Excalidraw Diagrams

Create diagrams using the Excalidraw canvas server and verify output with screenshots.

Prerequisites

1. Canvas server running at http://localhost:3000

   cd ${MCP_EXCALIDRAW_PATH:-~/code/mcp_excalidraw} && npm run canvas
   

   Set MCP_EXCALIDRAW_PATH in your global mise config (~/.config/mise/config.toml) if the path differs.

2. MCP server configured in project .mcp.json (for MCP tool access)

Creating Diagrams

Via HTTP API (Recommended)

Create elements directly via the canvas server API:

curl -X POST http://localhost:3000/api/elements/batch \
  -H "Content-Type: application/json" \
  -d @/tmp/elements.json

Element JSON structure:

{
  "elements": [
    {"type": "rectangle", "x": 100, "y": 100, "width": 150, "height": 80, "backgroundColor": "#a5d8ff", "strokeColor": "#1971c2"},
    {"type": "text", "x": 120, "y": 130, "text": "Label", "fontSize": 20}
  ]
}

Element Types

Type	Required Props	Optional Props
rectangle	x, y, width, height	backgroundColor, strokeColor, strokeWidth
ellipse	x, y, width, height	backgroundColor, strokeColor
diamond	x, y, width, height	backgroundColor, strokeColor
text	x, y, text	fontSize, fontFamily
arrow	x, y, width, height	strokeColor, strokeWidth
line	x, y, width, height	strokeColor, strokeWidth

Color Palette

• Blue: #a5d8ff (bg), #1971c2 (stroke)
• Green: #b2f2bb (bg), #2f9e44 (stroke)
• Red: #ffc9c9 (bg), #e03131 (stroke)
• Yellow: #ffec99 (bg), #f08c00 (stroke)
• Purple: #d0bfff (bg), #7950f2 (stroke)
• Gray: #dee2e6 (bg), #495057 (stroke)

Verifying Output

Take a screenshot to verify the diagram rendered correctly:

from playwright.sync_api import sync_playwright

with sync_playwright() as p:
    browser = p.chromium.launch(headless=True)
    page = browser.new_page()
    page.goto('http://localhost:3000')
    page.wait_for_load_state('networkidle')
    page.wait_for_timeout(2000)
    page.screenshot(path='/tmp/diagram.png', full_page=True)
    browser.close()

Run with: uv run python /tmp/screenshot_script.py

Then read the screenshot to verify: Read /tmp/diagram.png

Common Patterns

Flowchart (Vertical)

{
  "elements": [
    {"type": "rectangle", "x": 100, "y": 50, "width": 120, "height": 60, "backgroundColor": "#a5d8ff", "strokeColor": "#1971c2"},
    {"type": "text", "x": 130, "y": 70, "text": "Start", "fontSize": 16},
    {"type": "arrow", "x": 160, "y": 110, "width": 0, "height": 40, "strokeColor": "#495057"},
    {"type": "rectangle", "x": 100, "y": 150, "width": 120, "height": 60, "backgroundColor": "#b2f2bb", "strokeColor": "#2f9e44"},
    {"type": "text", "x": 120, "y": 170, "text": "Process", "fontSize": 16},
    {"type": "arrow", "x": 160, "y": 210, "width": 0, "height": 40, "strokeColor": "#495057"},
    {"type": "rectangle", "x": 100, "y": 250, "width": 120, "height": 60, "backgroundColor": "#ffc9c9", "strokeColor": "#e03131"},
    {"type": "text", "x": 135, "y": 270, "text": "End", "fontSize": 16}
  ]
}

Architecture Boxes (Horizontal)

{
  "elements": [
    {"type": "rectangle", "x": 50, "y": 100, "width": 100, "height": 80, "backgroundColor": "#a5d8ff", "strokeColor": "#1971c2"},
    {"type": "text", "x": 70, "y": 130, "text": "Client", "fontSize": 14},
    {"type": "arrow", "x": 150, "y": 140, "width": 50, "height": 0, "strokeColor": "#495057"},
    {"type": "rectangle", "x": 200, "y": 100, "width": 100, "height": 80, "backgroundColor": "#b2f2bb", "strokeColor": "#2f9e44"},
    {"type": "text", "x": 225, "y": 130, "text": "API", "fontSize": 14},
    {"type": "arrow", "x": 300, "y": 140, "width": 50, "height": 0, "strokeColor": "#495057"},
    {"type": "rectangle", "x": 350, "y": 100, "width": 100, "height": 80, "backgroundColor": "#d0bfff", "strokeColor": "#7950f2"},
    {"type": "text", "x": 385, "y": 130, "text": "DB", "fontSize": 14}
  ]
}

API Endpoints

Endpoint	Method	Description
/api/elements	GET	List all elements
/api/elements	POST	Create single element
/api/elements/batch	POST	Create multiple elements
/api/elements/:id	PUT	Update element
/api/elements/:id	DELETE	Delete element
/api/elements/from-mermaid	POST	Convert Mermaid to elements
/health	GET	Server health check

Workflow

1. Check canvas server is running: curl http://localhost:3000/health
2. Clear canvas if needed: Click "Clear Canvas" in browser or restart server
3. Create elements via API batch endpoint
4. Take screenshot to verify
5. Show screenshot to user for confirmation
6. Iterate if needed

Mermaid Conversion

Convert Mermaid diagrams to Excalidraw:

curl -X POST http://localhost:3000/api/elements/from-mermaid \
  -H "Content-Type: application/json" \
  -d '{"mermaidDiagram": "graph TD; A-->B; B-->C;"}'

Note: Mermaid conversion requires the browser canvas to be open for rendering.