Add UI to MCP Server
Enrich an existing MCP server's tools with interactive UIs using the MCP Apps SDK (@modelcontextprotocol/ext-apps).
How It Works
Existing tools get paired with HTML resources that render inline in the host's conversation. The tool continues to work for text-only clients — UI is an enhancement, not a replacement. Each tool that benefits from UI gets linked to a resource via _meta.ui.resourceUri, and the host renders that resource in a sandboxed iframe when the tool is called.
Getting Reference Code
Clone the SDK repository for working examples and API documentation:
git clone --branch "v$(npm view @modelcontextprotocol/ext-apps version)" --depth 1 https://github.com/modelcontextprotocol/ext-apps.git /tmp/mcp-ext-appsAPI Reference (Source Files)
Read JSDoc documentation directly from /tmp/mcp-ext-apps/src/:
| File | Contents |
|---|---|
src/app.ts | App class, handlers (ontoolinput, ontoolresult, onhostcontextchanged, onteardown), lifecycle |
src/server/index.ts | registerAppTool, registerAppResource, getUiCapability, tool visibility options |
src/spec.types.ts | All type definitions: McpUiHostContext, CSS variable keys, display modes |
src/styles.ts | applyDocumentTheme, applyHostStyleVariables, applyHostFonts |
src/react/useApp.tsx | useApp hook for React apps |
src/react/useHostStyles.ts | useHostStyles, useHostStyleVariables, useHostFonts hooks |
Key Examples (Mixed Tool Patterns)
These examples demonstrate servers with both App-enhanced and plain tools — the exact pattern you're adding:
| Example | Pattern |
|---|---|
examples/map-server/ | show-map (App tool) + geocode (plain tool) |
examples/pdf-server/ | display_pdf (App tool) + list_pdfs (plain tool) + read_pdf_bytes (app-only tool) |
examples/system-monitor-server/ | get-system-info (App tool) + poll-system-stats (app-only polling tool) |
Framework Templates
Learn and adapt from /tmp/mcp-ext-apps/examples/basic-server-{framework}/:
| Template | Key Files |
|---|---|
basic-server-vanillajs/ | server.ts, src/mcp-app.ts, mcp-app.html |
basic-server-react/ | server.ts, src/mcp-app.tsx (uses useApp hook) |
basic-server-vue/ | server.ts, src/App.vue |
basic-server-svelte/ | server.ts, src/App.svelte |
basic-server-preact/ | server.ts, src/mcp-app.tsx |
basic-server-solid/ | server.ts, src/mcp-app.tsx |
Step 1: Analyze Existing Tools
Before writing any code, analyze the server's existing tools and determine which ones benefit from UI.
- Read the server source and list all registered tools
- For each tool, assess whether it would benefit from UI (returns data that could be visualized, involves user interaction, etc.) vs. is fine as text-only (simple lookups, utility functions)
- Identify tools that could become app-only helpers (data the UI needs to poll/fetch but the model doesn't need to call directly)
- Present the analysis to the user and confirm which tools to enhance
Decision Framework
| Tool output type | UI benefit | Example |
|---|---|---|
| Structured data / lists / tables | High — interactive table, search, filtering | List of items, search results |
| Metrics / numbers over time | High — charts, gauges, dashboards | System stats, analytics |
| Media / rich content | High — viewer, player, renderer | Maps, PDFs, images, video |
| Simple text / confirmations | Low — text is fine | "File created", "Setting updated" |
| Data for other tools | Consider app-only | Polling endpoints, chunk loaders |
Step 2: Add Dependencies
npm install @modelcontextprotocol/ext-apps
npm install -D vite vite-plugin-singlefilePlus framework-specific dependencies if needed (e.g., react, react-dom, @vitejs/plugin-react for React).
Use npm install to add dependencies rather than manually writing version numbers. This lets npm resolve the latest compatible versions. Never specify version numbers from memory.
Step 3: Set Up the Build Pipeline
Vite Configuration
Create vite.config.ts with vite-plugin-singlefile to bundle the UI into a single HTML file:
import { defineConfig } from "vite";
import { viteSingleFile } from "vite-plugin-singlefile";
export default defineConfig({
plugins: [viteSingleFile()],
build: {
outDir: "dist",
rollupOptions: {
input: "mcp-app.html", // one per UI, or one shared entry
},
},
});HTML Entry Point
Create mcp-app.html (or one per distinct UI if tools need different views):
<!doctype html>
<html lang="en">
<head>
<meta charset="UTF-8" />
<meta name="viewport" content="width=device-width, initial-scale=1.0" />
<title>MCP App</title>
</head>
<body>
<div id="root"></div>
<script type="module" src="./src/mcp-app.ts"></script>
</body>
</html>Build Scripts
Add build scripts to package.json. The UI must be built before the server code bundles it:
{
"scripts": {
"build:ui": "vite build",
"build:server": "tsc",
"build": "npm run build:ui && npm run build:server",
"serve": "tsx server.ts"
}
}Step 4: Convert Tools to App Tools
Transform plain MCP tools into App tools with UI.
Before (plain MCP tool):
server.tool("my-tool", { param: z.string() }, async (args) => {
const data = await fetchData(args.param);
return { content: [{ type: "text", text: JSON.stringify(data) }] };
});After (App tool with UI):
import { registerAppTool, registerAppResource, RESOURCE_MIME_TYPE } from "@modelcontextprotocol/ext-apps/server";
const resourceUri = "ui://my-tool/mcp-app.html";
registerAppTool(server, "my-tool", {
description: "Shows data with an interactive UI",
inputSchema: { param: z.string() },
_meta: { ui: { resourceUri } },
}, async (args) => {
const data = await fetchData(args.param);
return {
content: [{ type: "text", text: JSON.stringify(data) }], // text fallback for non-UI hosts
structuredContent: { data }, // structured data for the UI
};
});Key guidance:
- Always keep the
contentarray with a text fallback for text-only clients - Add
structuredContentfor data the UI needs to render - Link the tool to its resource via
_meta.ui.resourceUri - Leave tools that don't benefit from UI unchanged — they stay as plain tools
Step 5: Register Resources
Register the HTML resource so the host can fetch it:
import fs from "node:fs/promises";
import path from "node:path";
const resourceUri = "ui://my-tool/mcp-app.html";
registerAppResource(server, {
uri: resourceUri,
name: "My Tool UI",
mimeType: RESOURCE_MIME_TYPE,
}, async () => {
const html = await fs.readFile(
path.resolve(import.meta.dirname, "dist", "mcp-app.html"),
"utf-8",
);
return { contents: [{ uri: resourceUri, mimeType: RESOURCE_MIME_TYPE, text: html }] };
});If multiple tools share the same UI, they can reference the same resourceUri and the same resource registration.
Step 6: Build the UI
Handler Registration
Register ALL handlers BEFORE calling app.connect():
import { App, PostMessageTransport, applyDocumentTheme, applyHostStyleVariables, applyHostFonts } from "@modelcontextprotocol/ext-apps";
const app = new App({ name: "My App", version: "1.0.0" });
app.ontoolinput = (params) => {
// Render the UI using params.arguments and/or params.structuredContent
};
app.ontoolresult = (result) => {
// Update UI with final tool result
};
app.onhostcontextchanged = (ctx) => {
if (ctx.theme) applyDocumentTheme(ctx.theme);
if (ctx.styles?.variables) applyHostStyleVariables(ctx.styles.variables);
if (ctx.styles?.css?.fonts) applyHostFonts(ctx.styles.css.fonts);
if (ctx.safeAreaInsets) {
const { top, right, bottom, left } = ctx.safeAreaInsets;
document.body.style.padding = `${top}px ${right}px ${bottom}px ${left}px`;
}
};
app.onteardown = async () => {
return {};
};
await app.connect(new PostMessageTransport());Host Styling
Use host CSS variables for theme integration:
.container {
background: var(--color-background-secondary);
color: var(--color-text-primary);
font-family: var(--font-sans);
border-radius: var(--border-radius-md);
}Key variable groups: --color-background-, --color-text-, --color-border-, --font-sans, --font-mono, --font-text--size, --font-heading--size, --border-radius-. See src/spec.types.ts for the full list.
For React apps, use the useApp and useHostStyles hooks instead — see basic-server-react/ for the pattern.
Optional Enhancements
App-Only Helper Tools
Tools the UI calls but the model doesn't need to invoke directly (polling, pagination, chunk loading):
registerAppTool(server, "poll-data", {
description: "Polls latest data for the UI",
_meta: { ui: { resourceUri, visibility: ["app"] } },
}, async () => {
const data = await getLatestData();
return { content: [{ type: "text", text: JSON.stringify(data) }] };
});The UI calls these via app.callServerTool("poll-data", {}).
CSP Configuration
If the UI needs to load external resources (fonts, APIs, CDNs), declare the domains:
registerAppResource(server, {
uri: resourceUri,
name: "My Tool UI",
mimeType: RESOURCE_MIME_TYPE,
_meta: {
ui: {
connectDomains: ["api.example.com"], // fetch/XHR targets
resourceDomains: ["cdn.example.com"], // scripts, styles, images
frameDomains: ["embed.example.com"], // nested iframes
},
},
}, async () => { /* ... */ });Streaming Partial Input
For large tool inputs, show progress during LLM generation:
app.ontoolinputpartial = (params) => {
const args = params.arguments; // Healed partial JSON - always valid
// Render preview with partial data
};
app.ontoolinput = (params) => {
// Final complete input - switch to full render
};Graceful Degradation with getUiCapability()
Conditionally register App tools only when the client supports UI, falling back to text-only tools:
import { getUiCapability, registerAppTool, RESOURCE_MIME_TYPE } from "@modelcontextprotocol/ext-apps/server";
server.server.oninitialized = () => {
const clientCapabilities = server.server.getClientCapabilities();
const uiCap = getUiCapability(clientCapabilities);
if (uiCap?.mimeTypes?.includes(RESOURCE_MIME_TYPE)) {
// Client supports UI — register App tool
registerAppTool(server, "my-tool", {
description: "Shows data with interactive UI",
_meta: { ui: { resourceUri } },
}, appToolHandler);
} else {
// Text-only client — register plain tool
server.tool("my-tool", "Shows data", { param: z.string() }, plainToolHandler);
}
};Fullscreen Mode
Allow the UI to expand to fullscreen:
app.onhostcontextchanged = (ctx) => {
if (ctx.availableDisplayModes?.includes("fullscreen")) {
fullscreenBtn.style.display = "block";
}
if (ctx.displayMode) {
container.classList.toggle("fullscreen", ctx.displayMode === "fullscreen");
}
};
async function toggleFullscreen() {
const newMode = currentMode === "fullscreen" ? "inline" : "fullscreen";
const result = await app.requestDisplayMode({ mode: newMode });
currentMode = result.mode;
}Common Mistakes to Avoid
- Forgetting text
contentfallback — Always includecontentarray with text for non-UI hosts - Registering handlers after
connect()— Register ALL handlers BEFORE callingapp.connect() - Missing
vite-plugin-singlefile— Without it, assets won't load in the sandboxed iframe - Forgetting resource registration — The tool references a
resourceUrithat must have a matching resource - Hardcoding styles — Use host CSS variables (
var(--color-*)) for theme integration - Not handling safe area insets — Always apply
ctx.safeAreaInsetsinonhostcontextchanged
Testing
Using basic-host
Test the enhanced server with the basic-host example:
# Terminal 1: Build and run your server
npm run build && npm run serve
# Terminal 2: Run basic-host (from cloned repo)
cd /tmp/mcp-ext-apps/examples/basic-host
npm install
SERVERS='["http://localhost:3001/mcp"]' npm run start
# Open http://localhost:8080Configure SERVERS with a JSON array of your server URLs (default: http://localhost:3001/mcp).
Verify
- Plain tools still work and return text output
- App tools render their UI in the iframe
ontoolinputhandler fires with tool argumentsontoolresulthandler fires with tool result- Host styling (theme, fonts, colors) applies correctly
