Porthole

What is this?

Porthole is a single-process MCP server that gives Claude Code a visual output channel. When Claude calls show_widget, the result appears instantly in your browser — no copy-paste, no screenshots.

One process, three roles:

Features

• Chart.js charts — bar, line, pie, radar, scatter with dark theme
• SVG diagrams — flow charts, architecture diagrams, trees
• Metric dashboards — cards, tables, badges, grids
• Interactive widgets — tabs, sliders, buttons that send data back to Claude
• Widget history — sidebar tracks all widgets, click to revisit
• Hook notifications — toast alerts when Claude finishes or needs attention
• Design system — CSS variables, consistent dark theme, no hardcoded colors

Quick start

git clone https://github.com/robbinfan/porthole.git
cd porthole
npm install
npm run build

Register as MCP server

claude mcp add --scope user porthole \
  -- node /path/to/porthole/dist/index.js

Or edit ~/.claude.json directly:

{
  "mcpServers": {
    "porthole": {
      "command": "node",
      "args": ["/path/to/porthole/dist/index.js"]
    }
  }
}

Open the canvas

open http://localhost:8080

That's it. Every show_widget call from Claude now renders in your browser.

Tell Claude to use it

Add to your CLAUDE.md (project root or ~/.claude/CLAUDE.md):

# Visualization

You have access to `show_widget` and `visualize_read_me` MCP tools.
Use them automatically when producing:
- Metrics comparisons, benchmarks, performance data
- Charts, time-series, A/B analysis
- Architecture or flow diagrams
- Any table with >3 numeric columns

Call procedure:
1. visualize_read_me(modules) — silent, don't mention to user
   - numbers/metrics → ["chart", "mockup"]
   - diagrams        → ["diagram"]
   - interactive     → ["interactive", "chart"]
2. show_widget(i_have_seen_read_me: true, title, widget_code, ...)
3. Brief text summary after — no markdown tables duplicating the widget.

Widget sizes: comparison=900×560, single chart=720×480, dashboard=1100×680

How it works

Claude Code ──stdio──▶ Porthole MCP Server ──ws──▶ Browser Canvas
                            │
                       HTTP :8080
                       serves UI

1. Claude calls visualize_read_me(["chart"]) → gets design guidelines
2. Claude calls show_widget({ title, widget_code }) → server broadcasts via WebSocket
3. Browser receives widget and renders Chart.js / SVG / HTML

Widget code is a raw HTML fragment: <style> → content → <script> (streaming-friendly order).

Project structure

porthole/
├── src/
│   ├── index.ts          # Entry point
│   ├── server.ts         # HTTP + WebSocket + MCP server
│   └── guidelines.ts     # Design system guidelines
├── public/
│   └── canvas.html       # Browser UI
├── assets/
│   ├── porthole-logo.svg
│   └── porthole-wordmark.svg
├── dist/                 # Compiled output
├── package.json
└── tsconfig.json

HTTP API

Hook notifications (optional)

Notifications appear as toasts in the canvas UI.

~/.claude/hooks/notify-porthole.sh:

#!/usr/bin/env bash
INPUT="$(cat)"
curl -sf --max-time 2 -X POST \
  -H 'Content-Type: application/json' \
  -d "$INPUT" \
  http://localhost:8080/hook > /dev/null 2>&1 || true
exit 0

~/.claude/settings.json:

{
  "hooks": {
    "Stop":         [{ "hooks": [{ "type": "command", "command": "bash ~/.claude/hooks/notify-porthole.sh" }] }],
    "Notification": [{ "hooks": [{ "type": "command", "command": "bash ~/.claude/hooks/notify-porthole.sh" }] }]
  }
}

Environment variables

Run on Coder

git clone https://github.com/robbinfan/porthole.git ~/porthole
cd ~/porthole
npm install && npm run build

claude mcp add --scope user porthole \
  -- node /home/coder/porthole/dist/index.js

# Expose port
coder port-forward <workspace-name> --tcp 8080:8080

Keep-alive with PM2 (optional)

npm install -g pm2
pm2 start dist/index.js --name porthole
pm2 save && pm2 startup

License

MIT