Add basic-host and basic-server-react examples

Introduce new example implementations to replace simple-host and
simple-server:

- basic-host: Reference host implementation with double-iframe sandbox
  pattern for secure UI isolation (outer iframe validates and relays
  messages, inner iframe renders untrusted tool UI via srcdoc)

- basic-server-react: MCP server example demonstrating tool registration
  with linked UI resources and React UI using the useApp() hook

Build infrastructure updates:
- Add build:all script and simplify CI/pre-commit to use it
- Add examples:dev for watch-mode development workflow

Also change PostMessageTransport logging from info to debug level.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

examples/basic-host/src/global.css

@@ -0,0 +1,12 @@
+* {
+  box-sizing: border-box;
+}
+
+html, body {
+  font-family: system-ui, -apple-system, sans-serif;
+  font-size: 1rem;
+}
+
+code {
+  font-size: 1em;
+}

examples/basic-host/src/implementation.ts

@@ -0,0 +1,257 @@
+import { RESOURCE_URI_META_KEY, type McpUiSandboxProxyReadyNotification } from "@modelcontextprotocol/ext-apps";
+import { AppBridge, PostMessageTransport } from "@modelcontextprotocol/ext-apps/app-bridge";
+import { Client } from "@modelcontextprotocol/sdk/client/index.js";
+import { StreamableHTTPClientTransport } from "@modelcontextprotocol/sdk/client/streamableHttp.js";
+import type { CallToolResult, Tool } from "@modelcontextprotocol/sdk/types.js";
+
+
+const SANDBOX_PROXY_URL = new URL("http://localhost:8081/sandbox.html");
+const IMPLEMENTATION = { name: "MCP Apps Host", version: "1.0.0" };
+
+
+export const log = {
+  info: console.log.bind(console, "[HOST]"),
+  warn: console.warn.bind(console, "[HOST]"),
+  error: console.error.bind(console, "[HOST]"),
+};
+
+
+export interface ServerInfo {
+  client: Client;
+  tools: Map<string, Tool>;
+  appHtmlCache: Map<string, string>;
+}
+
+
+export async function connectToServer(serverUrl: URL): Promise<ServerInfo> {
+  const client = new Client(IMPLEMENTATION);
+
+  log.info("Connecting to server:", serverUrl.href);
+  await client.connect(new StreamableHTTPClientTransport(serverUrl));
+  log.info("Connection successful");
+
+  const toolsList = await client.listTools();
+  const tools = new Map(toolsList.tools.map((tool) => [tool.name, tool]));
+  log.info("Server tools:", Array.from(tools.keys()));
+
+  return { client, tools, appHtmlCache: new Map() };
+}
+
+
+export interface ToolCallInfo {
+  serverInfo: ServerInfo;
+  tool: Tool;
+  input: Record<string, unknown>;
+  resultPromise: Promise<CallToolResult>;
+  appHtmlPromise?: Promise<string>;
+}
+
+
+export function hasAppHtml(toolCallInfo: ToolCallInfo): toolCallInfo is Required<ToolCallInfo> {
+  return !!toolCallInfo.appHtmlPromise;
+}
+
+
+export function callTool(
+  serverInfo: ServerInfo,
+  name: string,
+  input: Record<string, unknown>,
+): ToolCallInfo {
+  log.info("Calling tool", name, "with input", input);
+  const resultPromise = serverInfo.client.callTool({ name, arguments: input }) as Promise<CallToolResult>;
+
+  const tool = serverInfo.tools.get(name);
+  if (!tool) {
+    throw new Error(`Unknown tool: ${name}`);
+  }
+
+  const toolCallInfo: ToolCallInfo = { serverInfo, tool, input, resultPromise };
+
+  const uiResourceUri = getUiResourceUri(tool);
+  if (uiResourceUri) {
+    toolCallInfo.appHtmlPromise = getUiResourceHtml(serverInfo, uiResourceUri);
+  }
+
+  return toolCallInfo;
+}
+
+
+function getUiResourceUri(tool: Tool): string | undefined {
+  const uri = tool._meta?.[RESOURCE_URI_META_KEY];
+  if (typeof uri === "string" && uri.startsWith("ui://")) {
+    return uri;
+  } else if (uri !== undefined) {
+    throw new Error(`Invalid UI resource URI: ${JSON.stringify(uri)}`);
+  }
+}
+
+
+async function getUiResourceHtml(serverInfo: ServerInfo, uri: string): Promise<string> {
+  let html = serverInfo.appHtmlCache.get(uri);
+  if (html) {
+    log.info("Read UI resource from cache:", uri);
+    return html;
+  }
+
+  log.info("Reading UI resource:", uri);
+  const resource = await serverInfo.client.readResource({ uri });
+
+  if (!resource) {
+    throw new Error(`Resource not found: ${uri}`);
+  }
+
+  if (resource.contents.length !== 1) {
+    throw new Error(`Unexpected contents count: ${resource.contents.length}`);
+  }
+
+  const content = resource.contents[0];
+
+  // Per the MCP App specification, "text/html+mcp" signals this resource is
+  // indeed for an MCP App UI.
+  if (content.mimeType !== "text/html+mcp") {
+    throw new Error(`Unsupported MIME type: ${content.mimeType}`);
+  }
+
+  html = "blob" in content ? atob(content.blob) : content.text;
+  serverInfo.appHtmlCache.set(uri, html);
+  return html;
+}
+
+
+export function loadSandboxProxy(iframe: HTMLIFrameElement): Promise<boolean> {
+  // Prevent reload
+  if (iframe.src) return Promise.resolve(false);
+
+  iframe.setAttribute("sandbox", "allow-scripts allow-same-origin allow-forms");
+
+  const readyNotification: McpUiSandboxProxyReadyNotification["method"] =
+    "ui/notifications/sandbox-proxy-ready";
+
+  const readyPromise = new Promise<boolean>((resolve) => {
+    const listener = ({ source, data }: MessageEvent) => {
+      if (source === iframe.contentWindow && data?.method === readyNotification) {
+        log.info("Sandbox proxy loaded")
+        window.removeEventListener("message", listener);
+        resolve(true);
+      }
+    };
+    window.addEventListener("message", listener);
+  });
+
+  log.info("Loading sandbox proxy...");
+  iframe.src = SANDBOX_PROXY_URL.href;
+
+  return readyPromise;
+}
+
+
+export async function initializeApp(
+  iframe: HTMLIFrameElement,
+  appBridge: AppBridge,
+  { input, resultPromise, appHtmlPromise }: Required<ToolCallInfo>,
+): Promise<void> {
+  const appInitializedPromise = hookInitializedCallback(appBridge);
+
+  // Connect app bridge (triggers MCP initialization handshake)
+  //
+  // IMPORTANT: Pass `iframe.contentWindow` as BOTH target and source to ensure
+  // this proxy only responds to messages from its specific iframe.
+  await appBridge.connect(
+    new PostMessageTransport(iframe.contentWindow!, iframe.contentWindow!),
+  );
+
+  // Load inner iframe HTML
+  log.info("Sending UI resource HTML to MCP App");
+  await appBridge.sendSandboxResourceReady({ html: await appHtmlPromise });
+
+  // Wait for inner iframe to be ready
+  log.info("Waiting for MCP App to initialize...");
+  await appInitializedPromise;
+  log.info("MCP App initialized");
+
+  // Send tool call input to iframe
+  log.info("Sending tool call input to MCP App:", input);
+  appBridge.sendToolInput({ arguments: input });
+
+  // Schedule tool call result to be sent to MCP App
+  resultPromise.then((result) => {
+    log.info("Sending tool call result to MCP App:", result);
+    appBridge.sendToolResult(result);
+  });
+}
+
+/**
+ * Hooks into `AppBridge.oninitialized` and returns a Promise that resolves when
+ * the MCP App is initialized (i.e., when the inner iframe is ready).
+ */
+function hookInitializedCallback(appBridge: AppBridge): Promise<void> {
+  const oninitialized = appBridge.oninitialized;
+  return new Promise<void>((resolve) => {
+    appBridge.oninitialized = (...args) => {
+      resolve();
+      appBridge.oninitialized = oninitialized;
+      appBridge.oninitialized?.(...args);
+    };
+  });
+}
+
+
+export function newAppBridge(serverInfo: ServerInfo, iframe: HTMLIFrameElement): AppBridge {
+  const serverCapabilities = serverInfo.client.getServerCapabilities();
+  const appBridge = new AppBridge(serverInfo.client, IMPLEMENTATION, {
+    openLinks: {},
+    serverTools: serverCapabilities?.tools,
+    serverResources: serverCapabilities?.resources,
+  });
+
+  // Register all handlers before calling connect(). The Guest UI can start
+  // sending requests immediately after the initialization handshake, so any
+  // handlers registered after connect() might miss early requests.
+
+  appBridge.onmessage = async (params, _extra) => {
+    log.info("Message from MCP App:", params);
+    return {};
+  };
+
+  appBridge.onopenlink = async (params, _extra) => {
+    log.info("Open link request:", params);
+    window.open(params.url, "_blank", "noopener,noreferrer");
+    return {};
+  };
+
+  appBridge.onloggingmessage = (params) => {
+    log.info("Log message from MCP App:", params);
+  };
+
+  appBridge.onsizechange = async ({ width, height }) => {
+    // The MCP App has requested a `width` and `height`, but if
+    // `box-sizing: border-box` is applied to the outer iframe element, then we
+    // must add border thickness to `width` and `height` to compute the actual
+    // necessary width and height (in order to prevent a resize feedback loop).
+    const style = getComputedStyle(iframe);
+    const isBorderBox = style.boxSizing === "border-box";
+
+    // Animate the change for a smooth transition.
+    const from: Keyframe = {};
+    const to: Keyframe = {};
+
+    if (width !== undefined) {
+      if (isBorderBox) {
+        width += parseFloat(style.borderLeftWidth) + parseFloat(style.borderRightWidth);
+      }
+      from.width = `${iframe.offsetWidth}px`;
+      iframe.style.width = to.width = `${width}px`;
+    }
+    if (height !== undefined) {
+      if (isBorderBox) {
+        height += parseFloat(style.borderTopWidth) + parseFloat(style.borderBottomWidth);
+      }
+      from.height = `${iframe.offsetHeight}px`;
+      iframe.style.height = to.height = `${height}px`;
+    }
+
+    iframe.animate([from, to], { duration: 300, easing: "ease-out" });
+  };
+
+  return appBridge;
+}

examples/basic-host/src/index.module.css

@@ -0,0 +1,129 @@
+.callToolPanel, .toolCallInfoPanel {
+  margin: 0 auto;
+  padding: 1rem;
+  border: 1px solid #ddd;
+  border-radius: 4px;
+
+  * + & {
+    margin-top: 1rem;
+  }
+}
+
+.callToolPanel {
+  max-width: 480px;
+
+  form {
+    display: flex;
+    flex-direction: column;
+    gap: 1rem;
+  }
+
+  label {
+    display: flex;
+    flex-direction: column;
+    gap: 0.25rem;
+    font-weight: 600;
+  }
+
+  select,
+  textarea {
+    padding: 0.5rem;
+    border: 1px solid #ccc;
+    border-radius: 4px;
+    font-family: monospace;
+    font-size: inherit;
+  }
+
+  textarea {
+    min-height: 6rem;
+    resize: vertical;
+
+    &[aria-invalid="true"] {
+      background-color: #fdd;
+    }
+  }
+
+  button {
+    align-self: center;
+    min-width: 200px;
+    padding: 0.75rem 1.5rem;
+    border: none;
+    border-radius: 4px;
+    background-color: #1e3a5f;
+    font-size: inherit;
+    font-weight: 600;
+    color: white;
+    cursor: pointer;
+
+    &:hover:not(:disabled) {
+      background-color: #2d4a7c;
+    }
+
+    &:disabled {
+      opacity: 0.5;
+      cursor: not-allowed;
+    }
+  }
+}
+
+.toolCallInfoPanel {
+  display: flex;
+  gap: 1rem;
+  max-width: 1120px;
+  animation: slideDown 0.3s ease-out;
+}
+
+@keyframes slideDown {
+  from { opacity: 0; transform: translateY(-12px); }
+}
+
+.inputInfoPanel {
+  display: flex;
+  flex: 3;
+  flex-direction: column;
+  gap: 0.5rem;
+  min-width: 0;
+
+  .toolName {
+    margin: 0;
+    font-family: monospace;
+    font-size: 1.5rem;
+  }
+}
+
+.outputInfoPanel {
+  flex: 4;
+  min-width: 0;
+}
+
+.jsonBlock {
+  flex-grow: 1;
+  min-height: 0;
+  margin: 0;
+  padding: 1rem;
+  border-radius: 4px;
+  background-color: #f5f5f5;
+  overflow: auto;
+}
+
+.appIframePanel {
+  display: flex;
+  justify-content: center;
+  width: 100%;
+  min-height: 200px;
+
+  iframe {
+    flex-shrink: 0;
+    height: 100%;
+    max-width: 100%;
+    box-sizing: border-box;
+    border: 3px dashed #888;
+    border-radius: 4px;
+  }
+}
+
+.error {
+  padding: 1.5rem;
+  background-color: #ddd;
+  color: #d00;
+}

examples/basic-host/src/index.tsx

@@ -0,0 +1,195 @@
+import { Component, type ErrorInfo, type ReactNode, StrictMode, Suspense, use, useEffect, useMemo, useRef, useState } from "react";
+import { createRoot } from "react-dom/client";
+import { callTool, connectToServer, hasAppHtml, initializeApp, loadSandboxProxy, log, newAppBridge, type ServerInfo, type ToolCallInfo } from "./implementation";
+import styles from "./index.module.css";
+
+
+const MCP_SERVER_URL = new URL("http://localhost:3001/mcp");
+
+
+interface HostProps {
+  serverInfoPromise: Promise<ServerInfo>;
+}
+function Host({ serverInfoPromise }: HostProps) {
+  const serverInfo = use(serverInfoPromise);
+  const [toolCallInfos, setToolCallInfos] = useState<ToolCallInfo[]>([]);
+
+  return (
+    <>
+      {toolCallInfos.map((info, i) => (
+        <ToolCallInfoPanel key={i} toolCallInfo={info} />
+      ))}
+      <CallToolPanel
+        serverInfo={serverInfo}
+        addToolCallInfo={(info) => setToolCallInfos([...toolCallInfos, info])}
+      />
+    </>
+  );
+}
+
+
+interface CallToolPanelProps {
+  serverInfo: ServerInfo;
+  addToolCallInfo: (toolCallInfo: ToolCallInfo) => void;
+}
+function CallToolPanel({ serverInfo, addToolCallInfo }: CallToolPanelProps) {
+  const toolNames = Array.from(serverInfo.tools.keys());
+  const [selectedTool, setSelectedTool] = useState(toolNames[0] ?? "");
+  const [inputJson, setInputJson] = useState("{}");
+
+  const isValidJson = useMemo(() => {
+    try {
+      JSON.parse(inputJson);
+      return true;
+    } catch {
+      return false;
+    }
+  }, [inputJson]);
+
+  const handleSubmit = () => {
+    const toolCallInfo = callTool(serverInfo, selectedTool, JSON.parse(inputJson));
+    addToolCallInfo(toolCallInfo);
+  };
+
+  return (
+    <div className={styles.callToolPanel}>
+      <form onSubmit={(e) => { e.preventDefault(); handleSubmit(); }}>
+        <label>
+          Tool Name
+          <select
+            value={selectedTool}
+            onChange={(e) => setSelectedTool(e.target.value)}
+          >
+            {toolNames.map((name) => (
+              <option key={name} value={name}>{name}</option>
+            ))}
+          </select>
+        </label>
+        <label>
+          Tool Input
+          <textarea
+            aria-invalid={!isValidJson}
+            value={inputJson}
+            onChange={(e) => setInputJson(e.target.value)}
+          />
+        </label>
+        <button type="submit" disabled={!selectedTool || !isValidJson}>
+          Call Tool
+        </button>
+      </form>
+    </div>
+  );
+}
+
+
+interface ToolCallInfoPanelProps {
+  toolCallInfo: ToolCallInfo;
+}
+function ToolCallInfoPanel({ toolCallInfo }: ToolCallInfoPanelProps) {
+  return (
+    <div className={styles.toolCallInfoPanel}>
+      <div className={styles.inputInfoPanel}>
+        <h2 className={styles.toolName}>{toolCallInfo.tool.name}</h2>
+        <JsonBlock value={toolCallInfo.input} />
+      </div>
+      <div className={styles.outputInfoPanel}>
+        <ErrorBoundary>
+          <Suspense fallback="Loading...">
+            {
+              hasAppHtml(toolCallInfo)
+                ? <AppIFramePanel toolCallInfo={toolCallInfo} />
+                : <ToolResultPanel toolCallInfo={toolCallInfo} />
+            }
+          </Suspense>
+        </ErrorBoundary>
+      </div>
+    </div>
+  );
+}
+
+
+function JsonBlock({ value }: { value: object }) {
+  return (
+    <pre className={styles.jsonBlock}>
+      <code>{JSON.stringify(value, null, 2)}</code>
+    </pre>
+  );
+}
+
+
+interface AppIFramePanelProps {
+  toolCallInfo: Required<ToolCallInfo>;
+}
+function AppIFramePanel({ toolCallInfo }: AppIFramePanelProps) {
+  const iframeRef = useRef<HTMLIFrameElement | null>(null);
+
+  useEffect(() => {
+    const iframe = iframeRef.current!;
+    loadSandboxProxy(iframe).then((firstTime) => {
+      // The `firstTime` check guards against React Strict Mode's double
+      // invocation (mount → unmount → remount simulation in development).
+      // Outside of Strict Mode, this `useEffect` runs only once per
+      // `toolCallInfo`.
+      if (firstTime) {
+        const appBridge = newAppBridge(toolCallInfo.serverInfo, iframe);
+        initializeApp(iframe, appBridge, toolCallInfo);
+      }
+    });
+  }, [toolCallInfo]);
+
+  return (
+    <div className={styles.appIframePanel}>
+      <iframe ref={iframeRef} />
+    </div>
+  );
+}
+
+
+interface ToolResultPanelProps {
+  toolCallInfo: ToolCallInfo;
+}
+function ToolResultPanel({ toolCallInfo }: ToolResultPanelProps) {
+  const result = use(toolCallInfo.resultPromise);
+  return <JsonBlock value={result} />;
+}
+
+
+interface ErrorBoundaryProps {
+  children: ReactNode;
+}
+interface ErrorBoundaryState {
+  hasError: boolean;
+  error: unknown;
+}
+class ErrorBoundary extends Component<ErrorBoundaryProps, ErrorBoundaryState> {
+  state: ErrorBoundaryState = { hasError: false, error: undefined };
+
+  // Called during render phase - must be pure (no side effects)
+  // Note: error is `unknown` because JS allows throwing any value
+  static getDerivedStateFromError(error: unknown): ErrorBoundaryState {
+    return { hasError: true, error };
+  }
+
+  // Called during commit phase - can have side effects (logging, etc.)
+  componentDidCatch(error: unknown, errorInfo: ErrorInfo): void {
+    log.error("Caught:", error, errorInfo.componentStack);
+  }
+
+  render(): ReactNode {
+    if (this.state.hasError) {
+      const { error } = this.state;
+      const message = error instanceof Error ? error.message : String(error);
+      return <div className={styles.error}><strong>ERROR:</strong> {message}</div>;
+    }
+    return this.props.children;
+  }
+}
+
+
+createRoot(document.getElementById("root")!).render(
+  <StrictMode>
+    <Suspense fallback={<p>Connecting to server ({MCP_SERVER_URL.href})...</p>}>
+      <Host serverInfoPromise={connectToServer(MCP_SERVER_URL)} />
+    </Suspense>
+  </StrictMode>,
+);

examples/basic-host/src/sandbox.ts

@@ -0,0 +1,87 @@
+import type { McpUiSandboxProxyReadyNotification, McpUiSandboxResourceReadyNotification } from "../../../dist/src/types";
+
+const ALLOWED_REFERRER_PATTERN = /^http:\/\/(localhost|127\.0\.0\.1)(:|\/|$)/;
+
+if (window.self === window.top) {
+  throw new Error("This file is only to be used in an iframe sandbox.");
+}
+
+if (!document.referrer) {
+  throw new Error("No referrer, cannot validate embedding site.");
+}
+
+if (!document.referrer.match(ALLOWED_REFERRER_PATTERN)) {
+  throw new Error(
+    `Embedding domain not allowed in referrer ${document.referrer}. (Consider updating the validation logic to allow your domain.)`,
+  );
+}
+
+// Security self-test: verify iframe isolation is working correctly.
+// This MUST throw a SecurityError -- if `window.top` is accessible, the sandbox
+// configuration is dangerously broken and untrusted content could escape.
+try {
+  window.top!.alert("If you see this, the sandbox is not setup securely.");
+  throw "FAIL";
+} catch (e) {
+  if (e === "FAIL") {
+    throw new Error("The sandbox is not setup securely.");
+  }
+
+  // Expected: SecurityError confirms proper sandboxing.
+}
+
+// Double-iframe sandbox architecture: THIS file is the outer sandbox proxy
+// iframe on a separate origin. It creates an inner iframe for untrusted HTML
+// content. Per the specification, the Host and the Sandbox MUST have different
+// origins.
+const inner = document.createElement("iframe");
+inner.style = "width:100%; height:100%; border:none;";
+inner.setAttribute("sandbox", "allow-scripts allow-same-origin allow-forms");
+document.body.appendChild(inner);
+
+const RESOURCE_READY_NOTIFICATION: McpUiSandboxResourceReadyNotification["method"] =
+  "ui/notifications/sandbox-resource-ready";
+const PROXY_READY_NOTIFICATION: McpUiSandboxProxyReadyNotification["method"] =
+  "ui/notifications/sandbox-proxy-ready";
+
+// Message relay: This Sandbox (outer iframe) acts as a bidirectional bridge,
+// forwarding messages between:
+//
+//   Host (parent window) ↔ Sandbox (outer frame) ↔ Guest UI (inner iframe)
+//
+// Reason: the parent window and inner iframe have different origins and can't
+// communicate directly, so the outer iframe forwards messages in both
+// directions to connect them.
+//
+// Special case: The "ui/notifications/sandbox-proxy-ready" message is
+// intercepted here (not relayed) because the Sandbox uses it to configure and
+// load the inner iframe with the Guest UI HTML content.
+window.addEventListener("message", async (event) => {
+  if (event.source === window.parent) {
+    // NOTE: In production you'll also want to validate `event.origin` against
+    // your Host domain.
+    if (event.data && event.data.method === RESOURCE_READY_NOTIFICATION) {
+      const { html, sandbox } = event.data.params;
+      if (typeof sandbox === "string") {
+        inner.setAttribute("sandbox", sandbox);
+      }
+      if (typeof html === "string") {
+        inner.srcdoc = html;
+      }
+    } else {
+      if (inner && inner.contentWindow) {
+        inner.contentWindow.postMessage(event.data, "*");
+      }
+    }
+  } else if (event.source === inner.contentWindow) {
+    // Relay messages from inner frame to parent window.
+    window.parent.postMessage(event.data, "*");
+  }
+});
+
+// Notify the Host that the Sandbox is ready to receive Guest UI HTML.
+window.parent.postMessage({
+  jsonrpc: "2.0",
+  method: PROXY_READY_NOTIFICATION,
+  params: {},
+}, "*");

examples/basic-host/src/vite-env.d.ts

@@ -0,0 +1 @@
+/// <reference types="vite/client" />