diff --git a/src/ZB.MOM.WW.ScadaBridge.Communication/ClusterState/ActiveNodeEvaluator.cs b/src/ZB.MOM.WW.ScadaBridge.Communication/ClusterState/ActiveNodeEvaluator.cs
new file mode 100644
index 00000000..57d7b425
--- /dev/null
+++ b/src/ZB.MOM.WW.ScadaBridge.Communication/ClusterState/ActiveNodeEvaluator.cs
@@ -0,0 +1,39 @@
+using Akka.Cluster;
+
+namespace ZB.MOM.WW.ScadaBridge.Communication.ClusterState;
+
+/// <summary>
+/// THE single definition of "active node" (review 01 [High]; review 02 round 2 [Critical] N1):
+/// a node is active when it is the OLDEST Up member (optionally within a role scope) — i.e.
+/// the member the ClusterSingletonManager places singletons on. Cluster LEADERSHIP (lowest
+/// address) is an Akka-internal concept that diverges from singleton placement permanently
+/// once the original first node restarts and rejoins; every product-level active/standby
+/// decision must use this evaluator, never <c>cluster.State.Leader</c>.
+/// <para>
+/// Lives in Communication (not Host) so BOTH <c>SiteCommunicationActor</c> and
+/// <c>SiteReplicationActor</c> can default to it — Host cannot be referenced from either.
+/// The Host's <c>ClusterActivityEvaluator.SelfIsOldest</c> delegates here, so the S&amp;F
+/// delivery gate (<c>IClusterNodeProvider.SelfIsPrimary</c>), the resync authority checks,
+/// and the heartbeat IsActive stamp all share one implementation.
+/// </para>
+/// </summary>
+public static class ActiveNodeEvaluator
+{
+    /// <summary>True when self is Up and no other Up member (in the role scope) is older.</summary>
+    /// <param name="cluster">The Akka cluster to evaluate.</param>
+    /// <param name="role">Optional role scope; when set, only members with this role are considered.</param>
+    /// <returns><c>true</c> when self is Up and the oldest Up member in the role scope.</returns>
+    public static bool SelfIsOldestUp(Cluster cluster, string? role = null)
+    {
+        var self = cluster.SelfMember;
+        if (self.Status != MemberStatus.Up)
+            return false;
+        if (role != null && !self.HasRole(role))
+            return false;
+
+        return cluster.State.Members
+            .Where(m => m.Status == MemberStatus.Up)
+            .Where(m => role == null || m.HasRole(role))
+            .All(m => m.UniqueAddress.Equals(self.UniqueAddress) || self.IsOlderThan(m));
+    }
+}
diff --git a/src/ZB.MOM.WW.ScadaBridge.Host/Health/ClusterActivityEvaluator.cs b/src/ZB.MOM.WW.ScadaBridge.Host/Health/ClusterActivityEvaluator.cs
index 382ade94..b4729cb5 100644
--- a/src/ZB.MOM.WW.ScadaBridge.Host/Health/ClusterActivityEvaluator.cs
+++ b/src/ZB.MOM.WW.ScadaBridge.Host/Health/ClusterActivityEvaluator.cs
@@ -13,23 +13,15 @@ namespace ZB.MOM.WW.ScadaBridge.Host.Health;
 /// </summary>
 public static class ClusterActivityEvaluator
 {
-    /// <summary>True when self is Up and no other Up member (in the role scope) is older.</summary>
+    /// <summary>True when self is Up and no other Up member (in the role scope) is older.
+    /// Delegates to the shared <see cref="Communication.ClusterState.ActiveNodeEvaluator"/> —
+    /// one implementation for the delivery gate, the resync authority checks, and the
+    /// heartbeat IsActive stamp (review 02 round 2, N1).</summary>
     /// <param name="cluster">The Akka cluster to evaluate.</param>
     /// <param name="role">Optional role scope; when set, only members with this role are considered.</param>
     /// <returns><c>true</c> when self is Up and the oldest Up member in the role scope.</returns>
-    public static bool SelfIsOldest(Cluster cluster, string? role = null)
-    {
-        var self = cluster.SelfMember;
-        if (self.Status != MemberStatus.Up)
-            return false;
-        if (role != null && !self.HasRole(role))
-            return false;
-
-        return cluster.State.Members
-            .Where(m => m.Status == MemberStatus.Up)
-            .Where(m => role == null || m.HasRole(role))
-            .All(m => m.UniqueAddress.Equals(self.UniqueAddress) || self.IsOlderThan(m));
-    }
+    public static bool SelfIsOldest(Cluster cluster, string? role = null) =>
+        Communication.ClusterState.ActiveNodeEvaluator.SelfIsOldestUp(cluster, role);
 
     /// <summary>The oldest Up member in the role scope, or null while none is Up. Used for Primary/Standby labelling.</summary>
     /// <param name="cluster">The Akka cluster to evaluate.</param>
diff --git a/tests/ZB.MOM.WW.ScadaBridge.Communication.Tests/ActiveNodeEvaluatorTests.cs b/tests/ZB.MOM.WW.ScadaBridge.Communication.Tests/ActiveNodeEvaluatorTests.cs
new file mode 100644
index 00000000..07589409
--- /dev/null
+++ b/tests/ZB.MOM.WW.ScadaBridge.Communication.Tests/ActiveNodeEvaluatorTests.cs
@@ -0,0 +1,57 @@
+using System.Net;
+using System.Net.Sockets;
+using Akka.Actor;
+using Akka.Configuration;
+using Xunit;
+using ZB.MOM.WW.ScadaBridge.Communication.ClusterState;
+
+namespace ZB.MOM.WW.ScadaBridge.Communication.Tests;
+
+public class ActiveNodeEvaluatorTests : IAsyncLifetime
+{
+    private ActorSystem? _system;
+
+    public async Task InitializeAsync()
+    {
+        var port = FreePort();
+        var config = ConfigurationFactory.ParseString($@"
+akka {{
+    actor.provider = cluster
+    remote.dot-netty.tcp {{ hostname = ""127.0.0.1"", port = {port} }}
+    cluster {{
+        seed-nodes = [""akka.tcp://ane-test@127.0.0.1:{port}""]
+        roles = [""site-x""]
+        min-nr-of-members = 1
+    }}
+}}");
+        _system = ActorSystem.Create("ane-test", config);
+        var cluster = Akka.Cluster.Cluster.Get(_system);
+        var deadline = DateTime.UtcNow.AddSeconds(20);
+        while (cluster.SelfMember.Status != Akka.Cluster.MemberStatus.Up && DateTime.UtcNow < deadline)
+            await Task.Delay(100);
+    }
+
+    public async Task DisposeAsync() { if (_system != null) await _system.Terminate(); }
+
+    [Fact]
+    public void SelfIsOldestUp_SoleUpMember_ReturnsTrue()
+    {
+        var cluster = Akka.Cluster.Cluster.Get(_system!);
+        Assert.True(ActiveNodeEvaluator.SelfIsOldestUp(cluster));
+        Assert.True(ActiveNodeEvaluator.SelfIsOldestUp(cluster, "site-x"));
+    }
+
+    [Fact]
+    public void SelfIsOldestUp_RoleNotHeld_ReturnsFalse()
+    {
+        var cluster = Akka.Cluster.Cluster.Get(_system!);
+        Assert.False(ActiveNodeEvaluator.SelfIsOldestUp(cluster, "role-nonexistent"));
+    }
+
+    private static int FreePort()
+    {
+        using var l = new TcpListener(IPAddress.Loopback, 0);
+        l.Start();
+        return ((IPEndPoint)l.LocalEndpoint).Port;
+    }
+}