diff --git a/dotnet/src/Microsoft.Agents.AI.Workflows.Declarative/DeclarativeWorkflowBuilder.cs b/dotnet/src/Microsoft.Agents.AI.Workflows.Declarative/DeclarativeWorkflowBuilder.cs
index 054aa38b237..bec044cc8d7 100644
--- a/dotnet/src/Microsoft.Agents.AI.Workflows.Declarative/DeclarativeWorkflowBuilder.cs
+++ b/dotnet/src/Microsoft.Agents.AI.Workflows.Declarative/DeclarativeWorkflowBuilder.cs
@@ -2,6 +2,7 @@
 
 using System;
 using System.IO;
+using System.Linq;
 using Microsoft.Agents.AI.Workflows.Declarative.Extensions;
 using Microsoft.Agents.AI.Workflows.Declarative.Interpreter;
 using Microsoft.Agents.AI.Workflows.Declarative.PowerFx;
@@ -104,6 +105,24 @@ private static AdaptiveDialog ReadWorkflow(TextReader yamlReader)
             throw new DeclarativeModelException($"Unsupported root element: {rootElement.GetType().Name}. Expected an {nameof(Workflow)}.");
         }
 
+        // Unknown template properties are retained as extension data by the YAML reader.
+        // Reject them here rather than silently emitting an empty or incomplete message.
+        foreach (MessageActivityTemplate template in workflowElement.Descendants().OfType<MessageActivityTemplate>())
+        {
+            if (template.ExtensionData is { Properties.Count: > 0 } extensionData)
+            {
+                BotElement? owner = template.Parent;
+                while (owner is not null && owner is not DialogAction)
+                {
+                    owner = owner.Parent;
+                }
+
+                string action = owner is DialogAction dialogAction ? $" in action #{dialogAction.Id} ({dialogAction.GetType().Name})" : string.Empty;
+                string properties = string.Join(", ", extensionData.Properties.Keys.OrderBy(key => key, StringComparer.Ordinal));
+                throw new DeclarativeModelException($"Unknown message template properties{action}: {properties}. Check the YAML property names, such as 'text'.");
+            }
+        }
+
         return workflowElement;
     }
 }
diff --git a/dotnet/src/Microsoft.Agents.AI.Workflows.Declarative/Interpreter/DeclarativeActionExecutor.cs b/dotnet/src/Microsoft.Agents.AI.Workflows.Declarative/Interpreter/DeclarativeActionExecutor.cs
index 5d052c64d3d..f23055c5c1b 100644
--- a/dotnet/src/Microsoft.Agents.AI.Workflows.Declarative/Interpreter/DeclarativeActionExecutor.cs
+++ b/dotnet/src/Microsoft.Agents.AI.Workflows.Declarative/Interpreter/DeclarativeActionExecutor.cs
@@ -105,7 +105,7 @@ public override async ValueTask HandleAsync(ActionExecutorResult message, IWorkf
         catch (Exception exception)
         {
             Debug.WriteLine($"ERROR [{this.Id}] {exception.GetType().Name}\n{exception.Message}");
-            throw new DeclarativeActionException($"Unhandled workflow failure - #{this.Id} ({this.Model.GetType().Name})", exception);
+            throw new DeclarativeActionException($"Unhandled workflow failure - #{this.Id} ({this.Model.GetType().Name}): {exception.Message}", exception);
         }
         finally
         {
diff --git a/dotnet/tests/Microsoft.Agents.AI.Workflows.Declarative.UnitTests/DeclarativeValidationTests.cs b/dotnet/tests/Microsoft.Agents.AI.Workflows.Declarative.UnitTests/DeclarativeValidationTests.cs
new file mode 100644
index 00000000000..345f9a9b82a
--- /dev/null
+++ b/dotnet/tests/Microsoft.Agents.AI.Workflows.Declarative.UnitTests/DeclarativeValidationTests.cs
@@ -0,0 +1,119 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+using System.IO;
+using System.Linq;
+using System.Threading;
+using System.Threading.Tasks;
+using Microsoft.Extensions.AI;
+using Moq;
+
+namespace Microsoft.Agents.AI.Workflows.Declarative.UnitTests;
+
+public sealed class DeclarativeValidationTests
+{
+    [Theory]
+    [InlineData("SendActivity", "activity", "")]
+    [InlineData("Question", "prompt", "property: Local.answer\n      entity: StringPrebuiltEntity")]
+    public void Build_UnknownMessagePropertyReportsAction(string kind, string property, string extra)
+    {
+        // Arrange
+        using var reader = new StringReader($"""
+            kind: Workflow
+            trigger:
+              kind: OnConversationStart
+              id: start
+              actions:
+                - kind: {kind}
+                  id: invalid_message
+                  {property}:
+                    kind: Message
+                    test: [Hello]
+                  {extra}
+            """);
+        var options = new DeclarativeWorkflowOptions(Mock.Of<ResponseAgentProvider>());
+
+        // Act
+        var exception = Assert.Throws<DeclarativeModelException>(() => DeclarativeWorkflowBuilder.Build<string>(reader, options));
+
+        // Assert
+        Assert.Contains("test", exception.Message);
+        Assert.Contains("invalid_message", exception.Message);
+        Assert.Contains(kind, exception.Message);
+    }
+
+    [Fact]
+    public void Build_PreservesSupportedTemplatePropertiesAndActionExtensions()
+    {
+        // Arrange
+        using var reader = new StringReader("""
+            kind: Workflow
+            trigger:
+              kind: OnConversationStart
+              id: start
+              actions:
+                - kind: Question
+                  id: question
+                  autoSend: false
+                  property: Local.answer
+                  entity: StringPrebuiltEntity
+                  prompt:
+                    kind: Message
+                    text:
+                      - Hello
+                    summary: Summary
+            """);
+
+        // Act
+        var workflow = DeclarativeWorkflowBuilder.Build<string>(reader, new(Mock.Of<ResponseAgentProvider>()));
+
+        // Assert
+        Assert.NotNull(workflow);
+    }
+
+    [Theory]
+    [InlineData("Text", false)]
+    [InlineData("Test", true)]
+    public async Task Run_TemplateExpressionReportsUnderlyingErrorAsync(string member, bool fails)
+    {
+        // Arrange
+        using var reader = new StringReader($$"""
+            kind: Workflow
+            trigger:
+              kind: OnConversationStart
+              id: start
+              actions:
+                - kind: SetVariable
+                  id: set
+                  variable: Topic.ToolResponse
+                  value: =System.LastMessage
+                - kind: SendActivity
+                  id: output
+                  activity: "{Local.ToolResponse.{{member}}}"
+            """);
+        var provider = new Mock<ResponseAgentProvider>();
+        provider.Setup(p => p.CreateConversationAsync(It.IsAny<CancellationToken>())).ReturnsAsync("local");
+        provider.Setup(p => p.CreateMessageAsync(It.IsAny<string>(), It.IsAny<ChatMessage>(), It.IsAny<CancellationToken>()))
+            .ReturnsAsync((string _, ChatMessage message, CancellationToken _) => message);
+        var workflow = DeclarativeWorkflowBuilder.Build<string>(reader, new(provider.Object));
+
+        // Act
+        await using var run = await InProcessExecution.RunStreamingAsync(workflow, "Hello");
+        var events = await run.WatchStreamAsync().ToArrayAsync();
+
+        // Assert
+        if (fails)
+        {
+            var failure = Assert.Single(events.OfType<ExecutorFailedEvent>());
+            var exception = Assert.IsType<DeclarativeActionException>(failure.Data);
+            Assert.Contains("output", exception.Message);
+            Assert.Contains("SendActivity", exception.Message);
+            Assert.Contains("'Test'", exception.Message);
+            Assert.NotNull(exception.InnerException);
+        }
+        else
+        {
+            Assert.Empty(events.OfType<ExecutorFailedEvent>());
+            Assert.Equal("Hello", Assert.Single(events.OfType<AgentResponseEvent>()).Response.Text);
+        }
+    }
+}