fix(react): StrictMode cleanup + relax late-handler guard for re-reg (#631)

* fix(react): close App on useApp unmount; relax late-handler guard for re-registration

useApp's effect cleanup now closes the App (and its transport) instead
of only flipping a `mounted` flag. Under React StrictMode dev
double-invoke, the abandoned first instance's PostMessageTransport
otherwise keeps a window `message` listener and receives every host
postMessage alongside the second instance.

The late-handler guard from #629 now tracks `_everHadListener` per
event and only flags the *first* registration of a one-shot event
post-handshake. Re-registration (React useEffect cleanup + re-add on
dep change) of an event that had a pre-connect handler is silent,
including under `strict: true`.

* feat(app-bridge): warn on second ui/initialize

If a View double-mounts (e.g. React StrictMode in dev) without closing
the previous App, AppBridge now logs a console.warn when it receives a
second ui/initialize. Behavior is unchanged — it still responds and the
latest appInfo/appCapabilities replace the previous values — this is
purely a diagnostic for host implementers.

src/app-bridge.test.ts

@@ -15,6 +15,7 @@ import {
 import { z } from "zod/v4";
 
 import { App } from "./app";
+import { LATEST_PROTOCOL_VERSION } from "./types";
 import {
   AppBridge,
   getToolUiResourceUri,
@@ -2021,6 +2022,45 @@ describe("App <-> AppBridge integration", () => {
           expect(warnings()).toEqual([]);
         });
 
+        it("does not warn on re-registration when a handler existed pre-connect", async () => {
+          const h = () => {};
+          app.addEventListener("toolresult", h);
+          await bridge.connect(bridgeTransport);
+          await app.connect(appTransport);
+
+          // React-style: useEffect cleanup removes, dep change re-adds.
+          app.removeEventListener("toolresult", h);
+          app.addEventListener("toolresult", () => {});
+
+          expect(warnings().filter((m) => lateMsg.test(m))).toEqual([]);
+        });
+
+        it("warns once for the first late registration, not on subsequent ones", async () => {
+          await bridge.connect(bridgeTransport);
+          await app.connect(appTransport);
+
+          app.addEventListener("toolinput", () => {});
+          app.addEventListener("toolinput", () => {});
+          app.addEventListener("toolinput", () => {});
+
+          const late = warnings().filter((m) => lateMsg.test(m));
+          expect(late).toHaveLength(1);
+          expect(late[0]).toContain('"toolinput"');
+        });
+
+        it("tracks first-handler per event independently", async () => {
+          app.addEventListener("toolinput", () => {});
+          await bridge.connect(bridgeTransport);
+          await app.connect(appTransport);
+
+          app.addEventListener("toolinput", () => {}); // had pre-connect handler → silent
+          app.addEventListener("toolresult", () => {}); // first reg, late → warns
+
+          const late = warnings().filter((m) => lateMsg.test(m));
+          expect(late).toHaveLength(1);
+          expect(late[0]).toContain('"toolresult"');
+        });
+
         it("throws instead of warning when strict: true", async () => {
           const [strictAppT, strictBridgeT] =
             InMemoryTransport.createLinkedPair();
@@ -2034,6 +2074,9 @@ describe("App <-> AppBridge integration", () => {
             {},
             { autoResize: false, strict: true },
           );
+          // Pre-connect registration for toolcancelled — later swap must not throw.
+          strictApp.addEventListener("toolcancelled", () => {});
+
           await strictBridge.connect(strictBridgeT);
           await strictApp.connect(strictAppT);
 
@@ -2043,10 +2086,57 @@ describe("App <-> AppBridge integration", () => {
           expect(() => {
             strictApp.addEventListener("toolinput", () => {});
           }).toThrow(lateMsg);
-          expect(warnings()).toEqual([]);
+          // Swapping a handler that existed pre-connect is allowed under strict.
+          expect(() => {
+            strictApp.addEventListener("toolcancelled", () => {});
+          }).not.toThrow();
+          expect(warnings().filter((m) => lateMsg.test(m))).toEqual([]);
         });
       });
 
+      it("AppBridge warns on a second ui/initialize (View double-mount)", async () => {
+        await bridge.connect(bridgeTransport);
+        await app.connect(appTransport);
+        expect(warnings()).toEqual([]);
+
+        // Simulate a second View instance re-running the handshake.
+        appTransport.send({
+          jsonrpc: "2.0",
+          id: 99,
+          method: "ui/initialize",
+          params: {
+            protocolVersion: LATEST_PROTOCOL_VERSION,
+            appInfo: testAppInfo,
+            appCapabilities: {},
+          },
+        });
+        await flush();
+
+        const doubleInit = warnings().filter((m) =>
+          /second ui\/initialize/.test(m),
+        );
+        expect(doubleInit).toHaveLength(1);
+      });
+
+      it("close() stops further notification delivery (StrictMode cleanup relies on this)", async () => {
+        const received: unknown[] = [];
+        app.addEventListener("toolresult", (r) => received.push(r));
+        await bridge.connect(bridgeTransport);
+        await app.connect(appTransport);
+
+        await bridge.sendToolResult({
+          content: [{ type: "text", text: "before" }],
+        });
+        expect(received).toHaveLength(1);
+
+        await app.close();
+
+        await bridge
+          .sendToolResult({ content: [{ type: "text", text: "after" }] })
+          .catch(() => {});
+        expect(received).toHaveLength(1);
+      });
+
       it("AppBridge warns on tools/call from a View that skipped the handshake", async () => {
         bridge.oncalltool = async () => ({ content: [] });
         await bridge.connect(bridgeTransport);

src/app-bridge.ts

@@ -1463,6 +1463,15 @@ export class AppBridge extends ProtocolWithEvents<
   ): Promise<McpUiInitializeResult> {
     const requestedVersion = request.params.protocolVersion;
 
+    if (this._appInfo !== undefined) {
+      console.warn(
+        "[ext-apps] AppBridge received a second ui/initialize. The View may " +
+          "be double-mounting (e.g. React StrictMode in dev) without closing " +
+          "the previous App instance. Responding normally; the latest " +
+          "appInfo/appCapabilities replace the previous values.",
+      );
+    }
+
     this._appCapabilities = request.params.appCapabilities;
     this._appInfo = request.params.appInfo;
 

src/app.ts

@@ -395,14 +395,28 @@ export class App extends ProtocolWithEvents<
     new Set(["toolinput", "toolinputpartial", "toolresult", "toolcancelled"]);
 
   /**
-   * Warn (or throw under `strict`) when a one-shot event handler is registered
+   * One-shot events that have had at least one handler registered (via `on*`
-   * after the `ui/initialize` → `ui/notifications/initialized` handshake has
+   * setter or `addEventListener`) at any point. Once an event is in this set,
-   * completed. The host may have already fired the notification by then.
+   * subsequent late registrations are not flagged — only the *first* handler
+   * matters for the missed-notification race, and re-registration (e.g. React
+   * `useEffect` cleanup → re-add on dep change) is a legitimate pattern.
+   */
+  private readonly _everHadListener = new Set<keyof AppEventMap>();
+
+  /**
+   * Warn (or throw under `strict`) when the *first* handler for a one-shot
+   * event is registered after the `ui/initialize` → `ui/notifications/initialized`
+   * handshake has completed. The host may have already fired the notification
+   * by then. Subsequent registrations for the same event are not flagged.
    *
    * Mirrors {@link _assertInitialized `_assertInitialized`} (the outbound-side guard).
    */
   private _assertHandlerTiming(event: keyof AppEventMap): void {
-    if (!this._initializedSent || !App.ONE_SHOT_EVENTS.has(event)) return;
+    if (!App.ONE_SHOT_EVENTS.has(event) || this._everHadListener.has(event)) {
+      return;
+    }
+    this._everHadListener.add(event);
+    if (!this._initializedSent) return;
     const msg =
       `[ext-apps] "${String(event)}" handler registered after connect() ` +
       `completed the ui/initialize handshake. The host may have already sent ` +

src/react/useApp.tsx

@@ -138,6 +138,7 @@ export function useApp({
 
   useEffect(() => {
     let mounted = true;
+    let appInstance: App | undefined;
 
     async function connect() {
       try {
@@ -145,18 +146,24 @@ export function useApp({
           window.parent,
           window.parent,
         );
-        const app = new App(appInfo, capabilities, { autoResize, strict });
+        appInstance = new App(appInfo, capabilities, { autoResize, strict });
 
         // Register handlers BEFORE connecting
-        onAppCreated?.(app);
+        onAppCreated?.(appInstance);
 
-        await app.connect(transport);
+        await appInstance.connect(transport);
 
-        if (mounted) {
+        if (!mounted) {
-          setApp(app);
+          // Unmounted during the handshake (e.g. React StrictMode dev
-          setIsConnected(true);
+          // double-invoke). Close so the abandoned transport's window
-          setError(null);
+          // `message` listener doesn't keep receiving host postMessages
+          // alongside the second mount's instance.
+          void appInstance.close();
+          return;
         }
+        setApp(appInstance);
+        setIsConnected(true);
+        setError(null);
       } catch (error) {
         if (mounted) {
           setApp(null);
@@ -172,6 +179,7 @@ export function useApp({
 
     return () => {
       mounted = false;
+      void appInstance?.close();
     };
   }, []); // Intentionally not including options to avoid reconnection