---

name: ra-mcp-apps description: "MCP Apps guide: FastMCP 3.4+ server, Svelte 5 view, tool visibility, polling, sizing, fullscreen, view persistence. Use for any MCP App/UI work."

MCP Apps — Quick Reference

Server (FastMCP) ↔ Host (Claude/ChatGPT) ↔ View (sandboxed iframe). The View and Server never talk directly.

Supporting Files

• patterns.md — MCP Apps patterns: polling, chunked data, theming, fullscreen, view persistence (Brick Builder), per-view state, autoResize, command queues
• csp-cors.md — CSP configuration, CORS, stable origins, connectDomains vs resourceDomains
• testing.md — Testing with basic-host, Claude.ai, VS Code, tunnels

Data Flow

Field	LLM sees?	View sees?	Use for
content	Yes	Yes	Short text summary for model context
structuredContent	No*	Yes	Rich data for UI rendering
_meta	No	Yes	viewUUID, timestamps, metadata

*ChatGPT exposes structuredContent to both.

Tool Visibility

Visibility	LLM?	View?	Use case
["model", "app"] (default)	Yes	Yes	General tools
["model"]	Yes	No	LLM-only triggers
["app"]	No	Yes	Pagination, refresh, polling, dangerous actions

Server Quick Start (FastMCP 3.4+)

from fastmcp import FastMCP, Context
from fastmcp.apps import AppConfig, ResourceCSP, UI_EXTENSION_ID
from fastmcp.tools import ToolResult
from mcp import types

mcp = FastMCP("My App")
RESOURCE_URI = "ui://my-app/view.html"

# Tool with UI — creates an iframe
@mcp.tool(app=AppConfig(resource_uri=RESOURCE_URI))
async def show_data(query: str, ctx: Context) -> ToolResult:
    has_ui = ctx.client_supports_extension(UI_EXTENSION_ID)
    return ToolResult(
        content=[types.TextContent(type="text", text="Summary for LLM")],
        structured_content={"data": [...], "query": query},
    )

# App-only tool — View calls this, LLM never sees it
@mcp.tool(app=AppConfig(resource_uri=RESOURCE_URI, visibility=["app"]))
async def load_page(page: int) -> ToolResult: ...

# Resource — serves the bundled HTML
@mcp.resource(uri=RESOURCE_URI)
def get_ui() -> str:
    return Path("dist/mcp-app.html").read_text()

View Quick Start (Svelte 5)

<script lang="ts">
import { onMount } from "svelte";
import { App, applyDocumentTheme, applyHostStyleVariables, applyHostFonts, type McpUiHostContext } from "@modelcontextprotocol/ext-apps";

let app = $state<App | null>(null);
let hostContext = $state<McpUiHostContext | undefined>();
let data = $state.raw<MyData | null>(null);

$effect(() => {
  if (hostContext?.theme) applyDocumentTheme(hostContext.theme);
  if (hostContext?.styles?.variables) applyHostStyleVariables(hostContext.styles.variables);
  if (hostContext?.styles?.css?.fonts) applyHostFonts(hostContext.styles.css.fonts);
});

onMount(async () => {
  const instance = new App(
    { name: "My App", version: "1.0.0" },
    { availableDisplayModes: ["inline", "fullscreen"] },
    { autoResize: false },  // Required for flexible-height content — see patterns.md
  );
  instance.ontoolresult = (result) => { data = result.structuredContent; };
  instance.onhostcontextchanged = (ctx) => { hostContext = { ...hostContext, ...ctx }; };
  instance.onteardown = async () => { /* cleanup */ return {}; };
  await instance.connect();
  app = instance;
  hostContext = instance.getHostContext();
});
</script>

Decision Checklist

Question	Answer
What does the LLM need?	content
What does the UI need?	structuredContent
LLM triggers this?	visibility: ["model"] or default
UI-only action?	visibility: ["app"]
Data >100KB?	Chunked app-only tool — see patterns.md
External APIs?	Declare in ResourceCSP — see csp-cors.md
Fullscreen?	availableDisplayModes: ["inline", "fullscreen"]
View should persist across LLM tool calls?	Brick Builder pattern — see patterns.md
Multi-user server?	Per-view state with MemoryStore — see patterns.md

Gotchas

• Tool with resourceUri = new iframe. Never put AppConfig(resource_uri=...) on update tools.
• autoResize: false required for flexible-height content to prevent infinite resize loops.
• onteardown — always implement to stop polling and clean up.
• CSP deny-by-default. External images silently don't load without resource_domains.
• asyncio.gather for parallel fetches — sequential await calls wait for each other.
• Multi-user servers need per-view state (MemoryStore + view_id), not module-level globals.