Arc Diagrams

Arc is a visual diagram editor for creating architecture diagrams. This skill enables you to generate Arc-compatible diagram configs that can be rendered in React applications.

When to Use

Activate this skill when the user asks to:

• "Create an architecture diagram"
• "Draw a system diagram"
• "Visualize the architecture"
• "Make a diagram showing..."
• "Design a flow diagram"

Diagram Format

Arc diagrams are JSON objects with this structure:

interface ArcDiagramData {
  layout: { width: number; height: number }
  nodes: Record<string, NodePosition>
  nodeData: Record<string, NodeData>
  connectors: Connector[]
  connectorStyles: Record<string, ConnectorStyle>
  flows?: DiagramFlow[]
}

Creating a Diagram

1. Define Layout

Set canvas dimensions (default 1600x900 for editor, smaller for exports):

"layout": { "width": 860, "height": 400 }

2. Position Nodes

Each node needs an x, y position and size:

"nodes": {
  "frontend": { "x": 50,  "y": 100, "size": "l" },
  "api":      { "x": 300, "y": 100, "size": "m" },
  "database": { "x": 550, "y": 100, "size": "m" }
}

Size options (canonical — use these):

Size	Key	Dimensions (px)
Extra Small	xs	80 × 36
Small	s	110 × 48
Medium	m	160 × 75
Large	l	220 × 90

Source: src/utils/constants.ts → NODE_SIZES. Do not use large/normal/small.

3. Define Node Data

Each node needs an icon, name, and color:

"nodeData": {
  "frontend": {
    "icon": "Monitor",
    "name": "Frontend",
    "subtitle": "React",
    "description": "User interface",
    "color": "violet"
  },
  "api": {
    "icon": "Server",
    "name": "API",
    "subtitle": "Node.js",
    "description": "REST endpoints",
    "color": "emerald"
  },
  "database": {
    "icon": "Database",
    "name": "PostgreSQL",
    "subtitle": "Storage",
    "description": "Persistent data",
    "color": "blue"
  }
}

Available icons (from Lucide):

• Devices: Monitor, Server, Smartphone, Watch, Cloud, Cpu, Database, HardDrive
• Network: Wifi, Globe
• Users: User, Users
• Security: Lock, Key, Shield
• Media: Mic, Camera, Speaker, Headphones
• Code: Code, Terminal, FileCode, Folder
• Data: Zap, Activity, BarChart, PieChart
• Flow: ArrowRight, ArrowDown, RefreshCw, Repeat
• Structure: Box, Package, Layers, Grid
• UI: Settings, Bell, Mail, MessageSquare, Search, Filter
• Actions: Download, Upload, Play, Pause, Square, Circle

Available colors: violet | emerald | blue | amber | sky | zinc | rose | orange

4. Connect Nodes

Define connections between nodes:

"connectors": [
  {
    "from": "frontend",
    "to": "api",
    "fromAnchor": "right",
    "toAnchor": "left",
    "style": "http"
  },
  {
    "from": "api",
    "to": "database",
    "fromAnchor": "right",
    "toAnchor": "left",
    "style": "sql"
  }
]

Anchor positions: top | bottom | left | right | bottomLeft | bottomRight | topLeft | topRight

Optional: Add "curve": "natural" or "curve": "step" for curved connectors.

Stable identity: Give a connector an id when it may be referenced by a focus story or guided view, reviewed in a diff, or when another connector shares the same from/to pair. Without an id, a connector is only addressable by endpoint pair or array index.

Guided views: when the diagram explains a flow a reader should follow in order, add views[] — {id, title, node?, nodes?, connectors?, mode?, caption?, steps?} per chapter. The player draws a chapter rail (showViews) and /player/<session>?view=<id> deep-links a chapter. A view anchored on node inherits that node's focusTargets story; declare nodes/connectors/mode to override it.

5. Animate message flows

Add flows[] when a message or packet should travel the diagram rather than a connector merely having motion styling:

"flows": [
  {
    "id": "request",
    "label": "Request",
    "legs": [
      { "id": "client-api" },
      { "id": "api-db", "pause": 0.4 }
    ],
    "duration": 3,
    "delay": 0.2,
    "hold": 0.6,
    "repeat": "indefinite",
    "easing": "ease-in-out",
    "marker": "packet",
    "color": "emerald",
    "size": 11,
    "trail": "wake"
  }
]

• Each leg references a connector by stable id (preferred) or endpoint { "from": "...", "to": "..." }; add direction: "reverse" to run a leg backwards and pause to dwell between hops.
• Timing is deterministic: speed is px/s (default 160), duration is total route seconds including pauses and overrides speed, delay/hold are seconds, and repeat is a count or "indefinite".
• marker is dot (default), packet, pulse, or arrow; trail is none, fade, or wake; color/size tune the marker.
• Standalone SVG animates via SMIL, render_svg/render_png can sample a still at flowTime, and arc render --format gif|mp4 or MCP render_animation captures deterministic frames through Chrome and encodes with ffmpeg.

6. Style Connectors

Define appearance for each connector style:

"connectorStyles": {
  "http": { "color": "amber", "strokeWidth": 2, "label": "HTTP" },
  "sql": { "color": "blue", "strokeWidth": 2, "label": "SQL" },
  "async": { "color": "zinc", "strokeWidth": 1.5, "dashed": true }
}

Complete Example

Here's a full diagram for a typical web application:

{
  "layout": { "width": 700, "height": 300 },
  "nodes": {
    "client":   { "x": 50,  "y": 100, "size": "l" },
    "api":      { "x": 300, "y": 100, "size": "m" },
    "cache":    { "x": 300, "y": 220, "size": "s" },
    "database": { "x": 520, "y": 100, "size": "m" }
  },
  "nodeData": {
    "client":   { "icon": "Monitor",  "name": "Client",   "subtitle": "React",    "color": "violet" },
    "api":      { "icon": "Server",   "name": "API",      "subtitle": "Express",  "color": "emerald" },
    "cache":    { "icon": "Zap",      "name": "Cache",    "subtitle": "Redis",    "color": "amber" },
    "database": { "icon": "Database", "name": "Database", "subtitle": "Postgres", "color": "blue" }
  },
  "connectors": [
    { "from": "client", "to": "api",      "fromAnchor": "right",  "toAnchor": "left", "style": "http" },
    { "from": "api",    "to": "database", "fromAnchor": "right",  "toAnchor": "left", "style": "sql" },
    { "from": "api",    "to": "cache",    "fromAnchor": "bottom", "toAnchor": "top",  "style": "cache" }
  ],
  "connectorStyles": {
    "http":  { "color": "amber",   "strokeWidth": 2, "label": "HTTP" },
    "sql":   { "color": "blue",    "strokeWidth": 2, "label": "SQL" },
    "cache": { "color": "emerald", "strokeWidth": 2, "label": "cache" }
  }
}

Layout judgment

The contract past "does it validate": a good Arc diagram reads top-to-bottom or left-to-right in one glance. These are the composition rules validateDiagram partially enforces (composition/* warnings) and the rest is on you.

Budget. ≤12 nodes per flat view. Past ~16 without groups/layoutHints/views, composition/too-dense warns — the fix is chaptering, not a bigger canvas: split into groups + layoutHints for regions, or views[] for a walkthrough.

One spine. Pick one dominant flow direction and hold it: left→right for request/data paths, top→bottom for stacks and pipelines. Everything not on the spine (replicas, sidecars, observability taps) sits on a parallel lane — above/below for a horizontal spine — and reads as secondary: zinc color, dashed, thinner strokeWidth.

Spacing math. NODE_SIZES are xs 80×36, s 110×48, m 160×75, l 220×90. Budget ~150–200px of horizontal pitch between connected nodes (edge gap ≈ 40–80px of clear channel for the stroke + label), ~60–100px vertically. Keep sibling nodes on one baseline and aligned to a ~50px grid when hand-placing.

When to autoLayout. Call autoLayout() (or the editor's auto-layout) when node count exceeds ~8, when groups own their members via layoutHints, or when the graph is dense and any hand order will cross edges. Hand-place only when the shape itself carries meaning (a ring, a hub-and-spoke) and the node count is small.

Labels. The name row is what scans. Budget by size: xs ~5, s ~9, m ~17, l ~26 chars (composition/label-overflow warns past it, and suggests the next size up). Prefer subtitle for the qualifier instead of a long name — "API" + subtitle "Express" beats "Express API Server".

Routing. Anchors take the direction of travel: right→left on a horizontal spine, bottom→top on a vertical one. Never route a connector through an unrelated node (geometry/connector-through-node is an error). Diagonal corner anchors (bottomRight etc.) are for fan-out, not the spine. Give an id to any connector a focus story or view will reference, and to parallel edges sharing from/to.

Validation and repair

Always validate before handing off JSON.

• In the repo, call validateDiagram(value) from src/utils/diagramDiagnostics.ts.
• Through MCP, call validate_diagram; it returns { ok, diagnostics }.
• validateDiagramShape() remains the low-level shape gate; validateDiagram wraps it and adds repairable diagnostics.

Each diagnostic has a stable code, severity, subject, human message, optional evidence, and supportedFixes.

Repair loop:

1. Read diagnostics; fix severity: "error" before warnings.
2. Apply one supportedFixes entry when offered, or make the direct correction it names.
3. Re-validate the edited document.
4. Repeat until no error-severity diagnostics remain; leave warnings only when intentional.

Common codes:

• shape/* — malformed document or entries; fix these before anything else.
• semantic/dangling-connector-endpoint — retarget from/to or remove the connector.
• semantic/unknown-* and semantic/duplicate-connector-id — use the suggested valid value or unique connector id.
• geometry/node-overlap, geometry/connector-through-node, geometry/node-outside-layout — move nodes or run autoLayout().
• composition/too-dense, composition/label-overflow — advisory budget warnings: chapter the diagram, or resize the node.

Preview and export

• render_ascii previews a diagram in plain text without external dependencies.
• render_svg returns deterministic SVG markup; authored flows animate via SMIL by default, flowTime samples a deterministic still, and animateFlows: false disables them.
• render_png returns MCP image content when Chrome/Chromium is installed; set ARC_CHROME when it is not on PATH, and use flowTime to choose a deterministic animation frame. If it returns render/chrome-unavailable, continue with ASCII/SVG or ask the user to install Chrome/Chromium.
• render_animation writes a GIF or MP4 for diagrams with flows; set output to a .gif/.mp4 path, optionally duration, fps, scale, theme, and mode. It needs Chrome/Chromium plus ffmpeg (ARC_CHROME/ARC_FFMPEG).
• render_html emits paste-ready output: format=component for a React TSX component, format=iframe for a studio embed tag, or format=html for a standalone SVG page.
• The CLI equivalent is arc render diagram.json --out artifact.gif --duration 4 --fps 12.

Output

When generating a diagram, output:

1. The complete JSON config
2. Brief explanation of the architecture shown
3. Suggest the user save it as a .arc.json file or paste into the Arc editor

TypeScript Export

For React projects, diagrams can be exported as TypeScript:

import type { ArcDiagramData } from '@arach/arc'

const diagram: ArcDiagramData = {
  // ... config here
}

export default diagram