diff --git a/src/ZB.MOM.WW.ScadaBridge.StoreAndForward/StoreAndForwardService.cs b/src/ZB.MOM.WW.ScadaBridge.StoreAndForward/StoreAndForwardService.cs
index d2aad773..77527dd5 100644
--- a/src/ZB.MOM.WW.ScadaBridge.StoreAndForward/StoreAndForwardService.cs
+++ b/src/ZB.MOM.WW.ScadaBridge.StoreAndForward/StoreAndForwardService.cs
@@ -85,6 +85,21 @@ public class StoreAndForwardService
     private Timer? _retryTimer;
     private int _retryInProgress;
 
+    /// <summary>
+    /// Active-node delivery gate. When set, the retry sweep runs ONLY when the
+    /// gate reports true — the standby node applies replicated buffer operations
+    /// but must never deliver (Component-StoreAndForward.md "standby-passive"
+    /// model). Re-evaluated on every sweep tick so a failover resumes delivery
+    /// within one RetryTimerInterval without re-wiring. Null (tests, central
+    /// hosts) preserves the ungated legacy behaviour. A throwing gate is treated
+    /// as standby — safe-by-default, mirroring SiteCommunicationActor's
+    /// DefaultIsActiveCheck fallback.
+    /// </summary>
+    private Func<bool>? _deliveryGate;
+
+    /// <summary>Installs the active-node delivery gate (see <see cref="_deliveryGate"/>).</summary>
+    public void SetDeliveryGate(Func<bool> gate) => _deliveryGate = gate;
+
     /// <summary>
     /// The in-flight retry sweep <see cref="Task"/>, or
     /// <c>null</c> when no sweep is currently running. Captured when the timer
@@ -568,6 +583,24 @@ public class StoreAndForwardService
 
         try
         {
+            var gate = _deliveryGate;
+            if (gate != null)
+            {
+                bool isActive;
+                try { isActive = gate(); }
+                catch (Exception ex)
+                {
+                    _logger.LogWarning(ex,
+                        "S&F delivery gate threw; treating this node as standby for this sweep");
+                    isActive = false;
+                }
+                if (!isActive)
+                {
+                    _logger.LogDebug("S&F retry sweep skipped: this node is not the active site node");
+                    return;
+                }
+            }
+
             var messages = await _storage.GetMessagesForRetryAsync();
             if (messages.Count == 0) return;
 
diff --git a/tests/ZB.MOM.WW.ScadaBridge.StoreAndForward.Tests/StoreAndForwardServiceTests.cs b/tests/ZB.MOM.WW.ScadaBridge.StoreAndForward.Tests/StoreAndForwardServiceTests.cs
index 9d479afc..24454a27 100644
--- a/tests/ZB.MOM.WW.ScadaBridge.StoreAndForward.Tests/StoreAndForwardServiceTests.cs
+++ b/tests/ZB.MOM.WW.ScadaBridge.StoreAndForward.Tests/StoreAndForwardServiceTests.cs
@@ -582,4 +582,60 @@ public class StoreAndForwardServiceTests : IAsyncLifetime, IDisposable
         Assert.True(handlerCompleted,
             "Sweep handler must have finished before StopAsync returned.");
     }
+
+    // ── Task 3 (arch review 02, Stability #2): active-node delivery gate ──
+    // The standby site node applies replicated buffer operations but must never
+    // deliver; the sweep runs only when the gate reports the node is active.
+
+    [Fact]
+    public async Task RetrySweep_SkipsDelivery_WhenDeliveryGateReportsStandby()
+    {
+        var delivered = 0;
+        _service.RegisterDeliveryHandler(StoreAndForwardCategory.ExternalSystem,
+            _ => { delivered++; return Task.FromResult(true); });
+        _service.SetDeliveryGate(() => false); // standby
+
+        var result = await _service.EnqueueAsync(StoreAndForwardCategory.ExternalSystem, "target-x", "{}",
+            attemptImmediateDelivery: false, retryInterval: TimeSpan.Zero);
+        await _service.RetryPendingMessagesAsync();
+
+        Assert.Equal(0, delivered);
+        var row = await _service.GetMessageByIdAsync(result.MessageId);
+        Assert.NotNull(row);
+        Assert.Equal(StoreAndForwardMessageStatus.Pending, row!.Status); // row untouched
+    }
+
+    [Fact]
+    public async Task RetrySweep_ResumesDelivery_WhenGateFlipsActive()
+    {
+        var delivered = 0;
+        var active = false;
+        _service.RegisterDeliveryHandler(StoreAndForwardCategory.ExternalSystem,
+            _ => { delivered++; return Task.FromResult(true); });
+        _service.SetDeliveryGate(() => active);
+
+        await _service.EnqueueAsync(StoreAndForwardCategory.ExternalSystem, "target-x", "{}",
+            attemptImmediateDelivery: false, retryInterval: TimeSpan.Zero);
+
+        await _service.RetryPendingMessagesAsync();
+        Assert.Equal(0, delivered);
+
+        active = true; // failover: this node became active
+        await _service.RetryPendingMessagesAsync();
+        Assert.Equal(1, delivered);
+    }
+
+    [Fact]
+    public async Task RetrySweep_TreatsThrowingGateAsStandby()
+    {
+        var delivered = 0;
+        _service.RegisterDeliveryHandler(StoreAndForwardCategory.ExternalSystem,
+            _ => { delivered++; return Task.FromResult(true); });
+        _service.SetDeliveryGate(() => throw new InvalidOperationException("cluster not ready"));
+
+        await _service.EnqueueAsync(StoreAndForwardCategory.ExternalSystem, "t", "{}",
+            attemptImmediateDelivery: false, retryInterval: TimeSpan.Zero);
+        await _service.RetryPendingMessagesAsync(); // must not throw
+        Assert.Equal(0, delivered);
+    }
 }