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.

Author: ochafik

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,
@@ -2093,6 +2094,30 @@ describe("App <-> AppBridge integration", () => {
         });
       });
 
+      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));

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;