diff --git a/src/ZB.MOM.WW.MxGateway.Tests/Alarms/AlarmFailoverEndToEndTests.cs b/src/ZB.MOM.WW.MxGateway.Tests/Alarms/AlarmFailoverEndToEndTests.cs
new file mode 100644
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+++ b/src/ZB.MOM.WW.MxGateway.Tests/Alarms/AlarmFailoverEndToEndTests.cs
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+using System.Diagnostics.CodeAnalysis;
+using System.Runtime.CompilerServices;
+using System.Threading.Channels;
+using Google.Protobuf.WellKnownTypes;
+using Microsoft.Extensions.Logging.Abstractions;
+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 a single <see cref="GatewayAlarmMonitor"/> instance through the full
+///     alarm-provider failover/failback lifecycle and asserts the live feed reflects
+///     each stage. This complements the per-aspect tests in
+///     <c>GatewayAlarmMonitorProviderModeTests</c> with one cohesive scenario:
+///     subscribe (watch-list + forced mode) -> baseline alarmmgr status -> alarmmgr
+///     transition -> failover to subtag (degraded) -> subtag transition -> failback
+///     to alarmmgr (recovered).
+/// </summary>
+/// <remarks>
+///     The minimal session-manager / watch-list-resolver harness here is replicated
+///     (not shared) from the sibling <c>GatewayAlarmMonitorProviderModeTests</c>. The
+///     sibling's harness is a private nested type, and the task forbids changing that
+///     test's behaviour; replicating the few members this scenario needs keeps the
+///     sibling completely untouched and this file self-contained, at the cost of a
+///     small amount of duplication.
+/// </remarks>
+public sealed class AlarmFailoverEndToEndTests
+{
+    private static readonly TimeSpan WaitTimeout = TimeSpan.FromSeconds(30);
+
+    [Fact]
+    public async Task ProviderFailoverAndFailback_FullLifecycle_ReflectedInFeed()
+    {
+        using GatewayMetrics metrics = new();
+        FakeSessionManager sessions = new();
+
+        // Watch-list-bearing, Auto config: a non-empty resolved watch-list with the
+        // default ("Auto" -> Unspecified) forced mode, so step 1 can assert both the
+        // resolved watch-list and the forced mode/failover the SubscribeAlarms carries.
+        StubWatchListResolver resolver = new(
+        [
+            new AlarmSubtagTarget { AlarmFullReference = "Galaxy!Area.Tank01.Hi", ActiveSubtag = "Tank01.Hi.active" },
+            new AlarmSubtagTarget { AlarmFullReference = "Galaxy!Area.Tank02.Lo", ActiveSubtag = "Tank02.Lo.active" },
+        ]);
+
+        AlarmsOptions options = new()
+        {
+            Enabled = true,
+            SubscriptionExpression = @"\\NODE\Galaxy!Area",
+            Fallback = new AlarmFallbackOptions
+            {
+                Mode = "Auto",
+                ConsecutiveFailureThreshold = 3,
+                FailbackProbeIntervalSeconds = 9,
+                FailbackStableProbes = 2,
+            },
+        };
+
+        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);
+
+        // --- Step 1: SubscribeAlarms carries the resolved watch-list + forced mode/failover. ---
+        SubscribeAlarmsCommand sent = Assert.IsType<SubscribeAlarmsCommand>(sessions.LastSubscribeCommand);
+        Assert.Equal(AlarmProviderMode.Unspecified, sent.ForcedMode); // "Auto"
+        Assert.Equal(3, sent.Failover.ConsecutiveFailureThreshold);
+        Assert.Equal(9, sent.Failover.FailbackProbeIntervalSeconds);
+        Assert.Equal(2, sent.Failover.FailbackStableProbes);
+        Assert.NotEmpty(sent.WatchList);
+        Assert.Equal(2, sent.WatchList.Count);
+        Assert.Contains(sent.WatchList, t => t.AlarmFullReference == "Galaxy!Area.Tank01.Hi");
+
+        // Live feed reader collecting every message in order.
+        List<AlarmFeedMessage> received = [];
+        TaskCompletionSource baselineReceived = new(TaskCreationOptions.RunContinuationsAsynchronously);
+        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);
+                        if (received.Count == 1)
+                        {
+                            // The first message is the baseline ProviderStatus.
+                            baselineReceived.TrySetResult();
+                        }
+                    }
+                }
+            }
+            catch (OperationCanceledException)
+            {
+                // Expected when the test cancels the stream.
+            }
+        });
+
+        // --- Step 2: first message is ProviderStatus{Alarmmgr, Degraded=false}. ---
+        await baselineReceived.Task.WaitAsync(WaitTimeout);
+        AlarmFeedMessage baseline;
+        lock (received)
+        {
+            baseline = received[0];
+        }
+
+        Assert.Equal(AlarmFeedMessage.PayloadOneofCase.ProviderStatus, baseline.PayloadCase);
+        Assert.Equal(AlarmProviderMode.Alarmmgr, baseline.ProviderStatus.Mode);
+        Assert.False(baseline.ProviderStatus.Degraded);
+
+        // --- Step 3: alarmmgr-style transition (Degraded=false, SourceProvider=Alarmmgr, Raise). ---
+        sessions.EmitEvent(new MxEvent
+        {
+            OnAlarmTransition = new OnAlarmTransitionEvent
+            {
+                AlarmFullReference = "Galaxy!Area.Tank01.Hi",
+                SourceObjectReference = "Tank01",
+                AlarmTypeName = "AnalogLimitAlarm.Hi",
+                TransitionKind = AlarmTransitionKind.Raise,
+                Severity = 500,
+                Degraded = false,
+                SourceProvider = AlarmProviderMode.Alarmmgr,
+                TransitionTimestamp = Timestamp.FromDateTimeOffset(DateTimeOffset.UtcNow),
+            },
+        });
+
+        AlarmFeedMessage alarmmgrTransition = await WaitForAsync(
+            received,
+            m => m.PayloadCase == AlarmFeedMessage.PayloadOneofCase.Transition
+                && m.Transition.AlarmFullReference == "Galaxy!Area.Tank01.Hi"
+                && m.Transition.SourceProvider == AlarmProviderMode.Alarmmgr,
+            WaitTimeout);
+
+        Assert.Equal(AlarmTransitionKind.Raise, alarmmgrTransition.Transition.TransitionKind);
+        Assert.False(alarmmgrTransition.Transition.Degraded);
+        Assert.Equal(AlarmProviderMode.Alarmmgr, alarmmgrTransition.Transition.SourceProvider);
+
+        // --- Step 4: failover to subtag -> ProviderStatus{Subtag, Degraded=true}. ---
+        sessions.EmitEvent(new MxEvent
+        {
+            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);
+
+        // --- Step 5: subtag-style transition (Degraded=true, SourceProvider=Subtag, Raise on a different ref). ---
+        sessions.EmitEvent(new MxEvent
+        {
+            OnAlarmTransition = new OnAlarmTransitionEvent
+            {
+                AlarmFullReference = "Galaxy!Area.Tank02.Lo",
+                SourceObjectReference = "Tank02",
+                AlarmTypeName = "AnalogLimitAlarm.Lo",
+                TransitionKind = AlarmTransitionKind.Raise,
+                Severity = 250,
+                Degraded = true,
+                SourceProvider = AlarmProviderMode.Subtag,
+                TransitionTimestamp = Timestamp.FromDateTimeOffset(DateTimeOffset.UtcNow),
+            },
+        });
+
+        AlarmFeedMessage subtagTransition = await WaitForAsync(
+            received,
+            m => m.PayloadCase == AlarmFeedMessage.PayloadOneofCase.Transition
+                && m.Transition.AlarmFullReference == "Galaxy!Area.Tank02.Lo"
+                && m.Transition.SourceProvider == AlarmProviderMode.Subtag,
+            WaitTimeout);
+
+        Assert.Equal(AlarmTransitionKind.Raise, subtagTransition.Transition.TransitionKind);
+        Assert.True(subtagTransition.Transition.Degraded);
+        Assert.Equal(AlarmProviderMode.Subtag, subtagTransition.Transition.SourceProvider);
+
+        // --- Step 6: failback to alarmmgr -> ProviderStatus{Alarmmgr, Degraded=false} (recovery). ---
+        sessions.EmitEvent(new MxEvent
+        {
+            OnAlarmProviderModeChanged = new OnAlarmProviderModeChangedEvent
+            {
+                Mode = AlarmProviderMode.Alarmmgr,
+                Reason = "recovered",
+                Hresult = 0,
+                At = Timestamp.FromDateTimeOffset(DateTimeOffset.UtcNow),
+            },
+        });
+
+        // Match the recovery status specifically: an Alarmmgr ProviderStatus that
+        // carries the "recovered" reason, distinguishing it from the baseline at [0].
+        AlarmFeedMessage recovered = await WaitForAsync(
+            received,
+            m => m.PayloadCase == AlarmFeedMessage.PayloadOneofCase.ProviderStatus
+                && m.ProviderStatus.Mode == AlarmProviderMode.Alarmmgr
+                && m.ProviderStatus.Reason == "recovered",
+            WaitTimeout);
+
+        Assert.Equal(AlarmProviderMode.Alarmmgr, recovered.ProviderStatus.Mode);
+        Assert.False(recovered.ProviderStatus.Degraded);
+        Assert.Equal("recovered", recovered.ProviderStatus.Reason);
+
+        await streamCts.CancelAsync();
+        await reader;
+        await cts.CancelAsync();
+        await monitor.StopAsync(CancellationToken.None);
+    }
+
+    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.");
+    }
+
+    /// <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;
+    }
+}