fix(archreview): gateway core P0 remediation (GWC-01/02/03, TST-02, TST-12)

Interlocking changes across the gateway server (shared GatewaySession.cs /
SessionManager.cs), committed together:

- GWC-01 (Critical): alarm monitor now attaches as an internal
  (non-counted) distributor subscriber instead of a second raw drain of the
  single worker event channel; WorkerClient._events -> SingleReader with a
  claimed-once guard so a future dual-consumer regression throws loudly.
- GWC-02 (High): faulted sessions are swept in CloseExpiredLeasesAsync
  (IsFaultedReapable + FaultedReason); new FaultedGraceSeconds (default 0).
- GWC-03 (High): configurable MaxSparseArrayLength (default 1_000_000)
  enforced before allocation.
- TST-02 (High, security): StreamEvents attach now enforces the opening key
  id -> PermissionDenied on owner mismatch.
- TST-12 (Medium): CLAUDE.md retention-defaults sentence corrected.

Verified: NonWindows build clean; targeted tests 135/135 on macOS, plus
WorkerClientTests 18/18 on the Windows host.

src/ZB.MOM.WW.MxGateway.Server/Alarms/GatewayAlarmMonitor.cs

@@ -225,11 +225,14 @@ public sealed class GatewayAlarmMonitor : BackgroundService, IGatewayAlarmServic
             Task reconcileLoop = ReconcileLoopAsync(session.SessionId, linked.Token);
             try
             {
-                await foreach (WorkerEvent workerEvent in _sessionManager
-                    .ReadEventsAsync(session.SessionId, linked.Token)
+                // Consume mapped MxEvents through the session's single distributor pump (as an
+                // internal, non-counted subscriber) rather than opening a second raw drain of the
+                // worker event channel — a second drain would split events with the dashboard
+                // mirror pump and silently lose Acknowledge/mode-change transitions (GWC-01).
+                await foreach (MxEvent mxEvent in _sessionManager
+                    .ReadAlarmEventsAsync(session.SessionId, linked.Token)
                     .ConfigureAwait(false))
                 {
-                    MxEvent? mxEvent = workerEvent.Event;
                     if (mxEvent is { BodyCase: MxEvent.BodyOneofCase.OnAlarmTransition }
                         && mxEvent.OnAlarmTransition is not null)
                     {

src/ZB.MOM.WW.MxGateway.Server/Configuration/EventOptions.cs

@@ -27,4 +27,14 @@ public sealed class EventOptions
     ///     bounds the buffer).
     /// </summary>
     public double ReplayRetentionSeconds { get; init; } = 300;
+
+    /// <summary>
+    ///     Gets the maximum <c>total_length</c> a sparse-array write may declare before the
+    ///     gateway rejects it with <c>InvalidArgument</c>, enforced in
+    ///     <see cref="Sessions.SparseArrayExpander"/> before the full array is materialized.
+    ///     Guards against a single write forcing a multi-GB allocation. The default of
+    ///     1,000,000 elements is far above any realistic MXAccess array write yet well below
+    ///     the frame-size ceiling.
+    /// </summary>
+    public int MaxSparseArrayLength { get; init; } = 1_000_000;
 }

src/ZB.MOM.WW.MxGateway.Server/Configuration/GatewayOptionsValidator.cs

@@ -181,6 +181,10 @@ public sealed class GatewayOptionsValidator : OptionsValidatorBase<GatewayOption
             options.DetachGraceSeconds,
             "MxGateway:Sessions:DetachGraceSeconds must be zero or greater (0 disables detach-grace retention).",
             builder);
+        AddIfNegative(
+            options.FaultedGraceSeconds,
+            "MxGateway:Sessions:FaultedGraceSeconds must be zero or greater (0 reaps a faulted session on the next sweep).",
+            builder);
         AddIfNegative(
             options.WorkerReadyWaitTimeoutMs,
             "MxGateway:Sessions:WorkerReadyWaitTimeoutMs must be greater than or equal to zero.",
@@ -214,6 +218,10 @@ public sealed class GatewayOptionsValidator : OptionsValidatorBase<GatewayOption
         builder.RequireThat(
             options.ReplayRetentionSeconds >= 0,
             "MxGateway:Events:ReplayRetentionSeconds must be greater than or equal to zero.");
+
+        builder.RequireThat(
+            options.MaxSparseArrayLength >= 1 && options.MaxSparseArrayLength <= Array.MaxLength,
+            $"MxGateway:Events:MaxSparseArrayLength must be between 1 and {Array.MaxLength}.");
     }
 
     private static void ValidateDashboard(DashboardOptions options, ValidationBuilder builder)

src/ZB.MOM.WW.MxGateway.Server/Configuration/SessionOptions.cs

@@ -45,6 +45,18 @@ public sealed class SessionOptions
     /// </remarks>
     public int DetachGraceSeconds { get; init; } = 30;
 
+    /// <summary>
+    ///     Gets the grace period, in seconds, that a faulted session is retained before the
+    ///     lease monitor reaps it (killing its worker and freeing the session slot). A faulted
+    ///     session is otherwise permanently unusable — every command fails the readiness check —
+    ///     yet without this sweep it pins a session slot and a live x86 worker until its normal
+    ///     lease expires (up to <see cref="DefaultLeaseSeconds"/>). A value of <c>0</c> (the
+    ///     default) reaps a faulted session on the next sweep cycle, bounding the blast radius; a
+    ///     positive value keeps the faulted session observable via <c>GetSessionStatus</c> for the
+    ///     grace window before it is reclaimed. Must be greater than or equal to zero.
+    /// </summary>
+    public int FaultedGraceSeconds { get; init; }
+
     /// <summary>
     /// Gets a value indicating whether multiple event subscribers are allowed per session.
     /// </summary>

src/ZB.MOM.WW.MxGateway.Server/Grpc/EventStreamService.cs

@@ -48,6 +48,7 @@ public sealed class EventStreamService(
     /// </remarks>
     public async IAsyncEnumerable<MxEvent> StreamEventsAsync(
         StreamEventsRequest request,
+        string? callerKeyId,
         [EnumeratorCancellation] CancellationToken cancellationToken)
     {
         if (!sessionManager.TryGetSession(request.SessionId, out GatewaySession? session) || session is null)
@@ -57,6 +58,19 @@ public sealed class EventStreamService(
                 $"Session {request.SessionId} was not found.");
         }
 
+        // Owner-scoped attach (TST-02, security control): a session's event stream may be
+        // attached or reattached ONLY by the API key that opened the session. The detach-grace
+        // and fan-out retention windows are on by default, so without this check any event-scoped
+        // key that learns a session id could attach to another key's retained session and receive
+        // its replayed and live data. Ordinal comparison; null owner (session opened with no auth)
+        // matches only a null caller key.
+        if (!string.Equals(session.OwnerKeyId, callerKeyId, StringComparison.Ordinal))
+        {
+            throw new SessionManagerException(
+                SessionManagerErrorCode.PermissionDenied,
+                $"Session {request.SessionId} is owned by a different API key; event-stream attach is owner-scoped.");
+        }
+
         // No `using` here — subscriber.Dispose() is called exactly once in the finally
         // block below, which also disposes the reader. A `using` declaration would add a
         // second Dispose on the same path and double-decrement the session subscriber count.

src/ZB.MOM.WW.MxGateway.Server/Grpc/IEventStreamService.cs

@@ -11,9 +11,16 @@ public interface IEventStreamService
     /// Streams events for the specified session to the caller.
     /// </summary>
     /// <param name="request">Request payload.</param>
+    /// <param name="callerKeyId">
+    ///     The API key id of the calling client, used to enforce that only the key that opened a
+    ///     session may attach or reattach its event stream. <see langword="null"/> when the call
+    ///     is unauthenticated (e.g. auth disabled), which only matches a session opened with no
+    ///     owner key.
+    /// </param>
     /// <param name="cancellationToken">Token to cancel the asynchronous operation.</param>
     /// <returns>The events emitted for the requested session.</returns>
     IAsyncEnumerable<MxEvent> StreamEventsAsync(
         StreamEventsRequest request,
+        string? callerKeyId,
         CancellationToken cancellationToken);
 }

src/ZB.MOM.WW.MxGateway.Server/Grpc/MxAccessGatewayService.cs

@@ -148,7 +148,7 @@ public sealed class MxAccessGatewayService(
         {
             requestValidator.ValidateStreamEvents(request);
             await foreach (MxEvent publicEvent in eventStreamService
-                .StreamEventsAsync(request, context.CancellationToken)
+                .StreamEventsAsync(request, identityAccessor.Current?.KeyId, context.CancellationToken)
                 .WithCancellation(context.CancellationToken)
                 .ConfigureAwait(false))
             {
@@ -931,6 +931,7 @@ public sealed class MxAccessGatewayService(
             SessionManagerErrorCode.SessionLimitExceeded => StatusCode.ResourceExhausted,
             SessionManagerErrorCode.OpenFailed => StatusCode.Unavailable,
             SessionManagerErrorCode.CloseFailed => StatusCode.Unavailable,
+            SessionManagerErrorCode.PermissionDenied => StatusCode.PermissionDenied,
             _ => StatusCode.Unavailable,
         };
 

src/ZB.MOM.WW.MxGateway.Server/Sessions/GatewaySession.cs

@@ -23,8 +23,10 @@ public sealed class GatewaySession
     private bool _closeStarted;
     private int _activeEventSubscriberCount;
     private readonly TimeSpan _detachGrace;
+    private readonly TimeSpan _faultedGrace;
     private readonly TimeSpan _workerReadyWaitTimeout;
     private DateTimeOffset? _detachedAtUtc;
+    private DateTimeOffset? _faultedAtUtc;
     // True once at least one external subscriber attached SUCCESSFULLY. Detach-grace's
     // "last subscriber dropped" stamp (see DetachEventSubscriber) is gated on this so a
     // FAILED first attach — which still runs the rollback DetachEventSubscriber from the
@@ -139,6 +141,14 @@ public sealed class GatewaySession
     ///     When <see langword="null"/> (legacy unit-construction paths that do not exercise Galaxy
     ///     metadata), addresses pass through unchanged.
     /// </param>
+    /// <param name="faultedGrace">
+    ///     Grace window kept after the session faults before the lease monitor reaps it. When the
+    ///     window is positive the faulted session stays observable via <c>GetSessionStatus</c> for
+    ///     that long before it is reclaimed; <see cref="TimeSpan.Zero"/> (the default) makes the
+    ///     session reapable on the next sweep. The fault timestamp is stamped in
+    ///     <see cref="MarkFaulted"/> using <paramref name="eventStreaming"/>'s clock so the timer
+    ///     is unit-testable.
+    /// </param>
     public GatewaySession(
         string sessionId,
         string backendName,
@@ -156,7 +166,8 @@ public sealed class GatewaySession
         SessionEventStreaming? eventStreaming = null,
         TimeSpan detachGrace = default,
         TimeSpan workerReadyWaitTimeout = default,
-        ArrayAddressNormalizer? addressNormalizer = null)
+        ArrayAddressNormalizer? addressNormalizer = null,
+        TimeSpan faultedGrace = default)
     {
         if (string.IsNullOrWhiteSpace(sessionId))
         {
@@ -195,6 +206,7 @@ public sealed class GatewaySession
         _leaseExpiresAt = openedAt + leaseDuration;
         _eventStreaming = eventStreaming ?? SessionEventStreaming.Default;
         _detachGrace = detachGrace > TimeSpan.Zero ? detachGrace : TimeSpan.Zero;
+        _faultedGrace = faultedGrace > TimeSpan.Zero ? faultedGrace : TimeSpan.Zero;
         _workerReadyWaitTimeout = workerReadyWaitTimeout > TimeSpan.Zero ? workerReadyWaitTimeout : TimeSpan.Zero;
         _addressNormalizer = addressNormalizer;
     }
@@ -522,6 +534,33 @@ public sealed class GatewaySession
         }
     }
 
+    /// <summary>
+    ///     Registers a gateway-owned <em>internal</em> (non-counted) distributor subscriber and
+    ///     returns its lease. The lease's <see cref="IEventSubscriberLease.Reader"/> yields the
+    ///     same mapped <see cref="MxEvent"/>s the single distributor pump fans to every
+    ///     subscriber; disposing the lease unregisters it.
+    /// </summary>
+    /// <remarks>
+    ///     Used by the central alarm monitor so it consumes events through the one distributor
+    ///     pump instead of opening a second raw drain of the single worker event channel (which
+    ///     would split events between the two readers). Mirrors the dashboard-mirror lease:
+    ///     <c>isInternal: true</c> keeps this subscriber out of the
+    ///     <c>MaxEventSubscribersPerSession</c> accounting and out of the single-subscriber
+    ///     overflow-fault path, so a slow alarm reconcile can never fault the session — it only
+    ///     disconnects this internal subscriber.
+    /// </remarks>
+    /// <returns>The internal subscriber's lease; dispose it to unregister.</returns>
+    public IEventSubscriberLease AttachInternalEventSubscriber()
+    {
+        // Same sequence StartDashboardMirror uses: create the distributor (claiming the pump
+        // start if we are first), register the internal subscriber BEFORE the pump starts so a
+        // subscriber is always present at pump start, then start the pump if requested.
+        SessionEventDistributor distributor = EnsureDistributorCreated(out bool startNow);
+        IEventSubscriberLease lease = distributor.Register(isInternal: true);
+        StartPumpIfRequested(distributor, startNow);
+        return lease;
+    }
+
     private static void StartPumpIfRequested(SessionEventDistributor distributor, bool startNow)
     {
         if (!startNow)
@@ -724,6 +763,11 @@ public sealed class GatewaySession
 
             _finalFault = reason;
             _state = SessionState.Faulted;
+
+            // Stamp the fault time once, on the first fault, so the sweeper can apply
+            // FaultedGraceSeconds. A subsequent MarkFaulted (already-faulted session) keeps the
+            // original timestamp so the grace window is measured from the first fault.
+            _faultedAtUtc ??= _eventStreaming.TimeProvider.GetUtcNow();
         }
     }
 
@@ -790,6 +834,24 @@ public sealed class GatewaySession
         }
     }
 
+    /// <summary>
+    ///     Determines whether a faulted session is now eligible for reaping by the lease monitor.
+    ///     A faulted session is permanently unusable (every command fails the readiness check),
+    ///     so the sweeper closes it exactly as it closes an expired lease — but no sooner than the
+    ///     configured <c>FaultedGraceSeconds</c> after the fault, so a monitoring client can still
+    ///     observe the fault before the slot is reclaimed. Always returns <see langword="false"/>
+    ///     for a non-faulted session.
+    /// </summary>
+    /// <param name="now">Current timestamp for comparison.</param>
+    /// <returns><see langword="true"/> if the session is faulted and past its fault-grace window; otherwise <see langword="false"/>.</returns>
+    public bool IsFaultedReapable(DateTimeOffset now)
+    {
+        lock (_syncRoot)
+        {
+            return IsFaultedReapableCore(now);
+        }
+    }
+
     /// <summary>
     ///     Attaches an event subscriber and returns a lease whose
     ///     <see cref="IEventSubscriberLease.Reader"/> reads the fanned public
@@ -1053,12 +1115,13 @@ public sealed class GatewaySession
         _addressNormalizer?.Normalize(address) ?? address;
 
     // MXAccess writes replace the whole array; expand a sparse value in place so the worker only
-    // ever receives a whole-array MxValue. No-op for null or non-sparse values.
-    private static void ExpandValue(MxValue? value)
+    // ever receives a whole-array MxValue. No-op for null or non-sparse values. The configured
+    // MxGateway:Events:MaxSparseArrayLength cap is enforced before the full array is allocated.
+    private void ExpandValue(MxValue? value)
     {
         if (value is not null)
         {
-            SparseArrayExpander.Expand(value);
+            SparseArrayExpander.Expand(value, _eventStreaming.EventOptions.MaxSparseArrayLength);
         }
     }
 
@@ -1571,7 +1634,7 @@ public sealed class GatewaySession
 
             // Re-verify eligibility atomically. If a subscriber reattached between the sweep's
             // eligibility check and this point, neither condition holds and we decline.
-            bool eligible = IsLeaseExpiredCore(now) || IsDetachGraceExpiredCore(now);
+            bool eligible = IsLeaseExpiredCore(now) || IsFaultedReapableCore(now) || IsDetachGraceExpiredCore(now);
             if (!eligible)
             {
                 alreadyClosing = false;
@@ -1597,6 +1660,12 @@ public sealed class GatewaySession
             && _detachedAtUtc is not null
             && now - _detachedAtUtc.Value >= _detachGrace;
 
+    private bool IsFaultedReapableCore(DateTimeOffset now)
+        => _state is SessionState.Faulted
+            && (_faultedGrace <= TimeSpan.Zero
+                || _faultedAtUtc is null
+                || now - _faultedAtUtc.Value >= _faultedGrace);
+
     // Final terminal transition; under _syncRoot to keep _state writes single-lock.
     // Closed is unconditionally terminal — TransitionTo refuses to overwrite it —
     // so we don't need to re-check the precondition here.

src/ZB.MOM.WW.MxGateway.Server/Sessions/ISessionManager.cs

@@ -43,6 +43,18 @@ public interface ISessionManager
         string sessionId,
         CancellationToken cancellationToken);
 
+    /// <summary>
+    ///     Reads mapped events for the central alarm monitor by attaching an internal
+    ///     (non-counted) distributor subscriber, so the alarm feed shares the one worker-event
+    ///     pump instead of opening a second raw drain of the single worker event channel.
+    /// </summary>
+    /// <param name="sessionId">Identifier of the session.</param>
+    /// <param name="cancellationToken">Token to cancel the asynchronous operation.</param>
+    /// <returns>The mapped <see cref="MxEvent"/>s fanned by the session's distributor.</returns>
+    IAsyncEnumerable<MxEvent> ReadAlarmEventsAsync(
+        string sessionId,
+        CancellationToken cancellationToken);
+
     /// <summary>Closes a session and terminates its worker process.</summary>
     /// <param name="sessionId">Identifier of the session to close.</param>
     /// <param name="cancellationToken">Token to cancel the asynchronous operation.</param>

src/ZB.MOM.WW.MxGateway.Server/Sessions/SessionManager.cs

@@ -1,4 +1,5 @@
 using System.Diagnostics.CodeAnalysis;
+using System.Runtime.CompilerServices;
 using System.Security.Cryptography;
 using Google.Protobuf.WellKnownTypes;
 using Microsoft.Extensions.Logging;
@@ -18,6 +19,7 @@ public sealed class SessionManager : ISessionManager
     public const string GatewayShutdownReason = "gateway-shutdown";
     public const string LeaseExpiredReason = "lease-expired";
     public const string DetachGraceExpiredReason = "detach-grace-expired";
+    public const string FaultedReason = "faulted-reaped";
 
     private readonly ISessionRegistry _registry;
     private readonly ISessionWorkerClientFactory _workerClientFactory;
@@ -189,6 +191,22 @@ public sealed class SessionManager : ISessionManager
         return session.ReadEventsAsync(cancellationToken);
     }
 
+    /// <inheritdoc />
+    public async IAsyncEnumerable<MxEvent> ReadAlarmEventsAsync(
+        string sessionId,
+        [EnumeratorCancellation] CancellationToken cancellationToken)
+    {
+        GatewaySession session = GetRequiredSession(sessionId);
+        using IEventSubscriberLease lease = session.AttachInternalEventSubscriber();
+
+        await foreach (MxEvent mxEvent in lease.Reader
+            .ReadAllAsync(cancellationToken)
+            .ConfigureAwait(false))
+        {
+            yield return mxEvent;
+        }
+    }
+
     /// <inheritdoc />
     public async Task<SessionCloseResult> CloseSessionAsync(
         string sessionId,
@@ -259,22 +277,25 @@ public sealed class SessionManager : ISessionManager
         int closedCount = 0;
         foreach (GatewaySession session in _registry.Snapshot())
         {
-            // A session is swept when its normal lease has expired OR its detach-grace
-            // retention window has elapsed (last external subscriber dropped and no client
-            // reconnected within DetachGraceSeconds). The detach-grace close is the same
-            // teardown as a lease-expiry close; only the reason differs so operators can tell
-            // a short reconnect-window expiry from a long idle-lease expiry in logs/metrics.
-            // Lease-expiry takes PRECEDENCE over detach-grace when both conditions fire
-            // simultaneously (reason will be lease-expired, not detach-grace-expired).
+            // A session is swept when its normal lease has expired, it has FAULTED (a faulted
+            // session is permanently unusable yet otherwise pins a session slot and a live x86
+            // worker until its DefaultLeaseSeconds lease expires), OR its detach-grace retention
+            // window has elapsed (last external subscriber dropped and no client reconnected
+            // within DetachGraceSeconds). All three are the same teardown; only the reason differs
+            // so operators can tell a fault reap from a short reconnect-window expiry from a long
+            // idle-lease expiry in logs/metrics. Precedence when several fire simultaneously:
+            // lease-expired, then faulted, then detach-grace.
             //
             // TOCTOU note: eligibility is re-verified atomically inside TryBeginCloseIfExpired
             // under _syncRoot, so a client that reattaches a subscriber between the check above
             // and the close call wins the race and the session is left open and usable.
             string? reason = session.IsLeaseExpired(now)
                 ? LeaseExpiredReason
-                : session.IsDetachGraceExpired(now)
-                    ? DetachGraceExpiredReason
-                    : null;
+                : session.IsFaultedReapable(now)
+                    ? FaultedReason
+                    : session.IsDetachGraceExpired(now)
+                        ? DetachGraceExpiredReason
+                        : null;
             if (reason is null)
             {
                 continue;
@@ -457,7 +478,8 @@ public sealed class SessionManager : ISessionManager
             eventStreaming,
             TimeSpan.FromSeconds(Math.Max(0, _options.Sessions.DetachGraceSeconds)),
             TimeSpan.FromMilliseconds(Math.Max(0, _options.Sessions.WorkerReadyWaitTimeoutMs)),
-            _addressNormalizer);
+            _addressNormalizer,
+            TimeSpan.FromSeconds(Math.Max(0, _options.Sessions.FaultedGraceSeconds)));
     }
 
     private static string CreateClientCorrelationId(

src/ZB.MOM.WW.MxGateway.Server/Sessions/SessionManagerErrorCode.cs

@@ -10,4 +10,11 @@ public enum SessionManagerErrorCode
     SessionLimitExceeded,
     OpenFailed,
     CloseFailed,
+
+    /// <summary>
+    ///     The caller is not permitted to perform the operation on this session — for example,
+    ///     attaching a <c>StreamEvents</c> stream to a session opened by a different API key.
+    ///     Maps to gRPC <c>PermissionDenied</c>.
+    /// </summary>
+    PermissionDenied,
 }

src/ZB.MOM.WW.MxGateway.Server/Sessions/SparseArrayExpander.cs

@@ -33,13 +33,20 @@ internal static class SparseArrayExpander
     /// a sparse array this is a no-op, so callers may invoke it unconditionally.
     /// </summary>
     /// <param name="value">The value to expand in place.</param>
+    /// <param name="maxSparseArrayLength">
+    /// The maximum <c>total_length</c> the sparse array may declare before the write is
+    /// rejected, enforced before the full array is allocated (see
+    /// <c>MxGateway:Events:MaxSparseArrayLength</c>). Defaults to <see cref="int.MaxValue"/>
+    /// so the <see cref="Array.MaxLength"/> backstop is the only bound in test/unit-construction
+    /// paths that do not thread the configured cap.
+    /// </param>
     /// <exception cref="RpcException">
     /// <see cref="StatusCode.InvalidArgument"/> when the sparse payload is invalid: zero
-    /// total length, an index at or beyond the total length, a duplicate index, an
-    /// unsupported element type, or an element value whose kind does not match the declared
-    /// element type.
+    /// total length, a total length exceeding <paramref name="maxSparseArrayLength"/>, an index
+    /// at or beyond the total length, a duplicate index, an unsupported element type, or an
+    /// element value whose kind does not match the declared element type.
     /// </exception>
-    public static void Expand(MxValue value)
+    public static void Expand(MxValue value, int maxSparseArrayLength = int.MaxValue)
     {
         ArgumentNullException.ThrowIfNull(value);
 
@@ -62,6 +69,12 @@ internal static class SparseArrayExpander
             throw Invalid($"Sparse array element_data_type '{elementType}' is not a supported scalar element type.");
         }
 
+        if (totalLength > (uint)maxSparseArrayLength)
+        {
+            throw Invalid(
+                $"Sparse array total_length {totalLength} exceeds the configured maximum {maxSparseArrayLength} (MxGateway:Events:MaxSparseArrayLength).");
+        }
+
         if (totalLength > (uint)Array.MaxLength)
         {
             throw Invalid(

src/ZB.MOM.WW.MxGateway.Server/Workers/WorkerClient.cs

@@ -32,6 +32,7 @@ public sealed class WorkerClient : IWorkerClient
     private DateTimeOffset _lastHeartbeatAt;
     private int? _processId;
     private int _eventQueueDepth;
+    private int _eventsReaderClaimed;
     private Task? _readLoopTask;
     private Task? _writeLoopTask;
     private Task? _heartbeatLoopTask;
@@ -70,7 +71,13 @@ public sealed class WorkerClient : IWorkerClient
         _events = Channel.CreateBounded<WorkerEvent>(
             new BoundedChannelOptions(_options.EventChannelCapacity)
             {
-                SingleReader = false,
+                // The worker event channel has exactly ONE consumer: the per-session
+                // SessionEventDistributor pump. The alarm monitor and dashboard mirror both
+                // attach to the distributor rather than draining this channel directly, so a
+                // second concurrent reader would silently split events between the two
+                // enumerators. SingleReader=true asserts that invariant; ReadEventsAsync adds a
+                // claimed-once guard so a regression fails loudly instead of losing events.
+                SingleReader = true,
                 SingleWriter = true,
                 FullMode = BoundedChannelFullMode.Wait,
                 AllowSynchronousContinuations = false,
@@ -224,7 +231,24 @@ public sealed class WorkerClient : IWorkerClient
     }
 
     /// <inheritdoc />
-    public async IAsyncEnumerable<WorkerEvent> ReadEventsAsync(
+    public IAsyncEnumerable<WorkerEvent> ReadEventsAsync(CancellationToken cancellationToken)
+    {
+        // The event channel is SingleReader: only one enumerator may ever drain it, otherwise
+        // the two readers would each receive a random subset of events. Claim the reader at CALL
+        // time (not lazily on first MoveNext) and fail loudly on a second consumer rather than
+        // silently splitting the stream (see GWC-01). The distributor pump is the only intended
+        // caller; the alarm monitor and dashboard mirror attach to the distributor instead.
+        if (Interlocked.CompareExchange(ref _eventsReaderClaimed, 1, 0) != 0)
+        {
+            throw new InvalidOperationException(
+                "WorkerClient.ReadEventsAsync was already claimed by another consumer. The worker event "
+                + "channel is single-reader; attach to the SessionEventDistributor instead of draining it twice.");
+        }
+
+        return ReadEventsCoreAsync(cancellationToken);
+    }
+
+    private async IAsyncEnumerable<WorkerEvent> ReadEventsCoreAsync(
         [EnumeratorCancellation] CancellationToken cancellationToken)
     {
         await foreach (WorkerEvent workerEvent in _events.Reader.ReadAllAsync(cancellationToken).ConfigureAwait(false))

src/ZB.MOM.WW.MxGateway.Tests/Alarms/AlarmFailoverEndToEndTests.cs

@@ -470,6 +470,20 @@ public sealed class AlarmFailoverEndToEndTests
             }
         }
 
+        /// <inheritdoc />
+        public async IAsyncEnumerable<MxEvent> ReadAlarmEventsAsync(
+            string sessionId,
+            [EnumeratorCancellation] CancellationToken cancellationToken)
+        {
+            await foreach (WorkerEvent workerEvent in _events.Reader.ReadAllAsync(cancellationToken))
+            {
+                if (workerEvent.Event is not null)
+                {
+                    yield return workerEvent.Event;
+                }
+            }
+        }
+
         /// <inheritdoc />
         public bool TryGetSession(string sessionId, [MaybeNullWhen(false)] out GatewaySession session)
         {

src/ZB.MOM.WW.MxGateway.Tests/Alarms/GatewayAlarmMonitorProviderModeTests.cs

@@ -783,6 +783,20 @@ public sealed class GatewayAlarmMonitorProviderModeTests
             }
         }
 
+        /// <inheritdoc />
+        public async IAsyncEnumerable<MxEvent> ReadAlarmEventsAsync(
+            string sessionId,
+            [EnumeratorCancellation] CancellationToken cancellationToken)
+        {
+            await foreach (WorkerEvent workerEvent in _events.Reader.ReadAllAsync(cancellationToken))
+            {
+                if (workerEvent.Event is not null)
+                {
+                    yield return workerEvent.Event;
+                }
+            }
+        }
+
         /// <inheritdoc />
         public bool TryGetSession(string sessionId, [MaybeNullWhen(false)] out GatewaySession session)
         {

src/ZB.MOM.WW.MxGateway.Tests/Configuration/GatewayOptionsValidatorTests.cs

@@ -520,4 +520,32 @@ public sealed class GatewayOptionsValidatorTests
         ValidateOptionsResult result = new GatewayOptionsValidator().Validate(null, options);
         Assert.True(result.Succeeded);
     }
+
+    /// <summary>Verifies a <see cref="EventOptions.MaxSparseArrayLength"/> below one fails validation.</summary>
+    /// <param name="value">Sparse-array cap under test.</param>
+    [Theory]
+    [InlineData(0)]
+    [InlineData(-1)]
+    public void Validate_Fails_WhenMaxSparseArrayLengthBelowOne(int value)
+    {
+        GatewayOptions options = CloneWithEvents(
+            ValidOptions(),
+            new EventOptions { MaxSparseArrayLength = value });
+        ValidateOptionsResult result = new GatewayOptionsValidator().Validate(null, options);
+        Assert.True(result.Failed);
+        Assert.Contains(
+            result.Failures!,
+            f => f.Contains("MxGateway:Events:MaxSparseArrayLength"));
+    }
+
+    /// <summary>Verifies a positive <see cref="EventOptions.MaxSparseArrayLength"/> within range passes validation.</summary>
+    [Fact]
+    public void Validate_Succeeds_WhenMaxSparseArrayLengthWithinRange()
+    {
+        GatewayOptions options = CloneWithEvents(
+            ValidOptions(),
+            new EventOptions { MaxSparseArrayLength = 1 });
+        ValidateOptionsResult result = new GatewayOptionsValidator().Validate(null, options);
+        Assert.True(result.Succeeded);
+    }
 }

src/ZB.MOM.WW.MxGateway.Tests/Gateway/Dashboard/DashboardSessionAdminServiceTests.cs

@@ -281,6 +281,14 @@ public sealed class DashboardSessionAdminServiceTests
             throw new NotSupportedException();
         }
 
+        /// <inheritdoc />
+        public IAsyncEnumerable<MxEvent> ReadAlarmEventsAsync(
+            string sessionId,
+            CancellationToken cancellationToken)
+        {
+            throw new NotSupportedException();
+        }
+
         /// <inheritdoc />
         public Task<SessionCloseResult> CloseSessionAsync(
             string sessionId,

src/ZB.MOM.WW.MxGateway.Tests/Gateway/Grpc/EventStreamServiceTests.cs

@@ -38,6 +38,58 @@ public sealed class EventStreamServiceTests
         Assert.Equal(1, metrics.GetSnapshot().StreamDisconnects);
     }
 
+    /// <summary>
+    ///     TST-02 (owner-scoped attach): the API key that opened a session may attach its event
+    ///     stream — the caller key equals the session owner, so streaming proceeds normally.
+    /// </summary>
+    /// <returns>A task that represents the asynchronous operation.</returns>
+    [Fact]
+    public async Task StreamEventsAsync_WhenCallerKeyMatchesOwner_Streams()
+    {
+        FakeWorkerClient workerClient = new();
+        GatewaySession session = CreateReadySession(workerClient, ownerKeyId: "key-owner");
+        EventStreamService service = CreateService(new FakeSessionManager(session));
+        workerClient.Events.Add(CreateWorkerEvent(sequence: 5, MxEventFamily.OnDataChange));
+        workerClient.CompleteAfterConfiguredEvents = true;
+
+        List<MxEvent> events = [];
+        await foreach (MxEvent mxEvent in service
+            .StreamEventsAsync(CreateRequest(session.SessionId), callerKeyId: "key-owner", CancellationToken.None)
+            .WithCancellation(CancellationToken.None))
+        {
+            events.Add(mxEvent);
+        }
+
+        Assert.Equal([5UL], events.Select(mxEvent => mxEvent.WorkerSequence).ToArray());
+    }
+
+    /// <summary>
+    ///     TST-02 (owner-scoped attach, security control): a caller whose API key differs from
+    ///     the key that opened the session is rejected with a <see cref="SessionManagerErrorCode.PermissionDenied"/>
+    ///     fault before any events are streamed — closing the reconnect/fan-out trust-boundary hole.
+    /// </summary>
+    /// <returns>A task that represents the asynchronous operation.</returns>
+    [Fact]
+    public async Task StreamEventsAsync_WhenCallerKeyDiffersFromOwner_ThrowsPermissionDenied()
+    {
+        FakeWorkerClient workerClient = new();
+        GatewaySession session = CreateReadySession(workerClient, ownerKeyId: "key-owner");
+        EventStreamService service = CreateService(new FakeSessionManager(session));
+
+        SessionManagerException exception = await Assert.ThrowsAsync<SessionManagerException>(async () =>
+        {
+            await foreach (MxEvent _ in service
+                .StreamEventsAsync(CreateRequest(session.SessionId), callerKeyId: "key-intruder", CancellationToken.None)
+                .WithCancellation(CancellationToken.None))
+            {
+                // No event should be yielded — the owner check runs before the first attach.
+            }
+        });
+
+        Assert.Equal(SessionManagerErrorCode.PermissionDenied, exception.ErrorCode);
+        Assert.Equal(0, session.ActiveEventSubscriberCount);
+    }
+
     /// <summary>Verifies that a second event subscriber is rejected when one is already active.</summary>
     /// <returns>A task that represents the asynchronous operation.</returns>
     [Fact]
@@ -48,13 +100,13 @@ public sealed class EventStreamServiceTests
         EventStreamService service = CreateService(new FakeSessionManager(session));
         using CancellationTokenSource firstSubscriberCancellation = new();
         await using IAsyncEnumerator<MxEvent> firstSubscriber = service
-            .StreamEventsAsync(CreateRequest(session.SessionId), firstSubscriberCancellation.Token)
+            .StreamEventsAsync(CreateRequest(session.SessionId), callerKeyId: null, firstSubscriberCancellation.Token)
             .GetAsyncEnumerator(firstSubscriberCancellation.Token);
         Task<bool> firstMoveTask = firstSubscriber.MoveNextAsync().AsTask();
 
         await WaitUntilAsync(() => session.ActiveEventSubscriberCount == 1);
         await using IAsyncEnumerator<MxEvent> secondSubscriber = service
-            .StreamEventsAsync(CreateRequest(session.SessionId), CancellationToken.None)
+            .StreamEventsAsync(CreateRequest(session.SessionId), callerKeyId: null, CancellationToken.None)
             .GetAsyncEnumerator();
 
         SessionManagerException exception = await Assert.ThrowsAsync<SessionManagerException>(
@@ -78,7 +130,7 @@ public sealed class EventStreamServiceTests
         EventStreamService service = CreateService(new FakeSessionManager(session));
         using CancellationTokenSource cancellationTokenSource = new();
         await using IAsyncEnumerator<MxEvent> subscriber = service
-            .StreamEventsAsync(CreateRequest(session.SessionId), cancellationTokenSource.Token)
+            .StreamEventsAsync(CreateRequest(session.SessionId), callerKeyId: null, cancellationTokenSource.Token)
             .GetAsyncEnumerator(cancellationTokenSource.Token);
         Task<bool> moveTask = subscriber.MoveNextAsync().AsTask();
 
@@ -108,7 +160,7 @@ public sealed class EventStreamServiceTests
         workerClient.Events.Add(CreateWorkerEvent(sequence: 3, MxEventFamily.OnDataChange));
         workerClient.CompleteAfterConfiguredEvents = true;
         await using IAsyncEnumerator<MxEvent> subscriber = service
-            .StreamEventsAsync(CreateRequest(session.SessionId), CancellationToken.None)
+            .StreamEventsAsync(CreateRequest(session.SessionId), callerKeyId: null, CancellationToken.None)
             .GetAsyncEnumerator();
 
         Assert.True(await subscriber.MoveNextAsync().AsTask().WaitAsync(TestTimeout));
@@ -142,10 +194,10 @@ public sealed class EventStreamServiceTests
         firstWorkerClient.CompleteAfterConfiguredEvents = true;
         secondWorkerClient.CompleteAfterConfiguredEvents = true;
         await using IAsyncEnumerator<MxEvent> firstSubscriber = service
-            .StreamEventsAsync(CreateRequest(firstSession.SessionId), CancellationToken.None)
+            .StreamEventsAsync(CreateRequest(firstSession.SessionId), callerKeyId: null, CancellationToken.None)
             .GetAsyncEnumerator();
         await using IAsyncEnumerator<MxEvent> secondSubscriber = service
-            .StreamEventsAsync(CreateRequest(secondSession.SessionId), CancellationToken.None)
+            .StreamEventsAsync(CreateRequest(secondSession.SessionId), callerKeyId: null, CancellationToken.None)
             .GetAsyncEnumerator();
 
         Assert.True(await firstSubscriber.MoveNextAsync().AsTask().WaitAsync(TestTimeout));
@@ -192,7 +244,7 @@ public sealed class EventStreamServiceTests
 
         workerClient.CompleteAfterConfiguredEvents = true;
         await using IAsyncEnumerator<MxEvent> subscriber = service
-            .StreamEventsAsync(CreateRequest(session.SessionId), CancellationToken.None)
+            .StreamEventsAsync(CreateRequest(session.SessionId), callerKeyId: null, CancellationToken.None)
             .GetAsyncEnumerator();
 
         // The pump fans 50 events into a subscriber channel with capacity 1 faster than this
@@ -246,7 +298,7 @@ public sealed class EventStreamServiceTests
 
         workerClient.CompleteAfterConfiguredEvents = true;
         await using IAsyncEnumerator<MxEvent> subscriber = service
-            .StreamEventsAsync(CreateRequest(session.SessionId), CancellationToken.None)
+            .StreamEventsAsync(CreateRequest(session.SessionId), callerKeyId: null, CancellationToken.None)
             .GetAsyncEnumerator();
 
         SessionManagerException exception = await Assert.ThrowsAsync<SessionManagerException>(
@@ -298,7 +350,7 @@ public sealed class EventStreamServiceTests
         using GatewayMetrics metrics = new();
         EventStreamService service = CreateService(new FakeSessionManager(session), metrics);
         await using IAsyncEnumerator<MxEvent> subscriber = service
-            .StreamEventsAsync(CreateRequest(session.SessionId), CancellationToken.None)
+            .StreamEventsAsync(CreateRequest(session.SessionId), callerKeyId: null, CancellationToken.None)
             .GetAsyncEnumerator();
 
         WorkerClientException exception = await Assert.ThrowsAsync<WorkerClientException>(
@@ -333,7 +385,7 @@ public sealed class EventStreamServiceTests
 
         // Resume after sequence 2: retained window [1..5] covers it — replay 3,4,5 then live.
         await using IAsyncEnumerator<MxEvent> resume = service
-            .StreamEventsAsync(CreateRequest(session.SessionId, afterWorkerSequence: 2), CancellationToken.None)
+            .StreamEventsAsync(CreateRequest(session.SessionId, afterWorkerSequence: 2), callerKeyId: null, CancellationToken.None)
             .GetAsyncEnumerator();
 
         MxEvent r3 = await ReadNextAsync(resume);
@@ -374,7 +426,7 @@ public sealed class EventStreamServiceTests
         // Resume after 1: events 1,2 are below the oldest retained (3) and were evicted, so
         // they are unrecoverable => sentinel first, then the retained tail 3,4,5, then live.
         await using IAsyncEnumerator<MxEvent> realResume = service
-            .StreamEventsAsync(CreateRequest(session.SessionId, afterWorkerSequence: 1), CancellationToken.None)
+            .StreamEventsAsync(CreateRequest(session.SessionId, afterWorkerSequence: 1), callerKeyId: null, CancellationToken.None)
             .GetAsyncEnumerator();
 
         MxEvent sentinel = await ReadNextAsync(realResume);
@@ -419,7 +471,7 @@ public sealed class EventStreamServiceTests
         // Resume after 2: replay 3,4 then live 5,6,7. Collect across the boundary and assert
         // the full sequence is contiguous with no duplicate and no skip.
         await using IAsyncEnumerator<MxEvent> resume = service
-            .StreamEventsAsync(CreateRequest(session.SessionId, afterWorkerSequence: 2), CancellationToken.None)
+            .StreamEventsAsync(CreateRequest(session.SessionId, afterWorkerSequence: 2), callerKeyId: null, CancellationToken.None)
             .GetAsyncEnumerator();
 
         List<ulong> collected = [];
@@ -461,7 +513,7 @@ public sealed class EventStreamServiceTests
         // reads must be exactly 4 then 5 (no sentinel, no <=3 event); a live tag confirms the
         // stream resumed live strictly after 5.
         await using IAsyncEnumerator<MxEvent> resume = service
-            .StreamEventsAsync(CreateRequest(session.SessionId, afterWorkerSequence: 3), CancellationToken.None)
+            .StreamEventsAsync(CreateRequest(session.SessionId, afterWorkerSequence: 3), callerKeyId: null, CancellationToken.None)
             .GetAsyncEnumerator();
 
         MxEvent first = await ReadNextAsync(resume);
@@ -512,7 +564,7 @@ public sealed class EventStreamServiceTests
         int expectedCount)
     {
         await using IAsyncEnumerator<MxEvent> primer = service
-            .StreamEventsAsync(CreateRequest(sessionId), CancellationToken.None)
+            .StreamEventsAsync(CreateRequest(sessionId), callerKeyId: null, CancellationToken.None)
             .GetAsyncEnumerator();
         for (int i = 0; i < expectedCount; i++)
         {
@@ -551,7 +603,7 @@ public sealed class EventStreamServiceTests
     {
         List<MxEvent> events = [];
         await foreach (MxEvent mxEvent in service
-            .StreamEventsAsync(CreateRequest(sessionId), CancellationToken.None)
+            .StreamEventsAsync(CreateRequest(sessionId), callerKeyId: null, CancellationToken.None)
             .WithCancellation(CancellationToken.None))
         {
             events.Add(mxEvent);
@@ -575,7 +627,8 @@ public sealed class EventStreamServiceTests
         int queueCapacity = 8,
         GatewayMetrics? metrics = null,
         EventBackpressurePolicy backpressurePolicy = EventBackpressurePolicy.FailFast,
-        int replayBufferCapacity = 1024)
+        int replayBufferCapacity = 1024,
+        string? ownerKeyId = null)
     {
         // The per-subscriber overflow policy now lives in the session's
         // SessionEventDistributor, so the session must share the same metrics sink and
@@ -587,7 +640,7 @@ public sealed class EventStreamServiceTests
             "pipe",
             "nonce",
             "client",
-            ownerKeyId: null,
+            ownerKeyId: ownerKeyId,
             "client-session",
             "client-correlation",
             TimeSpan.FromSeconds(30),
@@ -702,6 +755,14 @@ public sealed class EventStreamServiceTests
             return _sessions[sessionId].ReadEventsAsync(cancellationToken);
         }
 
+        /// <inheritdoc />
+        public IAsyncEnumerable<MxEvent> ReadAlarmEventsAsync(
+            string sessionId,
+            CancellationToken cancellationToken)
+        {
+            throw new NotSupportedException();
+        }
+
         /// <inheritdoc />
         public Task<SessionCloseResult> CloseSessionAsync(
             string sessionId,

src/ZB.MOM.WW.MxGateway.Tests/Gateway/Grpc/MxAccessGatewayServiceConstraintTests.cs

@@ -948,6 +948,14 @@ public sealed class MxAccessGatewayServiceConstraintTests
             }
         }
 
+        /// <inheritdoc />
+        public IAsyncEnumerable<MxEvent> ReadAlarmEventsAsync(
+            string sessionId,
+            CancellationToken cancellationToken)
+        {
+            throw new NotSupportedException();
+        }
+
         /// <inheritdoc />
         public Task<SessionCloseResult> CloseSessionAsync(
             string sessionId,
@@ -994,6 +1002,7 @@ public sealed class MxAccessGatewayServiceConstraintTests
         /// <inheritdoc />
         public async IAsyncEnumerable<MxEvent> StreamEventsAsync(
             StreamEventsRequest request,
+            string? callerKeyId,
             [System.Runtime.CompilerServices.EnumeratorCancellation] CancellationToken cancellationToken)
         {
             foreach (WorkerEvent ev in sessionManager.Events)

src/ZB.MOM.WW.MxGateway.Tests/Gateway/Grpc/MxAccessGatewayServiceTests.cs

@@ -616,6 +616,14 @@ public sealed class MxAccessGatewayServiceTests
             }
         }
 
+        /// <inheritdoc />
+        public IAsyncEnumerable<MxEvent> ReadAlarmEventsAsync(
+            string sessionId,
+            CancellationToken cancellationToken)
+        {
+            throw new NotSupportedException();
+        }
+
         /// <inheritdoc />
         public Task<SessionCloseResult> CloseSessionAsync(
             string sessionId,
@@ -653,6 +661,7 @@ public sealed class MxAccessGatewayServiceTests
         /// <inheritdoc />
         public async IAsyncEnumerable<MxEvent> StreamEventsAsync(
             StreamEventsRequest request,
+            string? callerKeyId,
             [EnumeratorCancellation] CancellationToken cancellationToken)
         {
             sessionManager.RecordReadEventsSessionId(request.SessionId);

src/ZB.MOM.WW.MxGateway.Tests/Gateway/Sessions/GatewaySessionDashboardMirrorTests.cs

@@ -88,7 +88,7 @@ public sealed class GatewaySessionDashboardMirrorTests
         Task grpcReader = Task.Run(async () =>
         {
             await foreach (MxEvent mxEvent in service
-                .StreamEventsAsync(new StreamEventsRequest { SessionId = session.SessionId }, CancellationToken.None)
+                .StreamEventsAsync(new StreamEventsRequest { SessionId = session.SessionId }, callerKeyId: null, CancellationToken.None)
                 .WithCancellation(CancellationToken.None))
             {
                 grpcEvents.Add(mxEvent);
@@ -107,6 +107,65 @@ public sealed class GatewaySessionDashboardMirrorTests
         Assert.Equal([1UL, 2UL, 3UL], broadcaster.Captures.Select(capture => capture.MxEvent.WorkerSequence).ToArray());
     }
 
+    /// <summary>
+    ///     GWC-01 regression: with the internal dashboard mirror active, a second internal
+    ///     subscriber (the alarm monitor's feed, attached via
+    ///     <see cref="GatewaySession.AttachInternalEventSubscriber"/>) receives EVERY event —
+    ///     including the alarm <c>Acknowledge</c> transition — rather than the two consumers
+    ///     each getting a random half of the single worker channel. Before the fix the alarm
+    ///     monitor drained the worker channel directly, so with the dashboard mirror pump also
+    ///     draining it the two split the stream and Acknowledge transitions were silently lost.
+    /// </summary>
+    /// <returns>A task that represents the asynchronous operation.</returns>
+    [Fact]
+    public async Task InternalAlarmSubscriber_AndDashboardMirror_BothReceiveEveryEvent()
+    {
+        FakeWorkerClient workerClient = new();
+        workerClient.Events.Add(CreateWorkerEvent(1, MxEventFamily.OnDataChange));
+        workerClient.Events.Add(CreateAlarmTransitionEvent(2, AlarmTransitionKind.Raise));
+        workerClient.Events.Add(CreateAlarmTransitionEvent(3, AlarmTransitionKind.Acknowledge));
+        workerClient.Events.Add(CreateAlarmTransitionEvent(4, AlarmTransitionKind.Clear));
+        workerClient.CompleteAfterConfiguredEvents = true;
+        // Hold the finite stream until BOTH internal subscribers (dashboard mirror + alarm feed)
+        // are registered so neither misses an event before the pump drains.
+        workerClient.HoldEventsUntilReleased();
+        RecordingDashboardEventBroadcaster broadcaster = new();
+
+        await using GatewaySession session = CreateSession(workerClient, broadcaster);
+        session.AttachWorkerClient(workerClient);
+
+        // MarkReady registers the internal dashboard subscriber and starts the (gated) pump.
+        session.MarkReady();
+
+        // Attach the alarm monitor's internal subscriber and drain it concurrently. Registered
+        // BEFORE the stream is released, so it is present at pump start alongside the dashboard.
+        using IEventSubscriberLease alarmLease = session.AttachInternalEventSubscriber();
+        List<MxEvent> alarmEvents = [];
+        Task alarmReader = Task.Run(async () =>
+        {
+            await foreach (MxEvent mxEvent in alarmLease.Reader.ReadAllAsync(CancellationToken.None))
+            {
+                alarmEvents.Add(mxEvent);
+            }
+        });
+
+        workerClient.ReleaseEvents();
+
+        await WaitUntilAsync(() => broadcaster.Captures.Count == 4 && alarmEvents.Count == 4);
+        await alarmReader.WaitAsync(TestTimeout);
+
+        // Both internal consumers see the full, identical, ordered stream — no splitting.
+        Assert.Equal([1UL, 2UL, 3UL, 4UL], broadcaster.Captures.Select(capture => capture.MxEvent.WorkerSequence).ToArray());
+        Assert.Equal([1UL, 2UL, 3UL, 4UL], alarmEvents.Select(mxEvent => mxEvent.WorkerSequence).ToArray());
+
+        // The Acknowledge transition specifically reaches the alarm feed (the transition the
+        // pre-fix split silently dropped, leaving clients showing unacked alarms indefinitely).
+        Assert.Contains(
+            alarmEvents,
+            mxEvent => mxEvent.BodyCase == MxEvent.BodyOneofCase.OnAlarmTransition
+                && mxEvent.OnAlarmTransition.TransitionKind == AlarmTransitionKind.Acknowledge);
+    }
+
     /// <summary>
     ///     Hazard guard: starting the pump at Ready with a fast-completing worker stream
     ///     and zero subscribers used to drain into nothing and leave a later subscriber hanging.
@@ -222,6 +281,19 @@ public sealed class GatewaySessionDashboardMirrorTests
         return new WorkerEvent { Event = mxEvent };
     }
 
+    private static WorkerEvent CreateAlarmTransitionEvent(ulong sequence, AlarmTransitionKind kind)
+    {
+        MxEvent mxEvent = new()
+        {
+            SessionId = "session-dashboard-mirror",
+            Family = MxEventFamily.OnAlarmTransition,
+            WorkerSequence = sequence,
+            OnAlarmTransition = new OnAlarmTransitionEvent { TransitionKind = kind },
+        };
+
+        return new WorkerEvent { Event = mxEvent };
+    }
+
     private static async Task WaitUntilAsync(Func<bool> predicate, [CallerArgumentExpression(nameof(predicate))] string? condition = null)
     {
         using CancellationTokenSource cancellationTokenSource = new(TestTimeout);
@@ -280,6 +352,11 @@ public sealed class GatewaySessionDashboardMirrorTests
             string sessionId,
             CancellationToken cancellationToken) => session.ReadEventsAsync(cancellationToken);
 
+        /// <inheritdoc />
+        public IAsyncEnumerable<MxEvent> ReadAlarmEventsAsync(
+            string sessionId,
+            CancellationToken cancellationToken) => throw new NotSupportedException();
+
         /// <inheritdoc />
         public Task<SessionCloseResult> CloseSessionAsync(
             string sessionId,

src/ZB.MOM.WW.MxGateway.Tests/Gateway/Sessions/SessionManagerTests.cs

@@ -975,6 +975,68 @@ public sealed class SessionManagerTests
         Assert.Equal(0, workerClient.ShutdownCount);
     }
 
+    /// <summary>
+    ///     A faulted session is reaped by the lease sweep (default <c>FaultedGraceSeconds=0</c>)
+    ///     even though its normal lease is still far in the future, tearing down its worker and
+    ///     freeing the slot, while a healthy leased session in the same manager is untouched.
+    /// </summary>
+    /// <returns>A task that represents the asynchronous operation.</returns>
+    [Fact]
+    public async Task CloseExpiredLeasesAsync_ReapsFaultedSession()
+    {
+        FakeWorkerClient faultedClient = new();
+        FakeWorkerClient healthyClient = new();
+        QueueingSessionWorkerClientFactory factory = new(faultedClient, healthyClient);
+        SessionManager manager = CreateManager(factory);
+        GatewaySession faultedSession = await manager.OpenSessionAsync(CreateOpenRequest(), "client-1", ownerKeyId: null, CancellationToken.None);
+        GatewaySession healthySession = await manager.OpenSessionAsync(CreateOpenRequest(), "client-2", ownerKeyId: null, CancellationToken.None);
+        DateTimeOffset now = DateTimeOffset.UtcNow;
+
+        // Both leases are far in the future, so only the fault can reap the faulted session.
+        faultedSession.ExtendLease(now.AddMinutes(30));
+        healthySession.ExtendLease(now.AddMinutes(30));
+        faultedSession.MarkFaulted("test fault");
+
+        int closedCount = await manager.CloseExpiredLeasesAsync(now, CancellationToken.None);
+
+        Assert.Equal(1, closedCount);
+        Assert.Equal(SessionState.Closed, faultedSession.State);
+        Assert.Equal(1, faultedClient.ShutdownCount);
+        Assert.Equal(SessionState.Ready, healthySession.State);
+        Assert.Equal(0, healthyClient.ShutdownCount);
+    }
+
+    /// <summary>
+    ///     With a positive <c>FaultedGraceSeconds</c>, a faulted session stays observable until
+    ///     the grace window elapses, then the next sweep reaps it.
+    /// </summary>
+    /// <returns>A task that represents the asynchronous operation.</returns>
+    [Fact]
+    public async Task CloseExpiredLeasesAsync_RespectsFaultedGraceWindow()
+    {
+        FakeWorkerClient workerClient = new();
+        FakeTimeProvider clock = new(DateTimeOffset.UtcNow);
+        SessionManager manager = CreateManager(
+            new FakeSessionWorkerClientFactory(workerClient),
+            options: CreateOptions(defaultLeaseSeconds: 1800, faultedGraceSeconds: 30),
+            timeProvider: clock);
+        GatewaySession session = await manager.OpenSessionAsync(CreateOpenRequest(), "client-1", ownerKeyId: null, CancellationToken.None);
+        session.MarkFaulted("test fault");
+
+        // Before the grace window elapses: the faulted session is retained (still observable).
+        clock.Advance(TimeSpan.FromSeconds(29));
+        int closedBefore = await manager.CloseExpiredLeasesAsync(clock.GetUtcNow(), CancellationToken.None);
+        Assert.Equal(0, closedBefore);
+        Assert.Equal(SessionState.Faulted, session.State);
+
+        // After the grace window elapses: the sweep reaps it.
+        clock.Advance(TimeSpan.FromSeconds(1));
+        int closedAfter = await manager.CloseExpiredLeasesAsync(clock.GetUtcNow(), CancellationToken.None);
+        Assert.Equal(1, closedAfter);
+        Assert.Equal(SessionState.Closed, session.State);
+        Assert.Equal(1, workerClient.ShutdownCount);
+    }
+
     /// <summary>Verifies that shutdown closes all registered sessions.</summary>
     /// <returns>A task that represents the asynchronous operation.</returns>
     [Fact]
@@ -1023,7 +1085,8 @@ public sealed class SessionManagerTests
         int maxSessions = 64,
         int defaultLeaseSeconds = 1800,
         int detachGraceSeconds = 0,
-        int workerReadyWaitTimeoutMs = 0)
+        int workerReadyWaitTimeoutMs = 0,
+        int faultedGraceSeconds = 0)
     {
         return new GatewayOptions
         {
@@ -1034,6 +1097,7 @@ public sealed class SessionManagerTests
                 DefaultLeaseSeconds = defaultLeaseSeconds,
                 DetachGraceSeconds = detachGraceSeconds,
                 WorkerReadyWaitTimeoutMs = workerReadyWaitTimeoutMs,
+                FaultedGraceSeconds = faultedGraceSeconds,
             },
             Worker = new WorkerOptions
             {

src/ZB.MOM.WW.MxGateway.Tests/Gateway/Sessions/SparseArrayExpanderTests.cs

@@ -222,4 +222,35 @@ public sealed class SparseArrayExpanderTests
         RpcException ex = Assert.Throws<RpcException>(() => SparseArrayExpander.Expand(value));
         Assert.Equal(StatusCode.InvalidArgument, ex.StatusCode);
     }
+
+    /// <summary>Verifies that a total length above the configured cap throws <see cref="StatusCode.InvalidArgument"/> before the full array is allocated.</summary>
+    [Fact]
+    public void Expand_TotalLengthExceedsConfiguredCap_ThrowsBeforeAllocation()
+    {
+        // A total_length that would force a huge allocation, but well below Array.MaxLength,
+        // so only the configured cap can reject it (the Array.MaxLength backstop would not).
+        MxValue value = SparseValue(MxDataType.Integer, 500_000_000u);
+
+        RpcException ex = Assert.Throws<RpcException>(() => SparseArrayExpander.Expand(value, maxSparseArrayLength: 1_000_000));
+        Assert.Equal(StatusCode.InvalidArgument, ex.StatusCode);
+        Assert.Contains("MaxSparseArrayLength", ex.Status.Detail, StringComparison.Ordinal);
+
+        // The value must be untouched — expansion (allocation) never ran.
+        Assert.Equal(MxValue.KindOneofCase.SparseArrayValue, value.KindCase);
+    }
+
+    /// <summary>Verifies that a total length at or below the configured cap still expands normally.</summary>
+    [Fact]
+    public void Expand_TotalLengthAtConfiguredCap_Expands()
+    {
+        MxValue value = SparseValue(
+            MxDataType.Integer,
+            4,
+            (1, new MxValue { Int32Value = 7 }));
+
+        SparseArrayExpander.Expand(value, maxSparseArrayLength: 4);
+
+        Assert.Equal(MxValue.KindOneofCase.ArrayValue, value.KindCase);
+        Assert.Equal(new[] { 0, 7, 0, 0 }, value.ArrayValue.Int32Values.Values);
+    }
 }

src/ZB.MOM.WW.MxGateway.Tests/Gateway/Workers/WorkerClientTests.cs

@@ -122,6 +122,27 @@ public sealed class WorkerClientTests
         Assert.Equal(MxEventFamily.OperationComplete, events.Current.Event.Family);
     }
 
+    /// <summary>
+    ///     The worker event channel is single-reader: a second <see cref="WorkerClient.ReadEventsAsync"/>
+    ///     enumerator must throw rather than silently split events between two consumers (GWC-01).
+    /// </summary>
+    /// <returns>A task that represents the asynchronous operation.</returns>
+    [Fact]
+    public async Task ReadEventsAsync_SecondEnumerator_Throws()
+    {
+        await using PipePair pipePair = await PipePair.CreateAsync();
+        await using WorkerClient client = CreateClient(pipePair);
+        await CompleteHandshakeAsync(client, pipePair);
+        using CancellationTokenSource cancellationTokenSource = new(TestTimeout);
+
+        // First call claims the single reader.
+        IAsyncEnumerable<WorkerEvent> first = client.ReadEventsAsync(cancellationTokenSource.Token);
+        await using IAsyncEnumerator<WorkerEvent> firstEnumerator = first.GetAsyncEnumerator(cancellationTokenSource.Token);
+
+        // A second call must fail loudly at call time rather than racing the first for events.
+        Assert.Throws<InvalidOperationException>(() => client.ReadEventsAsync(cancellationTokenSource.Token));
+    }
+
     /// <summary>Verifies that the read loop faults the client when the event queue overflows.</summary>
     /// <returns>A task that represents the asynchronous operation.</returns>
     [Fact]

src/ZB.MOM.WW.MxGateway.Tests/Security/Authorization/GatewayGrpcAuthorizationInterceptorTests.cs

@@ -477,6 +477,14 @@ public sealed class GatewayGrpcAuthorizationInterceptorTests
             return AsyncEnumerable.Empty<WorkerEvent>();
         }
 
+        /// <inheritdoc />
+        public IAsyncEnumerable<MxEvent> ReadAlarmEventsAsync(
+            string sessionId,
+            CancellationToken cancellationToken)
+        {
+            return AsyncEnumerable.Empty<MxEvent>();
+        }
+
         /// <inheritdoc />
         public Task<SessionCloseResult> CloseSessionAsync(
             string sessionId,
@@ -515,6 +523,7 @@ public sealed class GatewayGrpcAuthorizationInterceptorTests
         /// <inheritdoc />
         public async IAsyncEnumerable<MxEvent> StreamEventsAsync(
             StreamEventsRequest request,
+            string? callerKeyId,
             [EnumeratorCancellation] CancellationToken cancellationToken)
         {
             await Task.CompletedTask;