fix(centralui): enable Test Bindings for MxGateway connections

The Test Bindings button was disabled (greyed out) for any attribute bound
to a non-OPC-UA connection. BuildTestableRows() filtered to protocol ==
"OpcUa", a stale gate left over from when OPC UA was the only protocol.
ReadTagValuesCommand is protocol-agnostic (routes through
IDataConnection.ReadBatchAsync, which MxGatewayDataConnection implements),
so the filter only blocked the UI — mirroring the already-fixed IsBrowsable.

Remove the OPC-UA-only filter and update the stale comments. Add a bUnit
regression test (theory over MxGateway + OpcUa) asserting the button is
enabled for a readable-protocol binding.

Verified live: dialog opens for an MxGateway binding and returns a
Good-quality read.

docs/requirements/Component-DataConnectionLayer.md

@@ -62,7 +62,7 @@ All protocols produce the same value tuple consumed by Instance Actors. Before t
 - Connects to the **MxAccess Gateway** (AVEVA/Wonderware MXAccess-backed Galaxy) over gRPC using the `ZB.MOM.WW.MxGateway.Client` NuGet package (from the Gitea feed); `ZB.MOM.WW.MxGateway.Contracts` is pulled in transitively.
 - Session-based: `OpenSession` + `Register` on connect; `AddItem` + `Advise` per subscription; value changes arrive on the gateway's server-streaming event feed (`StreamEvents`), resumable via `worker_sequence`.
 - Read/Write via `ReadBulk` / `WriteBulk`; writes carry a configurable `WriteUserId`. Quality maps the OPC-style quality byte (≥192 Good, ≥64 Uncertain, else Bad), with a failing MXAccess status proxy treated as Bad.
-- Galaxy hierarchy browse via the separate `GalaxyRepositoryClient` (`BrowseChildren`) — objects are navigable nodes (keyed by Galaxy gobject id), attributes are selectable leaves (keyed by full tag reference).
+- Galaxy hierarchy browse via the separate `GalaxyRepositoryClient` — objects are navigable nodes (keyed by Galaxy gobject id), attributes are selectable leaves (keyed by full tag reference). Browse is **lazy and attribute-light**: navigation uses `BrowseChildren` with `include_attributes=false` (child objects only), and an object's own attributes are fetched only when it is expanded, via `DiscoverHierarchy(root=<object>, max_depth=0)` scoped to that single object. This keeps each browse level's reply small; inlining every child's full attribute set could exceed the Akka remote frame and silently drop the reply.
 - Disconnect detection: a fault on the event stream raises `IDataConnection.Disconnected`, driving the same reconnection state machine as OPC UA.
 - Implemented as `MxGatewayDataConnection` over an `IMxGatewayClient` seam; the seam is decoupled from the generated gRPC types (only `RealMxGatewayClient` references them), so the adapter is fully unit-testable with a fake.
 
@@ -171,6 +171,7 @@ DCL is a clean data pipe on the hot path. Browse is an **opt-in capability** for
 - `DataConnectionManagerActor` handles `BrowseNodeCommand` (fields: `ConnectionName`, `ParentNodeId`) and replies with `BrowseNodeResult` (children + `Truncated` + structured `BrowseFailure?`). The Central UI facade is `IBrowseService`/`BrowseService`, backing the `NodeBrowserDialog` tag picker.
 - Node ids are opaque protocol-specific strings: OPC UA uses NodeIds; MxGateway uses Galaxy gobject ids for navigable objects and full tag references for selectable attribute leaves.
 - Browse runs against the live session; no caching at DCL.
+- **Frame-size guard**: the reply crosses the site→central Akka frame (default 128 KB) on a temp Ask actor; an oversized reply is silently discarded by remoting, hanging the picker. The child handler caps each `BrowseNodeResult` to a byte budget (~100 KB) before replying, OR-ing the adapter's own truncation signal into `Truncated`. This is protocol-agnostic (every adapter's reply funnels through it). Per-protocol upstream caps narrow the window first: OPC UA requests at most 500 references per node (continuation point → `Truncated`); MxGateway relies on the gateway's `BrowseChildren` page cap. A `Truncated` level prompts manual node-id entry in the picker rather than auto-paging.
 
 ## Value Update Message Format
 

src/ZB.MOM.WW.ScadaBridge.CentralUI/Components/Dialogs/NodeBrowserDialog.razor

@@ -6,7 +6,7 @@
 @if (_isVisible)
 {
     <div class="modal show d-block" tabindex="-1" role="dialog" style="background-color: rgba(0,0,0,0.5);">
-        <div class="modal-dialog modal-lg" role="document">
+        <div class="modal-dialog modal-xl" role="document">
             <div class="modal-content">
                 <div class="modal-header">
                     <h5 class="modal-title">Browse — @ConnectionName</h5>
@@ -21,7 +21,7 @@
                         </div>
                     }
 
-                    <div class="opcua-browser-tree">
+                    <div class="node-browser-tree">
                         @if (_rootNodes.Count == 0 && _failure is null)
                         {
                             <em class="text-muted">Loading…</em>
@@ -167,20 +167,25 @@
         _manualNodeId = node.NodeId;
     }
 
-    // Task 17: map each BrowseFailureKind to a friendly UI message. The raw
-    // failure.Message is surfaced verbatim only for ServerError (which carries
-    // the OPC UA SDK's own Bad_* text) and as the default fallback for any
-    // future failure kind added without a UI mapping.
+    // Task 17: map each BrowseFailureKind to a friendly UI message. Messages are
+    // protocol-agnostic (the dialog serves every browsable protocol — OPC UA,
+    // MxGateway, …). The raw failure.Message is surfaced verbatim for ServerError
+    // (which carries the underlying protocol SDK's own error text), for
+    // NotBrowsable when the adapter supplied a reason (e.g. a gateway build that
+    // lacks the browse RPC), and as the default fallback for any future failure
+    // kind added without a UI mapping.
     private void SetFailure(BrowseFailure failure)
     {
         _failure = failure;
         _failureMessage = failure.Kind switch
         {
             BrowseFailureKind.ConnectionNotFound      => "Connection no longer exists at the site.",
-            BrowseFailureKind.ConnectionNotConnected  => "OPC UA session not connected — retry shortly or use manual entry.",
-            BrowseFailureKind.NotBrowsable            => "This connection does not support browsing.",
+            BrowseFailureKind.ConnectionNotConnected  => "Connection not connected — retry shortly or use manual entry.",
+            BrowseFailureKind.NotBrowsable            => string.IsNullOrWhiteSpace(failure.Message)
+                                                            ? "This connection does not support browsing."
+                                                            : failure.Message,
             BrowseFailureKind.Timeout                 => "Browse timed out — the server may be slow. Try again or enter the node id manually.",
-            BrowseFailureKind.ServerError             => $"OPC UA server error: {failure.Message}",
+            BrowseFailureKind.ServerError             => $"Server error: {failure.Message}",
             _                                          => failure.Message
         };
         StateHasChanged();

src/ZB.MOM.WW.ScadaBridge.CentralUI/Components/Dialogs/NodeBrowserDialog.razor.css

@@ -0,0 +1,14 @@
+/* Scoped styles for the protocol-agnostic tag browse dialog. */
+
+/* Cap the tree's height and let it scroll independently so deep hierarchies
+   (e.g. a Galaxy with many objects/attributes) don't push the manual-entry
+   field and Select/Cancel buttons off-screen. Both axes scroll: vertical for
+   long sibling lists, horizontal for deeply-indented nested nodes. */
+.node-browser-tree {
+    max-height: 55vh;
+    overflow: auto;
+    border: 1px solid var(--bs-border-color, #dee2e6);
+    border-radius: 0.375rem;
+    padding: 0.5rem 0.75rem;
+    background-color: var(--bs-body-bg, #fff);
+}

src/ZB.MOM.WW.ScadaBridge.CentralUI/Components/Pages/Deployment/InstanceConfigure.razor

@@ -158,9 +158,10 @@
                         <button class="btn btn-success btn-sm" @onclick="SaveBindings" disabled="@_saving">Save Bindings</button>
                         @* Test Bindings: one-shot live read of every bound attribute
                            whose row has a connection picked AND an effective tag
-                           path. Disabled when no testable rows. Currently OPC UA
-                           only — other protocols (none yet) would need their own
-                           wire+adapter support to round-trip through ReadTagValuesCommand. *@
+                           path. Disabled when no testable rows. Protocol-agnostic —
+                           any connection whose adapter implements ReadBatchAsync
+                           (OPC UA and MxGateway today) round-trips through
+                           ReadTagValuesCommand. *@
                         <button class="btn btn-outline-primary btn-sm"
                                 @onclick="OpenTestBindings"
                                 disabled="@(!HasTestableBindings())">
@@ -612,8 +613,10 @@
 
     /// <summary>
     /// Builds the list of testable rows: attributes that have a connection
-    /// picked AND a non-empty effective tag path AND an OPC UA connection
-    /// (the only protocol routed through <c>ReadTagValuesCommand</c> today).
+    /// picked AND a non-empty effective tag path. Protocol-agnostic — every
+    /// data-connection adapter implements <c>ReadBatchAsync</c>, so the read
+    /// routes through <c>ReadTagValuesCommand</c> regardless of protocol
+    /// (OPC UA and MxGateway today).
     /// </summary>
     private List<TestBindingsDialog.BindingRowToTest> BuildTestableRows()
     {
@@ -626,10 +629,11 @@
             var conn = _siteConnections.FirstOrDefault(c => c.Id == connId);
             if (conn is null) continue;
 
-            // OPC UA only — other protocols don't have a site-side
-            // ReadTagValuesCommand handler wired up yet.
-            if (!string.Equals(conn.Protocol, "OpcUa", StringComparison.OrdinalIgnoreCase))
-                continue;
+            // Protocol-agnostic: ReadTagValuesCommand routes through the
+            // site-side IDataConnection.ReadBatchAsync contract, which every
+            // adapter implements (OPC UA and MxGateway today). A not-connected
+            // or unsupported connection short-circuits to a typed banner in the
+            // dialog rather than being filtered out here — mirrors IsBrowsable.
 
             var effectivePath = _bindingOverrides.GetValueOrDefault(attr.Name)
                                 ?? GetTemplateDefault(attr.Name);

src/ZB.MOM.WW.ScadaBridge.DataConnectionLayer/Actors/DataConnectionActor.cs

@@ -989,7 +989,7 @@ public class DataConnectionActor : UntypedActor, IWithStash, IWithTimers
     ///
     /// Failure mapping:
     /// <list type="bullet">
-    ///   <item><see cref="BrowseFailureKind.NotBrowsable"/> — adapter is not <see cref="IBrowsableDataConnection"/>.</item>
+    ///   <item><see cref="BrowseFailureKind.NotBrowsable"/> — adapter is not <see cref="IBrowsableDataConnection"/>, or it threw <see cref="NotSupportedException"/> (browsable adapter, but the server/protocol cannot browse — e.g. a gateway build predating the browse RPC); message carried verbatim in the latter case.</item>
     ///   <item><see cref="BrowseFailureKind.ConnectionNotConnected"/> — adapter threw <see cref="ConnectionNotConnectedException"/>.</item>
     ///   <item><see cref="BrowseFailureKind.Timeout"/> — adapter threw <see cref="OperationCanceledException"/>.</item>
     ///   <item><see cref="BrowseFailureKind.ServerError"/> — any other exception, message carried verbatim.</item>
@@ -1015,13 +1015,16 @@ public class DataConnectionActor : UntypedActor, IWithStash, IWithTimers
             return;
         }
 
-        _log.Debug("[{0}] Browsing OPC UA children of {1}", _connectionName, command.ParentNodeId ?? "(root)");
+        _log.Debug("[{0}] Browsing children of {1}", _connectionName, command.ParentNodeId ?? "(root)");
 
         browsable.BrowseChildrenAsync(command.ParentNodeId).ContinueWith(t =>
         {
             if (t.IsCompletedSuccessfully)
             {
-                return new BrowseNodeResult(t.Result.Children, t.Result.Truncated, Failure: null);
+                // Bound the reply to stay under Akka's remote frame size before it
+                // crosses the site→central boundary (see CapBrowseChildren).
+                var (children, truncated) = CapBrowseChildren(t.Result.Children, t.Result.Truncated);
+                return new BrowseNodeResult(children, truncated, Failure: null);
             }
 
             var baseEx = t.Exception?.GetBaseException();
@@ -1035,6 +1038,13 @@ public class DataConnectionActor : UntypedActor, IWithStash, IWithTimers
                     Array.Empty<BrowseNode>(),
                     Truncated: false,
                     new BrowseFailure(BrowseFailureKind.Timeout, "Browse cancelled.")),
+                // Adapter reachable but the protocol/server cannot browse (e.g. an
+                // MxGateway build that predates the BrowseChildren RPC). Carry the
+                // adapter's explanatory message through as NotBrowsable.
+                NotSupportedException notSupported => new BrowseNodeResult(
+                    Array.Empty<BrowseNode>(),
+                    Truncated: false,
+                    new BrowseFailure(BrowseFailureKind.NotBrowsable, notSupported.Message)),
                 _ => new BrowseNodeResult(
                     Array.Empty<BrowseNode>(),
                     Truncated: false,
@@ -1045,6 +1055,47 @@ public class DataConnectionActor : UntypedActor, IWithStash, IWithTimers
         }).PipeTo(sender);
     }
 
+    /// <summary>
+    /// Estimated-byte ceiling for a single <see cref="BrowseNodeResult"/>, kept
+    /// comfortably below Akka's default 128 KB remote frame size. A browse reply
+    /// crosses the site→central frame on a temp Ask actor; an oversized reply is
+    /// silently discarded by remoting (the picker then hangs on "loading…"). The
+    /// limit is a byte budget rather than a child count because the only thing
+    /// that actually consumes frame space is serialized size — OPC UA NodeIds and
+    /// MxGateway tag references vary widely in length, so a fixed count is not a
+    /// safe proxy.
+    /// </summary>
+    private const int BrowseResultByteBudget = 100 * 1024;
+
+    /// <summary>
+    /// Truncates a browse child list to <see cref="BrowseResultByteBudget"/> using
+    /// a conservative per-node size estimate (JSON structural overhead plus the two
+    /// variable-length strings — ASCII NodeId/DisplayName ≈ 1 byte/char). Returns
+    /// the kept prefix and a <c>Truncated</c> flag OR-ed with the adapter's own
+    /// truncation signal, so the picker shows its "use manual entry" hint when the
+    /// level is clipped. Protocol-agnostic: every adapter's reply funnels through
+    /// here regardless of how it paginates upstream.
+    /// </summary>
+    private static (IReadOnlyList<BrowseNode> Children, bool Truncated) CapBrowseChildren(
+        IReadOnlyList<BrowseNode> children, bool truncated)
+    {
+        var budget = 0;
+        var kept = new List<BrowseNode>(children.Count);
+        foreach (var node in children)
+        {
+            budget += 64 + (node.NodeId?.Length ?? 0) + (node.DisplayName?.Length ?? 0);
+            if (budget > BrowseResultByteBudget)
+            {
+                truncated = true;
+                break;
+            }
+
+            kept.Add(node);
+        }
+
+        return (kept, truncated);
+    }
+
     // ── Test Bindings (one-shot live read of bound tags) ──
 
     /// <summary>

src/ZB.MOM.WW.ScadaBridge.DataConnectionLayer/Adapters/RealMxGatewayClient.cs

@@ -138,39 +138,83 @@ public sealed class RealMxGatewayClient : IMxGatewayClient
     /// <inheritdoc />
     public async Task<(IReadOnlyList<MxBrowseChild> Children, bool Truncated)> BrowseChildrenAsync(string? parentNodeId, CancellationToken ct = default)
     {
-        var request = new BrowseChildrenRequest { IncludeAttributes = true };
+        // Navigation browse returns child OBJECTS only (IncludeAttributes = false).
+        // An object's own attributes are loaded lazily — via DiscoverHierarchy below,
+        // scoped to that single object — only when the object is expanded. This keeps
+        // each browse level's payload to a few KB. Inlining every child's full
+        // attribute set (the previous behaviour) produced replies that exceeded
+        // Akka's remote frame size (e.g. ~152 KB for one attribute-heavy area), and
+        // remoting silently discarded the oversized reply, hanging the picker.
+        var request = new BrowseChildrenRequest { IncludeAttributes = false };
+
         // Object NodeIds are the Galaxy gobject id (encoded as a string); attribute
         // NodeIds are FullTagReference leaves and never arrive here as a parent.
-        if (!string.IsNullOrEmpty(parentNodeId)
-            && int.TryParse(parentNodeId, NumberStyles.Integer, CultureInfo.InvariantCulture, out var gobjectId))
+        var parentGobjectId = 0;
+        var haveParentObject = !string.IsNullOrEmpty(parentNodeId)
+            && int.TryParse(parentNodeId, NumberStyles.Integer, CultureInfo.InvariantCulture, out parentGobjectId);
+        if (haveParentObject)
         {
-            request.ParentGobjectId = gobjectId;
+            request.ParentGobjectId = parentGobjectId;
         }
 
         BrowseChildrenReply reply;
+        GalaxyObject? parentObject = null;
         try
         {
             reply = await _galaxy!.BrowseChildrenRawAsync(request, ct).ConfigureAwait(false);
+
+            // When expanding a concrete object, fetch just that object (MaxDepth = 0)
+            // with its attributes so they can be listed as selectable leaves. The root
+            // level has no parent object, hence no attributes of its own.
+            if (haveParentObject)
+            {
+                var attrRequest = new DiscoverHierarchyRequest
+                {
+                    RootGobjectId = parentGobjectId,
+                    MaxDepth = 0,
+                    IncludeAttributes = true,
+                };
+                var attrReply = await _galaxy!.DiscoverHierarchyRawAsync(attrRequest, ct).ConfigureAwait(false);
+                parentObject = attrReply.Objects.Count > 0 ? attrReply.Objects[0] : null;
+            }
         }
         catch (RpcException ex) when (ex.StatusCode == StatusCode.Unavailable)
         {
             throw new ConnectionNotConnectedException($"MxGateway repository unavailable: {ex.Status.Detail}");
         }
+        catch (RpcException ex) when (ex.StatusCode == StatusCode.Unimplemented)
+        {
+            // The data pipe (read/subscribe/write) works against every gateway
+            // build, but Galaxy hierarchy browsing (the BrowseChildren RPC) was
+            // added later. An older gateway answers Unimplemented. Surface a
+            // clear, actionable reason instead of a raw gRPC fault — the actor
+            // maps NotSupportedException to BrowseFailureKind.NotBrowsable and
+            // carries this message through to the picker.
+            throw new NotSupportedException(
+                "The connected MxGateway build does not support hierarchy browsing. "
+                + "Update the gateway to a build that implements BrowseChildren, "
+                + "or enter the tag reference manually.");
+        }
 
         var children = new List<MxBrowseChild>();
-        for (var i = 0; i < reply.Children.Count; i++)
+
+        // Navigable child objects, keyed by gobject id. Always marked expandable —
+        // every Galaxy object carries attributes (and may host sub-objects), both
+        // resolved on demand when the node is expanded.
+        foreach (var obj in reply.Children)
         {
-            var obj = reply.Children[i];
-            var hasChildren = i < reply.ChildHasChildren.Count && reply.ChildHasChildren[i];
-            // Navigable container node, keyed by gobject id.
             children.Add(new MxBrowseChild(
                 obj.GobjectId.ToString(CultureInfo.InvariantCulture),
                 string.IsNullOrEmpty(obj.TagName) ? obj.ContainedName : obj.TagName,
                 BrowseNodeClass.Object,
-                hasChildren || obj.Attributes.Count > 0));
+                true));
+        }
 
-            // Selectable attribute leaves, keyed by their full tag reference.
-            foreach (var attr in obj.Attributes)
+        // The expanded object's own attributes, as selectable leaves keyed by their
+        // full tag reference.
+        if (parentObject is not null)
+        {
+            foreach (var attr in parentObject.Attributes)
             {
                 children.Add(new MxBrowseChild(
                     attr.FullTagReference,

src/ZB.MOM.WW.ScadaBridge.DataConnectionLayer/Adapters/RealOpcUaClient.cs

@@ -353,11 +353,16 @@ public class RealOpcUaClient : IOpcUaClient
         // Variables (selectable), Methods (display-only).
         var nodeClassMask = (uint)(NodeClass.Object | NodeClass.Variable | NodeClass.Method);
 
+        // requestedMaxReferencesPerNode: cap the server's per-call references so a
+        // huge flat folder cannot return an unbounded set. 500 leaves headroom for
+        // the downstream frame-size budget (DataConnectionActor.CapBrowseChildren)
+        // even with long string NodeIds; a non-empty continuation point surfaces as
+        // Truncated, prompting manual entry rather than auto-paging.
         var (_, continuationPoint, references) = await session.BrowseAsync(
             null,
             null,
             nodeToBrowse,
-            1000u,
+            500u,
             BrowseDirection.Forward,
             ReferenceTypeIds.HierarchicalReferences,
             true,

tests/ZB.MOM.WW.ScadaBridge.CentralUI.Tests/Deployment/InstanceConfigureAuditDrillinTests.cs

@@ -104,4 +104,63 @@ public class InstanceConfigureAuditDrillinTests : BunitContext
             Assert.Contains("Recent audit activity", link.TextContent);
         });
     }
+
+    /// <summary>
+    /// Regression: the Test Bindings button must be enabled for an attribute
+    /// bound to an MxGateway connection. The site-side ReadTagValuesCommand
+    /// path is protocol-agnostic (routes through IDataConnection.ReadBatchAsync,
+    /// which MxGateway implements), so the UI must not gate the button on
+    /// protocol == "OpcUa". Previously BuildTestableRows filtered to OPC UA
+    /// only, leaving the button greyed for MxGateway bindings.
+    /// </summary>
+    [Theory]
+    [InlineData("MxGateway")]
+    [InlineData("OpcUa")]
+    public void TestBindingsButton_Enabled_ForReadableProtocol(string protocol)
+    {
+        var instance = new Instance("Pump-42")
+        {
+            Id = 42,
+            TemplateId = 1,
+            SiteId = 1,
+            State = ZB.MOM.WW.ScadaBridge.Commons.Types.Enums.InstanceState.NotDeployed,
+        };
+
+        _templateRepo.GetInstanceByIdAsync(42, Arg.Any<CancellationToken>()).Returns(instance);
+        _templateRepo.GetTemplateByIdAsync(1, Arg.Any<CancellationToken>())
+            .Returns(new Template("Pump") { Id = 1 });
+        _siteRepo.GetAllSitesAsync(Arg.Any<CancellationToken>())
+            .Returns(new List<Site> { new("Plant A", "plant-a") { Id = 1 } });
+        _templateRepo.GetAreasBySiteIdAsync(1, Arg.Any<CancellationToken>())
+            .Returns(new List<Area>());
+        // One data-sourced attribute (non-empty DataSourceReference => testable row).
+        _templateRepo.GetAttributesByTemplateIdAsync(1, Arg.Any<CancellationToken>())
+            .Returns(new List<TemplateAttribute>
+            {
+                new("Speed") { Id = 1, DataSourceReference = "TestMachine_001.TestHistoryValue" },
+            });
+        // A connection on the attribute's chosen protocol.
+        _siteRepo.GetDataConnectionsBySiteIdAsync(1, Arg.Any<CancellationToken>())
+            .Returns(new List<DataConnection> { new("Shared", protocol, 1) { Id = 7 } });
+        _templateRepo.GetBindingsByInstanceIdAsync(42, Arg.Any<CancellationToken>())
+            .Returns(new List<InstanceConnectionBinding>
+            {
+                new("Speed") { Id = 1, InstanceId = 42, DataConnectionId = 7 },
+            });
+        _templateRepo.GetOverridesByInstanceIdAsync(42, Arg.Any<CancellationToken>())
+            .Returns(new List<InstanceAttributeOverride>());
+        _templateRepo.GetAlarmsByTemplateIdAsync(1, Arg.Any<CancellationToken>())
+            .Returns(new List<TemplateAlarm>());
+        _templateRepo.GetAlarmOverridesByInstanceIdAsync(42, Arg.Any<CancellationToken>())
+            .Returns(new List<InstanceAlarmOverride>());
+
+        var cut = Render<InstanceConfigurePage>(p => p.Add(c => c.Id, 42));
+
+        cut.WaitForAssertion(() =>
+        {
+            var testButton = cut.FindAll("button").Single(b => b.TextContent.Trim() == "Test Bindings");
+            Assert.False(testButton.HasAttribute("disabled"),
+                $"Test Bindings should be enabled for a {protocol} binding.");
+        });
+    }
 }

tests/ZB.MOM.WW.ScadaBridge.DataConnectionLayer.Tests/Actors/DataConnectionManagerBrowseHandlerTests.cs

@@ -162,4 +162,95 @@ public class DataConnectionManagerBrowseHandlerTests : TestKit
         Assert.Equal(BrowseFailureKind.ConnectionNotConnected, reply.Failure!.Kind);
         Assert.Empty(reply.Children);
     }
+
+    [Fact]
+    public void NotSupportedException_maps_to_NotBrowsable_carrying_message()
+    {
+        // A browsable adapter that is connected but whose server/protocol cannot
+        // browse (e.g. an MxGateway build predating the BrowseChildren RPC, which
+        // answers gRPC Unimplemented) throws NotSupportedException. The actor must
+        // surface this as NotBrowsable with the adapter's actionable message
+        // carried verbatim — distinct from the capability-check NotBrowsable
+        // (non-browsable adapter), which has no message.
+        const string reason =
+            "The connected MxGateway build does not support hierarchy browsing. "
+            + "Update the gateway to a build that implements BrowseChildren, "
+            + "or enter the tag reference manually.";
+
+        var adapter = Substitute.For<IDataConnection, IBrowsableDataConnection>();
+        ((IDataConnection)adapter).ConnectAsync(Arg.Any<IDictionary<string, string>>(), Arg.Any<CancellationToken>())
+            .Returns(Task.CompletedTask);
+        ((IDataConnection)adapter).Status.Returns(ConnectionHealth.Connected);
+
+        ((IBrowsableDataConnection)adapter)
+            .BrowseChildrenAsync(Arg.Any<string?>(), Arg.Any<CancellationToken>())
+            .Returns(Task.FromException<BrowseChildrenResult>(new NotSupportedException(reason)));
+
+        _factory.Create("MxGateway", Arg.Any<IDictionary<string, string>>())
+            .Returns((IDataConnection)adapter);
+
+        var manager = Sys.ActorOf(Props.Create(() =>
+            new DataConnectionManagerActor(_factory, _options, _healthCollector, null)));
+        manager.Tell(new CreateConnectionCommand(
+            "conn-old-gw", "MxGateway", new Dictionary<string, string>(), null, 3));
+
+        AwaitCondition(
+            () => _factory.ReceivedCalls().Any(c => c.GetMethodInfo().Name == "Create"),
+            TimeSpan.FromSeconds(2));
+
+        manager.Tell(new BrowseNodeCommand("conn-old-gw", ParentNodeId: null));
+
+        var reply = ExpectMsg<BrowseNodeResult>(TimeSpan.FromSeconds(3));
+        Assert.NotNull(reply.Failure);
+        Assert.Equal(BrowseFailureKind.NotBrowsable, reply.Failure!.Kind);
+        Assert.Equal(reason, reply.Failure.Message);
+        Assert.Empty(reply.Children);
+    }
+
+    [Fact]
+    public void Oversized_child_list_is_capped_to_frame_budget_and_marked_truncated()
+    {
+        // A level large enough to exceed Akka's 128 KB remote frame must be clipped
+        // by the actor BEFORE it crosses the site→central boundary — otherwise the
+        // reply is silently discarded and the picker hangs. This guards every
+        // protocol's reply, regardless of how the adapter paginates upstream. The
+        // adapter here reports Truncated=false; the actor must still truncate purely
+        // on serialized size.
+        const int byteBudget = 100 * 1024; // mirrors DataConnectionActor.BrowseResultByteBudget
+        var bigList = Enumerable.Range(0, 3000)
+            .Select(i => new BrowseNode($"ns=2;s=Item{i:D5}", $"Item{i:D5}", BrowseNodeClass.Variable, HasChildren: false))
+            .ToArray();
+
+        var adapter = Substitute.For<IDataConnection, IBrowsableDataConnection>();
+        ((IDataConnection)adapter).ConnectAsync(Arg.Any<IDictionary<string, string>>(), Arg.Any<CancellationToken>())
+            .Returns(Task.CompletedTask);
+        ((IDataConnection)adapter).Status.Returns(ConnectionHealth.Connected);
+        ((IBrowsableDataConnection)adapter)
+            .BrowseChildrenAsync(null, Arg.Any<CancellationToken>())
+            .Returns(new BrowseChildrenResult(bigList, Truncated: false));
+
+        _factory.Create("OpcUa", Arg.Any<IDictionary<string, string>>())
+            .Returns((IDataConnection)adapter);
+
+        var manager = Sys.ActorOf(Props.Create(() =>
+            new DataConnectionManagerActor(_factory, _options, _healthCollector, null)));
+        manager.Tell(new CreateConnectionCommand(
+            "conn-big", "OpcUa", new Dictionary<string, string>(), null, 3));
+
+        AwaitCondition(
+            () => _factory.ReceivedCalls().Any(c => c.GetMethodInfo().Name == "Create"),
+            TimeSpan.FromSeconds(2));
+
+        manager.Tell(new BrowseNodeCommand("conn-big", ParentNodeId: null));
+
+        var reply = ExpectMsg<BrowseNodeResult>(TimeSpan.FromSeconds(3));
+        Assert.Null(reply.Failure);
+        Assert.True(reply.Truncated, "an oversized level must be reported as truncated");
+        Assert.True(reply.Children.Count > 0, "the cap must still return a usable prefix");
+        Assert.True(reply.Children.Count < bigList.Length, "the level must actually be clipped");
+
+        // The kept prefix's estimated serialized size must respect the budget.
+        var keptBytes = reply.Children.Sum(n => 64 + n.NodeId.Length + n.DisplayName.Length);
+        Assert.True(keptBytes <= byteBudget, $"kept estimate {keptBytes} exceeds budget {byteBudget}");
+    }
 }