feat: add App.onteardown handler for graceful shutdown

Wire the guest UI side of ui/resource-teardown request:

- Add onteardown setter in App class for guest UIs to handle teardown
- Allow ui/resource-teardown in assertRequestHandlerCapability
- Import McpUiResourceTeardownRequest/Result types
- Add tests for sync and async cleanup handlers

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

Co-Authored-By: Claude <[email protected]>

Author: ochafik

src/app-bridge.test.ts

@@ -203,6 +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.ts

@@ -31,6 +31,9 @@ import {
   McpUiMessageResultSchema,
   McpUiOpenLinkRequest,
   McpUiOpenLinkResultSchema,
+  McpUiResourceTeardownRequest,
+  McpUiResourceTeardownRequestSchema,
+  McpUiResourceTeardownResult,
   McpUiSizeChangedNotification,
   McpUiToolCancelledNotification,
   McpUiToolCancelledNotificationSchema,
@@ -464,6 +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.
    *
@@ -570,6 +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`);