diff --git a/src/ZB.MOM.WW.ScadaBridge.CentralUI/Components/Pages/Monitoring/AlarmSummary.razor b/src/ZB.MOM.WW.ScadaBridge.CentralUI/Components/Pages/Monitoring/AlarmSummary.razor
index 536bdf1c..b3987202 100644
--- a/src/ZB.MOM.WW.ScadaBridge.CentralUI/Components/Pages/Monitoring/AlarmSummary.razor
+++ b/src/ZB.MOM.WW.ScadaBridge.CentralUI/Components/Pages/Monitoring/AlarmSummary.razor
@@ -291,6 +291,13 @@
         try
         {
             var result = await AlarmSummaryService.GetSiteAlarmsAsync(siteId);
+            // Stale-site guard (arch-review R2 N4): the operator may have switched sites
+            // while this fan-out was in flight — drop the result rather than labeling
+            // site A's alarms under site B's picker. Mirrors OnLiveAlarmsChanged (:374).
+            if (_selectedSiteId != siteId)
+            {
+                return;
+            }
             _rows = result.Alarms;
             _notReporting = result.NotReportingInstances;
             _rollup = AlarmSummaryService.ComputeRollup(_rows);
diff --git a/tests/ZB.MOM.WW.ScadaBridge.CentralUI.Tests/Monitoring/AlarmSummaryRenderTests.cs b/tests/ZB.MOM.WW.ScadaBridge.CentralUI.Tests/Monitoring/AlarmSummaryRenderTests.cs
index 51cfefc6..5d253e0b 100644
--- a/tests/ZB.MOM.WW.ScadaBridge.CentralUI.Tests/Monitoring/AlarmSummaryRenderTests.cs
+++ b/tests/ZB.MOM.WW.ScadaBridge.CentralUI.Tests/Monitoring/AlarmSummaryRenderTests.cs
@@ -208,6 +208,42 @@ public class AlarmSummaryRenderTests : BunitContext
         Assert.True(_liveCache.DisposeCount >= 1);
     }
 
+    // ── arch-review R2 N4: stale-site poll guard ───────────────────────────────
+
+    [Fact]
+    public void InFlightPollForPreviousSite_DoesNotOverwriteNewSitesRows()
+    {
+        // Site 1's fan-out hangs on a TCS; site 2 answers immediately (arch-review R2 N4).
+        var site1Tcs = new TaskCompletionSource<AlarmSummaryResult>();
+        _summary.GetSiteAlarmsAsync(1, Arg.Any<CancellationToken>()).Returns(site1Tcs.Task);
+        _summary.GetSiteAlarmsAsync(2, Arg.Any<CancellationToken>()).Returns(Task.FromResult(
+            new AlarmSummaryResult(
+                new List<AlarmSummaryRow>
+                {
+                    new("Site2Instance", new AlarmStateChanged("Site2Instance", "S2-alarm", AlarmState.Active, 100, DateTimeOffset.UtcNow)),
+                },
+                Array.Empty<string>())));
+
+        var cut = Render<AlarmSummaryPage>();
+        cut.Find("[data-test='alarm-summary-site']").Change("1"); // poll in flight, hung
+        cut.Find("[data-test='alarm-summary-site']").Change("2"); // site 2 renders
+        cut.WaitForAssertion(() => Assert.Equal("Site2Instance", FirstRowInstance(cut)));
+
+        // The stale site-1 fan-out now completes — it must be dropped, not rendered.
+        site1Tcs.SetResult(new AlarmSummaryResult(
+            new List<AlarmSummaryRow>
+            {
+                new("StaleSite1", new AlarmStateChanged("StaleSite1", "S1-alarm", AlarmState.Active, 999, DateTimeOffset.UtcNow)),
+            },
+            new[] { "StaleNotReporting" }));
+
+        cut.WaitForAssertion(() =>
+        {
+            Assert.Equal("Site2Instance", FirstRowInstance(cut));
+            Assert.DoesNotContain("StaleNotReporting", cut.Markup);
+        });
+    }
+
     /// <summary>
     /// Controllable in-memory fake of <see cref="ISiteAlarmLiveCache"/>: records
     /// subscribe/dispose counts and lets a test push a live snapshot (which flips