diff --git a/src/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer/Phase7Applier.cs b/src/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer/Phase7Applier.cs
index 7e3c64da..89e3643e 100644
--- a/src/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer/Phase7Applier.cs
+++ b/src/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer/Phase7Applier.cs
@@ -172,7 +172,7 @@ public sealed class Phase7Applier
         foreach (var tag in composition.EquipmentTags)
         {
             if (string.IsNullOrWhiteSpace(tag.FolderPath)) continue;
-            var folderNodeId = EquipmentSubFolderNodeId(tag.EquipmentId, tag.FolderPath);
+            var folderNodeId = EquipmentNodeIds.SubFolder(tag.EquipmentId, tag.FolderPath);
             if (!foldersCreated.Add(folderNodeId)) continue;
             SafeEnsureFolder(folderNodeId, parentNodeId: tag.EquipmentId, displayName: tag.FolderPath);
         }
@@ -187,8 +187,8 @@ public sealed class Phase7Applier
         {
             var parent = string.IsNullOrWhiteSpace(tag.FolderPath)
                 ? tag.EquipmentId
-                : EquipmentSubFolderNodeId(tag.EquipmentId, tag.FolderPath);
-            var nodeId = $"{parent}/{tag.Name}";
+                : EquipmentNodeIds.SubFolder(tag.EquipmentId, tag.FolderPath);
+            var nodeId = EquipmentNodeIds.Variable(tag.EquipmentId, tag.FolderPath, tag.Name);
             SafeEnsureVariable(nodeId, parent, tag.Name, tag.DataType);
         }
 
@@ -223,7 +223,7 @@ public sealed class Phase7Applier
         foreach (var v in composition.EquipmentVirtualTags)
         {
             if (string.IsNullOrWhiteSpace(v.FolderPath)) continue;
-            var folderNodeId = EquipmentSubFolderNodeId(v.EquipmentId, v.FolderPath);
+            var folderNodeId = EquipmentNodeIds.SubFolder(v.EquipmentId, v.FolderPath);
             if (!foldersCreated.Add(folderNodeId)) continue;
             SafeEnsureFolder(folderNodeId, parentNodeId: v.EquipmentId, displayName: v.FolderPath);
         }
@@ -234,8 +234,8 @@ public sealed class Phase7Applier
         {
             var parent = string.IsNullOrWhiteSpace(v.FolderPath)
                 ? v.EquipmentId
-                : EquipmentSubFolderNodeId(v.EquipmentId, v.FolderPath);
-            var nodeId = $"{parent}/{v.Name}";
+                : EquipmentNodeIds.SubFolder(v.EquipmentId, v.FolderPath);
+            var nodeId = EquipmentNodeIds.Variable(v.EquipmentId, v.FolderPath, v.Name);
             SafeEnsureVariable(nodeId, parent, v.Name, v.DataType);
         }
 
@@ -275,11 +275,6 @@ public sealed class Phase7Applier
                 .Select(a => a.EquipmentId).Distinct(StringComparer.Ordinal).Count());
     }
 
-    /// <summary>Deterministic NodeId for a tag's FolderPath sub-folder, scoped under its equipment
-    /// folder so two equipments' identically-named sub-folders never collide.</summary>
-    private static string EquipmentSubFolderNodeId(string equipmentId, string folderPath) =>
-        $"{equipmentId}/{folderPath}";
-
     private void SafeEnsureFolder(string nodeId, string? parentNodeId, string displayName)
     {
         try { _sink.EnsureFolder(nodeId, parentNodeId, displayName); }
diff --git a/src/Server/ZB.MOM.WW.OtOpcUa.Runtime/VirtualTags/VirtualTagHostActor.cs b/src/Server/ZB.MOM.WW.OtOpcUa.Runtime/VirtualTags/VirtualTagHostActor.cs
index 1015b315..fb92f253 100644
--- a/src/Server/ZB.MOM.WW.OtOpcUa.Runtime/VirtualTags/VirtualTagHostActor.cs
+++ b/src/Server/ZB.MOM.WW.OtOpcUa.Runtime/VirtualTags/VirtualTagHostActor.cs
@@ -17,10 +17,9 @@ namespace ZB.MOM.WW.OtOpcUa.Runtime.VirtualTags;
 ///     already-materialised Variable node (currently BadWaitingForInitialData) reflects the value.
 ///
 ///     <para>
-///         The published NodeId is computed with the <b>identical</b> formula
-///         <c>Phase7Applier.MaterialiseEquipmentVirtualTags</c> uses to materialise the variable —
-///         <c>{parent}/{Name}</c> where <c>parent = IsNullOrWhiteSpace(FolderPath) ? EquipmentId :
-///         {EquipmentId}/{FolderPath}</c> — or the value would land on a NodeId that does not exist.
+///         The published NodeId is computed by the shared <see cref="EquipmentNodeIds.Variable"/> —
+///         the single source of truth <c>Phase7Applier.MaterialiseEquipmentVirtualTags</c> also
+///         materialises against — so the value always lands on a NodeId that exists.
 ///     </para>
 /// </summary>
 public sealed class VirtualTagHostActor : ReceiveActor
@@ -143,14 +142,9 @@ public sealed class VirtualTagHostActor : ReceiveActor
         }
     }
 
-    /// <summary>Folder-scoped NodeId for a VirtualTag plan — MUST match
-    /// <c>Phase7Applier.MaterialiseEquipmentVirtualTags</c> exactly, or the published value lands on a
-    /// NodeId that was never materialised.</summary>
-    private static string NodeIdFor(EquipmentVirtualTagPlan p)
-    {
-        var parent = string.IsNullOrWhiteSpace(p.FolderPath)
-            ? p.EquipmentId
-            : $"{p.EquipmentId}/{p.FolderPath}";
-        return $"{parent}/{p.Name}";
-    }
+    /// <summary>Folder-scoped NodeId for a VirtualTag plan. The formula now lives in the shared
+    /// <see cref="EquipmentNodeIds"/> (the single source of truth that <c>Phase7Applier</c> also
+    /// materialises against), so the published value always lands on the NodeId that was materialised.</summary>
+    private static string NodeIdFor(EquipmentVirtualTagPlan p) =>
+        EquipmentNodeIds.Variable(p.EquipmentId, p.FolderPath, p.Name);
 }
diff --git a/tests/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer.Tests/Phase7ApplierTests.cs b/tests/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer.Tests/Phase7ApplierTests.cs
index 6820cd42..9a2e5f36 100644
--- a/tests/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer.Tests/Phase7ApplierTests.cs
+++ b/tests/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer.Tests/Phase7ApplierTests.cs
@@ -139,6 +139,8 @@ public sealed class Phase7ApplierTests
 
         sink.FolderCalls.ShouldBeEmpty(); // equipment folder already exists; no sub-folder needed
         sink.VariableCalls.ShouldHaveSingleItem().ShouldBe(("eq-1/Speed", "eq-1", "Speed", "Float"));
+        // Parity: the materialiser's NodeId is the shared EquipmentNodeIds formula (null/empty FolderPath).
+        sink.VariableCalls.Single().NodeId.ShouldBe(EquipmentNodeIds.Variable("eq-1", "", "Speed"));
     }
 
     /// <summary>Verifies a FolderPath on an equipment tag becomes a sub-folder UNDER the equipment
@@ -163,6 +165,8 @@ public sealed class Phase7ApplierTests
 
         sink.FolderCalls.ShouldHaveSingleItem().ShouldBe(("eq-1/Diagnostics", "eq-1", "Diagnostics"));
         sink.VariableCalls.ShouldHaveSingleItem().ShouldBe(("eq-1/Diagnostics/Temp", "eq-1/Diagnostics", "Temp", "Float"));
+        // Parity: the materialiser's NodeId is the shared EquipmentNodeIds formula (with FolderPath).
+        sink.VariableCalls.Single().NodeId.ShouldBe(EquipmentNodeIds.Variable("eq-1", "Diagnostics", "Temp"));
     }
 
     /// <summary>Regression for the FullName-as-NodeId collision: two identical machines exposing the
@@ -215,6 +219,47 @@ public sealed class Phase7ApplierTests
 
         sink.FolderCalls.ShouldBeEmpty(); // equipment folder already exists; no sub-folder needed
         sink.VariableCalls.ShouldHaveSingleItem().ShouldBe(("eq-1/speed-rpm", "eq-1", "speed-rpm", "Float64"));
+        // Parity: the vtag materialiser's NodeId is the shared EquipmentNodeIds formula.
+        sink.VariableCalls.Single().NodeId.ShouldBe(EquipmentNodeIds.Variable("eq-1", "", "speed-rpm"));
+    }
+
+    /// <summary>Golden/parity guard: the materialiser's Variable NodeId for BOTH the equipment-tag and
+    /// the equipment-VirtualTag pass is byte-identical to <see cref="EquipmentNodeIds.Variable"/> — the
+    /// single source of truth Phase7Applier + VirtualTagHostActor both point at. Covers null/empty
+    /// FolderPath (directly under equipment) and a non-empty FolderPath (sub-folder scoped). This test
+    /// LOCKS the formula against drift: any change to the materialiser NodeId that diverges from the
+    /// shared helper fails here.</summary>
+    [Fact]
+    public void Materialised_variable_node_ids_match_shared_EquipmentNodeIds_formula()
+    {
+        var sink = new RecordingSink();
+        var applier = new Phase7Applier(sink, NullLogger<Phase7Applier>.Instance);
+
+        var composition = new Phase7CompositionResult(
+            Array.Empty<EquipmentNode>(), Array.Empty<DriverInstancePlan>(), Array.Empty<ScriptedAlarmPlan>())
+        {
+            EquipmentTags = new[]
+            {
+                new EquipmentTagPlan("tag-flat", "eq-1", "drv", FolderPath: "", Name: "Speed", DataType: "Float", FullName: "40001"),
+                new EquipmentTagPlan("tag-nested", "eq-1", "drv", FolderPath: "Diagnostics", Name: "Temp", DataType: "Float", FullName: "40002"),
+            },
+            EquipmentVirtualTags = new[]
+            {
+                new EquipmentVirtualTagPlan("vt-flat", "eq-2", FolderPath: "", Name: "Efficiency", DataType: "Float64",
+                    Expression: "ctx.GetTag(\"a\")", DependencyRefs: new[] { "a" }),
+                new EquipmentVirtualTagPlan("vt-nested", "eq-2", FolderPath: "Calc", Name: "Avg", DataType: "Float64",
+                    Expression: "ctx.GetTag(\"b\")", DependencyRefs: new[] { "b" }),
+            },
+        };
+
+        applier.MaterialiseEquipmentTags(composition);
+        applier.MaterialiseEquipmentVirtualTags(composition);
+
+        var nodeIds = sink.VariableCalls.Select(v => v.NodeId).ToList();
+        nodeIds.ShouldContain(EquipmentNodeIds.Variable("eq-1", "", "Speed"));
+        nodeIds.ShouldContain(EquipmentNodeIds.Variable("eq-1", "Diagnostics", "Temp"));
+        nodeIds.ShouldContain(EquipmentNodeIds.Variable("eq-2", "", "Efficiency"));
+        nodeIds.ShouldContain(EquipmentNodeIds.Variable("eq-2", "Calc", "Avg"));
     }
 
     /// <summary>Two VirtualTags under the SAME equipment produce two distinct folder-scoped variables