Excalidraw Skill

Step 0: Detect Connection Mode

Before doing anything, determine which mode is available. Run these checks in order:

Check 1: MCP Server (Best experience)

mcp-cli tools | grep excalidraw

If you see tools like excalidraw/batch_create_elements → use MCP mode. Call MCP tools directly.

Check 2: REST API (Fallback — works without MCP server)

curl -s http://localhost:3000/health

If you get {"status":"ok"} → use REST API mode. Use HTTP endpoints (curl / fetch) from the cheatsheet.

Check 3: Nothing works → Guide user to install

If neither works, tell the user:

The Excalidraw canvas server is not running. To set up:

1. Clone: git clone https://github.com/yctimlin/mcp_excalidraw && cd mcp_excalidraw
2. Build: npm ci && npm run build
3. Start canvas: HOST=0.0.0.0 PORT=3000 npm run canvas
4. Open http://localhost:3000 in a browser
5. (Recommended) Install the MCP server for the best experience:

   claude mcp add excalidraw -s user -e EXPRESS_SERVER_URL=http://localhost:3000 -- node /path/to/mcp_excalidraw/dist/index.js
   

MCP vs REST API Quick Reference

REST API Gotchas (Critical — read before using REST API)

1. Labels: Use "label": {"text": "My Label"} (not "text": "My Label"). MCP tools auto-convert, REST API does not.
2. Arrow binding: Use "start": {"id": "svc-a"}, "end": {"id": "svc-b"} (not "startElementId"/"endElementId"). MCP tools accept startElementId and convert, REST API requires the start/end object format directly.
3. fontFamily: Must be a string (e.g. "1") or omit it entirely. Do NOT pass a number like 1.
4. Updating labels: When updating a shape via PUT /api/elements/:id, include the full label in the update body to preserve it. Omitting label from the update won't delete it, but re-sending ensures it renders correctly.
5. Screenshot in REST mode: POST /api/export/image returns {"data": "<base64>"}. Save to file and read it back for visual verification. Requires browser open.

Quality Gate (MANDATORY — read before creating any diagram)

After EVERY iteration (each batch of elements added), you MUST run a quality check before proceeding. NEVER say "looks great" unless ALL checks pass.

Quality Checklist — verify ALL before adding more elements:

1. Text truncation: Is ALL text fully visible? Labels must fit inside their shapes. If text is cut off or wrapping badly → increase width and/or height.
2. Overlap: Do ANY elements overlap each other? Check that no rectangles, ellipses, or text elements share the same space. Background zones must fully contain their children with padding.
3. Arrow crossing: Do arrows cross through unrelated elements or overlap with text labels? If yes → use curved/elbowed arrows with waypoints to route around obstacles (see "Arrow Routing" section). Never accept crossing arrows.
4. Arrow-text overlap: Do any arrow labels ("charge", "event", etc.) overlap with shapes? Arrow labels are positioned at the midpoint — if they overlap, either remove the label, shorten it, or adjust the arrow path.
5. Spacing: Is there at least 40px gap between elements? Cramped layouts are unreadable.
6. Readability: Can all labels be read at normal zoom? Font size >= 16 for body text, >= 20 for titles.

If ANY issue is found:

• STOP adding new elements
• Fix the issue first (resize, reposition, delete and recreate)
• Re-verify with a new screenshot
• Only proceed to next iteration after ALL checks pass

Sizing Rules (prevent truncation):

• Shape width: max(160, labelTextLength * 9) pixels. For multi-word labels like "API Gateway (Kong)", count all characters.
• Shape height: 60px for single line, 80px for 2 lines, 100px for 3 lines.
• Background zones: Add 50px padding on ALL sides around contained elements.
• Element spacing: 60px vertical between tiers, 40px horizontal between siblings.
• Side panels: Place at least 80px away from main diagram elements.
• Arrow labels: Keep labels short (1-2 words). Long arrow labels overlap with other elements.

Layout Planning (prevent overlap):

Before creating elements, plan your coordinate grid on paper first:

• Tier 1 (y=50-130): Client apps
• Tier 2 (y=200-280): Gateway/Edge
• Tier 3 (y=350-440): Services (spread wide: each service ~180px apart)
• Tier 4 (y=510-590): Data stores
• Side panels: x < 0 (left) or x > mainDiagramRight + 80 (right)

Do NOT place side panels (observability, external APIs) at the same x-range as the main diagram — they WILL overlap.

Quick Start

1. Run Step 0 above to detect your connection mode.
2. Open the canvas URL in a browser (required for image export/screenshot).
3. MCP mode: Use MCP tools for all operations. REST mode: Use HTTP endpoints from cheatsheet.
4. For full tool/endpoint reference, read references/cheatsheet.md.

Workflow: Draw A Diagram

MCP Mode

1. Call read_diagram_guide first to load design best practices.
2. Plan your coordinate grid (see Quality Gate → Layout Planning) before writing any JSON.
3. Optional: clear_canvas to start fresh.
4. Use batch_create_elements with shapes AND arrows in one call.
5. Assign custom id to shapes (e.g. "id": "auth-svc"). Set text field to label shapes.
6. Size shapes for their text — use width: max(160, textLength * 9).
7. Bind arrows using startElementId / endElementId — arrows auto-route.
8. set_viewport with scrollToContent: true to auto-fit the diagram.
9. Run Quality Checklist — get_canvas_screenshot and critically evaluate. Fix issues before proceeding.

REST API Mode

1. Read references/cheatsheet.md for design guidelines.
2. Plan your coordinate grid (see Quality Gate → Layout Planning) before writing any JSON.
3. Optional: curl -X DELETE http://localhost:3000/api/elements/clear
4. Create elements in one call (use @file.json for large payloads):

   curl -X POST http://localhost:3000/api/elements/batch \
     -H "Content-Type: application/json" \
     -d '{"elements": [
       {"id": "svc-a", "type": "rectangle", "x": 0, "y": 0, "width": 160, "height": 60, "label": {"text": "Service A"}},
       {"id": "svc-b", "type": "rectangle", "x": 0, "y": 200, "width": 160, "height": 60, "label": {"text": "Service B"}},
       {"type": "arrow", "x": 0, "y": 0, "start": {"id": "svc-a"}, "end": {"id": "svc-b"}}
     ]}'
   

5. Use "label": {"text": "..."} for shape labels (not "text": "...").
6. Bind arrows with "start": {"id": "..."} / "end": {"id": "..."} — server auto-routes edges.
7. Size shapes for their text — use width: max(160, labelTextLength * 9).
8. Run Quality Checklist — take screenshot, critically evaluate. Fix issues before adding more elements.

Arrow Binding (Recommended)

Bind arrows to shapes for auto-routed edges. The format differs between MCP and REST API:

MCP Mode — use startElementId / endElementId:

{"elements": [
  {"id": "svc-a", "type": "rectangle", "x": 0, "y": 0, "width": 120, "height": 60, "text": "Service A"},
  {"id": "svc-b", "type": "rectangle", "x": 0, "y": 200, "width": 120, "height": 60, "text": "Service B"},
  {"type": "arrow", "x": 0, "y": 0, "startElementId": "svc-a", "endElementId": "svc-b", "text": "calls"}
]}

REST API Mode — use start: {id} / end: {id} and label: {text}:

{"elements": [
  {"id": "svc-a", "type": "rectangle", "x": 0, "y": 0, "width": 120, "height": 60, "label": {"text": "Service A"}},
  {"id": "svc-b", "type": "rectangle", "x": 0, "y": 200, "width": 120, "height": 60, "label": {"text": "Service B"}},
  {"type": "arrow", "x": 0, "y": 0, "start": {"id": "svc-a"}, "end": {"id": "svc-b"}, "label": {"text": "calls"}}
]}

Arrows without binding use manual x, y, points coordinates.

Arrow Routing — Avoid Overlaps (Critical for complex diagrams)

Straight arrows (2-point) cause crossing and overlap in complex diagrams. Use curved or elbowed arrows instead:

Option 1: Curved arrows — add intermediate waypoints + roundness:

{
  "type": "arrow", "x": 100, "y": 100,
  "points": [[0, 0], [50, -40], [200, 0]],
  "roundness": {"type": 2},
  "strokeColor": "#1971c2"
}

The waypoint [50, -40] pushes the arrow upward to arc over elements. roundness: {type: 2} makes it a smooth curve.

Option 2: Elbowed arrows — right-angle routing (L-shaped or Z-shaped):

{
  "type": "arrow", "x": 100, "y": 100,
  "points": [[0, 0], [0, -50], [200, -50], [200, 0]],
  "elbowed": true,
  "strokeColor": "#1971c2"
}

When to use which:

• Fan-out arrows (one source → many targets): Use curved arrows with waypoints spread vertically to avoid overlapping each other.
• Cross-lane arrows (connecting to side panels): Use elbowed arrows that route around the main diagram — go UP first, then ACROSS, then DOWN.
• Inter-service arrows (horizontal connections): Use curved arrows with a slight vertical offset to avoid crossing through adjacent elements.

Rule of thumb: If an arrow would cross through an unrelated element, add a waypoint to route around it. Never accept crossing arrows — always fix them.

Workflow: Iterative Refinement (Key Differentiator)

The feedback loop that makes this skill unique. Each iteration MUST include a quality check.

MCP Mode (full feedback loop)

1. Add elements (batch_create_elements, create_element).
2. set_viewport with scrollToContent: true.
3. get_canvas_screenshot — critically evaluate against the Quality Checklist.
4. If issues found → fix them (update_element, delete_element, resize, reposition).
5. get_canvas_screenshot again — re-verify fix.
6. Only proceed to next iteration when ALL quality checks pass.

REST API Mode (partial feedback loop)

1. Add elements via POST /api/elements/batch.
2. POST /api/viewport with {"scrollToContent": true}.
3. Take screenshot: POST /api/export/image → save PNG → critically evaluate against Quality Checklist.
4. If issues found → fix via PUT /api/elements/:id or delete and recreate.
5. Re-screenshot and re-verify.
6. Only proceed to next iteration when ALL quality checks pass.

How to critically evaluate a screenshot:

• Look at EVERY label — is any text cut off or overflowing its container?
• Look at EVERY arrow — does any arrow pass through an unrelated element?
• Look at ALL element pairs — do any overlap or touch?
• Look at spacing — is anything crammed together?
• Be honest. If you see ANY issue, say "I see [issue], fixing it" — not "looks great".

Example flow (MCP):

batch_create_elements → get_canvas_screenshot → "text truncated on 2 shapes"
→ update_element (increase widths) → get_canvas_screenshot → "overlap between X and Y"
→ update_element (reposition) → get_canvas_screenshot → "all checks pass"
→ proceed to next iteration

Workflow: Refine An Existing Diagram

1. describe_scene to understand current state.
2. Identify targets by id, type, or label text (not x/y coordinates).
3. update_element to move/resize/recolor, delete_element to remove.
4. get_canvas_screenshot to verify changes visually.
5. If updates fail: check element id exists (get_element), element isn't locked (unlock_elements).

Workflow: File I/O (Diagrams-as-Code)

• Export to .excalidraw format: export_scene with optional filePath.
• Import from .excalidraw: import_scene with mode: "replace" or "merge".
• Export to image: export_to_image with format: "png" or "svg" (requires browser open).
• CLI export: node scripts/export-elements.cjs --out diagram.elements.json
• CLI import: node scripts/import-elements.cjs --in diagram.elements.json --mode batch|sync

Workflow: Snapshots (Save/Restore Canvas State)

1. snapshot_scene with a name before risky changes.
2. Make changes, describe_scene / get_canvas_screenshot to evaluate.
3. restore_snapshot to rollback if needed.

Workflow: Duplication

• duplicate_elements with elementIds and optional offsetX/offsetY (default 20,20).
• Useful for creating repeated patterns or copying existing layouts.

Points Format for Arrows/Lines

The points field accepts both formats:

• Tuple: [[0, 0], [100, 50]]
• Object: [{"x": 0, "y": 0}, {"x": 100, "y": 50}]

Both are normalized to tuples automatically.

Workflow: Share Diagram (excalidraw.com URL)

1. Create your diagram using any of the above workflows.
2. export_to_excalidraw_url — uploads encrypted scene, returns a shareable URL.
3. Share the URL — anyone can open it in excalidraw.com to view and edit.

Workflow: Viewport Control

• set_viewport with scrollToContent: true — auto-fit all elements (zoom-to-fit).
• set_viewport with scrollToElementId: "my-element" — center view on a specific element.
• set_viewport with zoom: 1.5, offsetX: 100, offsetY: 200 — manual camera control.

References

• references/cheatsheet.md: Complete MCP tool list (26 tools) + REST API endpoints + payload shapes.

Related Skills

• See also: figure-generation, algorithm-design, slide-generation