Excalidraw Skill

Prompts

Use when user requests diagrams, flowcharts, architecture charts, or visualizations. Also use proactively when explaining systems with 3+ components, complex data flows, or relationships that benefit from visual representation. Generates .excalidraw files and exports to PNG/SVG via Kroki API or locally using excalidraw-brute-export-cli.

Install

openclaw skills install excalidraw-skill

Excalidraw Diagrams

Overview

Generate .excalidraw JSON files and export to PNG/SVG.

Two export options:

Kroki API (curl) — zero install, SVG output only
excalidraw-brute-export-cli — local Firefox-based, PNG + SVG

Supported formats: PNG (local CLI only), SVG (both options). PDF is NOT supported.

When to Use

Explicit triggers: user says "画图", "diagram", "visualize", "flowchart", "draw", "架构图", "流程图"

Proactive triggers:

Explaining a system with 3+ interacting components
Describing a multi-step process or decision tree
Comparing architectures or approaches side by side

Skip when: a simple list or table suffices, or user is in a quick Q&A flow

Prerequisites

Option A: Kroki API (recommended — zero install, SVG only)

# Just needs curl (pre-installed on macOS/Linux/Windows Git Bash)
curl --version

No additional setup. SVG rendered via https://kroki.io.

Option B: Local CLI (required for PNG)

The CLI uses Firefox (not Chromium). Check and install:

npm install -g excalidraw-brute-export-cli
npx playwright install firefox

macOS patch (one-time, required):

CLI_MAIN=$(npm root -g)/excalidraw-brute-export-cli/src/main.js
sed -i '' 's/keyboard.press("Control+O")/keyboard.press("Meta+O")/' "$CLI_MAIN"
sed -i '' 's/keyboard.press("Control+Shift+E")/keyboard.press("Meta+Shift+E")/' "$CLI_MAIN"

Windows/Linux: No patch needed.

Workflow

Check deps — use Kroki (curl) for SVG; use local CLI for PNG
Plan — pick the visual metaphor (see Relationship-to-layout map), then the diagram type and color palette
Generate — write .excalidraw JSON file (section-by-section for large diagrams)
Export — run Kroki or CLI command
Verify the render — view the exported PNG, fix any defects, re-export (see Verify the Render)
Report — tell user the output file path

Design Principles

Default style

roughness: 0 — clean, modern look for all technical diagrams (use 1 only when user requests hand-drawn/casual style)
fontFamily: 2 (Helvetica) — professional look; use 1 (Virgil) only for casual/sketch style, 3 (Cascadia) for code snippets
fillStyle: "solid" — default fill

Containers: prefer typography over boxes

A box around every label makes a diagram look like a wireframe. The cleanest Excalidraw diagrams use free-floating text and lines for structure and reserve filled boxes for things that are genuinely components.

Default to no container — use a standalone text element unless the box earns its place.
Add a box only when the element is a real system component, an arrow binds to it, the shape itself carries meaning (decision diamond, start/end ellipse), or it groups a zone.
Aim for under ~30% of text elements inside boxes.
For timelines, trees, and hierarchies, use a line/connector + free-floating labels, not a stack of rectangles. Size, weight, and color create hierarchy without boxes.

Font size hierarchy

Level	Size	Use for
Title	28px	Diagram title
Header	24px	Section/group headers
Label	20px	Primary element labels
Description	16px	Secondary text, descriptions
Note	14px	Annotations, fine print

Color palette

Follow the 60-30-10 rule: 60% whitespace/neutral, 30% primary accent, 10% highlight.

Semantic fill colors (use with strokeColor one shade darker):

Category	Fill	Stroke	Use for
Primary / Input	`#dbeafe`	`#1e40af`	Entry points, APIs, user-facing
Success / Data	`#dcfce7`	`#166534`	Data stores, success states
Warning / Decision	`#fef9c3`	`#854d0e`	Decision points, conditions
Error / Critical	`#fee2e2`	`#991b1b`	Errors, alerts, critical paths
External / Storage	`#f3e8ff`	`#6b21a8`	External services, databases, AI/ML
Process / Default	`#e0f2fe`	`#0369a1`	Standard process steps
Trigger / Start	`#fed7aa`	`#c2410c`	Start nodes, triggers, events
Neutral / Container	`#f1f5f9`	`#475569`	Groups, swimlanes, backgrounds

Text colors:

Level	Color
Title	`#1e293b`
Label	`#334155`
Description	`#64748b`

Rule: Do not invent new colors. Pick from this palette.

Arrow semantics

Style	Meaning
Solid (`strokeStyle: "solid"`)	Primary flow, main path
Dashed (`"dashed"`)	Response, async, callback
Dotted (`"dotted"`)	Optional, reference, weak dependency

Excalidraw JSON Structure

File skeleton

{
  "type": "excalidraw",
  "version": 2,
  "source": "claude-code",
  "elements": [],
  "appState": { "viewBackgroundColor": "#ffffff" }
}

Element types

type	use for
rectangle	boxes, components, modules
ellipse	start/end nodes, databases
diamond	decision points
arrow	directed connections
line	undirected connections
text	standalone labels

image, frame, and embeddable are not covered by this skill: image needs a separate files map plus a fileId, and frames/embeds render inconsistently through the export path. Stick to the six types above.

Element sizing

Calculate element width from label text to prevent truncation:

Latin text:  width = max(160, charCount * 9)
CJK text:   width = max(160, charCount * 18)
Mixed text:  estimate each character individually, sum up

Height: use 60 for single-line labels, add 24 per additional line.

Standalone text does NOT auto-wrap. For multi-line standalone labels, insert manual \n line breaks yourself — aim for ≤ ~30 Latin (≤ ~15 CJK) characters per line at 16px — and add 24 height per line. (Text bound inside a shape via containerId wraps to the container width automatically, so size the container instead of adding \n.)

Required properties (all elements)

{
  "id": "auth_service",
  "type": "rectangle",
  "x": 100, "y": 100,
  "width": 160, "height": 60,
  "angle": 0,
  "strokeColor": "#1e40af",
  "backgroundColor": "#dbeafe",
  "fillStyle": "solid",
  "strokeWidth": 2,
  "roughness": 0,
  "opacity": 100,
  "seed": 100001,
  "boundElements": [
    { "id": "arrow_to_db", "type": "arrow" },
    { "id": "label_auth", "type": "text" }
  ]
}

Use descriptive string IDs (e.g., "api_gateway", "arrow_gw_to_auth") instead of random strings.

Give each element a unique seed (integer). Namespace by section: 100xxx, 200xxx, 300xxx.

JSON field rules

boundElements: use null when empty, never []
updated: always use 1, never timestamps
Do NOT include: frameId, index, versionNonce, rawText
points in arrows: always start at [0, 0]
seed: must be a positive integer, unique per element

Property values

Use only these values — all verified to render via Kroki and the local CLI:

Property	Valid values
`fillStyle`	`"solid"`, `"hachure"`, `"cross-hatch"`, `"zigzag"`
`strokeStyle`	`"solid"` (or omit), `"dashed"`, `"dotted"`
`fontFamily`	`1` (Virgil, hand-drawn), `2` (Helvetica), `3` (Cascadia, code)
`textAlign`	`"left"`, `"center"`, `"right"`
`verticalAlign`	`"top"`, `"middle"`, `"bottom"`
`startArrowhead` / `endArrowhead`	`null`, `"arrow"`, `"triangle"`, `"bar"`, `"dot"`, `"circle"`, `"diamond"`, `"crowfoot_many"`

Arrows default to endArrowhead: "arrow" and startArrowhead: null — omit both for a standard one-way arrow. Use "triangle" for UML inheritance, "diamond" for composition, and "crowfoot_many" for ER cardinality.

Need copy-paste templates or the full property/arrowhead catalogue? Read references/schema-reference.md — complete element templates (component+label, bound arrow, arrow label, swimlane zone, mind-map connector) and every verified property value.

Text inside shapes (contained text)

When text belongs inside a shape, bind them bidirectionally:

{
  "id": "label_auth",
  "type": "text",
  "text": "Auth Service",
  "fontSize": 20,
  "fontFamily": 2,
  "textAlign": "center",
  "verticalAlign": "middle",
  "strokeColor": "#1e293b",
  "containerId": "auth_service"
}

CRITICAL: Text strokeColor is the text color. Always set it explicitly to a dark color from the text color palette. Never omit it — omitting strokeColor on text can cause invisible text that blends with the shape background.

The parent shape must list the text in its boundElements:

"boundElements": [{ "id": "label_auth", "type": "text" }]

Arrow binding (bidirectional)

Arrows must bind to shapes, and shapes must reference bound arrows:

{
  "id": "arrow_gw_to_auth",
  "type": "arrow",
  "points": [[0, 0], [200, 0]],
  "startBinding": { "elementId": "api_gateway", "gap": 5, "focus": 0 },
  "endBinding": { "elementId": "auth_service", "gap": 5, "focus": 0 }
}

Both api_gateway and auth_service must include in their boundElements:

"boundElements": [{ "id": "arrow_gw_to_auth", "type": "arrow" }]

Endpoints must reach the shape borders. startBinding/endBinding (and their gap) only affect interactive editing on excalidraw.com — they do NOT clip the line when exporting via Kroki or the local CLI. The exporter draws your points literally. So compute endpoints edge-to-edge: set the arrow's x/y to the source shape's border (the side facing the target) and the last point to the target's border. Center-to-center points draw the line straight through both shapes.

Arrow labels

To label an arrow, bind a text element to it exactly like shape text: set the label's containerId to the arrow's id, and add the label to the arrow's boundElements. Excalidraw then centers the label on the arrow and masks the line behind the text, so it stays readable (no strike-through).

{
  "id": "arrow_valid_to_grant",
  "type": "arrow",
  "points": [[0, 0], [0, 120]],
  "boundElements": [{ "id": "lbl_yes", "type": "text" }]
}

{
  "id": "lbl_yes",
  "type": "text",
  "text": "Yes",
  "fontSize": 14,
  "width": 36,
  "strokeColor": "#1e293b",
  "containerId": "arrow_valid_to_grant"
}

CRITICAL: the label width must fit the text (charCount * 9), NOT the arrow length. Excalidraw masks the line behind the label's full bounding box — a label as wide as the arrow masks the entire arrow, so the line disappears and only floating text remains. Keep label widths small.

Arrow routing

L-shaped (elbow) arrows — orthogonal routing with 3+ points:

"points": [[0, 0], [100, 0], [100, 150]]

Elbowed arrows — automatic right-angle routing:

{
  "type": "arrow",
  "points": [[0, 0], [0, -50], [200, -50], [200, 0]],
  "elbowed": true
}

Curved arrows — smooth routing with waypoints:

{
  "type": "arrow",
  "points": [[0, 0], [50, -40], [200, 0]],
  "roundness": { "type": 2 }
}

Grouping

Related elements share groupIds. Nested groups list IDs innermost-first:

"groupIds": ["inner_group", "outer_group"]

Diagram Patterns

Choose the right visual pattern for each diagram type.

Relationship-to-layout map

Before locking in a diagram type, pick the visual metaphor that matches the relationship in the idea — it drives the layout more than the type label does:

Relationship in the idea	Visual metaphor	Build with
One → many (broadcast, dispatch)	Fan-out	one node, arrows radiating outward
Many → one (aggregate, merge)	Convergence	several inputs, arrows into one node
Parent → children (hierarchy)	Tree	trunk + branch lines, free-floating text
Repeating cycle (loop, feedback)	Cycle	nodes in a ring, curved arrows back to start
Input → transform → output	Assembly line	left-to-right pipeline of steps
A vs B (comparison)	Side-by-side	two parallel columns on a shared baseline
Before / after, phase break	Gap	whitespace or a dashed divider between groups
Fuzzy / overlapping state	Cloud	overlapping ellipses, no hard borders

Spacing Reference

Scenario	Spacing
Labeled arrow gap (between shapes)	150–200px
Unlabeled arrow gap	100–120px
Column spacing (labeled arrows)	400px (220px box + 180px gap)
Column spacing (unlabeled arrows)	340px (220px box + 120px gap)
Row spacing	280–350px (150px box + 130–200px gap)
Zone/container padding	50–60px around children
Zone/container opacity	25–40
Minimum gap between any elements	40px

Flowchart (LR or TB)

Ellipse for start/end, diamond for decisions, rectangle for process
200px horizontal spacing, 150px vertical spacing
Decision branches: "Yes" goes forward, "No" goes down
3–10 steps (max 15)

Architecture / System Diagram

Column spacing per table above; use labeled arrow spacing when connections have labels
Group related services in dashed Neutral containers (opacity: 30, padding: 50px)
Gateway/entry at left or top, databases at right or bottom
3–8 entities (max 12)

Sequence Diagram

200px between participants (rectangles at top)
Vertical lifelines as dashed lines
Horizontal arrows for messages, 60px vertical spacing
Solid arrow = request, dashed arrow = response

Mind Map

Central node: largest (200x100), Trigger color
Level 1: 150x70, Primary color, radial around center
Level 2: 120x50, Process color
Level 3: 90x40, Neutral color
Use lines (not arrows) for connections
4–6 branches (max 8), 2–4 sub-topics per branch
Place level-1 branches on a circle of radius R ≈ 280 around the center (cx, cy): for branch i of n, angle = 2π·i/n, x = cx + R·cos(angle), y = cy + R·sin(angle). Even spacing prevents the crossed-line tangle that ad-hoc placement produces.

Swimlane

Large transparent rectangles (Neutral fill, "dashed" stroke, opacity: 30) as lane boundaries
Lane label as free-standing text at top-left of lane (not bound to rectangle), 28px font
Elements flow left-to-right within lanes
Arrows cross lanes for handoffs

Section-by-Section Construction

For diagrams with 10+ elements, do NOT generate the entire JSON at once. Build in sections:

Plan all sections first — list element IDs, positions, and cross-section bindings
Write section 1 — create the file with initial elements
Append section 2 — read the file, add new elements to the elements array
Repeat — continue until all sections are done
Final pass — verify all boundElements and startBinding/endBinding references are consistent

Namespace element seeds by section (100xxx, 200xxx, 300xxx) to avoid collisions.

Export

Option A: Kroki API (SVG only — zero install)

# SVG via Kroki API
curl -s -X POST https://kroki.io/excalidraw/svg \
  -H "Content-Type: text/plain" \
  --data-binary "@diagram.excalidraw" \
  -o diagram.svg

# Via local Kroki Docker (offline)
curl -s -X POST http://localhost:8000/excalidraw/svg \
  -H "Content-Type: text/plain" \
  --data-binary "@diagram.excalidraw" \
  -o diagram.svg

Option B: Local CLI (PNG + SVG)

# PNG at 2x scale, with background baked in (recommended)
excalidraw-brute-export-cli -i diagram.excalidraw -o diagram.png -f png -s 2 -b true

# PNG at 1x scale
excalidraw-brute-export-cli -i diagram.excalidraw -o diagram.png -f png -s 1 -b true

# SVG
excalidraw-brute-export-cli -i diagram.excalidraw -o diagram.svg -f svg -s 1 -b true

Required flags: -f (format: png or svg) and -s (scale: 1, 2, or 3).

Optional flags: -b true bakes the viewBackgroundColor into the image — the export is transparent by default, so omit -b (or pass -b false) only when you want a transparent background. -d true exports dark mode; -e true embeds the scene so the PNG/SVG reopens as an editable drawing in excalidraw.com. (Long forms also work: --background, --dark-mode, --embed-scene, --format, --scale, --input, --output.)

Verify the Render

You cannot judge a diagram from its JSON. The JSON can look perfect while the image has clipped text, overlapping boxes, or an arrow slicing through a shape. After exporting, look at the result and fix it — this is the single highest-leverage step.

Render to PNG (the image must be viewable — PNG, not SVG, even if the user ultimately wants SVG):
```
excalidraw-brute-export-cli -i diagram.excalidraw -o /tmp/check.png -f png -s 2 -b true
```
View /tmp/check.png (Claude can read PNGs directly). Visual audit needs the local CLI; with Kroki-only (SVG), fall back to the structural checks below.

Audit the image:

Look for	Fix
Text clipped / overflowing its shape	Widen the shape (`max(160, charCount * 9)`, ×2 for CJK) or pre-wrap with `\n`
Boxes or labels overlapping	Re-space using the Spacing Reference (≥40px gap)
Arrow cutting straight through a shape	Move endpoints to the shape borders, not centers
Arrow invisible — only its label shows	Shrink the label `width` to fit the text
Element off-canvas or floating with no connection	Reposition / connect it
Isomorphism Test: mentally delete all text — does the structure alone still convey the idea?	If not, the layout is wrong, not the labels — restructure

Fix the JSON and re-export. Repeat until clean — typically 1–3 passes. Skip only for trivial 2–3 element diagrams.

Anti-Patterns

Never put text on large background/zone rectangles. Excalidraw centers text in the middle of the shape, overlapping contained elements. Instead, use a free-standing text element positioned at the top of the zone.

Avoid cross-zone arrows. Long diagonal arrows create visual spaghetti. Route arrows within zones or along zone edges. If a cross-zone connection is unavoidable, route it along the perimeter.

Use arrow labels sparingly. Bind labels to the arrow (see Arrow labels) so the line is masked behind the text instead of striking through it — but keep the label width to the text, never the arrow length. Keep labels to ≤12 characters and ensure ≥120px clear space between connected shapes. Omit labels when the connection meaning is obvious from context.

Don't use filled backgrounds on containers that hold other elements. Use opacity: 30 (or 25-40 range) for zone/container rectangles so contained elements remain visible.

Always set explicit strokeColor on text elements. Text strokeColor is the rendered text color. If omitted, text may inherit the parent shape's background color and become invisible. Use #1e293b (title), #334155 (label), or #64748b (description) from the text color palette.

Common Mistakes

Mistake	Fix
Kroki returns HTTP 400	Send `-H "Content-Type: text/plain"` (NOT `application/json`, which Kroki reads as a `{"diagram_source": ...}` wrapper and rejects); ensure valid JSON with `"type": "excalidraw"` and `"elements"` array
Kroki only outputs SVG	Use local CLI (`excalidraw-brute-export-cli`) for PNG
Export fails with "Missing required flag"	Always pass `-f png` and `-s 2`
Export fails with "Executable doesn't exist"	Run `npx playwright install firefox`
macOS: timeout waiting for file chooser	Apply the macOS Meta patch above
Arrow `points` not relative to origin	`points` always start at `[0,0]`
Missing `id` on elements	Use descriptive string IDs per element
Overlapping elements	Use spacing reference table; minimum 40px gap
Arrows not interactive in excalidraw.com	Add `boundElements` to shapes referencing all bound arrows/text
Arrow/line cuts straight through the shapes	Compute endpoints at the shape borders, not centers — bindings don't clip the static export
Arrow invisible — only its label shows	Bound label `width` spans the whole arrow and masks the line; set label `width` to fit the text (`charCount * 9`)
Exported PNG/SVG has no background	CLI export is transparent by default; pass `-b true` to bake in `viewBackgroundColor`
Text not centered in shape	Set `containerId` on text AND add text to shape's `boundElements`
All text same size	Use font size hierarchy: 28 → 24 → 20 → 16 → 14
Diagram looks monotone	Apply semantic colors from the palette, follow 60-30-10 rule
Text invisible / same color as background	Always set `strokeColor` on text elements to a dark color (`#1e293b`, `#334155`, or `#64748b`)
Text overlaps inside zone/container	Don't bind text to zone rectangles; use free-standing text at top
Text truncated in shapes	Use width formula: `max(160, charCount * 9)`, double for CJK
`boundElements: []` causes issues	Use `null` for empty boundElements, never `[]`