth | rgba(136,19,55,0.4) |
#fb7185 |
gateways, auth, firewalls |
| External / generic | rgba(30,41,59,0.5) |
#94a3b8 |
third parties, DNS |
Full styling, arrow markers, masking, spacing, and legend rules are in
references/design-system.md.
Output rules
- One self-contained
.htmlfile: inline CSS, inline SVG, no external images. - Keep the two pinned CDN scripts (html2canvas + jsPDF) with their SRI hashes — they power Copy/PNG/PDF export and nothing else. The diagram renders fully without them; they're only needed for export.
- Avoid SVG
<foreignObject>(renders inconsistently in html2canvas) — use plain<rect>/<text>. - Keep it to ~8–15 components for a high-level view; zoom into one subsystem for a deep-dive rather than cramming everything into one picture.
Dos and don'ts
Do:
- Draw only after the design stabilizes; render the component list and connections the design doc already produced.
- Map every box to a semantic type from the color table so the diagram reads at a glance.
- Draw arrows before boxes (so they sit behind), number them for request order, and reserve dashed arrows for async/replication/fallback.
- Keep it to ~8–15 components; zoom into one subsystem for a deep-dive instead of one crowded picture.
- Keep the file self-contained — inline CSS/SVG, system monospace font, no external images — so it renders offline.
Don't:
- Don't embed Mermaid or invent a second diagram convention; this skill is the one engine.
- Don't use SVG
<foreignObject>(html2canvas renders it inconsistently) — use plain<rect>/<text>. - Don't strip or rewrite the pinned CDN scripts' SRI hashes; recompute them only when bumping versions.
- Don't place legends inside boundary boxes or stack components with under-40px gaps — both cause overlaps.
- Don't add a hard dependency on any send-to-Claude transport; the Copy-Prompt button is the self-contained default.
Interactive / collaborative mode (optional)
For iterating with the user, an interactive variant adds layer toggles,
connection-type filters, zoom, and click-to-comment that builds a feedback prompt
with a Copy button. See references/interactive.md. It degrades gracefully: the
Copy-Prompt button always works. A live "Send to Claude" button is possible only
if a separate prompt-transport plugin (e.g. playground-sync) is installed — this
plugin does not require or bundle one.
Related building blocks
system-design— invoked by the orchestrator at the "draw it" steps; its diagramming notes feed what to draw at each step.- Any building block — invoked by a block whose "Diagram" section says "render with
architecture-diagram". This skill renders for all of them; it owns diagram generation and they point here rather than ship their own.
References
references/design-system.md— colors, arrow markers, z-order/masking, spacing, legend placement, the export toolbar internals. Read before drawing.references/interactive.md— the optional interactive/commentable diagram pattern and the (optional) send-to-Claude transport. Read only if building an interactive map.
Assets
assets/template.html— the self-contained dark-theme HTML+SVG starting point with the export toolbar. Copy and customize it.