diff --git a/src/ZB.MOM.WW.ScadaBridge.Host/Health/ClusterActivityEvaluator.cs b/src/ZB.MOM.WW.ScadaBridge.Host/Health/ClusterActivityEvaluator.cs
new file mode 100644
index 00000000..30613b81
--- /dev/null
+++ b/src/ZB.MOM.WW.ScadaBridge.Host/Health/ClusterActivityEvaluator.cs
@@ -0,0 +1,42 @@
+using Akka.Cluster;
+
+namespace ZB.MOM.WW.ScadaBridge.Host.Health;
+
+/// <summary>
+/// THE single definition of "active node" (review 01 [High]): 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
+/// cluster.State.Leader.
+/// </summary>
+public static class ClusterActivityEvaluator
+{
+    /// <summary>True when self is Up and no other Up member (in the role scope) is older.</summary>
+    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));
+    }
+
+    /// <summary>The oldest Up member in the role scope, or null while none is Up. Used for Primary/Standby labelling.</summary>
+    public static Member? OldestUpMember(Cluster cluster, string? role = null)
+    {
+        Member? oldest = null;
+        foreach (var m in cluster.State.Members.Where(m => m.Status == MemberStatus.Up))
+        {
+            if (role != null && !m.HasRole(role)) continue;
+            if (oldest == null || m.IsOlderThan(oldest)) oldest = m;
+        }
+        return oldest;
+    }
+}
diff --git a/tests/ZB.MOM.WW.ScadaBridge.Host.Tests/ClusterActivityEvaluatorTests.cs b/tests/ZB.MOM.WW.ScadaBridge.Host.Tests/ClusterActivityEvaluatorTests.cs
new file mode 100644
index 00000000..a1d34a86
--- /dev/null
+++ b/tests/ZB.MOM.WW.ScadaBridge.Host.Tests/ClusterActivityEvaluatorTests.cs
@@ -0,0 +1,54 @@
+using Akka.Actor;
+using Akka.Configuration;
+using ZB.MOM.WW.ScadaBridge.Host.Health;
+
+namespace ZB.MOM.WW.ScadaBridge.Host.Tests;
+
+public class ClusterActivityEvaluatorTests : 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://eval-test@127.0.0.1:{port}""]
+        roles = [""Central""]
+        min-nr-of-members = 1
+    }}
+}}");
+        _system = ActorSystem.Create("eval-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 SelfIsOldest_SoleUpMember_ReturnsTrue()
+    {
+        var cluster = Akka.Cluster.Cluster.Get(_system!);
+        Assert.True(ClusterActivityEvaluator.SelfIsOldest(cluster));
+        Assert.True(ClusterActivityEvaluator.SelfIsOldest(cluster, "Central"));
+    }
+
+    [Fact]
+    public void SelfIsOldest_RoleNotHeld_ReturnsFalse()
+    {
+        var cluster = Akka.Cluster.Cluster.Get(_system!);
+        // Self doesn't carry the role => it can never host that role's singletons.
+        Assert.False(ClusterActivityEvaluator.SelfIsOldest(cluster, "site-nonexistent"));
+    }
+
+    private static int FreePort()
+    {
+        var l = new System.Net.Sockets.TcpListener(System.Net.IPAddress.Loopback, 0);
+        l.Start(); var p = ((System.Net.IPEndPoint)l.LocalEndpoint).Port; l.Stop(); return p;
+    }
+}