diff --git a/dotnet/src/Microsoft.Agents.AI.Hosting.OpenAI/Responses/AgentResponseUpdateExtensions.cs b/dotnet/src/Microsoft.Agents.AI.Hosting.OpenAI/Responses/AgentResponseUpdateExtensions.cs
index 7930cba8380..f6c0cb1ac19 100644
--- a/dotnet/src/Microsoft.Agents.AI.Hosting.OpenAI/Responses/AgentResponseUpdateExtensions.cs
+++ b/dotnet/src/Microsoft.Agents.AI.Hosting.OpenAI/Responses/AgentResponseUpdateExtensions.cs
@@ -305,7 +305,14 @@ private static StreamingWorkflowEventComplete CreateWorkflowEventResponse(Workfl
         if (JsonSerializer.IsReflectionEnabledByDefault)
         {
             JsonElement? dataElement = null;
-            if (workflowEvent.Data is not null)
+            if (workflowEvent.Data is Exception exception)
+            {
+                // Exceptions cannot go through System.Text.Json: Exception.TargetSite is a MethodBase,
+                // which throws NotSupportedException and would surface as a serialization error instead
+                // of the actual failure. Report the message only; the stack trace is not for clients.
+                dataElement = JsonSerializer.SerializeToElement(exception.Message, OpenAIHostingJsonContext.Default.String);
+            }
+            else if (workflowEvent.Data is not null)
             {
                 dataElement = JsonSerializer.SerializeToElement(workflowEvent.Data, OpenAIHostingJsonUtilities.DefaultOptions.GetTypeInfo(typeof(object)));
             }
diff --git a/dotnet/tests/Microsoft.Agents.AI.Hosting.OpenAI.UnitTests/WorkflowErrorEventStreamingTests.cs b/dotnet/tests/Microsoft.Agents.AI.Hosting.OpenAI.UnitTests/WorkflowErrorEventStreamingTests.cs
new file mode 100644
index 00000000000..6ac22ecf3a8
--- /dev/null
+++ b/dotnet/tests/Microsoft.Agents.AI.Hosting.OpenAI.UnitTests/WorkflowErrorEventStreamingTests.cs
@@ -0,0 +1,73 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text.Json;
+using System.Threading.Tasks;
+using Microsoft.Agents.AI.Hosting.OpenAI.Responses;
+using Microsoft.Agents.AI.Hosting.OpenAI.Responses.Models;
+using Microsoft.Agents.AI.Workflows;
+using Microsoft.Extensions.AI;
+
+namespace Microsoft.Agents.AI.Hosting.OpenAI.UnitTests;
+
+/// <summary>
+/// Regression tests for a failing workflow used as an agent: the emitted update carries a
+/// <see cref="WorkflowErrorEvent"/> as its raw representation, whose data is the original
+/// <see cref="Exception"/>. Streaming that update must not try to JSON-serialize the exception,
+/// which fails on <c>Exception.TargetSite</c> and hides the actual workflow failure.
+/// </summary>
+public sealed class WorkflowErrorEventStreamingTests
+{
+    [Fact]
+    public async Task WorkflowErrorEvent_IsStreamedWithoutSerializationFailureAsync()
+    {
+        // Arrange: the update a failing workflow-as-agent produces.
+        const string ErrorMessage = "Executor 'AggregatorExecutor' cannot send messages of type 'RawAggregate'.";
+        var update = new AgentResponseUpdate(ChatRole.Assistant, [new ErrorContent(ErrorMessage)])
+        {
+            RawRepresentation = new WorkflowErrorEvent(Thrown(ErrorMessage))
+        };
+
+        var request = new CreateResponse { Input = "Hello", Stream = true };
+        var context = new AgentInvocationContext(new IdGenerator("resp_1", "conv_1"));
+
+        // Act
+        List<StreamingResponseEvent> events = [];
+        await foreach (var evt in ToAsyncEnumerableAsync(update).ToStreamingResponseAsync(request, context))
+        {
+            events.Add(evt);
+        }
+
+        // Assert: the workflow event is streamed and carries the real failure message.
+        var workflowEvent = Assert.Single(events.OfType<StreamingWorkflowEventComplete>());
+        Assert.NotNull(workflowEvent.Data);
+        JsonElement data = workflowEvent.Data.Value;
+        Assert.Equal(nameof(WorkflowErrorEvent), data.GetProperty("event_type").GetString());
+        Assert.Equal(ErrorMessage, data.GetProperty("data").GetString());
+    }
+
+    /// <summary>Returns an actually-thrown exception, so <c>TargetSite</c> is populated.</summary>
+    private static InvalidOperationException Thrown(string message)
+    {
+        try
+        {
+            throw new InvalidOperationException(message);
+        }
+        catch (InvalidOperationException ex)
+        {
+            return ex;
+        }
+    }
+
+    private static async IAsyncEnumerable<AgentResponseUpdate> ToAsyncEnumerableAsync(params AgentResponseUpdate[] updates)
+    {
+        foreach (var update in updates)
+        {
+            yield return update;
+        }
+
+        await Task.CompletedTask;
+    }
+}