diff --git a/src/ZB.MOM.WW.MxGateway.Server/Alarms/AlarmsServiceCollectionExtensions.cs b/src/ZB.MOM.WW.MxGateway.Server/Alarms/AlarmsServiceCollectionExtensions.cs
index 26b7e83..8050e42 100644
--- a/src/ZB.MOM.WW.MxGateway.Server/Alarms/AlarmsServiceCollectionExtensions.cs
+++ b/src/ZB.MOM.WW.MxGateway.Server/Alarms/AlarmsServiceCollectionExtensions.cs
@@ -13,6 +13,7 @@ public static class AlarmsServiceCollectionExtensions
     /// <returns>The service collection for chaining.</returns>
     public static IServiceCollection AddGatewayAlarms(this IServiceCollection services)
     {
+        services.AddSingleton<IAlarmWatchListResolver, AlarmWatchListResolver>();
         services.AddSingleton<GatewayAlarmMonitor>();
         services.AddSingleton<IGatewayAlarmService>(provider => provider.GetRequiredService<GatewayAlarmMonitor>());
         services.AddHostedService(provider => provider.GetRequiredService<GatewayAlarmMonitor>());
diff --git a/src/ZB.MOM.WW.MxGateway.Server/Alarms/GatewayAlarmMonitor.cs b/src/ZB.MOM.WW.MxGateway.Server/Alarms/GatewayAlarmMonitor.cs
index 30461c9..c9ae21f 100644
--- a/src/ZB.MOM.WW.MxGateway.Server/Alarms/GatewayAlarmMonitor.cs
+++ b/src/ZB.MOM.WW.MxGateway.Server/Alarms/GatewayAlarmMonitor.cs
@@ -1,7 +1,9 @@
 using System.Threading.Channels;
+using Google.Protobuf.WellKnownTypes;
 using Microsoft.Extensions.Options;
 using ZB.MOM.WW.MxGateway.Contracts.Proto;
 using ZB.MOM.WW.MxGateway.Server.Configuration;
+using ZB.MOM.WW.MxGateway.Server.Metrics;
 using ZB.MOM.WW.MxGateway.Server.Sessions;
 
 namespace ZB.MOM.WW.MxGateway.Server.Alarms;
@@ -23,6 +25,8 @@ public sealed class GatewayAlarmMonitor : BackgroundService, IGatewayAlarmServic
     private static readonly TimeSpan StartupGrace = TimeSpan.FromSeconds(2);
 
     private readonly ISessionManager _sessionManager;
+    private readonly IAlarmWatchListResolver _watchListResolver;
+    private readonly GatewayMetrics _metrics;
     private readonly AlarmsOptions _options;
     private readonly ILogger<GatewayAlarmMonitor> _logger;
 
@@ -30,20 +34,34 @@ public sealed class GatewayAlarmMonitor : BackgroundService, IGatewayAlarmServic
     private readonly Dictionary<string, ActiveAlarmSnapshot> _alarms = new(StringComparer.Ordinal);
     private readonly List<Subscriber> _subscribers = [];
 
+    // Current provider status (mode + degraded + reason + since), guarded by _sync.
+    // Initialized to the alarm-manager, not-degraded baseline so a late joiner sees
+    // a sensible status even before any OnAlarmProviderModeChanged event arrives.
+    private AlarmProviderMode _providerMode = AlarmProviderMode.Alarmmgr;
+    private bool _providerDegraded;
+    private string _providerReason = string.Empty;
+    private DateTimeOffset _providerSince = DateTimeOffset.UtcNow;
+
     private volatile GatewayAlarmMonitorState _state = GatewayAlarmMonitorState.Disabled;
     private volatile string? _lastError;
     private GatewaySession? _session;
 
     /// <summary>Initializes the gateway alarm monitor.</summary>
     /// <param name="sessionManager">Gateway session manager.</param>
+    /// <param name="watchListResolver">Resolver for the subtag-fallback watch-list.</param>
+    /// <param name="metrics">Gateway metrics sink.</param>
     /// <param name="options">Gateway options carrying the alarm configuration.</param>
     /// <param name="logger">Diagnostic logger.</param>
     public GatewayAlarmMonitor(
         ISessionManager sessionManager,
+        IAlarmWatchListResolver watchListResolver,
+        GatewayMetrics metrics,
         IOptions<GatewayOptions> options,
         ILogger<GatewayAlarmMonitor> logger)
     {
         _sessionManager = sessionManager ?? throw new ArgumentNullException(nameof(sessionManager));
+        _watchListResolver = watchListResolver ?? throw new ArgumentNullException(nameof(watchListResolver));
+        _metrics = metrics ?? throw new ArgumentNullException(nameof(metrics));
         _options = (options ?? throw new ArgumentNullException(nameof(options))).Value.Alarms;
         _logger = logger ?? throw new ArgumentNullException(nameof(logger));
     }
@@ -139,6 +157,16 @@ public sealed class GatewayAlarmMonitor : BackgroundService, IGatewayAlarmServic
     private async Task RunMonitorAsync(string subscription, CancellationToken stoppingToken)
     {
         _state = GatewayAlarmMonitorState.Starting;
+        lock (_sync)
+        {
+            // Re-baseline the provider status for this lifecycle so a restarted
+            // monitor advertises alarm-manager/not-degraded until told otherwise.
+            _providerMode = AlarmProviderMode.Alarmmgr;
+            _providerDegraded = false;
+            _providerReason = string.Empty;
+            _providerSince = DateTimeOffset.UtcNow;
+        }
+
         GatewaySession session = await _sessionManager.OpenSessionAsync(
                 new SessionOpenRequest(BackendName, MonitorClientName, Guid.NewGuid().ToString("N"), CommandTimeout: null),
                 MonitorClientName,
@@ -173,6 +201,15 @@ public sealed class GatewayAlarmMonitor : BackgroundService, IGatewayAlarmServic
                     {
                         ApplyTransition(mxEvent.OnAlarmTransition);
                     }
+                    else if (mxEvent is { BodyCase: MxEvent.BodyOneofCase.OnAlarmProviderModeChanged }
+                        && mxEvent.OnAlarmProviderModeChanged is not null)
+                    {
+                        await ApplyProviderModeChangeAsync(
+                                session.SessionId,
+                                mxEvent.OnAlarmProviderModeChanged,
+                                linked.Token)
+                            .ConfigureAwait(false);
+                    }
                 }
             }
             finally
@@ -209,6 +246,29 @@ public sealed class GatewayAlarmMonitor : BackgroundService, IGatewayAlarmServic
 
     private async Task SubscribeAlarmsAsync(string sessionId, string subscription, CancellationToken cancellationToken)
     {
+        IReadOnlyList<AlarmSubtagTarget> watchList = await _watchListResolver
+            .ResolveAsync(_options, cancellationToken)
+            .ConfigureAwait(false);
+
+        AlarmProviderMode forcedMode = MapForcedMode(_options.Fallback.Mode);
+
+        // When the forced mode is Unspecified (the "Auto" case) and the resolved
+        // watch-list is empty — the common alarmmgr-only deployment — the command
+        // is identical-in-effect to the historical SubscribeAlarms (wnwrap only):
+        // the worker builds the wnwrap consumer and no subtag watch-list.
+        SubscribeAlarmsCommand command = new()
+        {
+            SubscriptionExpression = subscription,
+            ForcedMode = forcedMode,
+            Failover = new AlarmFailoverConfig
+            {
+                ConsecutiveFailureThreshold = _options.Fallback.ConsecutiveFailureThreshold,
+                FailbackProbeIntervalSeconds = _options.Fallback.FailbackProbeIntervalSeconds,
+                FailbackStableProbes = _options.Fallback.FailbackStableProbes,
+            },
+        };
+        command.WatchList.AddRange(watchList);
+
         WorkerCommandReply reply = await _sessionManager.InvokeAsync(
                 sessionId,
                 new WorkerCommand
@@ -216,7 +276,7 @@ public sealed class GatewayAlarmMonitor : BackgroundService, IGatewayAlarmServic
                     Command = new MxCommand
                     {
                         Kind = MxCommandKind.SubscribeAlarms,
-                        SubscribeAlarms = new SubscribeAlarmsCommand { SubscriptionExpression = subscription },
+                        SubscribeAlarms = command,
                     },
                 },
                 cancellationToken)
@@ -310,6 +370,94 @@ public sealed class GatewayAlarmMonitor : BackgroundService, IGatewayAlarmServic
         }
     }
 
+    // Handles the worker's provider-mode-change event: updates the stored provider
+    // status, broadcasts it to every subscriber (provider status is global, not
+    // alarm-scoped), records the switch metric, and forces a cache reconcile so the
+    // active-alarm set reflects whatever the new mode reports.
+    private async Task ApplyProviderModeChangeAsync(
+        string sessionId,
+        OnAlarmProviderModeChangedEvent change,
+        CancellationToken cancellationToken)
+    {
+        AlarmProviderMode toMode = change.Mode;
+        string reason = change.Reason ?? string.Empty;
+
+        AlarmProviderStatus status;
+        int fromModeInt;
+        lock (_sync)
+        {
+            fromModeInt = ModeToInt(_providerMode);
+            _providerMode = toMode;
+            _providerDegraded = toMode == AlarmProviderMode.Subtag;
+            _providerReason = reason;
+            _providerSince = DateTimeOffset.UtcNow;
+            status = BuildProviderStatus();
+            BroadcastToAll(new AlarmFeedMessage { ProviderStatus = status });
+        }
+
+        _metrics.AlarmProviderSwitched(fromModeInt, ModeToInt(toMode), reason);
+
+        _logger.LogInformation(
+            "Alarm provider mode changed to {Mode} (degraded={Degraded}): {Reason}",
+            toMode,
+            status.Degraded,
+            reason);
+
+        try
+        {
+            await ReconcileAsync(sessionId, cancellationToken).ConfigureAwait(false);
+        }
+        catch (OperationCanceledException)
+        {
+            throw;
+        }
+        catch (Exception exception)
+        {
+            _logger.LogDebug(
+                exception,
+                "Reconcile after alarm provider mode change failed; keeping the current cache.");
+        }
+    }
+
+    // Caller holds _sync. Builds an AlarmProviderStatus snapshot of the current state.
+    private AlarmProviderStatus BuildProviderStatus()
+    {
+        return new AlarmProviderStatus
+        {
+            Mode = _providerMode,
+            Degraded = _providerDegraded,
+            Reason = _providerReason,
+            Since = Timestamp.FromDateTimeOffset(_providerSince),
+        };
+    }
+
+    // Maps the configured fallback mode string to the forced provider mode the
+    // worker honours. Case-insensitive; anything other than the two force values
+    // (including the default "Auto") yields Unspecified ("let the worker decide").
+    private static AlarmProviderMode MapForcedMode(string? mode)
+    {
+        if (string.Equals(mode, "ForceAlarmManager", StringComparison.OrdinalIgnoreCase))
+        {
+            return AlarmProviderMode.Alarmmgr;
+        }
+
+        if (string.Equals(mode, "ForceSubtag", StringComparison.OrdinalIgnoreCase))
+        {
+            return AlarmProviderMode.Subtag;
+        }
+
+        return AlarmProviderMode.Unspecified;
+    }
+
+    // Maps the provider-mode enum to the integer the metric expects
+    // (alarmmgr=1, subtag=2, unknown/unspecified=0).
+    private static int ModeToInt(AlarmProviderMode mode) => mode switch
+    {
+        AlarmProviderMode.Alarmmgr => 1,
+        AlarmProviderMode.Subtag => 2,
+        _ => 0,
+    };
+
     // Replaces the cache with the worker's authoritative snapshot, broadcasting
     // a synthetic transition for any alarm the live stream missed.
     private void ApplyReconcile(IEnumerable<ActiveAlarmSnapshot> snapshots)
@@ -374,6 +522,23 @@ public sealed class GatewayAlarmMonitor : BackgroundService, IGatewayAlarmServic
         }
     }
 
+    // Caller holds _sync. Pushes a feed message to every subscriber regardless of
+    // its alarm-filter prefix. Used for provider-status messages, which are global
+    // rather than scoped to a single alarm reference.
+    private void BroadcastToAll(AlarmFeedMessage message)
+    {
+        for (int index = _subscribers.Count - 1; index >= 0; index--)
+        {
+            Subscriber subscriber = _subscribers[index];
+            if (!subscriber.Channel.Writer.TryWrite(message))
+            {
+                subscriber.Channel.Writer.TryComplete(new InvalidOperationException(
+                    "Alarm feed subscriber fell behind and was dropped; reconnect to re-snapshot."));
+                _subscribers.RemoveAt(index);
+            }
+        }
+    }
+
     private void ClearCache()
     {
         lock (_sync)
@@ -398,11 +563,14 @@ public sealed class GatewayAlarmMonitor : BackgroundService, IGatewayAlarmServic
         Subscriber subscriber = new(channel, prefix);
 
         ActiveAlarmSnapshot[] snapshot;
+        AlarmProviderStatus providerStatus;
         lock (_sync)
         {
-            // Register before snapshotting under the same lock so no transition
-            // can slip between the snapshot and the live stream.
+            // Register before snapshotting under the same lock so neither a
+            // transition nor a provider-mode change can slip between the snapshot
+            // and the live stream.
             _subscribers.Add(subscriber);
+            providerStatus = BuildProviderStatus();
             snapshot = _alarms.Values
                 .Where(alarm => prefix.Length == 0
                     || alarm.AlarmFullReference.StartsWith(prefix, StringComparison.Ordinal))
@@ -412,6 +580,10 @@ public sealed class GatewayAlarmMonitor : BackgroundService, IGatewayAlarmServic
 
         try
         {
+            // Emit the current provider status first so a late joiner immediately
+            // learns the mode (and whether the feed is degraded) before any alarms.
+            yield return new AlarmFeedMessage { ProviderStatus = providerStatus };
+
             foreach (ActiveAlarmSnapshot alarm in snapshot)
             {
                 yield return new AlarmFeedMessage { ActiveAlarm = alarm };
diff --git a/src/ZB.MOM.WW.MxGateway.Tests/Alarms/GatewayAlarmMonitorProviderModeTests.cs b/src/ZB.MOM.WW.MxGateway.Tests/Alarms/GatewayAlarmMonitorProviderModeTests.cs
new file mode 100644
index 0000000..ca67e91
--- /dev/null
+++ b/src/ZB.MOM.WW.MxGateway.Tests/Alarms/GatewayAlarmMonitorProviderModeTests.cs
@@ -0,0 +1,398 @@
+using System.Diagnostics.CodeAnalysis;
+using System.Diagnostics.Metrics;
+using System.Runtime.CompilerServices;
+using System.Threading.Channels;
+using Google.Protobuf.WellKnownTypes;
+using Microsoft.Extensions.Logging.Abstractions;
+using Microsoft.Extensions.Options;
+using ZB.MOM.WW.MxGateway.Contracts.Proto;
+using ZB.MOM.WW.MxGateway.Server.Alarms;
+using ZB.MOM.WW.MxGateway.Server.Configuration;
+using ZB.MOM.WW.MxGateway.Server.Metrics;
+using ZB.MOM.WW.MxGateway.Server.Sessions;
+
+namespace ZB.MOM.WW.MxGateway.Tests.Alarms;
+
+/// <summary>
+///     Drives <see cref="GatewayAlarmMonitor"/> with a fake session manager to
+///     verify it reflects the worker's <c>OnAlarmProviderModeChanged</c> event into
+///     the alarm feed and the switch metric, and that a new subscriber receives the
+///     provider status as its first message. Also covers the watch-list / forced-mode
+///     wiring of the <c>SubscribeAlarms</c> command and the Mode→enum mapping.
+/// </summary>
+public sealed class GatewayAlarmMonitorProviderModeTests
+{
+    private static readonly TimeSpan WaitTimeout = TimeSpan.FromSeconds(15);
+
+    [Fact]
+    public async Task ProviderModeChange_BroadcastsDegradedStatus_AndIncrementsSwitchMetric()
+    {
+        using GatewayMetrics metrics = new();
+        long switchCount = 0;
+        using MeterListener listener = new();
+        listener.InstrumentPublished = (instrument, meterListener) =>
+        {
+            if (ReferenceEquals(instrument.Meter, metrics.Meter)
+                && instrument.Name == "mxgateway.alarms.provider_switches")
+            {
+                meterListener.EnableMeasurementEvents(instrument);
+            }
+        };
+        listener.SetMeasurementEventCallback<long>(
+            (instrument, measurement, _, _) =>
+            {
+                if (ReferenceEquals(instrument.Meter, metrics.Meter)
+                    && instrument.Name == "mxgateway.alarms.provider_switches")
+                {
+                    Interlocked.Add(ref switchCount, measurement);
+                }
+            });
+        listener.Start();
+
+        FakeSessionManager sessions = new();
+        using GatewayAlarmMonitor monitor = CreateMonitor(sessions, metrics);
+
+        using CancellationTokenSource cts = new();
+        await monitor.StartAsync(cts.Token);
+        await sessions.WaitForSubscribeAsync(WaitTimeout);
+
+        // Subscribe a live feed reader, drain its first (provider status) message.
+        List<AlarmFeedMessage> received = [];
+        using CancellationTokenSource streamCts = new();
+        Task reader = Task.Run(async () =>
+        {
+            try
+            {
+                await foreach (AlarmFeedMessage message in monitor.StreamAsync(null, streamCts.Token))
+                {
+                    lock (received) { received.Add(message); }
+                }
+            }
+            catch (OperationCanceledException)
+            {
+                // Expected when the test cancels the stream.
+            }
+        });
+
+        // Emit the worker event that flips the provider into subtag mode.
+        sessions.EmitEvent(new MxEvent
+        {
+            Family = MxEventFamily.OnAlarmProviderModeChanged,
+            OnAlarmProviderModeChanged = new OnAlarmProviderModeChangedEvent
+            {
+                Mode = AlarmProviderMode.Subtag,
+                Reason = "alarmmgr failed",
+                Hresult = unchecked((int)0x80004005),
+                At = Timestamp.FromDateTimeOffset(DateTimeOffset.UtcNow),
+            },
+        });
+
+        AlarmFeedMessage degraded = await WaitForAsync(
+            received,
+            m => m.PayloadCase == AlarmFeedMessage.PayloadOneofCase.ProviderStatus
+                && m.ProviderStatus.Mode == AlarmProviderMode.Subtag,
+            WaitTimeout);
+
+        Assert.Equal(AlarmProviderMode.Subtag, degraded.ProviderStatus.Mode);
+        Assert.True(degraded.ProviderStatus.Degraded);
+        Assert.Equal("alarmmgr failed", degraded.ProviderStatus.Reason);
+
+        await WaitUntilAsync(() => Interlocked.Read(ref switchCount) >= 1, WaitTimeout);
+        Assert.Equal(1, Interlocked.Read(ref switchCount));
+
+        await streamCts.CancelAsync();
+        await reader;
+        await cts.CancelAsync();
+        await monitor.StopAsync(CancellationToken.None);
+    }
+
+    [Fact]
+    public async Task NewSubscriber_ReceivesProviderStatusAsFirstMessage()
+    {
+        using GatewayMetrics metrics = new();
+        FakeSessionManager sessions = new();
+        using GatewayAlarmMonitor monitor = CreateMonitor(sessions, metrics);
+
+        using CancellationTokenSource cts = new();
+        await monitor.StartAsync(cts.Token);
+        await sessions.WaitForSubscribeAsync(WaitTimeout);
+
+        using CancellationTokenSource streamCts = new();
+        AlarmFeedMessage? first = null;
+        Task reader = Task.Run(async () =>
+        {
+            await foreach (AlarmFeedMessage message in monitor.StreamAsync(null, streamCts.Token))
+            {
+                first = message;
+                break;
+            }
+        });
+
+        await WaitUntilAsync(() => first is not null, WaitTimeout);
+
+        Assert.NotNull(first);
+        Assert.Equal(AlarmFeedMessage.PayloadOneofCase.ProviderStatus, first!.PayloadCase);
+        // Baseline before any provider-mode event: alarm-manager, not degraded.
+        Assert.Equal(AlarmProviderMode.Alarmmgr, first.ProviderStatus.Mode);
+        Assert.False(first.ProviderStatus.Degraded);
+
+        await streamCts.CancelAsync();
+        await reader;
+        await cts.CancelAsync();
+        await monitor.StopAsync(CancellationToken.None);
+    }
+
+    [Fact]
+    public async Task SubscribeAlarms_SendsForcedModeAndWatchList_FromConfiguration()
+    {
+        using GatewayMetrics metrics = new();
+        FakeSessionManager sessions = new();
+        StubWatchListResolver resolver = new(
+        [
+            new AlarmSubtagTarget { AlarmFullReference = "Galaxy!Area.Tank01.Hi", ActiveSubtag = "Tank01.Hi.active" },
+        ]);
+
+        AlarmsOptions options = new()
+        {
+            Enabled = true,
+            SubscriptionExpression = @"\\NODE\Galaxy!Area",
+            Fallback = new AlarmFallbackOptions
+            {
+                Mode = "ForceSubtag",
+                ConsecutiveFailureThreshold = 7,
+                FailbackProbeIntervalSeconds = 11,
+                FailbackStableProbes = 4,
+            },
+        };
+
+        using GatewayAlarmMonitor monitor = new(
+            sessions,
+            resolver,
+            metrics,
+            Microsoft.Extensions.Options.Options.Create(new GatewayOptions { Alarms = options }),
+            NullLogger<GatewayAlarmMonitor>.Instance);
+
+        using CancellationTokenSource cts = new();
+        await monitor.StartAsync(cts.Token);
+        await sessions.WaitForSubscribeAsync(WaitTimeout);
+
+        SubscribeAlarmsCommand sent = Assert.IsType<SubscribeAlarmsCommand>(sessions.LastSubscribeCommand);
+        Assert.Equal(AlarmProviderMode.Subtag, sent.ForcedMode);
+        Assert.Equal(7, sent.Failover.ConsecutiveFailureThreshold);
+        Assert.Equal(11, sent.Failover.FailbackProbeIntervalSeconds);
+        Assert.Equal(4, sent.Failover.FailbackStableProbes);
+        AlarmSubtagTarget target = Assert.Single(sent.WatchList);
+        Assert.Equal("Galaxy!Area.Tank01.Hi", target.AlarmFullReference);
+
+        await cts.CancelAsync();
+        await monitor.StopAsync(CancellationToken.None);
+    }
+
+    [Theory]
+    [InlineData("ForceAlarmManager", AlarmProviderMode.Alarmmgr)]
+    [InlineData("forcealarmmanager", AlarmProviderMode.Alarmmgr)]
+    [InlineData("ForceSubtag", AlarmProviderMode.Subtag)]
+    [InlineData("forcesubtag", AlarmProviderMode.Subtag)]
+    [InlineData("Auto", AlarmProviderMode.Unspecified)]
+    [InlineData("", AlarmProviderMode.Unspecified)]
+    [InlineData("nonsense", AlarmProviderMode.Unspecified)]
+    public async Task ModeString_MapsToForcedProviderMode(string mode, AlarmProviderMode expected)
+    {
+        using GatewayMetrics metrics = new();
+        FakeSessionManager sessions = new();
+
+        AlarmsOptions options = new()
+        {
+            Enabled = true,
+            SubscriptionExpression = @"\\NODE\Galaxy!Area",
+            Fallback = new AlarmFallbackOptions { Mode = mode },
+        };
+
+        using GatewayAlarmMonitor monitor = new(
+            sessions,
+            new StubWatchListResolver([]),
+            metrics,
+            Microsoft.Extensions.Options.Options.Create(new GatewayOptions { Alarms = options }),
+            NullLogger<GatewayAlarmMonitor>.Instance);
+
+        using CancellationTokenSource cts = new();
+        await monitor.StartAsync(cts.Token);
+        await sessions.WaitForSubscribeAsync(WaitTimeout);
+
+        Assert.Equal(expected, sessions.LastSubscribeCommand!.ForcedMode);
+        // Auto + empty watch-list preserves historical alarmmgr-only behaviour.
+        if (expected == AlarmProviderMode.Unspecified)
+        {
+            Assert.Empty(sessions.LastSubscribeCommand!.WatchList);
+        }
+
+        await cts.CancelAsync();
+        await monitor.StopAsync(CancellationToken.None);
+    }
+
+    private static GatewayAlarmMonitor CreateMonitor(FakeSessionManager sessions, GatewayMetrics metrics)
+    {
+        AlarmsOptions options = new()
+        {
+            Enabled = true,
+            SubscriptionExpression = @"\\NODE\Galaxy!Area",
+        };
+        return new GatewayAlarmMonitor(
+            sessions,
+            new StubWatchListResolver([]),
+            metrics,
+            Microsoft.Extensions.Options.Options.Create(new GatewayOptions { Alarms = options }),
+            NullLogger<GatewayAlarmMonitor>.Instance);
+    }
+
+    private static async Task<AlarmFeedMessage> WaitForAsync(
+        List<AlarmFeedMessage> received,
+        Func<AlarmFeedMessage, bool> predicate,
+        TimeSpan timeout)
+    {
+        DateTime deadline = DateTime.UtcNow + timeout;
+        while (DateTime.UtcNow < deadline)
+        {
+            lock (received)
+            {
+                AlarmFeedMessage? match = received.FirstOrDefault(predicate);
+                if (match is not null)
+                {
+                    return match;
+                }
+            }
+
+            await Task.Delay(25);
+        }
+
+        throw new TimeoutException("No matching AlarmFeedMessage was received in time.");
+    }
+
+    private static async Task WaitUntilAsync(Func<bool> condition, TimeSpan timeout)
+    {
+        DateTime deadline = DateTime.UtcNow + timeout;
+        while (DateTime.UtcNow < deadline)
+        {
+            if (condition())
+            {
+                return;
+            }
+
+            await Task.Delay(25);
+        }
+
+        throw new TimeoutException("Condition was not met in time.");
+    }
+
+    /// <summary><see cref="IAlarmWatchListResolver"/> that returns a fixed watch-list.</summary>
+    private sealed class StubWatchListResolver(IReadOnlyList<AlarmSubtagTarget> targets) : IAlarmWatchListResolver
+    {
+        /// <inheritdoc />
+        public Task<IReadOnlyList<AlarmSubtagTarget>> ResolveAsync(
+            AlarmsOptions options,
+            CancellationToken cancellationToken = default) => Task.FromResult(targets);
+    }
+
+    /// <summary>
+    ///     Minimal <see cref="ISessionManager"/> for driving the monitor: opens a
+    ///     constructed session, records the SubscribeAlarms command, replies OK to
+    ///     every command, and exposes a channel for pushing worker events.
+    /// </summary>
+    private sealed class FakeSessionManager : ISessionManager
+    {
+        private readonly Channel<WorkerEvent> _events = Channel.CreateUnbounded<WorkerEvent>();
+        private readonly TaskCompletionSource _subscribed =
+            new(TaskCreationOptions.RunContinuationsAsynchronously);
+
+        /// <summary>The most recent SubscribeAlarms command the monitor sent.</summary>
+        public SubscribeAlarmsCommand? LastSubscribeCommand { get; private set; }
+
+        /// <summary>Pushes a worker event onto the monitor's event stream.</summary>
+        public void EmitEvent(MxEvent mxEvent) =>
+            _events.Writer.TryWrite(new WorkerEvent { Event = mxEvent });
+
+        /// <summary>Completes once the monitor has issued its SubscribeAlarms command.</summary>
+        public Task WaitForSubscribeAsync(TimeSpan timeout) => _subscribed.Task.WaitAsync(timeout);
+
+        /// <inheritdoc />
+        public Task<GatewaySession> OpenSessionAsync(
+            SessionOpenRequest request,
+            string? clientIdentity,
+            CancellationToken cancellationToken)
+        {
+            GatewaySession session = new(
+                Guid.NewGuid().ToString("N"),
+                "Galaxy",
+                "pipe-test",
+                "nonce-test",
+                clientIdentity,
+                null,
+                null,
+                TimeSpan.FromSeconds(30),
+                TimeSpan.FromSeconds(30),
+                TimeSpan.FromSeconds(30),
+                DateTimeOffset.UtcNow);
+            return Task.FromResult(session);
+        }
+
+        /// <inheritdoc />
+        public Task<WorkerCommandReply> InvokeAsync(
+            string sessionId,
+            WorkerCommand command,
+            CancellationToken cancellationToken)
+        {
+            if (command.Command?.Kind == MxCommandKind.SubscribeAlarms)
+            {
+                LastSubscribeCommand = command.Command.SubscribeAlarms;
+                _subscribed.TrySetResult();
+            }
+
+            MxCommandReply reply = new()
+            {
+                ProtocolStatus = new ProtocolStatus { Code = ProtocolStatusCode.Ok },
+            };
+            if (command.Command?.Kind == MxCommandKind.QueryActiveAlarms)
+            {
+                reply.QueryActiveAlarms = new QueryActiveAlarmsReplyPayload();
+            }
+
+            return Task.FromResult(new WorkerCommandReply { Reply = reply });
+        }
+
+        /// <inheritdoc />
+        public async IAsyncEnumerable<WorkerEvent> ReadEventsAsync(
+            string sessionId,
+            [EnumeratorCancellation] CancellationToken cancellationToken)
+        {
+            await foreach (WorkerEvent workerEvent in _events.Reader.ReadAllAsync(cancellationToken))
+            {
+                yield return workerEvent;
+            }
+        }
+
+        /// <inheritdoc />
+        public bool TryGetSession(string sessionId, [MaybeNullWhen(false)] out GatewaySession session)
+        {
+            session = null;
+            return false;
+        }
+
+        /// <inheritdoc />
+        public Task<SessionCloseResult> CloseSessionAsync(string sessionId, CancellationToken cancellationToken)
+        {
+            _events.Writer.TryComplete();
+            return Task.FromResult(new SessionCloseResult(sessionId, SessionState.Closed, AlreadyClosed: false));
+        }
+
+        /// <inheritdoc />
+        public Task<SessionCloseResult> KillWorkerAsync(string sessionId, string reason, CancellationToken cancellationToken) =>
+            Task.FromResult(new SessionCloseResult(sessionId, SessionState.Closed, AlreadyClosed: false));
+
+        /// <inheritdoc />
+        public Task<int> CloseExpiredLeasesAsync(DateTimeOffset now, CancellationToken cancellationToken) =>
+            Task.FromResult(0);
+
+        /// <inheritdoc />
+        public Task ShutdownAsync(CancellationToken cancellationToken) => Task.CompletedTask;
+    }
+}