src/ZB.MOM.WW.OtOpcUa.Core.Abstractions/DriverCapability.cs

@@ -25,7 +25,7 @@ public enum DriverCapability
     /// <summary><see cref="ITagDiscovery.DiscoverAsync"/>. Retries by default.</summary>
     Discover,
 
-    /// <summary><see cref="ISubscribable.SubscribeAsync"/> and unsubscribe. Retries by default.</summary>
+    /// <summary><see cref="ISubscribable.SubscribeAsync(IReadOnlyList{string}, TimeSpan, CancellationToken)"/> and unsubscribe. Retries by default.</summary>
     Subscribe,
 
     /// <summary><see cref="IHostConnectivityProbe"/> probe loop. Retries by default.</summary>

src/ZB.MOM.WW.OtOpcUa.Core.Abstractions/ISubscribable.cs

@@ -20,7 +20,29 @@ public interface ISubscribable
         TimeSpan publishingInterval,
         CancellationToken cancellationToken);
 
-    /// <summary>Cancel a subscription returned by <see cref="SubscribeAsync"/>.</summary>
+    /// <summary>
+    ///     Subscribe to data changes with per-tag advanced tuning (sampling interval, queue
+    ///     size, monitoring mode, deadband filter). Drivers that don't have a native concept
+    ///     of these knobs (e.g. polled drivers like Modbus) MAY ignore the per-tag knobs and
+    ///     delegate to the simple
+    ///     <see cref="SubscribeAsync(IReadOnlyList{string}, TimeSpan, CancellationToken)"/>
+    ///     overload — the default implementation does exactly that, so existing implementers
+    ///     compile unchanged.
+    /// </summary>
+    /// <param name="tags">Per-tag subscription specs. <see cref="MonitoredTagSpec.TagName"/> is the driver-side full reference.</param>
+    /// <param name="publishingInterval">Subscription publishing interval, applied to the whole batch.</param>
+    /// <param name="cancellationToken">Cancellation.</param>
+    /// <returns>Opaque subscription handle for <see cref="UnsubscribeAsync"/>.</returns>
+    Task<ISubscriptionHandle> SubscribeAsync(
+        IReadOnlyList<MonitoredTagSpec> tags,
+        TimeSpan publishingInterval,
+        CancellationToken cancellationToken)
+        => SubscribeAsync(
+            tags.Select(t => t.TagName).ToList(),
+            publishingInterval,
+            cancellationToken);
+
+    /// <summary>Cancel a subscription returned by either <c>SubscribeAsync</c> overload.</summary>
     Task UnsubscribeAsync(ISubscriptionHandle handle, CancellationToken cancellationToken);
 
     /// <summary>
@@ -30,7 +52,7 @@ public interface ISubscribable
     event EventHandler<DataChangeEventArgs>? OnDataChange;
 }
 
-/// <summary>Opaque subscription identity returned by <see cref="ISubscribable.SubscribeAsync"/>.</summary>
+/// <summary>Opaque subscription identity returned by <see cref="ISubscribable.SubscribeAsync(IReadOnlyList{string}, TimeSpan, CancellationToken)"/>.</summary>
 public interface ISubscriptionHandle
 {
     /// <summary>Driver-internal subscription identifier (for diagnostics + post-mortem).</summary>
@@ -38,10 +60,99 @@ public interface ISubscriptionHandle
 }
 
 /// <summary>Event payload for <see cref="ISubscribable.OnDataChange"/>.</summary>
-/// <param name="SubscriptionHandle">The handle returned by the original <see cref="ISubscribable.SubscribeAsync"/> call.</param>
+/// <param name="SubscriptionHandle">The handle returned by the original <see cref="ISubscribable.SubscribeAsync(IReadOnlyList{string}, TimeSpan, CancellationToken)"/> call.</param>
 /// <param name="FullReference">Driver-side full reference of the changed attribute.</param>
 /// <param name="Snapshot">New value + quality + timestamps.</param>
 public sealed record DataChangeEventArgs(
     ISubscriptionHandle SubscriptionHandle,
     string FullReference,
     DataValueSnapshot Snapshot);
+
+/// <summary>
+///     Per-tag subscription tuning. Maps onto OPC UA <c>MonitoredItem</c> properties for the
+///     OpcUaClient driver; non-OPC-UA drivers either map a subset (e.g. ADS picks up
+///     <see cref="SamplingIntervalMs"/>) or ignore the knobs entirely and fall back to the
+///     simple <see cref="ISubscribable.SubscribeAsync(IReadOnlyList{string}, TimeSpan, CancellationToken)"/>.
+/// </summary>
+/// <param name="TagName">Driver-side full reference (e.g. <c>ns=2;s=Foo</c> for OPC UA).</param>
+/// <param name="SamplingIntervalMs">
+///     Server-side sampling rate in milliseconds. <c>null</c> = use the publishing interval.
+///     Sub-publish-interval values let a server sample faster than it publishes (queue +
+///     coalesce), useful for events that change between publish ticks.
+/// </param>
+/// <param name="QueueSize">Server-side notification queue depth. <c>null</c> = driver default (1).</param>
+/// <param name="DiscardOldest">
+///     When the server-side queue overflows: <c>true</c> drops oldest, <c>false</c> drops newest.
+///     <c>null</c> = driver default (true — preserve recency).
+/// </param>
+/// <param name="MonitoringMode">
+///     Per-item monitoring mode. <c>Reporting</c> = sample + publish, <c>Sampling</c> = sample
+///     but suppress publishing (useful with triggering), <c>Disabled</c> = neither.
+/// </param>
+/// <param name="DataChangeFilter">
+///     Optional data-change filter (deadband + trigger semantics). <c>null</c> = no filter
+///     (every change publishes regardless of magnitude).
+/// </param>
+public sealed record MonitoredTagSpec(
+    string TagName,
+    double? SamplingIntervalMs = null,
+    uint? QueueSize = null,
+    bool? DiscardOldest = null,
+    SubscriptionMonitoringMode? MonitoringMode = null,
+    DataChangeFilterSpec? DataChangeFilter = null);
+
+/// <summary>
+///     OPC UA <c>DataChangeFilter</c> spec. Mirrors the OPC UA Part 4 §7.17.2 structure but
+///     lives in Core.Abstractions so non-OpcUaClient drivers (e.g. Modbus, S7) can accept it
+///     as metadata even if they ignore the deadband mechanics.
+/// </summary>
+/// <param name="Trigger">When to fire: status only / status+value / status+value+timestamp.</param>
+/// <param name="DeadbandType">Deadband mode: none / absolute (engineering units) / percent of EURange.</param>
+/// <param name="DeadbandValue">
+///     Magnitude of the deadband. For <see cref="OtOpcUa.Core.Abstractions.DeadbandType.Absolute"/>
+///     this is in the variable's engineering units; for <see cref="OtOpcUa.Core.Abstractions.DeadbandType.Percent"/>
+///     it's a 0..100 percentage of EURange (server returns BadFilterNotAllowed if EURange isn't set).
+/// </param>
+public sealed record DataChangeFilterSpec(
+    DataChangeTrigger Trigger,
+    DeadbandType DeadbandType,
+    double DeadbandValue);
+
+/// <summary>
+///     OPC UA <c>DataChangeTrigger</c> values. Wraps the SDK enum so Core.Abstractions doesn't
+///     leak an OPC-UA-stack reference into every driver project.
+/// </summary>
+public enum DataChangeTrigger
+{
+    /// <summary>Fire only when StatusCode changes.</summary>
+    Status = 0,
+    /// <summary>Fire when StatusCode or Value changes (the OPC UA default).</summary>
+    StatusValue = 1,
+    /// <summary>Fire when StatusCode, Value, or SourceTimestamp changes.</summary>
+    StatusValueTimestamp = 2,
+}
+
+/// <summary>OPC UA deadband-filter modes.</summary>
+public enum DeadbandType
+{
+    /// <summary>No deadband — every value change publishes.</summary>
+    None = 0,
+    /// <summary>Deadband expressed in the variable's engineering units.</summary>
+    Absolute = 1,
+    /// <summary>Deadband expressed as 0..100 percent of the variable's EURange.</summary>
+    Percent = 2,
+}
+
+/// <summary>
+///     Per-item subscription monitoring mode. Wraps the OPC UA SDK's <c>MonitoringMode</c>
+///     so Core.Abstractions stays SDK-free.
+/// </summary>
+public enum SubscriptionMonitoringMode
+{
+    /// <summary>Item is created but neither sampling nor publishing.</summary>
+    Disabled = 0,
+    /// <summary>Item samples and queues but does not publish (useful with triggering).</summary>
+    Sampling = 1,
+    /// <summary>Item samples and publishes — the OPC UA default.</summary>
+    Reporting = 2,
+}

src/ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient/OpcUaClientDriver.cs

@@ -851,8 +851,20 @@ public sealed class OpcUaClientDriver(OpcUaClientDriverOptions options, string d
 
     // ---- ISubscribable ----
 
-    public async Task<ISubscriptionHandle> SubscribeAsync(
+    public Task<ISubscriptionHandle> SubscribeAsync(
         IReadOnlyList<string> fullReferences, TimeSpan publishingInterval, CancellationToken cancellationToken)
+    {
+        // Route the simple-string overload through the per-tag overload with all knobs at
+        // their defaults. Single code path for subscription create — keeps the wire-side
+        // identical for callers that don't need per-tag tuning.
+        var specs = new MonitoredTagSpec[fullReferences.Count];
+        for (var i = 0; i < fullReferences.Count; i++)
+            specs[i] = new MonitoredTagSpec(fullReferences[i]);
+        return SubscribeAsync(specs, publishingInterval, cancellationToken);
+    }
+
+    public async Task<ISubscriptionHandle> SubscribeAsync(
+        IReadOnlyList<MonitoredTagSpec> tags, TimeSpan publishingInterval, CancellationToken cancellationToken)
     {
         var session = RequireSession();
         var id = Interlocked.Increment(ref _nextSubscriptionId);
@@ -885,29 +897,28 @@ public sealed class OpcUaClientDriver(OpcUaClientDriverOptions options, string d
             session.AddSubscription(subscription);
             await subscription.CreateAsync(cancellationToken).ConfigureAwait(false);
 
-            foreach (var fullRef in fullReferences)
+            foreach (var spec in tags)
             {
-                if (!TryParseNodeId(session, fullRef, out var nodeId)) continue;
+                if (!TryParseNodeId(session, spec.TagName, out var nodeId)) continue;
-                // The tag string is routed through MonitoredItem.Handle so the Notification
+
-                // handler can identify which tag changed without an extra lookup.
+                var monItem = BuildMonitoredItem(spec, nodeId, intervalMs);
-                var item = new MonitoredItem(telemetry: null!, new MonitoredItemOptions
+                monItem.Notification += (mi, args) => OnMonitoredItemNotification(handle, mi, args);
-                {
+                subscription.AddItem(monItem);
-                    DisplayName = fullRef,
-                    StartNodeId = nodeId,
-                    AttributeId = Attributes.Value,
-                    MonitoringMode = MonitoringMode.Reporting,
-                    SamplingInterval = intervalMs,
-                    QueueSize = 1,
-                    DiscardOldest = true,
-                })
-                {
-                    Handle = fullRef,
-                };
-                item.Notification += (mi, args) => OnMonitoredItemNotification(handle, mi, args);
-                subscription.AddItem(item);
             }
 
-            await subscription.CreateItemsAsync(cancellationToken).ConfigureAwait(false);
+            try
+            {
+                await subscription.CreateItemsAsync(cancellationToken).ConfigureAwait(false);
+            }
+            catch (Opc.Ua.ServiceResultException sre)
+            {
+                // PercentDeadband requires the server to expose EURange on the variable; if
+                // it isn't set the server returns BadFilterNotAllowed during item creation.
+                // We swallow the exception here so other items in the batch still get created
+                // — per-item failure surfaces through MonitoredItem.Status.Error rather than
+                // tearing down the whole subscription.
+                if (sre.StatusCode != StatusCodes.BadFilterNotAllowed) throw;
+            }
             _subscriptions[id] = new RemoteSubscription(subscription, handle);
         }
         finally { _gate.Release(); }
@@ -915,6 +926,84 @@ public sealed class OpcUaClientDriver(OpcUaClientDriverOptions options, string d
         return handle;
     }
 
+    /// <summary>
+    ///     Map a <see cref="MonitoredTagSpec"/> to a SDK <see cref="MonitoredItem"/> with the
+    ///     per-tag knobs applied. Defaults match the original hard-coded values
+    ///     (Reporting / SamplingInterval=publishInterval / QueueSize=1 / DiscardOldest=true)
+    ///     so a spec with all knobs <c>null</c> behaves identically to the legacy path.
+    /// </summary>
+    internal static MonitoredItem BuildMonitoredItem(MonitoredTagSpec spec, NodeId nodeId, int defaultIntervalMs)
+    {
+        var sampling = spec.SamplingIntervalMs.HasValue ? (int)spec.SamplingIntervalMs.Value : defaultIntervalMs;
+        var queueSize = spec.QueueSize ?? 1u;
+        var discardOldest = spec.DiscardOldest ?? true;
+        var monitoringMode = spec.MonitoringMode is { } mm ? MapMonitoringMode(mm) : MonitoringMode.Reporting;
+        var filter = BuildDataChangeFilter(spec.DataChangeFilter);
+
+        var options = new MonitoredItemOptions
+        {
+            DisplayName = spec.TagName,
+            StartNodeId = nodeId,
+            AttributeId = Attributes.Value,
+            MonitoringMode = monitoringMode,
+            SamplingInterval = sampling,
+            QueueSize = queueSize,
+            DiscardOldest = discardOldest,
+            Filter = filter,
+        };
+
+        return new MonitoredItem(telemetry: null!, options)
+        {
+            // The tag string is routed through MonitoredItem.Handle so the Notification
+            // handler can identify which tag changed without an extra lookup.
+            Handle = spec.TagName,
+        };
+    }
+
+    /// <summary>
+    ///     Build the OPC UA <see cref="DataChangeFilter"/> from a <see cref="DataChangeFilterSpec"/>,
+    ///     or return <c>null</c> if the caller didn't supply a filter. PercentDeadband requires
+    ///     server-side EURange — if the server rejects with BadFilterNotAllowed, the caller's
+    ///     <c>SubscribeAsync</c> swallows it so other items in the batch still get created.
+    /// </summary>
+    internal static DataChangeFilter? BuildDataChangeFilter(DataChangeFilterSpec? spec)
+    {
+        if (spec is null) return null;
+        return new DataChangeFilter
+        {
+            Trigger = MapTrigger(spec.Trigger),
+            DeadbandType = (uint)MapDeadbandType(spec.DeadbandType),
+            DeadbandValue = spec.DeadbandValue,
+        };
+    }
+
+    /// <summary>Map our SDK-free <see cref="SubscriptionMonitoringMode"/> to the OPC UA SDK's enum.</summary>
+    internal static MonitoringMode MapMonitoringMode(SubscriptionMonitoringMode mode) => mode switch
+    {
+        SubscriptionMonitoringMode.Disabled => MonitoringMode.Disabled,
+        SubscriptionMonitoringMode.Sampling => MonitoringMode.Sampling,
+        SubscriptionMonitoringMode.Reporting => MonitoringMode.Reporting,
+        _ => MonitoringMode.Reporting,
+    };
+
+    /// <summary>Map our <see cref="Core.Abstractions.DataChangeTrigger"/> to the SDK enum.</summary>
+    internal static Opc.Ua.DataChangeTrigger MapTrigger(Core.Abstractions.DataChangeTrigger trigger) => trigger switch
+    {
+        Core.Abstractions.DataChangeTrigger.Status => Opc.Ua.DataChangeTrigger.Status,
+        Core.Abstractions.DataChangeTrigger.StatusValue => Opc.Ua.DataChangeTrigger.StatusValue,
+        Core.Abstractions.DataChangeTrigger.StatusValueTimestamp => Opc.Ua.DataChangeTrigger.StatusValueTimestamp,
+        _ => Opc.Ua.DataChangeTrigger.StatusValue,
+    };
+
+    /// <summary>Map our <see cref="Core.Abstractions.DeadbandType"/> to the SDK enum.</summary>
+    internal static Opc.Ua.DeadbandType MapDeadbandType(Core.Abstractions.DeadbandType type) => type switch
+    {
+        Core.Abstractions.DeadbandType.None => Opc.Ua.DeadbandType.None,
+        Core.Abstractions.DeadbandType.Absolute => Opc.Ua.DeadbandType.Absolute,
+        Core.Abstractions.DeadbandType.Percent => Opc.Ua.DeadbandType.Percent,
+        _ => Opc.Ua.DeadbandType.None,
+    };
+
     public async Task UnsubscribeAsync(ISubscriptionHandle handle, CancellationToken cancellationToken)
     {
         if (handle is not OpcUaSubscriptionHandle h) return;

tests/ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient.Tests/OpcUaClientMonitoredTagSpecTests.cs

@@ -0,0 +1,175 @@
+using Opc.Ua;
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.OpcUaClient.Tests;
+
+/// <summary>
+///     Unit tests for <see cref="MonitoredTagSpec"/> -> SDK <c>MonitoredItem</c> mapping.
+///     Assertion-only — no live SDK session required, so the tests run on every CI without
+///     a real OPC UA server fixture.
+/// </summary>
+[Trait("Category", "Unit")]
+public sealed class OpcUaClientMonitoredTagSpecTests
+{
+    private static readonly NodeId SampleNodeId = new("Demo", 2);
+
+    [Fact]
+    public void BuildMonitoredItem_with_all_defaults_matches_legacy_hard_coded_values()
+    {
+        // Spec with every per-tag knob null should behave identically to the legacy
+        // string-only SubscribeAsync path: Reporting / SamplingInterval=publishInterval /
+        // QueueSize=1 / DiscardOldest=true / no filter.
+        var spec = new MonitoredTagSpec("ns=2;s=Demo");
+        var item = OpcUaClientDriver.BuildMonitoredItem(spec, SampleNodeId, defaultIntervalMs: 250);
+
+        item.SamplingInterval.ShouldBe(250);
+        item.QueueSize.ShouldBe(1u);
+        item.DiscardOldest.ShouldBeTrue();
+        item.MonitoringMode.ShouldBe(MonitoringMode.Reporting);
+        item.Filter.ShouldBeNull();
+        item.Handle.ShouldBe("ns=2;s=Demo",
+            "the tag string is routed through Handle so the Notification callback can identify the changed tag without re-parsing DisplayName");
+    }
+
+    [Fact]
+    public void BuildMonitoredItem_applies_per_tag_sampling_interval_independent_of_publish_interval()
+    {
+        // Per-tag SamplingInterval lets the server sample faster than it publishes — useful
+        // for events that change between publish ticks. If the spec sets it explicitly, the
+        // mapping uses that value, not the publish-interval default.
+        var spec = new MonitoredTagSpec("ns=2;s=Fast", SamplingIntervalMs: 50);
+        var item = OpcUaClientDriver.BuildMonitoredItem(spec, SampleNodeId, defaultIntervalMs: 1000);
+        item.SamplingInterval.ShouldBe(50);
+    }
+
+    [Fact]
+    public void BuildMonitoredItem_applies_queue_size_and_discard_oldest_overrides()
+    {
+        var spec = new MonitoredTagSpec("ns=2;s=DeepQueue", QueueSize: 100, DiscardOldest: false);
+        var item = OpcUaClientDriver.BuildMonitoredItem(spec, SampleNodeId, defaultIntervalMs: 250);
+        item.QueueSize.ShouldBe(100u);
+        item.DiscardOldest.ShouldBeFalse(
+            "discard-oldest=false preserves earliest values — useful for audit-trail subscriptions where the first overflow sample is the most diagnostic");
+    }
+
+    [Theory]
+    [InlineData(SubscriptionMonitoringMode.Disabled, MonitoringMode.Disabled)]
+    [InlineData(SubscriptionMonitoringMode.Sampling, MonitoringMode.Sampling)]
+    [InlineData(SubscriptionMonitoringMode.Reporting, MonitoringMode.Reporting)]
+    public void BuildMonitoredItem_maps_each_monitoring_mode(SubscriptionMonitoringMode input, MonitoringMode expected)
+    {
+        var spec = new MonitoredTagSpec("ns=2;s=Mode", MonitoringMode: input);
+        var item = OpcUaClientDriver.BuildMonitoredItem(spec, SampleNodeId, defaultIntervalMs: 250);
+        item.MonitoringMode.ShouldBe(expected);
+    }
+
+    [Fact]
+    public void BuildMonitoredItem_with_absolute_deadband_emits_DataChangeFilter()
+    {
+        var spec = new MonitoredTagSpec(
+            "ns=2;s=Analog",
+            DataChangeFilter: new DataChangeFilterSpec(
+                Core.Abstractions.DataChangeTrigger.StatusValue,
+                Core.Abstractions.DeadbandType.Absolute,
+                DeadbandValue: 0.5));
+        var item = OpcUaClientDriver.BuildMonitoredItem(spec, SampleNodeId, defaultIntervalMs: 250);
+        var filter = item.Filter.ShouldBeOfType<DataChangeFilter>();
+        filter.Trigger.ShouldBe(Opc.Ua.DataChangeTrigger.StatusValue);
+        filter.DeadbandType.ShouldBe((uint)Opc.Ua.DeadbandType.Absolute);
+        filter.DeadbandValue.ShouldBe(0.5);
+    }
+
+    [Fact]
+    public void BuildMonitoredItem_with_percent_deadband_emits_percent_filter()
+    {
+        // PercentDeadband is calculated server-side as a fraction of EURange; the driver
+        // emits the filter unconditionally and lets the server return BadFilterNotAllowed
+        // if EURange isn't set on the variable. SubscribeAsync's catch-block swallows that
+        // status so other items in the batch still get created.
+        var spec = new MonitoredTagSpec(
+            "ns=2;s=Pct",
+            DataChangeFilter: new DataChangeFilterSpec(
+                Core.Abstractions.DataChangeTrigger.StatusValueTimestamp,
+                Core.Abstractions.DeadbandType.Percent,
+                DeadbandValue: 5.0));
+        var item = OpcUaClientDriver.BuildMonitoredItem(spec, SampleNodeId, defaultIntervalMs: 250);
+        var filter = item.Filter.ShouldBeOfType<DataChangeFilter>();
+        filter.Trigger.ShouldBe(Opc.Ua.DataChangeTrigger.StatusValueTimestamp);
+        filter.DeadbandType.ShouldBe((uint)Opc.Ua.DeadbandType.Percent);
+        filter.DeadbandValue.ShouldBe(5.0);
+    }
+
+    [Theory]
+    [InlineData(Core.Abstractions.DataChangeTrigger.Status, Opc.Ua.DataChangeTrigger.Status)]
+    [InlineData(Core.Abstractions.DataChangeTrigger.StatusValue, Opc.Ua.DataChangeTrigger.StatusValue)]
+    [InlineData(Core.Abstractions.DataChangeTrigger.StatusValueTimestamp, Opc.Ua.DataChangeTrigger.StatusValueTimestamp)]
+    public void MapTrigger_round_trips_each_enum_value(
+        Core.Abstractions.DataChangeTrigger input, Opc.Ua.DataChangeTrigger expected)
+        => OpcUaClientDriver.MapTrigger(input).ShouldBe(expected);
+
+    [Theory]
+    [InlineData(Core.Abstractions.DeadbandType.None, Opc.Ua.DeadbandType.None)]
+    [InlineData(Core.Abstractions.DeadbandType.Absolute, Opc.Ua.DeadbandType.Absolute)]
+    [InlineData(Core.Abstractions.DeadbandType.Percent, Opc.Ua.DeadbandType.Percent)]
+    public void MapDeadbandType_round_trips_each_enum_value(
+        Core.Abstractions.DeadbandType input, Opc.Ua.DeadbandType expected)
+        => OpcUaClientDriver.MapDeadbandType(input).ShouldBe(expected);
+
+    [Fact]
+    public async Task DefaultInterfaceImplementation_routes_through_legacy_overload()
+    {
+        // ISubscribable's default interface impl of the per-tag overload delegates to the
+        // simple-string overload, ignoring per-tag knobs. Drivers that DON'T override the
+        // new overload (Modbus / S7 / Galaxy / TwinCAT / FOCAS / AbCip / AbLegacy) still
+        // accept MonitoredTagSpec lists and just pass through the tag names — back-compat
+        // for ISubscribable consumers.
+        var stub = new StubSubscribableDriver();
+        var specs = new[]
+        {
+            new MonitoredTagSpec("Tag1", SamplingIntervalMs: 50, QueueSize: 5),
+            new MonitoredTagSpec("Tag2", DataChangeFilter: new DataChangeFilterSpec(
+                Core.Abstractions.DataChangeTrigger.StatusValue,
+                Core.Abstractions.DeadbandType.Absolute,
+                1.0)),
+        };
+
+        ISubscribable iface = stub;
+        _ = await iface.SubscribeAsync(specs, TimeSpan.FromMilliseconds(250), TestContext.Current.CancellationToken);
+
+        stub.LastTagNames.ShouldBe(["Tag1", "Tag2"]);
+        stub.LastPublishingInterval.ShouldBe(TimeSpan.FromMilliseconds(250));
+    }
+
+    /// <summary>
+    ///     Test-double <see cref="ISubscribable"/> that records whatever the legacy
+    ///     <c>SubscribeAsync(IReadOnlyList&lt;string&gt;, ...)</c> overload was called with.
+    ///     Used to verify the default-impl per-tag overload routes correctly without needing
+    ///     a real OPC UA session.
+    /// </summary>
+    private sealed class StubSubscribableDriver : ISubscribable
+    {
+        public IReadOnlyList<string>? LastTagNames { get; private set; }
+        public TimeSpan LastPublishingInterval { get; private set; }
+
+        public Task<ISubscriptionHandle> SubscribeAsync(
+            IReadOnlyList<string> fullReferences, TimeSpan publishingInterval, CancellationToken cancellationToken)
+        {
+            LastTagNames = fullReferences;
+            LastPublishingInterval = publishingInterval;
+            return Task.FromResult<ISubscriptionHandle>(new StubHandle());
+        }
+
+        public Task UnsubscribeAsync(ISubscriptionHandle handle, CancellationToken cancellationToken) => Task.CompletedTask;
+
+#pragma warning disable CS0067 // event never used — the test only asserts the SubscribeAsync call routing
+        public event EventHandler<DataChangeEventArgs>? OnDataChange;
+#pragma warning restore CS0067
+    }
+
+    private sealed record StubHandle() : ISubscriptionHandle
+    {
+        public string DiagnosticId => "stub";
+    }
+}