diff --git a/src/Persistence/MartenTests/AncillaryStores/multi_stream_projection_with_side_effects_on_ancillary_store.cs b/src/Persistence/MartenTests/AncillaryStores/multi_stream_projection_with_side_effects_on_ancillary_store.cs
new file mode 100644
index 000000000..ba834532e
--- /dev/null
+++ b/src/Persistence/MartenTests/AncillaryStores/multi_stream_projection_with_side_effects_on_ancillary_store.cs
@@ -0,0 +1,200 @@
+using System.Diagnostics;
+using IntegrationTests;
+using JasperFx;
+using JasperFx.Core;
+using JasperFx.Events;
+using JasperFx.Events.Daemon;
+using JasperFx.Events.Projections;
+using JasperFx.Resources;
+using Marten;
+using Marten.Events;
+using Marten.Events.Aggregation;
+using Marten.Events.Projections;
+using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.Hosting;
+using Shouldly;
+using Wolverine;
+using Wolverine.Marten;
+using Wolverine.Tracking;
+
+namespace MartenTests.AncillaryStores;
+
+/// <summary>
+/// GH-2529: Investigation test for the silent-hang concern when combining
+/// (a) a Marten ancillary document store with IntegrateWithWolverine,
+/// (b) the async daemon running on that ancillary store,
+/// (c) a multi-stream projection that publishes Wolverine messages via
+/// RaiseSideEffects().
+///
+/// If this test passes consistently, the suspected silent failure is not
+/// reproducible at the framework integration level. If it hangs or fails,
+/// we have a precise reproducer to drive the fix.
+/// </summary>
+public class multi_stream_projection_with_side_effects_on_ancillary_store : IAsyncLifetime
+{
+    private IHost _host = null!;
+
+    public async Task InitializeAsync()
+    {
+        _host = await Host.CreateDefaultBuilder()
+            .UseWolverine(opts =>
+            {
+                // Primary Marten store + Wolverine integration
+                opts.Services.AddMarten(m =>
+                {
+                    m.Connection(Servers.PostgresConnectionString);
+                    m.DatabaseSchemaName = "issue2529_main";
+                    m.Events.DatabaseSchemaName = "issue2529_main";
+                    m.DisableNpgsqlLogging = true;
+                }).IntegrateWithWolverine();
+
+                opts.Durability.Mode = DurabilityMode.Solo;
+                opts.Policies.AutoApplyTransactions();
+
+                // Ancillary store with async daemon + multi-stream projection
+                opts.Services.AddMartenStore<IIssue2529Store>(sp =>
+                    {
+                        var storeOptions = new StoreOptions();
+                        storeOptions.Connection(Servers.PostgresConnectionString);
+                        storeOptions.DatabaseSchemaName = "issue2529_ancillary";
+                        storeOptions.Events.DatabaseSchemaName = "issue2529_ancillary";
+                        storeOptions.DisableNpgsqlLogging = true;
+
+                        // The projection that calls RaiseSideEffects → publishes a Wolverine message
+                        storeOptions.Projections.Add<Issue2529CounterProjection>(ProjectionLifecycle.Async);
+                        return storeOptions;
+                    })
+                    .IntegrateWithWolverine()
+                    .AddAsyncDaemon(DaemonMode.Solo);
+
+                opts.Services.AddResourceSetupOnStartup(StartupAction.ResetState);
+
+                opts.Discovery.DisableConventionalDiscovery()
+                    .IncludeType<Issue2529SideEffectHandler>();
+            }).StartAsync();
+    }
+
+    public async Task DisposeAsync()
+    {
+        await _host.StopAsync();
+        _host.Dispose();
+    }
+
+    private async Task AppendAndWaitForProjectionAsync(params Guid[] streamIds)
+    {
+        using var scope = _host.Services.CreateScope();
+        var store = scope.ServiceProvider.GetRequiredService<IIssue2529Store>();
+
+        await using (var session = store.LightweightSession())
+        {
+            foreach (var id in streamIds)
+            {
+                session.Events.StartStream<Issue2529Counter>(id, new IncrementCounter());
+            }
+            await session.SaveChangesAsync();
+        }
+
+        // Make sure the async daemon has caught up to all the events we just wrote.
+        // This rules out "the daemon is just slow" as the cause of side-effect non-delivery.
+        using var daemon = await store.BuildProjectionDaemonAsync();
+        await daemon.WaitForNonStaleData(60.Seconds());
+    }
+
+    [Fact]
+    public async Task projection_side_effect_message_reaches_wolverine_handler()
+    {
+        var streamId = Guid.NewGuid();
+
+        Issue2529SideEffectHandler.SeenStreamIds.Clear();
+
+        var tracked = await _host
+            .TrackActivity()
+            .Timeout(60.Seconds())
+            .IncludeExternalTransports()
+            .WaitForMessageToBeReceivedAt<CounterIncremented>(_host)
+            .ExecuteAndWaitAsync((Func<IMessageContext, Task>)(_ => AppendAndWaitForProjectionAsync(streamId)));
+
+        tracked.Executed.MessagesOf<CounterIncremented>()
+            .Where(m => m.StreamId == streamId)
+            .ShouldHaveSingleItem();
+    }
+
+    // GH-2529: Was failing because concurrent slice processing in Marten's
+    // AggregationRunner corrupted MessageContext._outstanding (concurrent
+    // List<Envelope>.Add is not thread-safe), dropping most side-effect messages
+    // with no exception. Fixed by adding a lock around _outstanding mutations
+    // in MessageBus / MessageContext.
+    [Fact]
+    public async Task multiple_side_effects_in_one_batch_all_reach_wolverine()
+    {
+        var streamIds = new[] { Guid.NewGuid(), Guid.NewGuid(), Guid.NewGuid() };
+
+        Issue2529SideEffectHandler.SeenStreamIds.Clear();
+
+        var tracked = await _host
+            .TrackActivity()
+            .Timeout(60.Seconds())
+            .IncludeExternalTransports()
+            .ExecuteAndWaitAsync((Func<IMessageContext, Task>)(_ => AppendAndWaitForProjectionAsync(streamIds)));
+
+        var ourStreamIds = streamIds.ToHashSet();
+        var seen = tracked.Executed.MessagesOf<CounterIncremented>()
+            .Where(m => ourStreamIds.Contains(m.StreamId))
+            .Select(m => m.StreamId)
+            .OrderBy(x => x)
+            .ToArray();
+
+        seen.ShouldBe(streamIds.OrderBy(x => x).ToArray());
+    }
+}
+
+// ── Marker interface for the ancillary store ──
+public interface IIssue2529Store : IDocumentStore;
+
+// ── Domain ──
+public record IncrementCounter;
+
+public class Issue2529Counter
+{
+    public Guid Id { get; set; }
+    public int Count { get; set; }
+}
+
+// ── Multi-stream projection that publishes side effects ──
+public class Issue2529CounterProjection : MultiStreamProjection<Issue2529Counter, Guid>
+{
+    public Issue2529CounterProjection()
+    {
+        Identity<IEvent>(x => x.StreamId);
+    }
+
+    public static Issue2529Counter Create(IncrementCounter @event, IEvent metadata) =>
+        new() { Id = metadata.StreamId, Count = 1 };
+
+    public void Apply(Issue2529Counter counter, IncrementCounter _) => counter.Count++;
+
+    public override ValueTask RaiseSideEffects(IDocumentOperations operations, IEventSlice<Issue2529Counter> slice)
+    {
+        if (slice.Aggregate is not null)
+        {
+            slice.PublishMessage(new CounterIncremented(slice.Aggregate.Id, slice.Aggregate.Count));
+        }
+        return ValueTask.CompletedTask;
+    }
+}
+
+public record CounterIncremented(Guid StreamId, int Count);
+
+// ── Wolverine handler — records what it saw so the test can assert ──
+public class Issue2529SideEffectHandler
+{
+    public static Guid LastSeenStreamId = Guid.Empty;
+    public static readonly List<Guid> SeenStreamIds = new();
+
+    public static void Handle(CounterIncremented msg)
+    {
+        LastSeenStreamId = msg.StreamId;
+        lock (SeenStreamIds) SeenStreamIds.Add(msg.StreamId);
+        Debug.WriteLine($"Side effect received for stream {msg.StreamId} (count={msg.Count})");
+    }
+}
diff --git a/src/Wolverine/Runtime/MessageBus.cs b/src/Wolverine/Runtime/MessageBus.cs
index 13db506f5..dba4a3b8b 100644
--- a/src/Wolverine/Runtime/MessageBus.cs
+++ b/src/Wolverine/Runtime/MessageBus.cs
@@ -14,6 +14,14 @@ public static MessageBus Build(IWolverineRuntime runtime, string correlationId)
     
     // ReSharper disable once InconsistentNaming
     protected readonly List<Envelope> _outstanding = new();
+
+    // Protects _outstanding from concurrent mutation. The Marten async daemon's
+    // multi-stream projection runner can call PublishAsync on the same MessageContext
+    // from many slices in parallel (Block parallelism = 10). Without this lock,
+    // concurrent List<Envelope>.Add silently corrupts the list and drops messages.
+    // GH-2529.
+    // ReSharper disable once InconsistentNaming
+    protected readonly object _outstandingLock = new();
     private string? _tenantId;
 
     public MessageBus(IWolverineRuntime runtime) : this(runtime, Activity.Current?.RootId ?? Guid.NewGuid().ToString())
@@ -57,7 +65,17 @@ public virtual Task ReScheduleCurrentAsync(DateTimeOffset rescheduledAt)
     public IWolverineRuntime Runtime { get; }
     public IMessageStore Storage { get; internal set; }
 
-    public IEnumerable<Envelope> Outstanding => _outstanding;
+    /// <summary>
+    /// Snapshot of envelopes published in this context that have not yet been flushed.
+    /// Returns a copy to avoid concurrent-enumeration issues — see <see cref="_outstandingLock"/>.
+    /// </summary>
+    public IEnumerable<Envelope> Outstanding
+    {
+        get
+        {
+            lock (_outstandingLock) return _outstanding.ToArray();
+        }
+    }
 
     public IEnvelopeTransaction? Transaction { get; protected set; }
     public Guid ConversationId { get; protected set; }
@@ -267,7 +285,10 @@ internal async ValueTask PersistOrSendAsync(Envelope envelope)
 
         if (Transaction is not null)
         {
-            _outstanding.Fill(envelope);
+            lock (_outstandingLock)
+            {
+                _outstanding.Fill(envelope);
+            }
 
             await envelope.PersistAsync(Transaction);
 
@@ -366,7 +387,10 @@ internal async ValueTask PersistOrSendAsync(params Envelope[] outgoing)
 
             await Transaction.PersistAsync(envelopes);
 
-            _outstanding.Fill(outgoing);
+            lock (_outstandingLock)
+            {
+                _outstanding.Fill(outgoing);
+            }
         }
         else
         {
diff --git a/src/Wolverine/Runtime/MessageContext.cs b/src/Wolverine/Runtime/MessageContext.cs
index 2bf20ad0a..25f29f85b 100644
--- a/src/Wolverine/Runtime/MessageContext.cs
+++ b/src/Wolverine/Runtime/MessageContext.cs
@@ -151,12 +151,17 @@ public async Task FlushOutgoingMessagesAsync()
 
         await AssertAnyRequiredResponseWasGenerated();
 
-        if (_outstanding.Count == 0)
+        // Snapshot under lock so concurrent publishes from a Marten projection
+        // (Block parallelism = 10 in AggregationRunner) cannot corrupt the list
+        // while we're iterating it. GH-2529.
+        Envelope[] outgoing;
+        lock (_outstandingLock)
         {
-            return;
+            if (_outstanding.Count == 0) return;
+            outgoing = _outstanding.ToArray();
         }
 
-        foreach (var envelope in Outstanding)
+        foreach (var envelope in outgoing)
         {
             // https://github.com/JasperFx/wolverine/issues/2006
             if (envelope == null) continue;
@@ -209,8 +214,11 @@ public async Task FlushOutgoingMessagesAsync()
         }
 
         _sent ??= new();
-        _sent.AddRange(_outstanding);
-        _outstanding.Clear();
+        lock (_outstandingLock)
+        {
+            _sent.AddRange(_outstanding);
+            _outstanding.Clear();
+        }
 
         _hasFlushed = true;
 
@@ -236,10 +244,17 @@ public async Task AssertAnyRequiredResponseWasGenerated()
             if (isMissingRequestedReply())
             {
                 var failureDescription = $"No response was created for expected response '{Envelope!.ReplyRequested}' back to reply-uri {Envelope.ReplyUri}. ";
-                if (_outstanding.Count > 0)
+
+                Envelope[] outstandingSnapshot;
+                lock (_outstandingLock)
                 {
-                    var types = new List<string>(_outstanding.Count + (_sent?.Count ?? 0));
-                    foreach (var e in _outstanding) types.Add(e.MessageType!);
+                    outstandingSnapshot = _outstanding.ToArray();
+                }
+
+                if (outstandingSnapshot.Length > 0)
+                {
+                    var types = new List<string>(outstandingSnapshot.Length + (_sent?.Count ?? 0));
+                    foreach (var e in outstandingSnapshot) types.Add(e.MessageType!);
                     if (_sent != null)
                     {
                         foreach (var e in _sent) types.Add(e.MessageType!);
@@ -458,13 +473,19 @@ public async ValueTask SendFailureAcknowledgementAsync(string failureDescription
 
     Task IEnvelopeTransaction.PersistOutgoingAsync(Envelope envelope)
     {
-        _outstanding.Fill(envelope);
+        lock (_outstandingLock)
+        {
+            _outstanding.Fill(envelope);
+        }
         return Task.CompletedTask;
     }
 
     Task IEnvelopeTransaction.PersistOutgoingAsync(Envelope[] envelopes)
     {
-        _outstanding.Fill(envelopes);
+        lock (_outstandingLock)
+        {
+            _outstanding.Fill(envelopes);
+        }
         return Task.CompletedTask;
     }
 
@@ -519,7 +540,12 @@ public override async Task ReScheduleCurrentAsync(DateTimeOffset rescheduledAt)
 
     internal async Task CopyToAsync(IEnvelopeTransaction other)
     {
-        await other.PersistOutgoingAsync(_outstanding.ToArray());
+        Envelope[] snapshot;
+        lock (_outstandingLock)
+        {
+            snapshot = _outstanding.ToArray();
+        }
+        await other.PersistOutgoingAsync(snapshot);
 
         foreach (var envelope in Scheduled) await other.PersistIncomingAsync(envelope);
     }
@@ -530,7 +556,10 @@ internal async Task CopyToAsync(IEnvelopeTransaction other)
     public async ValueTask ClearAllAsync()
     {
         Scheduled.Clear();
-        _outstanding.Clear();
+        lock (_outstandingLock)
+        {
+            _outstanding.Clear();
+        }
 
         if (Transaction != null)
         {
@@ -649,7 +678,10 @@ internal void ClearState()
         _hasFlushed = false;
 
         _sent?.Clear();
-        _outstanding.Clear();
+        lock (_outstandingLock)
+        {
+            _outstanding.Clear();
+        }
         Scheduled.Clear();
         Envelope = null;
         Transaction = null;
@@ -679,7 +711,10 @@ internal void ReadEnvelope(Envelope? originalEnvelope, IChannelCallback channel)
             var ackEnvelope = Runtime.RoutingFor(typeof(Acknowledgement))
                 .RouteToDestination(ack, Envelope.ReplyUri, null);
             TrackEnvelopeCorrelation(ackEnvelope, Activity.Current);
-            _outstanding.Add(ackEnvelope);
+            lock (_outstandingLock)
+            {
+                _outstanding.Add(ackEnvelope);
+            }
         }
     }