| name | create-mcp-app |
| description | This skill should be used when the user asks to "create an MCP App", "build an MCP server", "add a UI to an MCP tool", "develop an interactive dashboard", "register an MCP server", or needs guidance on building MCP servers in the MemStack sandbox with optional Canvas UI rendering. Covers the full lifecycle from development to registration to UI activation. |
| tools | ["register_mcp_server","bash","read","write","edit","glob","grep"] |
Create MCP App in MemStack Sandbox
Build MCP servers in the sandbox environment with optional interactive UI that renders in the Canvas panel. This guide covers the complete lifecycle: develop, register, and activate.
Architecture Overview
You (Agent) write code in /workspace
|
v
register_mcp_server(server_name, server_type="stdio", command="python", args=["server.py"])
|
v
Platform starts your server as a child process (stdio transport)
|
v
Platform discovers tools via tools/list JSON-RPC call
|
v
Tools with _meta.ui.resourceUri → Platform auto-detects as "MCP App"
|
v
When tool is called → Platform fetches HTML via resources/read → Renders in Canvas panel
Key insight: The platform handles ALL the wiring. You just write a correct MCP server, register it, and the UI appears automatically.
Two Approaches to Build MCP Apps
Approach 1: FastMCP Framework (Recommended)
FastMCP is a modern, higher-level framework that simplifies MCP server development. It supports both tool-only servers and interactive UI apps.
Install FastMCP:
pip install "fastmcp[apps]"
Minimal FastMCP Server with UI:
"""MCP Server using FastMCP framework."""
from fastmcp import FastMCP
mcp = FastMCP("my-dashboard")
@mcp.tool()
def render_dashboard(query: str = "default") -> str:
"""Render the dashboard with current data."""
return "Dashboard rendered"
@mcp.resource("ui://my-dashboard/dashboard")
def dashboard_ui() -> str:
"""Serve the dashboard HTML."""
return """<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>My Dashboard</title>
<style>
body { font-family: system-ui, sans-serif; padding: 20px; }
</style>
</head>
<body>
<h1>My Dashboard</h1>
<div id="content">Dashboard content here</div>
</body>
</html>"""
if __name__ == "__main__":
mcp.run()
FastMCP with Tool-UI Binding:
from fastmcp import FastMCP
from fastmcp.server.apps import AppConfig
mcp = FastMCP("my-app")
@mcp.tool(app=AppConfig(resource_uri="ui://my-app/view.html"))
def show_data(data: str) -> str:
"""Show data with interactive UI."""
return f"Data: {data}"
@mcp.resource("ui://my-app/view.html")
def view_html() -> str:
return "<html>...</html>"
Approach 2: Native MCP SDK (Lower-level)
Use the official mcp package for full control. This is the approach documented in the main sections below.
Step-by-Step Workflow
Step 1: Create the MCP Server Code
Write a Python MCP server in /workspace/{server_name}/server.py using the mcp package.
Minimal example with UI:
"""MCP Server: my-dashboard - Interactive Dashboard."""
from mcp.server import Server
from mcp.server.stdio import stdio_server
import asyncio
server = Server("my-dashboard")
@server.list_tools()
async def list_tools():
"""List available tools."""
return [
{
"name": "render_dashboard",
"description": "Render the dashboard with current data",
"inputSchema": {
"type": "object",
"properties": {
"query": {"type": "string", "description": "Data query"}
}
},
"_meta": {
"ui": {
"resourceUri": "ui://my-dashboard/dashboard",
"title": "My Dashboard"
}
}
}
]
@server.call_tool()
async def call_tool(name: str, arguments: dict):
"""Handle tool calls."""
if name == "render_dashboard":
return {"content": [{"type": "text", : }]}
ValueError()
():
[
{
: ,
: ,
:
}
]
():
uri = (uri)
uri == :
html =
{: [{: uri, : , : html}]}
ValueError()
():
stdio_server() (read_stream, write_stream):
server.run(read_stream, write_stream)
__name__ == :
asyncio.run(main())
Step 2: Create pyproject.toml
[project]
name = "my-dashboard"
version = "0.1.0"
dependencies = ["mcp"]
[build-system]
requires = ["setuptools"]
build-backend = "setuptools.build_meta"
Step 3: Install Dependencies
cd /workspace/my-dashboard && pip install -e .
Step 4: Test the Server Locally (Optional but Recommended)
Before registering, verify the server starts without errors:
cd /workspace/my-dashboard && timeout 5 python server.py 2>&1 || true
If the server starts and waits for input (no error output), it is working correctly. The timeout command prevents it from hanging.
Step 5: Register the Server
Use the register_mcp_server tool:
register_mcp_server(
server_name="my-dashboard",
server_type="stdio",
command="python",
args=["/workspace/my-dashboard/server.py"]
)
After registration:
- Platform starts your server process
- Discovers tools via
tools/list
- Auto-detects
_meta.ui on tools → marks as MCP App
- Tools become available as
mcp__my_dashboard__render_dashboard
Step 6: Verify and Use
Call the registered tool. If it has _meta.ui, the Canvas panel will automatically render the HTML from your read_resource handler.
Critical Rules
1. read_resource Return Format (MUST be exact)
The read_resource handler MUST return a dict with this exact structure:
return {"contents": [{"uri": uri, "mimeType": "text/html", "text": html_string}]}
Common mistakes:
return (uri, "text/html", html_string)
return html_string
return {"uri": uri, "mimeType": "text/html", "text": html_string}
return {"content": [{"uri": uri, "mimeType": "text/html", "text": html_string}]}
The platform's MCP manager calls result.get("contents", []) and iterates to find item.get("text"). Any other format will silently fail.
2. _meta.ui Format (MUST use ui:// scheme)
"_meta": {
"ui": {
"resourceUri": "ui://server-name/resource-path",
"title": "Display Title"
}
}
- The
resourceUri MUST use the ui:// scheme
- The server name in the URI MUST match your registered server name
- The URI MUST match exactly what
list_resources and read_resource handle
3. stdio Transport is the Default and Correct Choice
The sandbox runs MCP servers as child processes using stdio (stdin/stdout). This is:
- The standard transport for container-local MCP servers
- Well-engineered with process lifecycle management, request-response correlation, and idle timeout with auto-restart
- Used by all built-in templates
Do NOT switch to HTTP/SSE/WebSocket unless you have a specific reason (e.g., the server must be shared across multiple clients). stdio is simpler and more reliable for sandbox use.
4. Server Process Must Stay Alive
The async with stdio_server() pattern keeps the server running. Your server MUST:
- Use
asyncio.run(main()) as the entry point
- NOT exit after handling one request
- NOT print to stdout (stdout is the JSON-RPC transport)
- Use stderr for logging:
import sys; print("debug", file=sys.stderr)
5. list_resources Must Declare All UI Resources
Every resourceUri referenced in _meta.ui must also appear in list_resources():
@server.list_resources()
async def list_resources():
return [
{
"uri": "ui://my-server/dashboard",
"name": "Dashboard UI",
"mimeType": "text/html"
}
]
6. Use Absolute Paths in args
When registering, always use absolute paths for the server script:
register_mcp_server(server_name="my-app", server_type="stdio",
command="python", args=["/workspace/my-app/server.py"])
register_mcp_server(server_name="my-app", server_type="stdio",
command="python", args=["server.py"])
Built-in Templates
The platform provides 3 server templates you can reference:
| Template | Description | Key Pattern |
|---|
web-dashboard | Interactive dashboard with metrics | _meta.ui + read_resource HTML |
api-wrapper | REST API integration | Tool-only, no UI |
data-processor | Data transformation pipeline | Tool-only, no UI |
The web-dashboard template is the canonical example for MCP Apps with UI.
FastMCP Prefab Apps (Recommended for Interactive UIs)
Prefab is a declarative UI framework for Python. You describe layouts, charts, tables, forms using Python DSL, and FastMCP compiles them to JSON for the Canvas renderer.
Install Prefab:
pip install "fastmcp[apps]" "prefab-ui"
Example: Interactive Chart:
from prefab_ui.components import Column, Heading, BarChart, ChartSeries
from prefab_ui.app import PrefabApp
from fastmcp import FastMCP
mcp = FastMCP("Dashboard")
@mcp.tool(app=True)
def sales_chart(year: int) -> PrefabApp:
"""Show sales data as an interactive chart."""
data = [
{"month": "Jan", "revenue": 10000},
{"month": "Feb", "revenue": 15000},
{"month": "Mar", "revenue": 12000},
]
with Column(gap=4, css_class="p-6") as view:
Heading(f"{year} Sales")
BarChart(
data=data,
series=[ChartSeries(data_key="revenue", label="Revenue")],
x_axis="month",
)
return PrefabApp(view=view)
Available Prefab Components:
- Layout:
Column, Row, Stack
- Typography:
Heading, Text, Label
- Interactive:
Button, IconButton, Link
- Charts:
BarChart, LineChart, PieChart, ScatterChart
- Data:
Table, DataGrid, List
- Forms:
TextField, Select, Checkbox, Radio, Slider, DatePicker
- Containers:
Card, Modal, Drawer, Tabs, Accordion
Example: Interactive Form:
from prefab_ui.components import Column, TextField, Button, Heading
from prefab_ui.app import PrefabApp
from fastmcp import FastMCP
mcp = FastMCP("FormApp")
@mcp.tool(app=True)
def user_form() -> PrefabApp:
"""Show a user input form."""
with Column(gap=3, css_class="p-4") as view:
Heading("User Information")
TextField(label="Name", placeholder="Enter your name")
TextField(label="Email", placeholder="Enter your email")
Button(label="Submit", on_click="submit_form")
return PrefabApp(view=view)
Debugging
Server won't start after registration
- Check stderr output in the registration response
- Verify dependencies are installed:
pip list | grep mcp
- Test the server manually:
cd /workspace/my-server && python server.py
- Check for syntax errors:
python -c "import server" (from the server directory)
UI doesn't appear in Canvas
- Verify
_meta.ui.resourceUri uses ui:// scheme
- Verify
list_resources() includes the same URI
- Verify
read_resource() handles the URI and returns the correct dict format
- Check that the HTML is valid and non-empty
Tools not discovered
- Verify
list_tools() returns a proper list of tool dicts
- Each tool must have
name, description, and inputSchema
- Check that the server doesn't crash during
tools/list
"Unknown tool" errors
After registration, tools are namespaced as mcp__{server_name}__{tool_name} (with hyphens replaced by underscores). Use this full name when calling.
Advanced Patterns
Multiple Tools with Shared UI
Multiple tools can reference the same resourceUri. The Canvas will render the same HTML resource regardless of which tool triggered it:
@server.list_tools()
async def list_tools():
return [
{
"name": "show_chart",
"description": "Show chart view",
"inputSchema": {"type": "object", "properties": {}},
"_meta": {"ui": {"resourceUri": "ui://my-app/view", "title": "Chart"}}
},
{
"name": "show_table",
"description": "Show table view",
"inputSchema": {"type": "object", "properties": {}},
"_meta": {"ui": {"resourceUri": "ui://my-app/view", "title": "Table"}}
}
]
Tools Without UI
Not every tool needs a UI. Tools without _meta.ui work normally as text-only tools:
{
"name": "fetch_data",
"description": "Fetch raw data",
"inputSchema": {"type": "object", "properties": {"query": {"type": "string"}}}
}
Dynamic HTML Content
The read_resource handler is called each time the UI needs to render. You can generate dynamic HTML based on server state:
import json
_current_data = {}
@server.call_tool()
async def call_tool(name: str, arguments: dict):
global _current_data
if name == "update_data":
_current_data = arguments.get("data", {})
return {"content": [{"type": "text", "text": "Data updated"}]}
@server.read_resource()
async def read_resource(uri):
uri = str(uri)
if uri == "ui://my-app/view":
html_content = json.dumps(_current_data, indent=2)
html = f"""<!DOCTYPE html>
<html><body>
<pre>{html_content}</pre>
</body></html>"""
return {"contents": [{"uri": uri, "mimeType": "text/html", "text": html}]}
Bidirectional Communication (Guest UI to Agent Chat)
MCP Apps can send messages FROM the Canvas UI BACK to the agent conversation. This enables interactive workflows where the user makes choices in the UI and the agent reacts.
Architecture:
Guest iframe (your HTML)
| window.parent.postMessage(jsonRpcMsg, '*')
v
sandbox_proxy.html (relay)
| window.parent.postMessage(jsonRpcMsg, '*')
v
Host @mcp-ui/client Transport
| Parses JSON-RPC, validates ui/message schema
v
StandardMCPAppRenderer.handleMessage
| Extracts text from content array
v
CanvasPanel.onSendPrompt(text)
| Sends as user message to conversation
v
Agent receives and processes the message
CRITICAL: Use the exact JSON-RPC format below. The host @mcp-ui/client Transport validates incoming messages against the JSON-RPC schema. Any deviation will be silently dropped.
let msgIdCounter = 1;
function sendToAgent(text) {
window.parent.postMessage({
jsonrpc: "2.0",
id: msgIdCounter++,
method: "ui/message",
params: {
role: "user",
content: [{ type: "text", text: text }]
}
}, "*");
}
document.getElementById('confirm-btn').addEventListener('click', () => {
const value = document.getElementById('result').textContent;
sendToAgent('User selected: ' + value);
});
Key requirements:
jsonrpc MUST be "2.0"id MUST be a unique incrementing number (not a string)method MUST be exactly "ui/message"params.content MUST be an array of ContentBlock objects: [{ type: "text", text: "..." }]params.role should be "user"- Use
window.parent.postMessage(msg, "*") — NOT window.postMessage
DO NOT USE these hallucinated/non-existent APIs:
window.mcpApp.sendMessage('result');
window.parent.postMessage({ type: 'mcp-tool-result', result: 'value' }, '*');
import { sendMessage } from 'https://cdn.example.com/@modelcontextprotocol/ext-apps/0.1.0/dist/index.js';
Complete example with bidirectional communication:
@server.read_resource()
async def read_resource(uri):
uri = str(uri)
if uri == "ui://my-app/picker":
html = """<!DOCTYPE html>
<html>
<head><meta charset="utf-8"><title>Picker</title></head>
<body>
<h1>Pick a Value</h1>
<div id="value">0</div>
<button onclick="change(1)">+</button>
<button onclick="change(-1)">-</button>
<button onclick="confirm()">Confirm</button>
<script>
let val = 0, msgId = 1;
function change(d) {
val += d;
document.getElementById('value').textContent = val;
}
function confirm() {
window.parent.postMessage({
jsonrpc: '2.0',
id: msgId++,
method: 'ui/message',
params: {
role: 'user',
content: [{ type: 'text', text: 'Selected value: ' + val }]
}
}, '*');
}
</script>
</body>
</html>"""
return {"contents": [{"uri": uri, "mimeType": "text/html", "text": html}]}
Checklist Before Registration
