Canvas Skill

Display HTML content on connected Brigade nodes (Mac app, iOS, Android).

Overview

The canvas tool lets you present web content on any connected node's canvas view. Great for:

• Displaying games, visualizations, dashboards
• Showing generated HTML content
• Interactive demos

How It Works

Architecture

┌─────────────────┐     ┌──────────────────┐     ┌─────────────┐
│  Canvas Host    │────▶│   Node Bridge    │────▶│  Node App   │
│  (HTTP Server)  │     │  (TCP Server)    │     │ (Mac/iOS/   │
│  Port 18793     │     │  Port 18790      │     │  Android)   │
└─────────────────┘     └──────────────────┘     └─────────────┘

1. Canvas Host Server: Serves static HTML/CSS/JS files from canvasHost.root directory
2. Node Bridge: Communicates canvas URLs to connected nodes
3. Node Apps: Render the content in a WebView

Tailscale Integration

The canvas host server binds based on gateway.bind setting:

Bind Mode	Server Binds To	Canvas URL Uses
loopback	127.0.0.1	localhost (local only)
lan	LAN interface	LAN IP address
tailnet	Tailscale interface	Tailscale hostname
auto	Best available	Tailscale > LAN > loopback

Key insight: The canvasHostHostForBridge is derived from bridgeHost. When bound to Tailscale, nodes receive URLs like:

http://<tailscale-hostname>:18793/__brigade__/canvas/<file>.html

This is why localhost URLs don't work - the node receives the Tailscale hostname from the bridge!

Actions

Action	Description
present	Show canvas with optional target URL
hide	Hide the canvas
navigate	Navigate to a new URL
eval	Execute JavaScript in the canvas
snapshot	Capture screenshot of canvas

Configuration

In the active Brigade config file ($BRIGADE_CONFIG_PATH, default ~/.brigade/brigade.json):

{
  "canvasHost": {
    "enabled": true,
    "port": 18793,
    "root": "/Users/you/brigade/canvas",
    "liveReload": true
  },
  "gateway": {
    "bind": "auto"
  }
}

Live Reload

When liveReload: true (default), the canvas host:

• Watches the root directory for changes (via chokidar)
• Injects a WebSocket client into HTML files
• Automatically reloads connected canvases when files change

Great for development!

Workflow

1. Create HTML content

Place files in the canvas root directory (default ~/brigade/canvas/):

cat > ~/brigade/canvas/my-game.html << 'HTML'
<!DOCTYPE html>
<html>
<head><title>My Game</title></head>
<body>
  <h1>Hello Canvas!</h1>
</body>
</html>
HTML

2. Find your canvas host URL

Check how your gateway is bound:

CONFIG_PATH="${BRIGADE_CONFIG_PATH:-${BRIGADE_STATE_DIR:-$HOME/.brigade}/brigade.json}"
cat "$CONFIG_PATH" | jq '.gateway.bind'

Then construct the URL:

• loopback: http://127.0.0.1:18793/__brigade__/canvas/<file>.html
• lan/tailnet/auto: http://<hostname>:18793/__brigade__/canvas/<file>.html

Find your Tailscale hostname:

tailscale status --json | jq -r '.Self.DNSName' | sed 's/\.$//'

3. Find connected nodes

brigade nodes list

Look for Mac/iOS/Android nodes with canvas capability.

4. Present content

canvas action:present node:<node-id> target:<full-url>

Example:

canvas action:present node:mac-63599bc4-b54d-4392-9048-b97abd58343a target:http://peters-mac-studio-1.sheep-coho.ts.net:18793/__brigade__/canvas/snake.html

5. Navigate, snapshot, or hide

canvas action:navigate node:<node-id> url:<new-url>
canvas action:snapshot node:<node-id>
canvas action:hide node:<node-id>

Debugging

White screen / content not loading

Cause: URL mismatch between server bind and node expectation.

Debug steps:

1. Check server bind: CONFIG_PATH="${BRIGADE_CONFIG_PATH:-${BRIGADE_STATE_DIR:-$HOME/.brigade}/brigade.json}"; cat "$CONFIG_PATH" | jq '.gateway.bind'
2. Check what port canvas is on: lsof -i :18793
3. Test URL directly: curl http://<hostname>:18793/__brigade__/canvas/<file>.html

Solution: Use the full hostname matching your bind mode, not localhost.

"node required" error

Always specify node:<node-id> parameter.

"node not connected" error

Node is offline. Use brigade nodes list to find online nodes.

Content not updating

If live reload isn't working:

1. Check liveReload: true in config
2. Ensure file is in the canvas root directory
3. Check for watcher errors in logs

URL Path Structure

The canvas host serves from /__brigade__/canvas/ prefix:

http://<host>:18793/__brigade__/canvas/index.html  → ~/brigade/canvas/index.html
http://<host>:18793/__brigade__/canvas/games/snake.html → ~/brigade/canvas/games/snake.html

The /__brigade__/canvas/ prefix is defined by CANVAS_HOST_PATH constant.

Tips

• Keep HTML self-contained (inline CSS/JS) for best results
• Use the default index.html as a test page (has bridge diagnostics)
• The canvas persists until you hide it or navigate away
• Live reload makes development fast - just save and it updates!
• A2UI JSON push is WIP - use HTML files for now