showcase-mcp

An MCP server that displays rich content (Markdown, HTML, Mermaid diagrams, images) in a native desktop window.

When an MCP client like Claude Code calls the show tool, a Neutralino.js-based window opens and renders content in real time via SSE.

Features

• Markdown — Rendered via marked
• HTML — Raw HTML display
• Mermaid — SVG diagram rendering
• Images — Display by URL
• Window ID management — Update existing windows by ID, or open new ones
• Real-time updates via SSE — Content is pushed to the window without reopening
• Non-blocking — Tool calls return immediately

Requirements

• Node.js >= 20
• pnpm

Setup

pnpm install

# Download Neutralino binary
pnpm run setup

# Build React bundle + Tailwind CSS
pnpm run build:assets

# Compile TypeScript
pnpm run build:ts

Usage

With Claude Code

Add the following to your .mcp.json:

{
  "mcpServers": {
    "showcase-mcp": {
      "type": "stdio",
      "command": "npx",
      "args": ["tsx", "<path-to>/showcase-mcp/src/index.ts"]
    }
  }
}

Then call the show tool from Claude Code to open a window.

Development

pnpm dev

show Tool

Parameters

Parameter	Type	Description
title	string	Title displayed at the top of the window (required)
message	string?	Markdown-formatted text
html	string?	Raw HTML content
mermaid	string?	Mermaid diagram source code
imageUrl	string?	URL of an image to display
windowId	string?	Window ID to update. Omit to open a new window.

Response

{ "windowId": "xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxx" }

Behavior

windowId	Action
Omitted	Opens a new window. Returns a new windowId.
Valid (window is open)	Updates the existing window's content. Returns the same windowId.
Invalid (window closed / unknown)	Opens a new window. Returns a new windowId.

Architecture

MCP Client (stdio) → MCP Server → Hono HTTP Server (port=0) → Neutralino Window
                                         ↕ SSE
                                    React UI (App.tsx)

1. MCP client calls show
2. Hono HTTP server starts on a random port and a Neutralino window opens
3. React UI connects via SSE (/events) and renders content
4. Subsequent calls with the same windowId update content through SSE
5. Calls without windowId always open a new window
6. Closing a window cleans up its HTTP server