diff --git a/docs/requirements/Component-DataConnectionLayer.md b/docs/requirements/Component-DataConnectionLayer.md
index 036d064c..857cffa6 100644
--- a/docs/requirements/Component-DataConnectionLayer.md
+++ b/docs/requirements/Component-DataConnectionLayer.md
@@ -468,4 +468,4 @@ The DCL reports the following metrics to the Health Monitoring component via the
OPC UA connection and endpoint verification select exactly the configured security mode. If a protected mode is unavailable, or discovery fails while protection was requested, the operation fails with the requested mode in its diagnostic; it never falls back to an unsecured session.
-DCL retains each subscribed alarm source's current standing state and its in-progress snapshot. A new subscriber sharing a feed receives a scoped replay, or the already-received prefix of an in-progress frame followed by the remaining frame. Cache state is discarded on reconnect and last-owner unsubscribe. A new Mx source reopens the shared stream even when the existing prefix covers it, obtaining a fresh standing-state snapshot; callbacks from the canceled stream are ignored. Mx refresh boundaries remain connection-wide but share a distinct per-stream epoch. Both replay cache and runtime mirror ignore stale-epoch rows and apply newer live raises/clears over a concurrent refresh before committing it.
+DCL retains each subscribed alarm source's current standing state and its in-progress snapshot. A new subscriber sharing a feed receives a scoped replay, or the already-received prefix of an in-progress frame followed by the remaining frame. Cache state is discarded on reconnect and last-owner unsubscribe. A new Mx source reopens the shared stream even when the existing prefix covers it, obtaining a fresh standing-state snapshot; callbacks from the canceled stream are ignored. Mx refresh boundaries remain connection-wide but share a distinct per-stream epoch. Both replay cache and runtime mirror ignore stale-epoch rows and apply newer live raises/clears over a concurrent refresh before committing it. The OPC UA client's per-condition `(active, acked)` memory used to derive transition kinds follows the same removal rule as the replay cache — a condition that returns to inactive + acknowledged (the default prior state) is forgotten — so it stays bounded on servers whose condition names carry a per-occurrence component (review 2026-10-21).
diff --git a/src/ZB.MOM.WW.ScadaBridge.DataConnectionLayer/Adapters/RealOpcUaClient.cs b/src/ZB.MOM.WW.ScadaBridge.DataConnectionLayer/Adapters/RealOpcUaClient.cs
index 6a7b4dd4..edd12df9 100644
--- a/src/ZB.MOM.WW.ScadaBridge.DataConnectionLayer/Adapters/RealOpcUaClient.cs
+++ b/src/ZB.MOM.WW.ScadaBridge.DataConnectionLayer/Adapters/RealOpcUaClient.cs
@@ -1236,8 +1236,18 @@ public class RealOpcUaClient : IOpcUaClient
var kind = inRefresh
? AlarmTransitionKind.Snapshot
: OpcUaAlarmMapper.DeriveKind(prevAcked, acked, prevActive, active);
- lastState?.TryAdd(sourceRef, (active, acked));
- if (lastState != null) lastState[sourceRef] = (active, acked);
+ if (lastState != null)
+ {
+ // Inactive + acknowledged IS the default prior state above, so forgetting it changes
+ // no derived kind — and keeps the map bounded on servers whose condition names carry
+ // a per-occurrence component (batch, lot, counter), where every occurrence is a new
+ // key for the life of the session (review 2026-10-21 02/L3; mirrors the actor's
+ // CacheAlarmTransition removal rule).
+ if (!active && acked)
+ lastState.Remove(sourceRef);
+ else
+ lastState[sourceRef] = (active, acked);
+ }
onTransition(new NativeAlarmTransition(
SourceReference: sourceRef,
diff --git a/tests/ZB.MOM.WW.ScadaBridge.DataConnectionLayer.Tests/Adapters/RealOpcUaAlarmLastStateTests.cs b/tests/ZB.MOM.WW.ScadaBridge.DataConnectionLayer.Tests/Adapters/RealOpcUaAlarmLastStateTests.cs
new file mode 100644
index 00000000..cc0aa8c0
--- /dev/null
+++ b/tests/ZB.MOM.WW.ScadaBridge.DataConnectionLayer.Tests/Adapters/RealOpcUaAlarmLastStateTests.cs
@@ -0,0 +1,89 @@
+using System.Collections.Concurrent;
+using System.Reflection;
+using Opc.Ua;
+using ZB.MOM.WW.ScadaBridge.Commons.Types.Alarms;
+using ZB.MOM.WW.ScadaBridge.DataConnectionLayer.Adapters;
+
+namespace ZB.MOM.WW.ScadaBridge.DataConnectionLayer.Tests.Adapters;
+
+///
+/// Review 2026-10-21 02/L3: the per-condition (active, acked) memory behind
+/// DeriveKind must not grow for the life of a session. A condition that returns to
+/// inactive + acknowledged is indistinguishable from one never seen (the default prior state), so
+/// its key is dropped — otherwise a server whose condition names carry a per-occurrence component
+/// (batch, lot, counter) adds a key per occurrence until the next reconnect.
+///
+public class RealOpcUaAlarmLastStateTests
+{
+ private const string Handle = "h1";
+
+ private static readonly MethodInfo Handler =
+ typeof(RealOpcUaClient).GetMethod("HandleAlarmEvent", BindingFlags.Instance | BindingFlags.NonPublic)!;
+
+ private static Dictionary SeedLastState(RealOpcUaClient client)
+ {
+ var field = typeof(RealOpcUaClient).GetField("_alarmLastState", BindingFlags.Instance | BindingFlags.NonPublic)!;
+ var map = (ConcurrentDictionary>)field.GetValue(client)!;
+ var perHandle = new Dictionary(StringComparer.Ordinal);
+ map[Handle] = perHandle;
+ return perHandle;
+ }
+
+ private static void Emit(RealOpcUaClient client, string condition, bool active, bool acked, List sink)
+ {
+ var fields = new VariantCollection
+ {
+ new(ObjectTypeIds.AlarmConditionType), // 0 EventType
+ new(new NodeId("Tank01", 2)), // 1 SourceNode
+ new("Tank01"), // 2 SourceName
+ new(DateTime.UtcNow), // 3 Time
+ new(new LocalizedText("msg")), // 4 Message
+ new((ushort)500), // 5 Severity
+ new(active), // 6 ActiveState/Id
+ new(acked), // 7 AckedState/Id
+ Variant.Null, // 8 ConfirmedState/Id
+ new(false), // 9 SuppressedOrShelved
+ Variant.Null, // 10 ShelvingState
+ new(condition), // 11 ConditionName
+ };
+ Handler.Invoke(client, [Handle, "ns=2;s=Tank01", new EventFieldList { EventFields = fields },
+ (Action)sink.Add]);
+ }
+
+ /// A per-occurrence condition that clears and is acknowledged leaves no key behind.
+ [Fact]
+ public void ReturnToNormal_DropsTheConditionKey()
+ {
+ var client = new RealOpcUaClient();
+ var lastState = SeedLastState(client);
+ var sink = new List();
+
+ for (var lot = 1; lot <= 50; lot++)
+ {
+ Emit(client, $"Hi.Lot{lot}", active: true, acked: false, sink);
+ Emit(client, $"Hi.Lot{lot}", active: false, acked: true, sink);
+ }
+
+ Assert.Empty(lastState);
+ Assert.Equal(100, sink.Count);
+ }
+
+ ///
+ /// Dropping the key does not change the transition kinds derived for a condition that comes
+ /// back — the dropped state equals the default prior state.
+ ///
+ [Fact]
+ public void ReRaiseAfterReturnToNormal_StillDerivesARaise()
+ {
+ var client = new RealOpcUaClient();
+ var lastState = SeedLastState(client);
+ var sink = new List();
+
+ Emit(client, "Hi", active: true, acked: false, sink);
+ Emit(client, "Hi", active: false, acked: true, sink);
+ Emit(client, "Hi", active: true, acked: false, sink);
+
+ Assert.Equal(sink[0].Kind, sink[2].Kind);
+ Assert.Single(lastState); // the re-raised condition is tracked again
+ }
+}