feat: implement ui/notifications/tool-cancelled + add missing App.onteardown

examples/basic-host/src/implementation.ts

@@ -186,11 +186,19 @@ export async function initializeApp(
   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);
-  });
+  // Schedule tool call result (or cancellation) to be sent to MCP App
+  resultPromise.then(
+    (result) => {
+      log.info("Sending tool call result to MCP App:", result);
+      appBridge.sendToolResult(result);
+    },
+    (error) => {
+      log.error("Tool call failed, sending cancellation to MCP App:", error);
+      appBridge.sendToolCancelled({
+        reason: error instanceof Error ? error.message : String(error),
+      });
+    },
+  );
 }
 
 /**

examples/basic-host/src/index.tsx

@@ -176,6 +176,7 @@ interface AppIFramePanelProps {
 }
 function AppIFramePanel({ toolCallInfo }: AppIFramePanelProps) {
   const iframeRef = useRef<HTMLIFrameElement | null>(null);
+  const appBridgeRef = useRef<ReturnType<typeof newAppBridge> | null>(null);
 
   useEffect(() => {
     const iframe = iframeRef.current!;
@@ -186,9 +187,20 @@ function AppIFramePanel({ toolCallInfo }: AppIFramePanelProps) {
       // `toolCallInfo`.
       if (firstTime) {
         const appBridge = newAppBridge(toolCallInfo.serverInfo, iframe);
+        appBridgeRef.current = appBridge;
         initializeApp(iframe, appBridge, toolCallInfo);
       }
     });
+
+    // Cleanup: send teardown notification before unmounting
+    return () => {
+      if (appBridgeRef.current) {
+        log.info("Sending teardown notification to MCP App");
+        appBridgeRef.current.sendResourceTeardown({}).catch((err) => {
+          log.warn("Teardown request failed (app may have already closed):", err);
+        });
+      }
+    };
   }, [toolCallInfo]);
 
   return (

src/app-bridge.test.ts

@@ -140,6 +140,36 @@ describe("App <-> AppBridge integration", () => {
       });
     });
 
+    it("sendToolCancelled triggers app.ontoolcancelled", async () => {
+      const receivedCancellations: unknown[] = [];
+      app.ontoolcancelled = (params) => {
+        receivedCancellations.push(params);
+      };
+
+      await app.connect(appTransport);
+      await bridge.sendToolCancelled({
+        reason: "User cancelled the operation",
+      });
+
+      expect(receivedCancellations).toHaveLength(1);
+      expect(receivedCancellations[0]).toEqual({
+        reason: "User cancelled the operation",
+      });
+    });
+
+    it("sendToolCancelled works without reason", async () => {
+      const receivedCancellations: unknown[] = [];
+      app.ontoolcancelled = (params) => {
+        receivedCancellations.push(params);
+      };
+
+      await app.connect(appTransport);
+      await bridge.sendToolCancelled({});
+
+      expect(receivedCancellations).toHaveLength(1);
+      expect(receivedCancellations[0]).toEqual({});
+    });
+
     it("setHostContext triggers app.onhostcontextchanged", async () => {
       const receivedContexts: unknown[] = [];
       app.onhostcontextchanged = (params) => {
@@ -173,6 +203,34 @@ describe("App <-> AppBridge integration", () => {
         { theme: "light" },
       ]);
     });
+
+    it("sendResourceTeardown triggers app.onteardown", async () => {
+      let teardownCalled = false;
+      app.onteardown = async () => {
+        teardownCalled = true;
+        return {};
+      };
+
+      await app.connect(appTransport);
+      await bridge.sendResourceTeardown({});
+
+      expect(teardownCalled).toBe(true);
+    });
+
+    it("sendResourceTeardown waits for async cleanup", async () => {
+      const cleanupSteps: string[] = [];
+      app.onteardown = async () => {
+        cleanupSteps.push("start");
+        await new Promise((resolve) => setTimeout(resolve, 10));
+        cleanupSteps.push("done");
+        return {};
+      };
+
+      await app.connect(appTransport);
+      await bridge.sendResourceTeardown({});
+
+      expect(cleanupSteps).toEqual(["start", "done"]);
+    });
   });
 
   describe("App -> Host notifications", () => {

src/app-bridge.ts

@@ -34,6 +34,7 @@ import {
 import {
   type McpUiSandboxResourceReadyNotification,
   type McpUiSizeChangedNotification,
+  type McpUiToolCancelledNotification,
   type McpUiToolInputNotification,
   type McpUiToolInputPartialNotification,
   type McpUiToolResultNotification,
@@ -737,6 +738,43 @@ export class AppBridge extends Protocol<Request, Notification, Result> {
     });
   }
 
+  /**
+   * Notify the Guest UI that tool execution was cancelled.
+   *
+   * The host MUST send this notification if tool execution was cancelled for any
+   * reason, including user action, sampling error, classifier intervention, or
+   * any other interruption. This allows the Guest UI to update its state and
+   * display appropriate feedback to the user.
+   *
+   * @param params - Optional cancellation details:
+   *   - `reason`: Human-readable explanation for why the tool was cancelled
+   *
+   * @example User-initiated cancellation
+   * ```typescript
+   * // User clicked "Cancel" button
+   * bridge.sendToolCancelled({ reason: "User cancelled the operation" });
+   * ```
+   *
+   * @example System-level cancellation
+   * ```typescript
+   * // Sampling error or timeout
+   * bridge.sendToolCancelled({ reason: "Request timeout after 30 seconds" });
+   *
+   * // Classifier intervention
+   * bridge.sendToolCancelled({ reason: "Content policy violation detected" });
+   * ```
+   *
+   * @see {@link McpUiToolCancelledNotification} for the notification type
+   * @see {@link sendToolResult} for sending successful results
+   * @see {@link sendToolInput} for sending tool arguments
+   */
+  sendToolCancelled(params: McpUiToolCancelledNotification["params"]) {
+    return this.notification(<McpUiToolCancelledNotification>{
+      method: "ui/notifications/tool-cancelled",
+      params,
+    });
+  }
+
   /**
    * Send HTML resource to the sandbox proxy for secure loading.
    *

src/app.ts

@@ -31,7 +31,12 @@ import {
   McpUiMessageResultSchema,
   McpUiOpenLinkRequest,
   McpUiOpenLinkResultSchema,
+  McpUiResourceTeardownRequest,
+  McpUiResourceTeardownRequestSchema,
+  McpUiResourceTeardownResult,
   McpUiSizeChangedNotification,
+  McpUiToolCancelledNotification,
+  McpUiToolCancelledNotificationSchema,
   McpUiToolInputNotification,
   McpUiToolInputNotificationSchema,
   McpUiToolInputPartialNotification,
@@ -388,6 +393,41 @@ export class App extends Protocol<Request, Notification, Result> {
     );
   }
 
+  /**
+   * Convenience handler for receiving tool cancellation notifications from the host.
+   *
+   * Set this property to register a handler that will be called when the host
+   * notifies that tool execution was cancelled. This can occur for various reasons
+   * including user action, sampling error, classifier intervention, or other
+   * interruptions. Apps should update their state and display appropriate feedback.
+   *
+   * This setter is a convenience wrapper around `setNotificationHandler()` that
+   * automatically handles the notification schema and extracts the params for you.
+   *
+   * Register handlers before calling {@link connect} to avoid missing notifications.
+   *
+   * @param callback - Function called when tool execution is cancelled
+   *
+   * @example Handle tool cancellation
+   * ```typescript
+   * app.ontoolcancelled = (params) => {
+   *   console.log("Tool cancelled:", params.reason);
+   *   showCancelledMessage(params.reason ?? "Operation was cancelled");
+   * };
+   * ```
+   *
+   * @see {@link setNotificationHandler} for the underlying method
+   * @see {@link McpUiToolCancelledNotification} for the notification structure
+   * @see {@link ontoolresult} for successful tool completion
+   */
+  set ontoolcancelled(
+    callback: (params: McpUiToolCancelledNotification["params"]) => void,
+  ) {
+    this.setNotificationHandler(McpUiToolCancelledNotificationSchema, (n) =>
+      callback(n.params),
+    );
+  }
+
   /**
    * Convenience handler for host context changes (theme, viewport, locale, etc.).
    *
@@ -427,6 +467,48 @@ export class App extends Protocol<Request, Notification, Result> {
     );
   }
 
+  /**
+   * Convenience handler for graceful shutdown requests from the host.
+   *
+   * Set this property to register a handler that will be called when the host
+   * requests the app to prepare for teardown. This allows the app to perform
+   * cleanup operations (save state, close connections, etc.) before being unmounted.
+   *
+   * The handler can be sync or async. The host will wait for the returned promise
+   * to resolve before proceeding with teardown.
+   *
+   * This setter is a convenience wrapper around `setRequestHandler()` that
+   * automatically handles the request schema.
+   *
+   * Register handlers before calling {@link connect} to avoid missing requests.
+   *
+   * @param callback - Function called when teardown is requested.
+   *   Can return void or a Promise that resolves when cleanup is complete.
+   *
+   * @example Perform cleanup before teardown
+   * ```typescript
+   * app.onteardown = async () => {
+   *   await saveState();
+   *   closeConnections();
+   *   console.log("App ready for teardown");
+   * };
+   * ```
+   *
+   * @see {@link setRequestHandler} for the underlying method
+   * @see {@link McpUiResourceTeardownRequest} for the request structure
+   */
+  set onteardown(
+    callback: (
+      params: McpUiResourceTeardownRequest["params"],
+      extra: RequestHandlerExtra,
+    ) => McpUiResourceTeardownResult | Promise<McpUiResourceTeardownResult>,
+  ) {
+    this.setRequestHandler(
+      McpUiResourceTeardownRequestSchema,
+      (request, extra) => callback(request.params, extra),
+    );
+  }
+
   /**
    * Convenience handler for tool call requests from the host.
    *
@@ -533,6 +615,7 @@ export class App extends Protocol<Request, Notification, Result> {
         }
         return;
       case "ping":
+      case "ui/resource-teardown":
         return;
       default:
         throw new Error(`No handler for method ${method} registered`);

src/generated/schema.json

@@ -1732,6 +1732,28 @@
         }
       ]
     },
+    "McpUiToolCancelledNotification": {
+      "$schema": "https://json-schema.org/draft/2020-12/schema",
+      "type": "object",
+      "properties": {
+        "method": {
+          "type": "string",
+          "const": "ui/notifications/tool-cancelled"
+        },
+        "params": {
+          "type": "object",
+          "properties": {
+            "reason": {
+              "description": "Optional reason for the cancellation (e.g., \"user action\", \"timeout\").",
+              "type": "string"
+            }
+          },
+          "additionalProperties": false
+        }
+      },
+      "required": ["method", "params"],
+      "additionalProperties": false
+    },
     "McpUiToolInputNotification": {
       "$schema": "https://json-schema.org/draft/2020-12/schema",
       "type": "object",

src/generated/schema.test.ts

@@ -51,6 +51,10 @@ export type McpUiToolInputPartialNotificationSchemaInferredType = z.infer<
   typeof generated.McpUiToolInputPartialNotificationSchema
 >;
 
+export type McpUiToolCancelledNotificationSchemaInferredType = z.infer<
+  typeof generated.McpUiToolCancelledNotificationSchema
+>;
+
 export type McpUiResourceTeardownRequestSchemaInferredType = z.infer<
   typeof generated.McpUiResourceTeardownRequestSchema
 >;
@@ -151,6 +155,12 @@ expectType<spec.McpUiToolInputPartialNotification>(
 expectType<McpUiToolInputPartialNotificationSchemaInferredType>(
   {} as spec.McpUiToolInputPartialNotification,
 );
+expectType<spec.McpUiToolCancelledNotification>(
+  {} as McpUiToolCancelledNotificationSchemaInferredType,
+);
+expectType<McpUiToolCancelledNotificationSchemaInferredType>(
+  {} as spec.McpUiToolCancelledNotification,
+);
 expectType<spec.McpUiResourceTeardownRequest>(
   {} as McpUiResourceTeardownRequestSchemaInferredType,
 );

src/generated/schema.ts

@@ -160,6 +160,24 @@ export const McpUiToolInputPartialNotificationSchema = z.object({
   }),
 });
 
+/**
+ * @description Notification that tool execution was cancelled (Host -> Guest UI).
+ * Host MUST send this if tool execution was cancelled for any reason (user action,
+ * sampling error, classifier intervention, etc.).
+ */
+export const McpUiToolCancelledNotificationSchema = z.object({
+  method: z.literal("ui/notifications/tool-cancelled"),
+  params: z.object({
+    /** @description Optional reason for the cancellation (e.g., "user action", "timeout"). */
+    reason: z
+      .string()
+      .optional()
+      .describe(
+        'Optional reason for the cancellation (e.g., "user action", "timeout").',
+      ),
+  }),
+});
+
 /**
  * @description Request for graceful shutdown of the Guest UI (Host -> Guest UI).
  * @see {@link app-bridge.AppBridge.sendResourceTeardown} for the host method that sends this

src/spec.types.ts

@@ -167,6 +167,19 @@ export interface McpUiToolResultNotification {
   params: CallToolResult;
 }
 
+/**
+ * @description Notification that tool execution was cancelled (Host -> Guest UI).
+ * Host MUST send this if tool execution was cancelled for any reason (user action,
+ * sampling error, classifier intervention, etc.).
+ */
+export interface McpUiToolCancelledNotification {
+  method: "ui/notifications/tool-cancelled";
+  params: {
+    /** @description Optional reason for the cancellation (e.g., "user action", "timeout"). */
+    reason?: string;
+  };
+}
+
 /**
  * @description Rich context about the host environment provided to Guest UIs.
  */