feat(opcua): #85 UNS Area/Line/Equipment folder hierarchy in SDK

Phase7Composer now carries UnsAreaProjection + UnsLineProjection lists so
the applier can materialise the full UNS topology in the OPC UA address
space. New IOpcUaAddressSpaceSink.EnsureFolder(folderNodeId, parentNodeId,
displayName) seam (no-op default, recorded in tests, forwarded by
DeferredAddressSpaceSink, implemented by SdkAddressSpaceSink). The SDK-
side OtOpcUaNodeManager gains an EnsureFolder API that creates
FolderState nodes with proper parent linkage; RebuildAddressSpace now
clears folders too so re-applies don't accumulate stale topology.

Phase7Applier.MaterialiseHierarchy walks composition.UnsAreas →
composition.UnsLines → composition.EquipmentNodes, calling EnsureFolder
with the correct parent at each level. Idempotent — calling twice with
the same composition is a no-op. OpcUaPublishActor.HandleRebuild invokes
it after Phase7Applier.Apply so OPC UA clients browsing the server now
see Area/Line/Equipment as proper folders rather than flat tag ids.

DeploymentArtifact.ParseComposition reads UnsAreas + UnsLines from the
JSON snapshot the ControlPlane emits, populating the new fields when
present.

Phase7Composer.Compose now accepts UnsAreas + UnsLines; a 3-arg overload
preserves the old signature for legacy callers + existing tests. The
Phase7CompositionResult convenience ctor likewise keeps the planner
tests working without UNS data.

3 new hierarchy tests (pure unit + boot-verify against a real
OtOpcUaSdkServer); OpcUaServer suite is 48/48 green (was 45, +3),
Runtime 74/74 unchanged.

Closes #85.

src/Core/ZB.MOM.WW.OtOpcUa.Commons/OpcUa/DeferredAddressSpaceSink.cs

@@ -27,5 +27,8 @@ public sealed class DeferredAddressSpaceSink : IOpcUaAddressSpaceSink
     public void WriteAlarmState(string alarmNodeId, bool active, bool acknowledged, DateTime sourceTimestampUtc)
         => _inner.WriteAlarmState(alarmNodeId, active, acknowledged, sourceTimestampUtc);
 
+    public void EnsureFolder(string folderNodeId, string? parentNodeId, string displayName)
+        => _inner.EnsureFolder(folderNodeId, parentNodeId, displayName);
+
     public void RebuildAddressSpace() => _inner.RebuildAddressSpace();
 }

src/Core/ZB.MOM.WW.OtOpcUa.Commons/OpcUa/IOpcUaAddressSpaceSink.cs

@@ -14,6 +14,14 @@ public interface IOpcUaAddressSpaceSink
     /// <summary>Write an alarm-condition Variable's active/acknowledged state.</summary>
     void WriteAlarmState(string alarmNodeId, bool active, bool acknowledged, DateTime sourceTimestampUtc);
 
+    /// <summary>
+    ///     Ensure a folder node exists under the given parent. Used by <c>Phase7Applier</c> to
+    ///     materialise the UNS Area/Line/Equipment hierarchy in the address space. When
+    ///     <paramref name="parentNodeId"/> is null the folder is parented under the namespace
+    ///     root. Idempotent: calling twice with the same id is safe.
+    /// </summary>
+    void EnsureFolder(string folderNodeId, string? parentNodeId, string displayName);
+
     /// <summary>
     ///     Tear down + repopulate the address space. Called by <c>OpcUaPublishActor</c> after a
     ///     successful deployment apply so the node manager reflects the new config. Idempotent.
@@ -33,5 +41,6 @@ public sealed class NullOpcUaAddressSpaceSink : IOpcUaAddressSpaceSink
     private NullOpcUaAddressSpaceSink() { }
     public void WriteValue(string nodeId, object? value, OpcUaQuality quality, DateTime sourceTimestampUtc) { }
     public void WriteAlarmState(string alarmNodeId, bool active, bool acknowledged, DateTime sourceTimestampUtc) { }
+    public void EnsureFolder(string folderNodeId, string? parentNodeId, string displayName) { }
     public void RebuildAddressSpace() { }
 }

src/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer/OtOpcUaNodeManager.cs

@@ -27,6 +27,7 @@ public sealed class OtOpcUaNodeManager : CustomNodeManager2
     public const string DefaultNamespaceUri = "https://zb.com/otopcua/ns";
 
     private readonly ConcurrentDictionary<string, BaseDataVariableState> _variables = new(StringComparer.Ordinal);
+    private readonly ConcurrentDictionary<string, FolderState> _folders = new(StringComparer.Ordinal);
     private FolderState? _root;
 
     public OtOpcUaNodeManager(IServerInternal server, ApplicationConfiguration configuration)
@@ -36,6 +37,7 @@ public sealed class OtOpcUaNodeManager : CustomNodeManager2
     }
 
     public int VariableCount => _variables.Count;
+    public int FolderCount => _folders.Count;
 
     /// <summary>
     ///     Apply a value write from <see cref="IOpcUaAddressSpaceSink.WriteValue"/>. Creates the
@@ -73,9 +75,43 @@ public sealed class OtOpcUaNodeManager : CustomNodeManager2
         }
     }
 
-    /// <summary>Clear every registered variable from the address space. Phase7Applier calls this
-    /// when Equipment/Alarm topology changes; the populator then re-adds via WriteValue on the
-    /// next pass.</summary>
+    /// <summary>
+    ///     Ensure a folder node exists at <paramref name="folderNodeId"/> with the given display
+    ///     name, parented under <paramref name="parentNodeId"/> (or the namespace root when null).
+    ///     #85 — used by <see cref="Phase7Applier"/> to materialise the UNS Area/Line/Equipment
+    ///     folder hierarchy. Idempotent: the second call with the same id returns the cached
+    ///     folder so adding child variables under it still works.
+    /// </summary>
+    public void EnsureFolder(string folderNodeId, string? parentNodeId, string displayName)
+    {
+        ArgumentException.ThrowIfNullOrEmpty(folderNodeId);
+        ArgumentException.ThrowIfNullOrEmpty(displayName);
+
+        if (_folders.ContainsKey(folderNodeId)) return;
+
+        lock (Lock)
+        {
+            if (_folders.ContainsKey(folderNodeId)) return;
+
+            var parent = ResolveParentFolder(parentNodeId);
+            var folder = new FolderState(parent)
+            {
+                NodeId = new NodeId(folderNodeId, NamespaceIndex),
+                BrowseName = new QualifiedName(folderNodeId, NamespaceIndex),
+                DisplayName = displayName,
+                EventNotifier = EventNotifiers.None,
+                TypeDefinitionId = ObjectTypeIds.FolderType,
+                ReferenceTypeId = ReferenceTypeIds.Organizes,
+            };
+            parent.AddChild(folder);
+            AddPredefinedNode(SystemContext, folder);
+            _folders[folderNodeId] = folder;
+        }
+    }
+
+    /// <summary>Clear every registered variable + folder from the address space. Phase7Applier
+    /// calls this when Equipment/Alarm topology changes; the populator then re-adds via
+    /// EnsureFolder + WriteValue on the next pass.</summary>
     public void RebuildAddressSpace()
     {
         lock (Lock)
@@ -86,9 +122,22 @@ public sealed class OtOpcUaNodeManager : CustomNodeManager2
                 PredefinedNodes?.Remove(v.NodeId);
             }
             _variables.Clear();
+
+            foreach (var f in _folders.Values)
+            {
+                f.Parent?.RemoveChild(f);
+                PredefinedNodes?.Remove(f.NodeId);
+            }
+            _folders.Clear();
         }
     }
 
+    private FolderState ResolveParentFolder(string? parentNodeId)
+    {
+        if (string.IsNullOrEmpty(parentNodeId)) return _root!;
+        return _folders.TryGetValue(parentNodeId, out var existing) ? existing : _root!;
+    }
+
     /// <inheritdoc />
     public override void CreateAddressSpace(IDictionary<NodeId, IList<IReference>> externalReferences)
     {

src/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer/Phase7Applier.cs

@@ -94,6 +94,43 @@ public sealed class Phase7Applier
         return new Phase7ApplyOutcome(removedCount, addedCount, changedCount, needsRebuild);
     }
 
+    /// <summary>
+    ///     #85 — build the UNS Area/Line/Equipment folder hierarchy in the address space from a
+    ///     composition snapshot. Called by <c>OpcUaPublishActor</c> after a rebuild so OPC UA
+    ///     clients browsing the server see proper folder structure instead of flat tag ids.
+    ///     Idempotent: each <c>EnsureFolder</c> call returns the existing folder if already
+    ///     present, so re-applies are cheap.
+    /// </summary>
+    public void MaterialiseHierarchy(Phase7CompositionResult composition)
+    {
+        ArgumentNullException.ThrowIfNull(composition);
+
+        foreach (var area in composition.UnsAreas)
+        {
+            SafeEnsureFolder(area.UnsAreaId, parentNodeId: null, displayName: area.DisplayName);
+        }
+        foreach (var line in composition.UnsLines)
+        {
+            SafeEnsureFolder(line.UnsLineId, parentNodeId: line.UnsAreaId, displayName: line.DisplayName);
+        }
+        foreach (var equipment in composition.EquipmentNodes)
+        {
+            // Equipment with no UnsLineId (legacy / dev rows) hang under the root.
+            var parent = string.IsNullOrWhiteSpace(equipment.UnsLineId) ? null : equipment.UnsLineId;
+            SafeEnsureFolder(equipment.EquipmentId, parentNodeId: parent, displayName: equipment.DisplayName);
+        }
+
+        _logger.LogInformation(
+            "Phase7Applier: hierarchy materialised (areas={Areas}, lines={Lines}, equipment={Equipment})",
+            composition.UnsAreas.Count, composition.UnsLines.Count, composition.EquipmentNodes.Count);
+    }
+
+    private void SafeEnsureFolder(string nodeId, string? parentNodeId, string displayName)
+    {
+        try { _sink.EnsureFolder(nodeId, parentNodeId, displayName); }
+        catch (Exception ex) { _logger.LogWarning(ex, "Phase7Applier: EnsureFolder threw for {Node}", nodeId); }
+    }
+
     private void SafeWriteAlarmState(string nodeId, bool active, bool acknowledged, DateTime ts)
     {
         try { _sink.WriteAlarmState(nodeId, active, acknowledged, ts); }

src/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer/Phase7Composer.cs

@@ -2,12 +2,30 @@ using ZB.MOM.WW.OtOpcUa.Configuration.Entities;
 
 namespace ZB.MOM.WW.OtOpcUa.OpcUaServer;
 
-/// <summary>Outcome of <see cref="Phase7Composer.Compose"/> — pure value tuple, no side effects.</summary>
+/// <summary>Outcome of <see cref="Phase7Composer.Compose"/> — pure value tuple, no side effects.
+/// <see cref="UnsAreas"/> + <see cref="UnsLines"/> carry the UNS topology so the applier can
+/// materialise the Area/Line/Equipment folder hierarchy in the address space; equipment carries
+/// its parent line id so the applier knows where to hang each equipment folder.</summary>
 public sealed record Phase7CompositionResult(
+    IReadOnlyList<UnsAreaProjection> UnsAreas,
+    IReadOnlyList<UnsLineProjection> UnsLines,
     IReadOnlyList<EquipmentNode> EquipmentNodes,
     IReadOnlyList<DriverInstancePlan> DriverInstancePlans,
-    IReadOnlyList<ScriptedAlarmPlan> ScriptedAlarmPlans);
+    IReadOnlyList<ScriptedAlarmPlan> ScriptedAlarmPlans)
+{
+    /// <summary>Convenience constructor for tests + earlier callers that don't yet carry UNS topology.</summary>
+    public Phase7CompositionResult(
+        IReadOnlyList<EquipmentNode> equipmentNodes,
+        IReadOnlyList<DriverInstancePlan> driverInstancePlans,
+        IReadOnlyList<ScriptedAlarmPlan> scriptedAlarmPlans)
+        : this(Array.Empty<UnsAreaProjection>(), Array.Empty<UnsLineProjection>(),
+               equipmentNodes, driverInstancePlans, scriptedAlarmPlans)
+    {
+    }
+}
 
+public sealed record UnsAreaProjection(string UnsAreaId, string DisplayName);
+public sealed record UnsLineProjection(string UnsLineId, string UnsAreaId, string DisplayName);
 public sealed record EquipmentNode(string EquipmentId, string DisplayName, string UnsLineId);
 public sealed record DriverInstancePlan(string DriverInstanceId, string DriverType, string ConfigJson);
 public sealed record ScriptedAlarmPlan(string ScriptedAlarmId, string EquipmentId, string PredicateScriptId, string MessageTemplate);
@@ -17,18 +35,38 @@ public sealed record ScriptedAlarmPlan(string ScriptedAlarmId, string EquipmentI
 /// driver-role host needs. Same inputs → same outputs, no logging, no DB writes. The driver-role
 /// startup (Task 53) consumes the result and hands it to the node-manager factory.
 ///
-/// Full migration of the legacy <c>Server.Phase7.Phase7Composer</c> (which mutates a server-side
-/// node cache, emits trace logs, and calls into <c>EquipmentNodeWalker</c>) is tracked as
-/// follow-up F14. This pure version handles the projection step; the side-effecting wiring
-/// stays in the legacy code until F14 lands.
+/// #85 — the composer now carries UNS topology (<see cref="UnsAreaProjection"/> +
+/// <see cref="UnsLineProjection"/>) so <c>Phase7Applier</c> can build the
+/// <c>Area/Line/Equipment</c> folder hierarchy in the SDK's address space. The legacy
+/// <c>EquipmentNodeWalker</c> integration that did this server-side is fully replaced by the
+/// (composer → applier → sink → node manager) chain.
 /// </summary>
 public static class Phase7Composer
 {
+    /// <summary>Convenience overload for legacy callers + tests that don't yet supply UNS topology.</summary>
     public static Phase7CompositionResult Compose(
+        IReadOnlyList<Equipment> equipment,
+        IReadOnlyList<DriverInstance> driverInstances,
+        IReadOnlyList<ScriptedAlarm> scriptedAlarms) =>
+        Compose(Array.Empty<UnsArea>(), Array.Empty<UnsLine>(), equipment, driverInstances, scriptedAlarms);
+
+    public static Phase7CompositionResult Compose(
+        IReadOnlyList<UnsArea> unsAreas,
+        IReadOnlyList<UnsLine> unsLines,
         IReadOnlyList<Equipment> equipment,
         IReadOnlyList<DriverInstance> driverInstances,
         IReadOnlyList<ScriptedAlarm> scriptedAlarms)
     {
+        var areas = unsAreas
+            .OrderBy(a => a.UnsAreaId, StringComparer.Ordinal)
+            .Select(a => new UnsAreaProjection(a.UnsAreaId, a.Name))
+            .ToList();
+
+        var lines = unsLines
+            .OrderBy(l => l.UnsLineId, StringComparer.Ordinal)
+            .Select(l => new UnsLineProjection(l.UnsLineId, l.UnsAreaId, l.Name))
+            .ToList();
+
         var nodes = equipment
             .OrderBy(e => e.EquipmentId, StringComparer.Ordinal)
             .Select(e => new EquipmentNode(e.EquipmentId, e.MachineCode, e.UnsLineId))
@@ -44,6 +82,6 @@ public static class Phase7Composer
             .Select(a => new ScriptedAlarmPlan(a.ScriptedAlarmId, a.EquipmentId, a.PredicateScriptId, a.MessageTemplate))
             .ToList();
 
-        return new Phase7CompositionResult(nodes, plans, alarms);
+        return new Phase7CompositionResult(areas, lines, nodes, plans, alarms);
     }
 }

src/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer/SdkAddressSpaceSink.cs

@@ -24,5 +24,8 @@ public sealed class SdkAddressSpaceSink : IOpcUaAddressSpaceSink
     public void WriteAlarmState(string alarmNodeId, bool active, bool acknowledged, DateTime sourceTimestampUtc)
         => _nodeManager.WriteAlarmState(alarmNodeId, active, acknowledged, sourceTimestampUtc);
 
+    public void EnsureFolder(string folderNodeId, string? parentNodeId, string displayName)
+        => _nodeManager.EnsureFolder(folderNodeId, parentNodeId, displayName);
+
     public void RebuildAddressSpace() => _nodeManager.RebuildAddressSpace();
 }

src/Server/ZB.MOM.WW.OtOpcUa.Runtime/Drivers/DeploymentArtifact.cs

@@ -89,34 +89,34 @@ public static class DeploymentArtifact
     /// </summary>
     public static Phase7CompositionResult ParseComposition(ReadOnlySpan<byte> blob)
     {
-        if (blob.IsEmpty)
-        {
-            return new Phase7CompositionResult(
-                Array.Empty<EquipmentNode>(),
-                Array.Empty<DriverInstancePlan>(),
-                Array.Empty<ScriptedAlarmPlan>());
-        }
+        if (blob.IsEmpty) return Empty();
 
         try
         {
             using var doc = JsonDocument.Parse(blob.ToArray());
             var root = doc.RootElement;
 
+            var areas = ReadArray(root, "UnsAreas", ReadAreaProjection);
+            var lines = ReadArray(root, "UnsLines", ReadLineProjection);
             var equipment = ReadArray(root, "Equipment", ReadEquipmentNode);
             var drivers = ReadArray(root, "DriverInstances", ReadDriverPlan);
             var alarms = ReadArray(root, "ScriptedAlarms", ReadAlarmPlan);
 
-            return new Phase7CompositionResult(equipment, drivers, alarms);
+            return new Phase7CompositionResult(areas, lines, equipment, drivers, alarms);
         }
         catch (JsonException)
         {
-            return new Phase7CompositionResult(
-                Array.Empty<EquipmentNode>(),
-                Array.Empty<DriverInstancePlan>(),
-                Array.Empty<ScriptedAlarmPlan>());
+            return Empty();
         }
     }
 
+    private static Phase7CompositionResult Empty() => new(
+        Array.Empty<UnsAreaProjection>(),
+        Array.Empty<UnsLineProjection>(),
+        Array.Empty<EquipmentNode>(),
+        Array.Empty<DriverInstancePlan>(),
+        Array.Empty<ScriptedAlarmPlan>());
+
     private static IReadOnlyList<T> ReadArray<T>(JsonElement root, string propertyName, Func<JsonElement, T?> reader)
         where T : class
     {
@@ -137,12 +137,31 @@ public static class DeploymentArtifact
 
     private static string IdentityOf<T>(T item) where T : class => item switch
     {
+        UnsAreaProjection a => a.UnsAreaId,
+        UnsLineProjection l => l.UnsLineId,
         EquipmentNode e => e.EquipmentId,
         DriverInstancePlan d => d.DriverInstanceId,
         ScriptedAlarmPlan a => a.ScriptedAlarmId,
         _ => string.Empty,
     };
 
+    private static UnsAreaProjection? ReadAreaProjection(JsonElement el)
+    {
+        var id = el.TryGetProperty("UnsAreaId", out var idEl) ? idEl.GetString() : null;
+        var name = el.TryGetProperty("Name", out var nameEl) ? nameEl.GetString() : null;
+        if (string.IsNullOrWhiteSpace(id)) return null;
+        return new UnsAreaProjection(id!, name ?? id!);
+    }
+
+    private static UnsLineProjection? ReadLineProjection(JsonElement el)
+    {
+        var id = el.TryGetProperty("UnsLineId", out var idEl) ? idEl.GetString() : null;
+        var areaId = el.TryGetProperty("UnsAreaId", out var areaEl) ? areaEl.GetString() : null;
+        var name = el.TryGetProperty("Name", out var nameEl) ? nameEl.GetString() : null;
+        if (string.IsNullOrWhiteSpace(id) || string.IsNullOrWhiteSpace(areaId)) return null;
+        return new UnsLineProjection(id!, areaId!, name ?? id!);
+    }
+
     private static EquipmentNode? ReadEquipmentNode(JsonElement el)
     {
         var id = el.TryGetProperty("EquipmentId", out var idEl) ? idEl.GetString() : null;

src/Server/ZB.MOM.WW.OtOpcUa.Runtime/OpcUa/OpcUaPublishActor.cs

@@ -185,6 +185,12 @@ public sealed class OpcUaPublishActor : ReceiveActor
 
             var outcome = _applier.Apply(plan);
             _lastApplied = composition;
+
+            // #85 — after the plan diff lands, rebuild the UNS folder hierarchy so OPC UA
+            // clients see Area/Line/Equipment as proper folders. Idempotent; Phase7Applier
+            // skips folders that already exist with the same node id.
+            _applier.MaterialiseHierarchy(composition);
+
             OtOpcUaTelemetry.OpcUaSinkWrite.Add(1, new KeyValuePair<string, object?>("kind", "rebuild"));
             _log.Info("OpcUaPublish: applied rebuild (correlation={Correlation}, added={Added}, removed={Removed}, changed={Changed}, rebuild={Rebuild})",
                 msg.Correlation, outcome.AddedNodes, outcome.RemovedNodes, outcome.ChangedNodes, outcome.RebuildCalled);

tests/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer.Tests/DeferredAddressSpaceSinkTests.cs

@@ -72,6 +72,8 @@ public sealed class DeferredAddressSpaceSinkTests
             => CallQueue.Enqueue($"WV:{nodeId}");
         public void WriteAlarmState(string alarmNodeId, bool active, bool acknowledged, DateTime sourceTimestampUtc)
             => CallQueue.Enqueue($"WA:{alarmNodeId}");
+        public void EnsureFolder(string folderNodeId, string? parentNodeId, string displayName)
+            => CallQueue.Enqueue($"EF:{folderNodeId}");
         public void RebuildAddressSpace() => CallQueue.Enqueue("RB");
     }
 }

tests/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer.Tests/Phase7ApplierHierarchyTests.cs

@@ -0,0 +1,135 @@
+using System.Collections.Concurrent;
+using Microsoft.Extensions.Logging.Abstractions;
+using Opc.Ua.Server;
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Commons.OpcUa;
+
+namespace ZB.MOM.WW.OtOpcUa.OpcUaServer.Tests;
+
+/// <summary>
+/// #85 — verifies <see cref="Phase7Applier.MaterialiseHierarchy"/> builds the UNS
+/// Area/Line/Equipment folder tree through <see cref="IOpcUaAddressSpaceSink.EnsureFolder"/>.
+/// One pure unit test (recording sink) confirms ordering + parenting; one boot-verify test
+/// drives a real <see cref="OtOpcUaNodeManager"/> and inspects the resulting predefined-node
+/// count to prove the folders land in the SDK address space.
+/// </summary>
+public sealed class Phase7ApplierHierarchyTests : IDisposable
+{
+    private static CancellationToken Ct => TestContext.Current.CancellationToken;
+
+    private readonly string _pkiRoot = Path.Combine(
+        Path.GetTempPath(),
+        $"otopcua-pki-{Guid.NewGuid():N}");
+
+    [Fact]
+    public void MaterialiseHierarchy_creates_areas_then_lines_then_equipment_with_correct_parents()
+    {
+        var sink = new RecordingFolderSink();
+        var applier = new Phase7Applier(sink, NullLogger<Phase7Applier>.Instance);
+
+        var composition = new Phase7CompositionResult(
+            UnsAreas: new[] { new UnsAreaProjection("area-1", "Plant North") },
+            UnsLines: new[] { new UnsLineProjection("line-1", "area-1", "Cell A") },
+            EquipmentNodes: new[] { new EquipmentNode("eq-1", "Pump-1", "line-1") },
+            DriverInstancePlans: Array.Empty<DriverInstancePlan>(),
+            ScriptedAlarmPlans: Array.Empty<ScriptedAlarmPlan>());
+
+        applier.MaterialiseHierarchy(composition);
+
+        var calls = sink.Calls;
+        calls.Count.ShouldBe(3);
+        calls[0].ShouldBe(("area-1", null, "Plant North"));
+        calls[1].ShouldBe(("line-1", "area-1", "Cell A"));
+        calls[2].ShouldBe(("eq-1", "line-1", "Pump-1"));
+    }
+
+    [Fact]
+    public void MaterialiseHierarchy_orphan_equipment_hangs_under_root()
+    {
+        var sink = new RecordingFolderSink();
+        var applier = new Phase7Applier(sink, NullLogger<Phase7Applier>.Instance);
+
+        var composition = new Phase7CompositionResult(
+            UnsAreas: Array.Empty<UnsAreaProjection>(),
+            UnsLines: Array.Empty<UnsLineProjection>(),
+            EquipmentNodes: new[] { new EquipmentNode("eq-orphan", "Orphan", UnsLineId: "") },
+            DriverInstancePlans: Array.Empty<DriverInstancePlan>(),
+            ScriptedAlarmPlans: Array.Empty<ScriptedAlarmPlan>());
+
+        applier.MaterialiseHierarchy(composition);
+
+        sink.Calls.Single().ShouldBe(("eq-orphan", null, "Orphan"));
+    }
+
+    [Fact]
+    public async Task MaterialiseHierarchy_against_real_SDK_node_manager_creates_folder_nodes()
+    {
+        await using var host = new OpcUaApplicationHost(
+            new OpcUaApplicationHostOptions
+            {
+                ApplicationName = "OtOpcUa.Hierarchy",
+                ApplicationUri = $"urn:OtOpcUa.Hierarchy:{Guid.NewGuid():N}",
+                OpcUaPort = AllocateFreePort(),
+                PublicHostname = "localhost",
+                PkiStoreRoot = _pkiRoot,
+            },
+            NullLogger<OpcUaApplicationHost>.Instance);
+
+        var sdkServer = new OtOpcUaSdkServer();
+        await host.StartAsync(sdkServer, Ct);
+        sdkServer.NodeManager.ShouldNotBeNull();
+
+        var sink = new SdkAddressSpaceSink(sdkServer.NodeManager!);
+        var applier = new Phase7Applier(sink, NullLogger<Phase7Applier>.Instance);
+
+        applier.MaterialiseHierarchy(new Phase7CompositionResult(
+            UnsAreas: new[] { new UnsAreaProjection("area-A", "Area A"), new UnsAreaProjection("area-B", "Area B") },
+            UnsLines: new[] { new UnsLineProjection("line-1", "area-A", "Line 1") },
+            EquipmentNodes: new[] { new EquipmentNode("eq-1", "Eq 1", "line-1"), new EquipmentNode("eq-2", "Eq 2", "line-1") },
+            DriverInstancePlans: Array.Empty<DriverInstancePlan>(),
+            ScriptedAlarmPlans: Array.Empty<ScriptedAlarmPlan>()));
+
+        sdkServer.NodeManager!.FolderCount.ShouldBe(5); // 2 areas + 1 line + 2 equipment
+
+        // Idempotent: re-applying with the same composition doesn't create duplicates.
+        applier.MaterialiseHierarchy(new Phase7CompositionResult(
+            UnsAreas: new[] { new UnsAreaProjection("area-A", "Area A"), new UnsAreaProjection("area-B", "Area B") },
+            UnsLines: new[] { new UnsLineProjection("line-1", "area-A", "Line 1") },
+            EquipmentNodes: new[] { new EquipmentNode("eq-1", "Eq 1", "line-1"), new EquipmentNode("eq-2", "Eq 2", "line-1") },
+            DriverInstancePlans: Array.Empty<DriverInstancePlan>(),
+            ScriptedAlarmPlans: Array.Empty<ScriptedAlarmPlan>()));
+
+        sdkServer.NodeManager!.FolderCount.ShouldBe(5);
+    }
+
+    private static int AllocateFreePort()
+    {
+        using var listener = new System.Net.Sockets.TcpListener(System.Net.IPAddress.Loopback, 0);
+        listener.Start();
+        var port = ((System.Net.IPEndPoint)listener.LocalEndpoint).Port;
+        listener.Stop();
+        return port;
+    }
+
+    public void Dispose()
+    {
+        if (Directory.Exists(_pkiRoot))
+        {
+            try { Directory.Delete(_pkiRoot, recursive: true); }
+            catch { /* best-effort */ }
+        }
+    }
+
+    private sealed class RecordingFolderSink : IOpcUaAddressSpaceSink
+    {
+        private readonly ConcurrentQueue<(string NodeId, string? Parent, string DisplayName)> _calls = new();
+        public List<(string NodeId, string? Parent, string DisplayName)> Calls => _calls.ToList();
+
+        public void WriteValue(string nodeId, object? value, OpcUaQuality quality, DateTime sourceTimestampUtc) { }
+        public void WriteAlarmState(string alarmNodeId, bool active, bool acknowledged, DateTime sourceTimestampUtc) { }
+        public void EnsureFolder(string folderNodeId, string? parentNodeId, string displayName)
+            => _calls.Enqueue((folderNodeId, parentNodeId, displayName));
+        public void RebuildAddressSpace() { }
+    }
+}

tests/Server/ZB.MOM.WW.OtOpcUa.OpcUaServer.Tests/Phase7ApplierTests.cs

@@ -125,13 +125,17 @@ public sealed class Phase7ApplierTests
     private sealed class RecordingSink : IOpcUaAddressSpaceSink
     {
         public ConcurrentQueue<(string NodeId, bool Active, bool Acknowledged)> AlarmQueue { get; } = new();
+        public ConcurrentQueue<(string NodeId, string? Parent, string DisplayName)> FolderQueue { get; } = new();
         public int RebuildCalls;
 
         public List<(string NodeId, bool Active, bool Acknowledged)> AlarmWrites => AlarmQueue.ToList();
+        public List<(string NodeId, string? Parent, string DisplayName)> FolderCalls => FolderQueue.ToList();
 
         public void WriteValue(string nodeId, object? value, OpcUaQuality quality, DateTime sourceTimestampUtc) { }
         public void WriteAlarmState(string alarmNodeId, bool active, bool acknowledged, DateTime sourceTimestampUtc)
             => AlarmQueue.Enqueue((alarmNodeId, active, acknowledged));
+        public void EnsureFolder(string folderNodeId, string? parentNodeId, string displayName)
+            => FolderQueue.Enqueue((folderNodeId, parentNodeId, displayName));
         public void RebuildAddressSpace() => Interlocked.Increment(ref RebuildCalls);
     }
 
@@ -145,6 +149,7 @@ public sealed class Phase7ApplierTests
         {
             if (_throwOnAlarmWrite) throw new InvalidOperationException("simulated sink fault");
         }
+        public void EnsureFolder(string folderNodeId, string? parentNodeId, string displayName) { }
         public void RebuildAddressSpace() { }
     }
 }

tests/Server/ZB.MOM.WW.OtOpcUa.Runtime.Tests/Observability/OtOpcUaTelemetryHookTests.cs

@@ -172,6 +172,7 @@ public sealed class OtOpcUaTelemetryHookTests : RuntimeActorTestBase
         public int Writes { get; private set; }
         public void WriteValue(string nodeId, object? value, OpcUaQuality quality, DateTime sourceTimestampUtc) => Writes++;
         public void WriteAlarmState(string alarmNodeId, bool active, bool acknowledged, DateTime occurredUtc) => Writes++;
+        public void EnsureFolder(string folderNodeId, string? parentNodeId, string displayName) { }
         public void RebuildAddressSpace() { /* recorded via span */ }
     }
 }

tests/Server/ZB.MOM.WW.OtOpcUa.Runtime.Tests/OpcUa/OpcUaPublishActorRebuildTests.cs

@@ -139,6 +139,8 @@ public sealed class OpcUaPublishActorRebuildTests : RuntimeActorTestBase
             => Calls.Enqueue($"WV:{nodeId}");
         public void WriteAlarmState(string alarmNodeId, bool active, bool acknowledged, DateTime ts)
             => Calls.Enqueue($"WA:{alarmNodeId}");
+        public void EnsureFolder(string folderNodeId, string? parentNodeId, string displayName)
+            => Calls.Enqueue($"EF:{folderNodeId}");
         public void RebuildAddressSpace() => Interlocked.Increment(ref RebuildCalls);
     }
 }

tests/Server/ZB.MOM.WW.OtOpcUa.Runtime.Tests/OpcUa/OpcUaPublishActorTests.cs

@@ -152,6 +152,8 @@ public sealed class OpcUaPublishActorTests : RuntimeActorTestBase
         public void WriteAlarmState(string alarmNodeId, bool active, bool acknowledged, DateTime ts) =>
             AlarmQueue.Enqueue((alarmNodeId, active, acknowledged, ts));
 
+        public void EnsureFolder(string folderNodeId, string? parentNodeId, string displayName) { }
+
         public void RebuildAddressSpace() => Interlocked.Increment(ref RebuildCalls);
     }