diff --git a/dotnet/src/Microsoft.Agents.AI.Workflows/Executor.cs b/dotnet/src/Microsoft.Agents.AI.Workflows/Executor.cs
index 6987c6aca3..df9afd5df7 100644
--- a/dotnet/src/Microsoft.Agents.AI.Workflows/Executor.cs
+++ b/dotnet/src/Microsoft.Agents.AI.Workflows/Executor.cs
@@ -133,7 +133,18 @@ internal sealed class ExecutorProtocol(MessageRouter router, ISet<Type> sendType
 
     public bool CanHandle(Type type) => router.CanHandle(type);
 
-    public bool CanOutput(Type type) => this._yieldTypes.Contains(new(type));
+    public bool CanOutput(Type type)
+    {
+        foreach (TypeId yieldType in this._yieldTypes)
+        {
+            if (yieldType.IsMatchPolymorphic(type))
+            {
+                return true;
+            }
+        }
+
+        return false;
+    }
 
     public ProtocolDescriptor Describe() => new(this.Router.IncomingTypes, yieldTypes, sendTypes, this.Router.HasCatchAll);
 }
diff --git a/dotnet/tests/Microsoft.Agents.AI.Workflows.UnitTests/PolymorphicOutputTests.cs b/dotnet/tests/Microsoft.Agents.AI.Workflows.UnitTests/PolymorphicOutputTests.cs
new file mode 100644
index 0000000000..040975e6a0
--- /dev/null
+++ b/dotnet/tests/Microsoft.Agents.AI.Workflows.UnitTests/PolymorphicOutputTests.cs
@@ -0,0 +1,276 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+using System.Collections.Generic;
+using System.Linq;
+using System.Threading;
+using System.Threading.Tasks;
+using FluentAssertions;
+
+namespace Microsoft.Agents.AI.Workflows.UnitTests;
+
+/// <summary>
+/// Regression tests for polymorphic output type handling in workflows.
+/// Verifies that executors can return derived types when the declared output type is a base class.
+/// </summary>
+/// <remarks>
+/// This addresses GitHub issue #4134: InvalidOperationException when returning derived type as workflow output.
+/// </remarks>
+public partial class PolymorphicOutputTests
+{
+    #region Test Type Hierarchy
+
+    /// <summary>
+    /// Base class used as declared output type.
+    /// </summary>
+    public class BaseOutput
+    {
+        public virtual string Name => "BaseOutput";
+    }
+
+    /// <summary>
+    /// Derived class returned at runtime.
+    /// </summary>
+    public class DerivedOutput : BaseOutput
+    {
+        public override string Name => "DerivedOutput";
+    }
+
+    /// <summary>
+    /// Second-level derived class for testing multiple inheritance levels.
+    /// </summary>
+    public class GrandchildOutput : DerivedOutput
+    {
+        public override string Name => "GrandchildOutput";
+    }
+
+    /// <summary>
+    /// Unrelated class that should NOT be accepted as output.
+    /// </summary>
+    public class UnrelatedOutput
+    {
+        public string Name => "UnrelatedOutput";
+    }
+
+    #endregion
+
+    #region Test Executors
+
+    /// <summary>
+    /// Executor that declares BaseOutput as yield type but returns DerivedOutput.
+    /// </summary>
+    internal sealed class DerivedOutputExecutor() : Executor(nameof(DerivedOutputExecutor))
+    {
+        protected override ProtocolBuilder ConfigureProtocol(ProtocolBuilder protocolBuilder)
+        {
+            return protocolBuilder.ConfigureRoutes(routeBuilder =>
+                routeBuilder.AddHandler<string, BaseOutput>(this.HandleAsync));
+        }
+
+        private async ValueTask<BaseOutput> HandleAsync(string input, IWorkflowContext context, CancellationToken cancellationToken)
+        {
+            await Task.Delay(10, cancellationToken);
+
+            // Arrange: Return a derived type where the method signature declares the base type
+            return new DerivedOutput();
+        }
+    }
+
+    /// <summary>
+    /// Executor that declares BaseOutput as yield type but returns GrandchildOutput (two levels deep).
+    /// </summary>
+    internal sealed class GrandchildOutputExecutor() : Executor(nameof(GrandchildOutputExecutor))
+    {
+        protected override ProtocolBuilder ConfigureProtocol(ProtocolBuilder protocolBuilder)
+        {
+            return protocolBuilder.ConfigureRoutes(routeBuilder =>
+                routeBuilder.AddHandler<string, BaseOutput>(this.HandleAsync));
+        }
+
+        private async ValueTask<BaseOutput> HandleAsync(string input, IWorkflowContext context, CancellationToken cancellationToken)
+        {
+            await Task.Delay(10, cancellationToken);
+
+            // Arrange: Return a grandchild type (two inheritance levels)
+            return new GrandchildOutput();
+        }
+    }
+
+    /// <summary>
+    /// Executor that attempts to return an unrelated type - should fail validation.
+    /// This executor intentionally bypasses type safety to test runtime validation.
+    /// </summary>
+    internal sealed class UnrelatedOutputExecutor() : Executor(nameof(UnrelatedOutputExecutor))
+    {
+        protected override ProtocolBuilder ConfigureProtocol(ProtocolBuilder protocolBuilder)
+        {
+            return protocolBuilder.ConfigureRoutes(routeBuilder =>
+                routeBuilder.AddHandler<string, BaseOutput>(this.HandleAsync));
+        }
+
+        private async ValueTask<BaseOutput> HandleAsync(string input, IWorkflowContext context, CancellationToken cancellationToken)
+        {
+            // Arrange: Attempt to yield an unrelated type - should throw
+            UnrelatedOutput unrelated = new();
+            await context.YieldOutputAsync(unrelated, cancellationToken).ConfigureAwait(false);
+
+            // This line should not be reached
+            return new BaseOutput();
+        }
+    }
+
+    /// <summary>
+    /// Executor that returns the exact declared type (baseline test).
+    /// </summary>
+    internal sealed class ExactTypeExecutor() : Executor(nameof(ExactTypeExecutor))
+    {
+        protected override ProtocolBuilder ConfigureProtocol(ProtocolBuilder protocolBuilder)
+        {
+            return protocolBuilder.ConfigureRoutes(routeBuilder =>
+                routeBuilder.AddHandler<string, BaseOutput>(this.HandleAsync));
+        }
+
+        private ValueTask<BaseOutput> HandleAsync(string input, IWorkflowContext context, CancellationToken cancellationToken)
+        {
+            BaseOutput result = new();
+            return new ValueTask<BaseOutput>(result);
+        }
+    }
+
+    #endregion
+
+    #region Tests
+
+    /// <summary>
+    /// Verifies that returning a derived type when the declared output type is a base class succeeds.
+    /// This is the main regression test for GitHub issue #4134.
+    /// </summary>
+    [Fact]
+    public async Task ReturningDerivedType_WhenBaseTypeIsDeclared_ShouldSucceedAsync()
+    {
+        // Arrange
+        DerivedOutputExecutor executor = new();
+        WorkflowBuilder builder = new WorkflowBuilder(executor).WithOutputFrom(executor);
+        Workflow workflow = builder.Build();
+
+        // Act
+        List<WorkflowEvent> events = [];
+        await using StreamingRun run = await InProcessExecution.RunStreamingAsync(workflow, "test input");
+        await foreach (WorkflowEvent evt in run.WatchStreamAsync())
+        {
+            events.Add(evt);
+        }
+
+        // Assert
+        events.Should().NotBeEmpty("workflow should produce events");
+
+        List<WorkflowOutputEvent> outputEvents = events.OfType<WorkflowOutputEvent>().ToList();
+        outputEvents.Should().ContainSingle("workflow should produce exactly one output event");
+
+        WorkflowOutputEvent outputEvent = outputEvents.Single();
+        outputEvent.Data.Should().BeOfType<DerivedOutput>("output should be the derived type");
+        ((DerivedOutput)outputEvent.Data!).Name.Should().Be("DerivedOutput");
+
+        // Verify no error events
+        List<WorkflowErrorEvent> errorEvents = events.OfType<WorkflowErrorEvent>().ToList();
+        errorEvents.Should().BeEmpty("workflow should not produce error events");
+    }
+
+    /// <summary>
+    /// Verifies that returning a grandchild type (multiple inheritance levels) succeeds.
+    /// </summary>
+    [Fact]
+    public async Task ReturningGrandchildType_WhenBaseTypeIsDeclared_ShouldSucceedAsync()
+    {
+        // Arrange
+        GrandchildOutputExecutor executor = new();
+        WorkflowBuilder builder = new WorkflowBuilder(executor).WithOutputFrom(executor);
+        Workflow workflow = builder.Build();
+
+        // Act
+        List<WorkflowEvent> events = [];
+        await using StreamingRun run = await InProcessExecution.RunStreamingAsync(workflow, "test input");
+        await foreach (WorkflowEvent evt in run.WatchStreamAsync())
+        {
+            events.Add(evt);
+        }
+
+        // Assert
+        events.Should().NotBeEmpty("workflow should produce events");
+
+        List<WorkflowOutputEvent> outputEvents = events.OfType<WorkflowOutputEvent>().ToList();
+        outputEvents.Should().ContainSingle("workflow should produce exactly one output event");
+
+        WorkflowOutputEvent outputEvent = outputEvents.Single();
+        outputEvent.Data.Should().BeOfType<GrandchildOutput>("output should be the grandchild type");
+        ((GrandchildOutput)outputEvent.Data!).Name.Should().Be("GrandchildOutput");
+
+        // Verify no error events
+        List<WorkflowErrorEvent> errorEvents = events.OfType<WorkflowErrorEvent>().ToList();
+        errorEvents.Should().BeEmpty("workflow should not produce error events");
+    }
+
+    /// <summary>
+    /// Verifies that returning an unrelated type still throws InvalidOperationException.
+    /// This ensures the fix doesn't break the existing validation for truly incompatible types.
+    /// </summary>
+    [Fact]
+    public async Task ReturningUnrelatedType_WhenBaseTypeIsDeclared_ShouldFailAsync()
+    {
+        // Arrange
+        UnrelatedOutputExecutor executor = new();
+        WorkflowBuilder builder = new WorkflowBuilder(executor).WithOutputFrom(executor);
+        Workflow workflow = builder.Build();
+
+        // Act
+        List<WorkflowEvent> events = [];
+        await using StreamingRun run = await InProcessExecution.RunStreamingAsync(workflow, "test input");
+        await foreach (WorkflowEvent evt in run.WatchStreamAsync())
+        {
+            events.Add(evt);
+        }
+
+        // Assert: Should have an error event with InvalidOperationException message
+        List<WorkflowErrorEvent> errorEvents = events.OfType<WorkflowErrorEvent>().ToList();
+        errorEvents.Should().ContainSingle("workflow should produce exactly one error event");
+
+        WorkflowErrorEvent errorEvent = errorEvents.Single();
+        string errorMessage = errorEvent.Data?.ToString() ?? string.Empty;
+        errorMessage.Should().Contain("Cannot output object of type UnrelatedOutput");
+        errorMessage.Should().Contain("BaseOutput");
+    }
+
+    /// <summary>
+    /// Verifies that returning the exact declared type still works (baseline test).
+    /// </summary>
+    [Fact]
+    public async Task ReturningExactType_WhenSameTypeIsDeclared_ShouldSucceedAsync()
+    {
+        // Arrange: Create an executor that returns the exact declared type
+        ExactTypeExecutor executor = new();
+        WorkflowBuilder builder = new WorkflowBuilder(executor).WithOutputFrom(executor);
+        Workflow workflow = builder.Build();
+
+        // Act
+        List<WorkflowEvent> events = [];
+        await using StreamingRun run = await InProcessExecution.RunStreamingAsync(workflow, "test input");
+        await foreach (WorkflowEvent evt in run.WatchStreamAsync())
+        {
+            events.Add(evt);
+        }
+
+        // Assert
+        events.Should().NotBeEmpty("workflow should produce events");
+
+        List<WorkflowOutputEvent> outputEvents = events.OfType<WorkflowOutputEvent>().ToList();
+        outputEvents.Should().ContainSingle("workflow should produce exactly one output event");
+
+        WorkflowOutputEvent outputEvent = outputEvents.Single();
+        outputEvent.Data.Should().BeOfType<BaseOutput>("output should be the exact base type");
+
+        // Verify no error events
+        List<WorkflowErrorEvent> errorEvents = events.OfType<WorkflowErrorEvent>().ToList();
+        errorEvents.Should().BeEmpty("workflow should not produce error events");
+    }
+
+    #endregion
+}