diff --git a/src/app.test.ts b/src/app.test.ts
index 8420e81ea..7081bd0ee 100644
--- a/src/app.test.ts
+++ b/src/app.test.ts
@@ -234,6 +234,43 @@ describe("App base MCP SDK v2 Protocol migration", () => {
     expect(setupCalls).toBe(1);
   });
 
+  for (const closingSide of ["app", "server"] as const) {
+    it(`cleans up automatic resize when ${closingSide} closes`, async () => {
+      const app = new App({ name: "view-app", version: "1.0.0" });
+      let activeObservers = 0;
+      let cleanupCalls = 0;
+      app.setupSizeChangedNotifications = () => {
+        activeObservers += 1;
+        return () => {
+          activeObservers -= 1;
+          cleanupCalls += 1;
+        };
+      };
+      let closeCallbacks = 0;
+      app.onclose = () => {
+        closeCallbacks += 1;
+      };
+
+      const first = await connectPair(app);
+      connected.push(first);
+      expect(activeObservers).toBe(1);
+      await first[closingSide].close();
+      expect(activeObservers).toBe(0);
+      expect(cleanupCalls).toBe(1);
+      expect(closeCallbacks).toBe(1);
+      await app.close();
+      expect(cleanupCalls).toBe(1);
+
+      const second = await connectPair(app);
+      connected.push(second);
+      expect(activeObservers).toBe(1);
+      await second[closingSide].close();
+      expect(activeObservers).toBe(0);
+      expect(cleanupCalls).toBe(2);
+      expect(closeCallbacks).toBe(2);
+    });
+  }
+
   it("preserves strict late-handler timing errors", async () => {
     const app = new App(
       { name: "view-app", version: "1.0.0" },
diff --git a/src/app.ts b/src/app.ts
index 89c6c35e7..870084cb9 100644
--- a/src/app.ts
+++ b/src/app.ts
@@ -565,6 +565,18 @@ export class App extends Protocol<BaseContext> {
     this.setEventHandler("hostcontextchanged", undefined);
   }
 
+  private _autoResizeCleanup?: () => void;
+
+  protected override _onclose(): void {
+    const cleanup = this._autoResizeCleanup;
+    this._autoResizeCleanup = undefined;
+    try {
+      cleanup?.();
+    } finally {
+      super._onclose();
+    }
+  }
+
   /** @internal */
   protected buildContext(ctx: BaseContext): BaseContext {
     return ctx;
@@ -2045,7 +2057,7 @@ export class App extends Protocol<BaseContext> {
       this._initializedSent = true;
 
       if (this.options?.autoResize) {
-        this.setupSizeChangedNotifications();
+        this._autoResizeCleanup = this.setupSizeChangedNotifications();
       }
     } catch (error) {
       // Disconnect if initialization fails.