yctimlin/mcp_excalidraw/excalidraw-skill

Excalidraw Skill

Step 0: Pick an Interface

Three interfaces drive the same live canvas. Pick the first one that applies:

1. MCP tools — if excalidraw/* tools (e.g. batch_create_elements) are in your tool list, prefer them: results land directly in your context, and screenshots come back as images without touching disk.
2. CLI (default when no MCP tools are present):

``bash npx -y mcp-excalidraw-server <command> ` No setup needed — any canvas-touching command **auto-starts the canvas server** on http://127.0.0.1:3000 (first npx run downloads the package). If the CLI is installed globally (npm i -g mcp-excalidraw-server), the shorter alias excalidraw-canvas <command>` works too.

1. REST API (last resort, e.g. from application code): HTTP endpoints on http://127.0.0.1:3000 — see references/cheatsheet.md for payloads. The server must already be running.

The canvas URL comes from EXPRESS_SERVER_URL (default http://127.0.0.1:3000). Screenshots and image exports render headless inside the canvas server, so you never need a browser tab to check your own work. Only mermaid conversion and viewport control need an open tab (CLI exits with code 4 when it's missing). Still tell the user the URL — opening it lets them watch you draw.

CLI Quick Reference

Results are JSON on stdout — except describe (plain text) and raw-content output when --out is omitted (export scene JSON, screenshot --format svg). Diagnostics on stderr. Exit codes: 0 ok, 1 error, 2 usage, 3 canvas unreachable, 4 browser tab required (only mermaid and screenshot --renderer browser).

Element Format (CLI and MCP)

The CLI and MCP tools accept the same agent-friendly format and normalize it automatically:

• Labels: put "text": "My Label" on any shape — converted to Excalidraw's bound-label format for you.
• Arrow binding: "startElementId": "a" / "endElementId": "b" — arrows auto-route to element edges.
• fontFamily: pass a string name ("helvetica", "cascadia", "excalifont", ...) or string number "1"–"8".
• points: both [[x,y], ...] tuples and [{"x":..,"y":..}] objects are accepted.
• Patch updates: in apply, update entries can use either direct fields ({"id":"a","x":120}) or a set object ({"id":"a","set":{"x":120}}). Do not mix both forms in one update entry.

Raw REST is stricter: labels must be "label": {"text": "..."}, bindings must be "start": {"id": "..."} / "end": {"id": "..."}. Only worry about this when POSTing to the API directly.

Coordinate System

The canvas uses a 2D coordinate grid: (0, 0) is the origin, x increases rightward, y increases downward. Plan your layout before writing any JSON.

General spacing guidelines:

• Vertical spacing between tiers: 80–120px (enough that arrows don't crowd labels)
• Horizontal spacing between siblings: 40–60px minimum; give labeled arrows 120px+
• Shape width: max(160, labelCharCount * 12) to keep the label on one line
• Shape height: 60px single-line, 80px two-line labels
• Background/zone padding: 50px on all sides around contained elements

Styling for a professional look:

• "fillStyle": "solid" on shapes gives crisp flat fills — the default is a sketchy hachure pattern
• Pair pastel backgroundColor fills with their darker strokeColor (palette in the cheatsheet)
• "strokeStyle": "dashed" on zone borders and async arrows reads as "boundary / background"

Layout Anti-Patterns (Critical for Complex Diagrams)

These are the most common mistakes that produce unreadable diagrams. Avoid all of them.

1. Do NOT use label.text (or text) on large background zone rectangles

When you put a label on a background rectangle, Excalidraw creates a bound text element centered in the middle of that shape — right where your service boxes will be placed. The text overlaps everything inside the zone and cannot be repositioned.

Wrong:

{"id": "vpc-zone", "type": "rectangle", "x": 50, "y": 50, "width": 800, "height": 400, "text": "VPC (10.0.0.0/16)"}

Right — use a free-standing text element anchored at the top of the zone:

{"id": "vpc-zone", "type": "rectangle", "x": 50, "y": 50, "width": 800, "height": 400, "backgroundColor": "#e3f2fd"},
{"id": "vpc-label", "type": "text", "x": 70, "y": 60, "width": 300, "height": 30, "text": "VPC (10.0.0.0/16)", "fontSize": 18}

The free-standing text element sits at the top corner of the zone and doesn't interfere with elements placed inside.

2. Avoid cross-zone arrows in complex diagrams

An arrow from an element in one layout zone to an element in a distant zone will draw a long diagonal line crossing through everything in between. In a multi-zone infra diagram this produces an unreadable tangle of spaghetti.

Design rule: Keep arrows within the same zone or tier. To show cross-zone relationships, use annotation text or separate the zones so their edges are adjacent (no elements between them), and route the arrow along the edge.

If you must connect across zones, use an elbowed arrow that travels along the perimeter — never through the middle of another zone.

3. Use arrow labels sparingly

Arrow labels are placed at the midpoint of the arrow. On short arrows, they overlap the shapes at both ends. On crowded diagrams, they collide with nearby elements.

• Only add an arrow label when the relationship name is genuinely essential (e.g., protocol, port number, data direction).
• If you're adding a label to every arrow, reconsider — it usually adds visual noise, not clarity.
• Keep arrow labels to ≤ 12 characters. Prefer omitting them entirely on dense diagrams.

Quality: Why It Matters (and How to Check)

Excalidraw diagrams are visual communication. If text is cut off, elements overlap, or arrows cross through unrelated shapes, the diagram becomes confusing and unprofessional — it defeats the whole purpose of drawing it. So after every batch of elements, verify before adding more.

Quality Checklist

After each add / apply / batch_create_elements, take a screenshot and check:

1. Text truncation — Is all label text fully visible? Truncated text means the shape is too small. Increase width and/or height.
2. Overlap — Do any shapes share the same space? Background zones must fully contain children with padding.
3. Arrow crossing — Do arrows cut through unrelated elements? If yes, route them around using curved or elbowed arrows (see Arrow Routing below).
4. Arrow-label overlap — Arrow labels sit at the midpoint. If they overlap a shape, shorten the label or adjust the arrow path.
5. Spacing — At least 40px gap between elements. Cramped layouts are hard to read.
6. Readability — Font size ≥ 16 for body text, ≥ 20 for titles.
7. Zone label placement — If you used text/label.text on a background zone rectangle, the zone label will be centered in the middle of the zone, overlapping everything inside. Fix: delete the bound text element and add a free-standing text element at the top of the zone instead (see Layout Anti-Patterns above).

If you find any issue: stop, fix it, re-screenshot, then continue. Say "I see [issue], fixing it" rather than glossing over problems. Only proceed once all checks pass.

Workflow: Drawing a New Diagram

Mermaid vs. Direct Creation — Which to Use?

Use `mermaid` / `create_from_mermaid` when: the user already has a Mermaid diagram, or the structure maps cleanly to a flowchart/sequence/ER diagram with standard Mermaid syntax. It's fast and handles conversion automatically, though you get less control over exact layout.

Create elements directly when: you need precise layout control, the diagram type doesn't map to Mermaid well (e.g., custom architecture, annotated cloud diagrams), or you want elements positioned in a specific coordinate grid.

Steps (CLI shown; MCP tools are 1:1 — see cheatsheet)

1. Plan your coordinate grid — map out tiers and x-positions before writing JSON. (MCP mode: call read_diagram_guide for colors/sizing; the same guidance lives in references/cheatsheet.md.)
2. Optional fresh start: npx -y mcp-excalidraw-server clear --yes
3. Create shapes and arrows in one call. Custom id fields (e.g. "id": "auth-svc") make later updates easy:

``bash npx -y mcp-excalidraw-server add - <<'EOF' [ {"id": "lb", "type": "rectangle", "x": 300, "y": 50, "width": 180, "height": 60, "text": "Load Balancer"}, {"id": "svc-a", "type": "rectangle", "x": 100, "y": 200, "width": 160, "height": 60, "text": "Web Server 1"}, {"id": "svc-b", "type": "rectangle", "x": 450, "y": 200, "width": 160, "height": 60, "text": "Web Server 2"}, {"id": "db", "type": "rectangle", "x": 275, "y": 350, "width": 210, "height": 60, "text": "PostgreSQL"}, {"type": "arrow", "x": 0, "y": 0, "startElementId": "lb", "endElementId": "svc-a"}, {"type": "arrow", "x": 0, "y": 0, "startElementId": "lb", "endElementId": "svc-b"}, {"type": "arrow", "x": 0, "y": 0, "startElementId": "svc-a", "endElementId": "db"}, {"type": "arrow", "x": 0, "y": 0, "startElementId": "svc-b", "endElementId": "db"} ] EOF ` (The - positional is optional — with no file argument, add` reads stdin.)

1. Set shape widths using max(160, labelLength * 12).
2. screenshot → view the file → run the Quality Checklist → fix issues before the next batch.

Arrow Routing — Avoid Overlaps

Straight arrows can cross through elements in complex diagrams. Use curved or elbowed arrows when needed:

Curved arrows (smooth arc over obstacles):

{
  "type": "arrow", "x": 100, "y": 100,
  "points": [[0, 0], [50, -40], [200, 0]],
  "roundness": {"type": 2}
}

The intermediate waypoint [50, -40] lifts the arrow upward. roundness: {type: 2} makes it smooth.

Elbowed arrows (right-angle / L-shaped routing):

{
  "type": "arrow", "x": 100, "y": 100,
  "points": [[0, 0], [0, -50], [200, -50], [200, 0]],
  "elbowed": true
}

When to use which:

• Fan-out (one source → many targets): curved arrows with waypoints spread to avoid overlapping
• Cross-lane (connecting to side panels): elbowed arrows that go up, then across, then down
• Long horizontal connections: curved arrows with a slight vertical offset

Rule: If an arrow would pass through an unrelated shape, add a waypoint to route around it.

Workflow: Iterative Refinement

Pairing describe with screenshot is what makes this skill powerful.