diff --git a/src/ZB.MOM.WW.OtOpcUa.Driver.Galaxy/Browse/AlarmRefBuilder.cs b/src/ZB.MOM.WW.OtOpcUa.Driver.Galaxy/Browse/AlarmRefBuilder.cs
new file mode 100644
index 0000000..bd0d4b6
--- /dev/null
+++ b/src/ZB.MOM.WW.OtOpcUa.Driver.Galaxy/Browse/AlarmRefBuilder.cs
@@ -0,0 +1,51 @@
+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Browse;
+
+/// <summary>
+///     Populates the five sub-attribute references on <see cref="AlarmConditionInfo"/>
+///     by Galaxy convention. The server-level <c>AlarmConditionService</c> (PR 2.2) uses
+///     these to subscribe to live alarm-state attributes and to route ack writes back to
+///     the alarm tag.
+/// </summary>
+/// <remarks>
+///     Galaxy alarms expose four runtime attributes plus a write-only ack target,
+///     consistently named on every alarm-bearing object:
+///     <list type="bullet">
+///       <item><c>&lt;tag&gt;.&lt;attr&gt;.InAlarm</c></item>
+///       <item><c>&lt;tag&gt;.&lt;attr&gt;.Priority</c></item>
+///       <item><c>&lt;tag&gt;.&lt;attr&gt;.DescAttrName</c></item>
+///       <item><c>&lt;tag&gt;.&lt;attr&gt;.Acked</c></item>
+///       <item><c>&lt;tag&gt;.&lt;attr&gt;.AckMsg</c></item>
+///     </list>
+///     This is the same convention the legacy <c>GalaxyAlarmTracker</c> hard-coded; we
+///     concentrate it here so PR 2.2's service receives complete <c>AlarmConditionInfo</c>
+///     rows during discovery without the server needing to know the convention.
+/// </remarks>
+internal static class AlarmRefBuilder
+{
+    private const string InAlarmSuffix = ".InAlarm";
+    private const string PrioritySuffix = ".Priority";
+    private const string DescAttrNameSuffix = ".DescAttrName";
+    private const string AckedSuffix = ".Acked";
+    private const string AckMsgSuffix = ".AckMsg";
+
+    /// <summary>
+    ///     Build an <see cref="AlarmConditionInfo"/> for an alarm-bearing attribute with all
+    ///     five sub-attribute references populated. <paramref name="fullReference"/> is the
+    ///     attribute's full reference (e.g. <c>"Tank1.Level.HiHi"</c>); the convention prefixes
+    ///     each suffix to it.
+    /// </summary>
+    public static AlarmConditionInfo Build(
+        string fullReference,
+        AlarmSeverity initialSeverity = AlarmSeverity.Medium,
+        string? initialDescription = null) => new(
+            SourceName: fullReference,
+            InitialSeverity: initialSeverity,
+            InitialDescription: initialDescription,
+            InAlarmRef: fullReference + InAlarmSuffix,
+            PriorityRef: fullReference + PrioritySuffix,
+            DescAttrNameRef: fullReference + DescAttrNameSuffix,
+            AckedRef: fullReference + AckedSuffix,
+            AckMsgWriteRef: fullReference + AckMsgSuffix);
+}
diff --git a/src/ZB.MOM.WW.OtOpcUa.Driver.Galaxy/Browse/DataTypeMap.cs b/src/ZB.MOM.WW.OtOpcUa.Driver.Galaxy/Browse/DataTypeMap.cs
new file mode 100644
index 0000000..83456c5
--- /dev/null
+++ b/src/ZB.MOM.WW.OtOpcUa.Driver.Galaxy/Browse/DataTypeMap.cs
@@ -0,0 +1,23 @@
+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Browse;
+
+/// <summary>
+///     Maps Galaxy <c>mx_data_type</c> integer codes to <see cref="DriverDataType"/>.
+///     Ported from the legacy <c>GalaxyProxyDriver.MapDataType</c> with the same fallback
+///     to <see cref="DriverDataType.String"/> for unknown codes — keeps wire compatibility
+///     with deployed configs while we tighten this through the parity matrix.
+/// </summary>
+internal static class DataTypeMap
+{
+    public static DriverDataType Map(int mxDataType) => mxDataType switch
+    {
+        0 => DriverDataType.Boolean,
+        1 => DriverDataType.Int32,
+        2 => DriverDataType.Float32,
+        3 => DriverDataType.Float64,
+        4 => DriverDataType.String,
+        5 => DriverDataType.DateTime,
+        _ => DriverDataType.String,
+    };
+}
diff --git a/src/ZB.MOM.WW.OtOpcUa.Driver.Galaxy/Browse/GalaxyDiscoverer.cs b/src/ZB.MOM.WW.OtOpcUa.Driver.Galaxy/Browse/GalaxyDiscoverer.cs
new file mode 100644
index 0000000..7987956
--- /dev/null
+++ b/src/ZB.MOM.WW.OtOpcUa.Driver.Galaxy/Browse/GalaxyDiscoverer.cs
@@ -0,0 +1,80 @@
+using MxGateway.Contracts.Proto.Galaxy;
+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Browse;
+
+/// <summary>
+///     Translates a Galaxy object hierarchy (from <see cref="IGalaxyHierarchySource"/>) into
+///     <see cref="IAddressSpaceBuilder"/> calls — folders for each gobject, variables for
+///     each dynamic attribute. Alarm-bearing attributes get all five sub-attribute refs
+///     populated via <see cref="AlarmRefBuilder"/> so the server-level alarm subsystem
+///     (PR 2.2) can subscribe + ack without help from the driver.
+/// </summary>
+/// <remarks>
+///     Hierarchy materialisation rules (mirror legacy <c>MxAccessGalaxyBackend.DiscoverAsync</c>):
+///     <list type="bullet">
+///       <item>Browse name = <c>contained_name</c> when present; falls back to <c>tag_name</c>.</item>
+///       <item>Folder per gobject; variables placed inside their owner folder.</item>
+///       <item>Variable's full reference = <c>tag_name.attribute_name</c> — the format MXAccess
+///         expects for read/write addressing (translated from the contained-name browse path).</item>
+///       <item>Hierarchy is rendered flat (one folder per gobject under the driver root) for
+///         this PR. PR 4.W's address-space wiring revisits whether to nest under
+///         <c>parent_gobject_id</c> for a true tree shape.</item>
+///     </list>
+/// </remarks>
+public sealed class GalaxyDiscoverer
+{
+    private readonly IGalaxyHierarchySource _source;
+
+    public GalaxyDiscoverer(IGalaxyHierarchySource source)
+    {
+        _source = source ?? throw new ArgumentNullException(nameof(source));
+    }
+
+    /// <summary>
+    ///     Drive the supplied builder with one folder + N variables per Galaxy object the
+    ///     gateway returns. Idempotent — caller can re-invoke after a redeploy event.
+    /// </summary>
+    public async Task DiscoverAsync(IAddressSpaceBuilder builder, CancellationToken cancellationToken)
+    {
+        ArgumentNullException.ThrowIfNull(builder);
+        var objects = await _source.GetHierarchyAsync(cancellationToken).ConfigureAwait(false);
+
+        foreach (var obj in objects)
+        {
+            var browseName = string.IsNullOrEmpty(obj.ContainedName) ? obj.TagName : obj.ContainedName;
+            if (string.IsNullOrEmpty(browseName)) continue; // skip objects with no usable identity
+
+            var folder = builder.Folder(browseName, browseName);
+
+            foreach (var attr in obj.Attributes)
+            {
+                if (string.IsNullOrEmpty(attr.AttributeName)) continue;
+
+                var fullReference = !string.IsNullOrEmpty(attr.FullTagReference)
+                    ? attr.FullTagReference
+                    : obj.TagName + "." + attr.AttributeName;
+
+                var info = new DriverAttributeInfo(
+                    FullName: fullReference,
+                    DriverDataType: DataTypeMap.Map(attr.MxDataType),
+                    IsArray: attr.IsArray,
+                    ArrayDim: attr.IsArray && attr.ArrayDimensionPresent && attr.ArrayDimension > 0
+                        ? (uint)attr.ArrayDimension
+                        : null,
+                    SecurityClass: SecurityMap.Map(attr.SecurityClassification),
+                    IsHistorized: attr.IsHistorized,
+                    IsAlarm: attr.IsAlarm);
+
+                var handle = folder.Variable(attr.AttributeName, attr.AttributeName, info);
+
+                // Alarm-bearing attributes ship the full sub-attribute ref set so the server's
+                // AlarmConditionService can subscribe + ack-write without re-deriving the names.
+                if (attr.IsAlarm)
+                {
+                    handle.MarkAsAlarmCondition(AlarmRefBuilder.Build(fullReference));
+                }
+            }
+        }
+    }
+}
diff --git a/src/ZB.MOM.WW.OtOpcUa.Driver.Galaxy/Browse/GatewayGalaxyHierarchySource.cs b/src/ZB.MOM.WW.OtOpcUa.Driver.Galaxy/Browse/GatewayGalaxyHierarchySource.cs
new file mode 100644
index 0000000..850fc7d
--- /dev/null
+++ b/src/ZB.MOM.WW.OtOpcUa.Driver.Galaxy/Browse/GatewayGalaxyHierarchySource.cs
@@ -0,0 +1,21 @@
+using MxGateway.Client;
+using MxGateway.Contracts.Proto.Galaxy;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Browse;
+
+/// <summary>
+///     Default <see cref="IGalaxyHierarchySource"/> wrapping the gateway's
+///     <see cref="GalaxyRepositoryClient"/>. Pages internally via the client's overload.
+/// </summary>
+public sealed class GatewayGalaxyHierarchySource : IGalaxyHierarchySource
+{
+    private readonly GalaxyRepositoryClient _client;
+
+    public GatewayGalaxyHierarchySource(GalaxyRepositoryClient client)
+    {
+        _client = client ?? throw new ArgumentNullException(nameof(client));
+    }
+
+    public Task<IReadOnlyList<GalaxyObject>> GetHierarchyAsync(CancellationToken cancellationToken)
+        => _client.DiscoverHierarchyAsync(cancellationToken);
+}
diff --git a/src/ZB.MOM.WW.OtOpcUa.Driver.Galaxy/Browse/IGalaxyHierarchySource.cs b/src/ZB.MOM.WW.OtOpcUa.Driver.Galaxy/Browse/IGalaxyHierarchySource.cs
new file mode 100644
index 0000000..b81dfbe
--- /dev/null
+++ b/src/ZB.MOM.WW.OtOpcUa.Driver.Galaxy/Browse/IGalaxyHierarchySource.cs
@@ -0,0 +1,19 @@
+using MxGateway.Contracts.Proto.Galaxy;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Browse;
+
+/// <summary>
+///     Driver-side seam between <see cref="GalaxyDiscoverer"/> and the gateway. Production
+///     wraps <c>GalaxyRepositoryClient</c>; tests substitute a fake returning canned
+///     <see cref="GalaxyObject"/> rows so the discoverer's translation logic can be exercised
+///     without a real gRPC channel.
+/// </summary>
+public interface IGalaxyHierarchySource
+{
+    /// <summary>
+    ///     Returns the full materialised Galaxy hierarchy. The gateway client pages
+    ///     internally; this interface deliberately exposes only the post-paging shape so
+    ///     callers don't reimplement paging.
+    /// </summary>
+    Task<IReadOnlyList<GalaxyObject>> GetHierarchyAsync(CancellationToken cancellationToken);
+}
diff --git a/src/ZB.MOM.WW.OtOpcUa.Driver.Galaxy/Browse/SecurityMap.cs b/src/ZB.MOM.WW.OtOpcUa.Driver.Galaxy/Browse/SecurityMap.cs
new file mode 100644
index 0000000..2ec7129
--- /dev/null
+++ b/src/ZB.MOM.WW.OtOpcUa.Driver.Galaxy/Browse/SecurityMap.cs
@@ -0,0 +1,25 @@
+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Browse;
+
+/// <summary>
+///     Maps Galaxy <c>security_classification</c> integer codes to
+///     <see cref="SecurityClassification"/>. Ported from the legacy
+///     <c>GalaxyProxyDriver.MapSecurity</c>; unknown codes fall back to
+///     <see cref="SecurityClassification.FreeAccess"/> so a forward-compatible Galaxy
+///     deployment with new classifications doesn't break discovery.
+/// </summary>
+internal static class SecurityMap
+{
+    public static SecurityClassification Map(int mxSec) => mxSec switch
+    {
+        0 => SecurityClassification.FreeAccess,
+        1 => SecurityClassification.Operate,
+        2 => SecurityClassification.SecuredWrite,
+        3 => SecurityClassification.VerifiedWrite,
+        4 => SecurityClassification.Tune,
+        5 => SecurityClassification.Configure,
+        6 => SecurityClassification.ViewOnly,
+        _ => SecurityClassification.FreeAccess,
+    };
+}
diff --git a/src/ZB.MOM.WW.OtOpcUa.Driver.Galaxy/GalaxyDriver.cs b/src/ZB.MOM.WW.OtOpcUa.Driver.Galaxy/GalaxyDriver.cs
index e09fb9f..e1aadf3 100644
--- a/src/ZB.MOM.WW.OtOpcUa.Driver.Galaxy/GalaxyDriver.cs
+++ b/src/ZB.MOM.WW.OtOpcUa.Driver.Galaxy/GalaxyDriver.cs
@@ -1,6 +1,8 @@
 using Microsoft.Extensions.Logging;
 using Microsoft.Extensions.Logging.Abstractions;
+using MxGateway.Client;
 using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+using ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Browse;
 using ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Config;
 
 namespace ZB.MOM.WW.OtOpcUa.Driver.Galaxy;
@@ -20,12 +22,19 @@ namespace ZB.MOM.WW.OtOpcUa.Driver.Galaxy;
 ///     <see cref="GalaxyDriverFactoryExtensions"/> registers under driver-type name
 ///     "GalaxyMxGateway" so both paths can be live simultaneously during parity testing.
 /// </remarks>
-public sealed class GalaxyDriver : IDriver, IDisposable
+public sealed class GalaxyDriver : IDriver, ITagDiscovery, IDisposable
 {
     private readonly string _driverInstanceId;
     private readonly GalaxyDriverOptions _options;
     private readonly ILogger<GalaxyDriver> _logger;
 
+    // PR 4.1 — IGalaxyHierarchySource is the test seam for browse. When null, the driver
+    // lazily builds a GatewayGalaxyHierarchySource around a GalaxyRepositoryClient on
+    // first DiscoverAsync. Tests inject a fake source via the internal ctor to exercise
+    // GalaxyDiscoverer's translation logic without a real gRPC channel.
+    private IGalaxyHierarchySource? _hierarchySource;
+    private GalaxyRepositoryClient? _ownedRepositoryClient;
+
     private DriverHealth _health = new(DriverState.Unknown, null, null);
     private bool _disposed;
 
@@ -33,12 +42,27 @@ public sealed class GalaxyDriver : IDriver, IDisposable
         string driverInstanceId,
         GalaxyDriverOptions options,
         ILogger<GalaxyDriver>? logger = null)
+        : this(driverInstanceId, options, hierarchySource: null, logger)
+    {
+    }
+
+    /// <summary>
+    ///     Test-visible ctor — inject a custom <see cref="IGalaxyHierarchySource"/> so
+    ///     <see cref="DiscoverAsync"/> can be exercised against canned hierarchies without
+    ///     building a real gRPC channel.
+    /// </summary>
+    internal GalaxyDriver(
+        string driverInstanceId,
+        GalaxyDriverOptions options,
+        IGalaxyHierarchySource? hierarchySource,
+        ILogger<GalaxyDriver>? logger = null)
     {
         _driverInstanceId = !string.IsNullOrWhiteSpace(driverInstanceId)
             ? driverInstanceId
             : throw new ArgumentException("Driver instance id required.", nameof(driverInstanceId));
         _options = options ?? throw new ArgumentNullException(nameof(options));
         _logger = logger ?? NullLogger<GalaxyDriver>.Instance;
+        _hierarchySource = hierarchySource;
     }
 
     /// <inheritdoc />
@@ -93,10 +117,52 @@ public sealed class GalaxyDriver : IDriver, IDisposable
     /// <inheritdoc />
     public Task FlushOptionalCachesAsync(CancellationToken cancellationToken) => Task.CompletedTask;
 
+    // ===== ITagDiscovery (PR 4.1) =====
+
+    /// <inheritdoc />
+    public async Task DiscoverAsync(IAddressSpaceBuilder builder, CancellationToken cancellationToken)
+    {
+        ObjectDisposedException.ThrowIf(_disposed, this);
+        ArgumentNullException.ThrowIfNull(builder);
+
+        var source = _hierarchySource ??= BuildDefaultHierarchySource();
+        var discoverer = new GalaxyDiscoverer(source);
+        await discoverer.DiscoverAsync(builder, cancellationToken).ConfigureAwait(false);
+    }
+
+    /// <summary>
+    ///     Lazily builds the default <see cref="IGalaxyHierarchySource"/> from
+    ///     <c>_options.Gateway</c>. Owned <see cref="GalaxyRepositoryClient"/> is disposed in
+    ///     <see cref="Dispose"/>. Tests bypass this by injecting their own source via the
+    ///     internal ctor.
+    /// </summary>
+    private IGalaxyHierarchySource BuildDefaultHierarchySource()
+    {
+        var gw = _options.Gateway;
+        var clientOptions = new MxGatewayClientOptions
+        {
+            Endpoint = new Uri(gw.Endpoint, UriKind.Absolute),
+            // PR 4.1 stub: ApiKeySecretRef is currently treated as the literal API key.
+            // PR 4.W (or a follow-up) wires up DPAPI-backed secret resolution.
+            ApiKey = gw.ApiKeySecretRef,
+            UseTls = gw.UseTls,
+            CaCertificatePath = gw.CaCertificatePath,
+            ConnectTimeout = TimeSpan.FromSeconds(gw.ConnectTimeoutSeconds),
+            DefaultCallTimeout = TimeSpan.FromSeconds(gw.DefaultCallTimeoutSeconds),
+            StreamTimeout = gw.StreamTimeoutSeconds > 0
+                ? TimeSpan.FromSeconds(gw.StreamTimeoutSeconds)
+                : null,
+        };
+        _ownedRepositoryClient = GalaxyRepositoryClient.Create(clientOptions);
+        return new GatewayGalaxyHierarchySource(_ownedRepositoryClient);
+    }
+
     public void Dispose()
     {
         if (_disposed) return;
         _disposed = true;
-        // No owned IDisposables until PR 4.2's GalaxyMxSession lands.
+        _ownedRepositoryClient?.DisposeAsync().AsTask().GetAwaiter().GetResult();
+        _ownedRepositoryClient = null;
+        _hierarchySource = null;
     }
 }
diff --git a/src/ZB.MOM.WW.OtOpcUa.Driver.Galaxy/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.csproj b/src/ZB.MOM.WW.OtOpcUa.Driver.Galaxy/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.csproj
index 4a2812d..dec2b68 100644
--- a/src/ZB.MOM.WW.OtOpcUa.Driver.Galaxy/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.csproj
+++ b/src/ZB.MOM.WW.OtOpcUa.Driver.Galaxy/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.csproj
@@ -15,6 +15,10 @@
     <ItemGroup>
         <ProjectReference Include="..\ZB.MOM.WW.OtOpcUa.Core.Abstractions\ZB.MOM.WW.OtOpcUa.Core.Abstractions.csproj"/>
         <ProjectReference Include="..\ZB.MOM.WW.OtOpcUa.Core\ZB.MOM.WW.OtOpcUa.Core.csproj"/>
+        <!-- mxaccessgw .NET client. Path-based ProjectReference because both repos sit
+             side-by-side on the dev box; long-term we'll consume MxGateway.Client as a
+             NuGet package. PR 4.W revisits the dependency shape before parity gating. -->
+        <ProjectReference Include="..\..\..\mxaccessgw\clients\dotnet\MxGateway.Client\MxGateway.Client.csproj"/>
     </ItemGroup>
 
     <ItemGroup>
diff --git a/tests/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Tests/Browse/GalaxyDiscovererTests.cs b/tests/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Tests/Browse/GalaxyDiscovererTests.cs
new file mode 100644
index 0000000..1585152
--- /dev/null
+++ b/tests/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Tests/Browse/GalaxyDiscovererTests.cs
@@ -0,0 +1,301 @@
+using MxGateway.Contracts.Proto.Galaxy;
+using Shouldly;
+using Xunit;
+using ZB.MOM.WW.OtOpcUa.Core.Abstractions;
+using ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Browse;
+
+namespace ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Tests.Browse;
+
+/// <summary>
+///     Tests <see cref="GalaxyDiscoverer"/>'s translation of <see cref="GalaxyObject"/> rows
+///     into <see cref="IAddressSpaceBuilder"/> calls. The fake builder records every Folder /
+///     Variable / MarkAsAlarmCondition call so assertions can target shape without booting
+///     a real OPC UA address space.
+/// </summary>
+public sealed class GalaxyDiscovererTests
+{
+    private sealed class FakeHierarchySource(IReadOnlyList<GalaxyObject> objects) : IGalaxyHierarchySource
+    {
+        public Task<IReadOnlyList<GalaxyObject>> GetHierarchyAsync(CancellationToken cancellationToken)
+            => Task.FromResult(objects);
+    }
+
+    private sealed record FolderCall(string BrowseName, string DisplayName);
+    private sealed record VariableCall(string FolderBrowseName, string AttributeName, DriverAttributeInfo Info);
+
+    private sealed class FakeBuilder : IAddressSpaceBuilder
+    {
+        public List<FolderCall> Folders { get; } = [];
+        public List<VariableCall> Variables { get; } = [];
+        public Dictionary<string, AlarmConditionInfo> AlarmDeclarations { get; } = [];
+
+        private readonly string? _currentFolder;
+
+        public FakeBuilder() : this(null) { }
+        private FakeBuilder(string? folder) { _currentFolder = folder; }
+
+        public IAddressSpaceBuilder Folder(string browseName, string displayName)
+        {
+            Folders.Add(new FolderCall(browseName, displayName));
+            // Return a child scoped to this folder; nested folders inherit the parent reference.
+            return new ChildBuilder(this, browseName);
+        }
+
+        public IVariableHandle Variable(string browseName, string displayName, DriverAttributeInfo attributeInfo)
+        {
+            var folder = _currentFolder ?? "<root>";
+            Variables.Add(new VariableCall(folder, browseName, attributeInfo));
+            return new FakeVariableHandle(this, attributeInfo.FullName);
+        }
+
+        public void AddProperty(string browseName, DriverDataType dataType, object? value) { }
+
+        // Child folder routes Variable calls back to the parent's lists with its own scope.
+        private sealed class ChildBuilder(FakeBuilder parent, string folderBrowseName) : IAddressSpaceBuilder
+        {
+            public IAddressSpaceBuilder Folder(string browseName, string displayName)
+            {
+                parent.Folders.Add(new FolderCall(browseName, displayName));
+                return new ChildBuilder(parent, browseName);
+            }
+
+            public IVariableHandle Variable(string browseName, string displayName, DriverAttributeInfo attributeInfo)
+            {
+                parent.Variables.Add(new VariableCall(folderBrowseName, browseName, attributeInfo));
+                return new FakeVariableHandle(parent, attributeInfo.FullName);
+            }
+
+            public void AddProperty(string browseName, DriverDataType dataType, object? value) { }
+        }
+
+        private sealed class FakeVariableHandle(FakeBuilder owner, string fullRef) : IVariableHandle
+        {
+            public string FullReference { get; } = fullRef;
+
+            public IAlarmConditionSink MarkAsAlarmCondition(AlarmConditionInfo info)
+            {
+                owner.AlarmDeclarations[FullReference] = info;
+                return new NoopSink();
+            }
+        }
+
+        private sealed class NoopSink : IAlarmConditionSink
+        {
+            public void OnTransition(AlarmEventArgs args) { }
+        }
+    }
+
+    private static GalaxyAttribute Attr(
+        string name, int mxDataType = 0, bool isArray = false, int arrayDim = 0,
+        int securityClass = 0, bool isHistorized = false, bool isAlarm = false,
+        string? fullTagReference = null)
+    {
+        var a = new GalaxyAttribute
+        {
+            AttributeName = name,
+            MxDataType = mxDataType,
+            IsArray = isArray,
+            ArrayDimension = arrayDim,
+            ArrayDimensionPresent = arrayDim > 0,
+            SecurityClassification = securityClass,
+            IsHistorized = isHistorized,
+            IsAlarm = isAlarm,
+        };
+        if (fullTagReference is not null) a.FullTagReference = fullTagReference;
+        return a;
+    }
+
+    private static GalaxyObject Obj(string tagName, string? containedName = null, params GalaxyAttribute[] attributes)
+    {
+        var o = new GalaxyObject
+        {
+            TagName = tagName,
+            ContainedName = containedName ?? tagName,
+        };
+        o.Attributes.AddRange(attributes);
+        return o;
+    }
+
+    [Fact]
+    public async Task DiscoverAsync_BuildsOneFolderPerObject_AndOneVariablePerAttribute()
+    {
+        var src = new FakeHierarchySource([
+            Obj("Tank1_Level", "Level", Attr("PV", mxDataType: 2 /*Float32*/), Attr("SP", mxDataType: 2)),
+            Obj("Tank1_Pump", "Pump", Attr("Running", mxDataType: 0 /*Boolean*/)),
+        ]);
+        var discoverer = new GalaxyDiscoverer(src);
+        var builder = new FakeBuilder();
+
+        await discoverer.DiscoverAsync(builder, CancellationToken.None);
+
+        builder.Folders.Count.ShouldBe(2);
+        builder.Folders[0].BrowseName.ShouldBe("Level");
+        builder.Folders[1].BrowseName.ShouldBe("Pump");
+        builder.Variables.Count.ShouldBe(3);
+        builder.Variables.ShouldContain(v => v.FolderBrowseName == "Level" && v.AttributeName == "PV");
+        builder.Variables.ShouldContain(v => v.FolderBrowseName == "Level" && v.AttributeName == "SP");
+        builder.Variables.ShouldContain(v => v.FolderBrowseName == "Pump"  && v.AttributeName == "Running");
+    }
+
+    [Fact]
+    public async Task DiscoverAsync_FullReference_DefaultsToTagDotAttribute()
+    {
+        var src = new FakeHierarchySource([
+            Obj("Tank1_Level", "Level", Attr("PV", mxDataType: 2)),
+        ]);
+        var discoverer = new GalaxyDiscoverer(src);
+        var builder = new FakeBuilder();
+
+        await discoverer.DiscoverAsync(builder, CancellationToken.None);
+
+        builder.Variables[0].Info.FullName.ShouldBe("Tank1_Level.PV");
+    }
+
+    [Fact]
+    public async Task DiscoverAsync_FullReference_PrefersGwSuppliedFullTagReference()
+    {
+        var src = new FakeHierarchySource([
+            Obj("Tank1_Level", "Level", Attr("PV", mxDataType: 2, fullTagReference: "explicit.full.ref")),
+        ]);
+        var discoverer = new GalaxyDiscoverer(src);
+        var builder = new FakeBuilder();
+
+        await discoverer.DiscoverAsync(builder, CancellationToken.None);
+
+        builder.Variables[0].Info.FullName.ShouldBe("explicit.full.ref");
+    }
+
+    [Fact]
+    public async Task DiscoverAsync_BrowseName_FallsBackToTagName_WhenContainedEmpty()
+    {
+        var src = new FakeHierarchySource([
+            Obj("Tank1_Level", containedName: "", attributes: Attr("PV")),
+        ]);
+        var discoverer = new GalaxyDiscoverer(src);
+        var builder = new FakeBuilder();
+
+        await discoverer.DiscoverAsync(builder, CancellationToken.None);
+
+        builder.Folders[0].BrowseName.ShouldBe("Tank1_Level");
+    }
+
+    [Fact]
+    public async Task DiscoverAsync_AttributeMetadata_PropagatesEveryField()
+    {
+        var src = new FakeHierarchySource([
+            Obj("T", "T", Attr("PV",
+                mxDataType: 3 /*Float64*/,
+                isArray: true, arrayDim: 16,
+                securityClass: 2 /*SecuredWrite*/,
+                isHistorized: true, isAlarm: false)),
+        ]);
+        var discoverer = new GalaxyDiscoverer(src);
+        var builder = new FakeBuilder();
+
+        await discoverer.DiscoverAsync(builder, CancellationToken.None);
+
+        var info = builder.Variables[0].Info;
+        info.DriverDataType.ShouldBe(DriverDataType.Float64);
+        info.IsArray.ShouldBeTrue();
+        info.ArrayDim.ShouldBe(16u);
+        info.SecurityClass.ShouldBe(SecurityClassification.SecuredWrite);
+        info.IsHistorized.ShouldBeTrue();
+        info.IsAlarm.ShouldBeFalse();
+    }
+
+    [Fact]
+    public async Task DiscoverAsync_AlarmAttribute_PopulatesAllFiveSubAttributeRefs()
+    {
+        var src = new FakeHierarchySource([
+            Obj("Tank1_Level", "Level", Attr("HiHi", mxDataType: 0, isAlarm: true)),
+        ]);
+        var discoverer = new GalaxyDiscoverer(src);
+        var builder = new FakeBuilder();
+
+        await discoverer.DiscoverAsync(builder, CancellationToken.None);
+
+        builder.AlarmDeclarations.ShouldContainKey("Tank1_Level.HiHi");
+        var info = builder.AlarmDeclarations["Tank1_Level.HiHi"];
+        info.SourceName.ShouldBe("Tank1_Level.HiHi");
+        info.InAlarmRef.ShouldBe("Tank1_Level.HiHi.InAlarm");
+        info.PriorityRef.ShouldBe("Tank1_Level.HiHi.Priority");
+        info.DescAttrNameRef.ShouldBe("Tank1_Level.HiHi.DescAttrName");
+        info.AckedRef.ShouldBe("Tank1_Level.HiHi.Acked");
+        info.AckMsgWriteRef.ShouldBe("Tank1_Level.HiHi.AckMsg");
+    }
+
+    [Fact]
+    public async Task DiscoverAsync_NonAlarmAttribute_DoesNotMarkCondition()
+    {
+        var src = new FakeHierarchySource([
+            Obj("T", "T", Attr("PV", isAlarm: false), Attr("HiHi", isAlarm: true)),
+        ]);
+        var discoverer = new GalaxyDiscoverer(src);
+        var builder = new FakeBuilder();
+
+        await discoverer.DiscoverAsync(builder, CancellationToken.None);
+
+        builder.AlarmDeclarations.Count.ShouldBe(1);
+        builder.AlarmDeclarations.ShouldContainKey("T.HiHi");
+        builder.AlarmDeclarations.ShouldNotContainKey("T.PV");
+    }
+
+    [Fact]
+    public async Task DiscoverAsync_SkipsObjectsWithEmptyIdentity()
+    {
+        var src = new FakeHierarchySource([
+            new GalaxyObject { TagName = "", ContainedName = "" }, // skip
+            Obj("Real", "Real", Attr("PV")),
+        ]);
+        var discoverer = new GalaxyDiscoverer(src);
+        var builder = new FakeBuilder();
+
+        await discoverer.DiscoverAsync(builder, CancellationToken.None);
+
+        builder.Folders.Count.ShouldBe(1);
+        builder.Folders[0].BrowseName.ShouldBe("Real");
+    }
+
+    [Fact]
+    public async Task DiscoverAsync_SkipsAttributesWithEmptyName()
+    {
+        var src = new FakeHierarchySource([
+            Obj("T", "T", new GalaxyAttribute { AttributeName = "", MxDataType = 0 }, Attr("PV")),
+        ]);
+        var discoverer = new GalaxyDiscoverer(src);
+        var builder = new FakeBuilder();
+
+        await discoverer.DiscoverAsync(builder, CancellationToken.None);
+
+        builder.Variables.Count.ShouldBe(1);
+        builder.Variables[0].AttributeName.ShouldBe("PV");
+    }
+
+    [Fact]
+    public async Task DriverDiscoverAsync_RoutesThroughInjectedSource()
+    {
+        var src = new FakeHierarchySource([Obj("T", "T", Attr("PV"))]);
+        var driver = new GalaxyDriverHelper().CreateWithFakeSource(src);
+        var builder = new FakeBuilder();
+
+        await driver.DiscoverAsync(builder, CancellationToken.None);
+
+        builder.Folders.Count.ShouldBe(1);
+        builder.Variables.Count.ShouldBe(1);
+    }
+
+    /// <summary>Helper that exercises the internal ctor (test seam) without exposing it publicly.</summary>
+    private sealed class GalaxyDriverHelper
+    {
+        public GalaxyDriver CreateWithFakeSource(IGalaxyHierarchySource source)
+            => new GalaxyDriver(
+                "galaxy-test",
+                new Config.GalaxyDriverOptions(
+                    new Config.GalaxyGatewayOptions("https://x", "k"),
+                    new Config.GalaxyMxAccessOptions("OtOpcUa-T"),
+                    new Config.GalaxyRepositoryOptions(),
+                    new Config.GalaxyReconnectOptions()),
+                source,
+                logger: null);
+    }
+}
diff --git a/tests/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Tests/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Tests.csproj b/tests/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Tests/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Tests.csproj
index e5c3a06..d63966c 100644
--- a/tests/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Tests/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Tests.csproj
+++ b/tests/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Tests/ZB.MOM.WW.OtOpcUa.Driver.Galaxy.Tests.csproj
@@ -21,6 +21,9 @@
 
     <ItemGroup>
         <ProjectReference Include="..\..\src\ZB.MOM.WW.OtOpcUa.Driver.Galaxy\ZB.MOM.WW.OtOpcUa.Driver.Galaxy.csproj"/>
+        <!-- Pulled in transitively via Driver.Galaxy → MxGateway.Client → MxGateway.Contracts;
+             explicit reference lets tests construct GalaxyObject / GalaxyAttribute fixtures. -->
+        <ProjectReference Include="..\..\..\mxaccessgw\src\MxGateway.Contracts\MxGateway.Contracts.csproj"/>
     </ItemGroup>
 
 </Project>