diff --git a/dotnet/src/Microsoft.Agents.AI.Workflows.Declarative.Foundry/AzureAgentProvider.cs b/dotnet/src/Microsoft.Agents.AI.Workflows.Declarative.Foundry/AzureAgentProvider.cs
index 98e8b7f53f..0a673e2aa5 100644
--- a/dotnet/src/Microsoft.Agents.AI.Workflows.Declarative.Foundry/AzureAgentProvider.cs
+++ b/dotnet/src/Microsoft.Agents.AI.Workflows.Declarative.Foundry/AzureAgentProvider.cs
@@ -4,7 +4,6 @@ using System;
 using System.ClientModel.Primitives;
 using System.Collections.Generic;
 using System.Collections.ObjectModel;
-using System.Linq;
 using System.Net.Http;
 using System.Runtime.CompilerServices;
 using System.Text.Json.Nodes;
@@ -70,7 +69,14 @@ public sealed class AzureAgentProvider(Uri projectEndpoint, TokenCredential proj
                 include: null,
                 cancellationToken).ConfigureAwait(false);
 
-        return newItems.AsChatMessages().Single();
+        ChatMessage[] createdMessages = [.. newItems.AsChatMessages()];
+        if (createdMessages.Length != 1)
+        {
+            throw new InvalidOperationException(
+                $"Expected exactly one chat message from created conversation item in conversation '{conversationId}', but got {createdMessages.Length}.");
+        }
+
+        return createdMessages[0];
 
         IEnumerable<ResponseItem> GetResponseItems()
         {
@@ -208,7 +214,14 @@ public sealed class AzureAgentProvider(Uri projectEndpoint, TokenCredential proj
     {
         AgentResponseItem responseItem = await this.GetConversationClient().GetProjectConversationItemAsync(conversationId, messageId, include: null, cancellationToken).ConfigureAwait(false);
         ResponseItem[] items = [responseItem.AsResponseResultItem()];
-        return items.AsChatMessages().Single();
+        ChatMessage[] messages = [.. items.AsChatMessages()];
+        if (messages.Length != 1)
+        {
+            throw new InvalidOperationException(
+                $"Expected exactly one chat message for message '{messageId}' in conversation '{conversationId}', but got {messages.Length}.");
+        }
+
+        return messages[0];
     }
 
     /// <inheritdoc/>
diff --git a/dotnet/src/Microsoft.Agents.AI.Workflows.Declarative/ObjectModel/InvokeAzureAgentExecutor.cs b/dotnet/src/Microsoft.Agents.AI.Workflows.Declarative/ObjectModel/InvokeAzureAgentExecutor.cs
index 86efa6ec43..322c460ee3 100644
--- a/dotnet/src/Microsoft.Agents.AI.Workflows.Declarative/ObjectModel/InvokeAzureAgentExecutor.cs
+++ b/dotnet/src/Microsoft.Agents.AI.Workflows.Declarative/ObjectModel/InvokeAzureAgentExecutor.cs
@@ -49,7 +49,11 @@ internal sealed class InvokeAzureAgentExecutor(InvokeAzureAgent model, ResponseA
 
     public async ValueTask ResumeAsync(IWorkflowContext context, ExternalInputResponse response, CancellationToken cancellationToken)
     {
-        await context.SetLastMessageAsync(response.Messages.Last()).ConfigureAwait(false);
+        ChatMessage? lastMessage = response.Messages.LastOrDefault();
+        if (lastMessage is not null)
+        {
+            await context.SetLastMessageAsync(lastMessage).ConfigureAwait(false);
+        }
         await this.InvokeAgentAsync(context, response.Messages, cancellationToken).ConfigureAwait(false);
     }
 
@@ -85,15 +89,19 @@ internal sealed class InvokeAzureAgentExecutor(InvokeAzureAgent model, ResponseA
         await this.AssignAsync(this.AgentOutput?.Messages?.Path, agentResponse.Messages.ToTable(), context).ConfigureAwait(false);
 
         // Attempt to parse the last message as JSON and assign to the response object variable.
-        try
+        string? lastMessageText = agentResponse.Messages.LastOrDefault()?.Text;
+        if (!string.IsNullOrEmpty(lastMessageText))
         {
-            JsonDocument jsonDocument = JsonDocument.Parse(agentResponse.Messages.Last().Text);
-            Dictionary<string, object?> objectProperties = jsonDocument.ParseRecord(VariableType.RecordType);
-            await this.AssignAsync(this.AgentOutput?.ResponseObject?.Path, objectProperties.ToFormula(), context).ConfigureAwait(false);
-        }
-        catch
-        {
-            // Not valid json, skip assignment.
+            try
+            {
+                using JsonDocument jsonDocument = JsonDocument.Parse(lastMessageText);
+                Dictionary<string, object?> objectProperties = jsonDocument.ParseRecord(VariableType.RecordType);
+                await this.AssignAsync(this.AgentOutput?.ResponseObject?.Path, objectProperties.ToFormula(), context).ConfigureAwait(false);
+            }
+            catch (JsonException)
+            {
+                // Not valid json, skip assignment.
+            }
         }
 
         if (this.Model.Input?.ExternalLoop?.When is not null)
diff --git a/dotnet/src/Microsoft.Agents.AI.Workflows.Declarative/ObjectModel/QuestionExecutor.cs b/dotnet/src/Microsoft.Agents.AI.Workflows.Declarative/ObjectModel/QuestionExecutor.cs
index 31cb82353e..32ce93c2af 100644
--- a/dotnet/src/Microsoft.Agents.AI.Workflows.Declarative/ObjectModel/QuestionExecutor.cs
+++ b/dotnet/src/Microsoft.Agents.AI.Workflows.Declarative/ObjectModel/QuestionExecutor.cs
@@ -122,10 +122,13 @@ internal sealed class QuestionExecutor(Question model, ResponseAgentProvider age
                 string? workflowConversationId = context.GetWorkflowConversation();
                 if (workflowConversationId is not null)
                 {
-                    // Input message always defined if values has been extracted.
-                    ChatMessage input = response.Messages.Last();
-                    await agentProvider.CreateMessageAsync(workflowConversationId, input, cancellationToken).ConfigureAwait(false);
-                    await context.SetLastMessageAsync(input).ConfigureAwait(false);
+                    // Input message expected to be defined when values have been extracted, but guard defensively.
+                    ChatMessage? input = response.Messages.LastOrDefault();
+                    if (input is not null)
+                    {
+                        await agentProvider.CreateMessageAsync(workflowConversationId, input, cancellationToken).ConfigureAwait(false);
+                        await context.SetLastMessageAsync(input).ConfigureAwait(false);
+                    }
                 }
             }
 
diff --git a/dotnet/src/Microsoft.Agents.AI.Workflows.Declarative/ObjectModel/RequestExternalInputExecutor.cs b/dotnet/src/Microsoft.Agents.AI.Workflows.Declarative/ObjectModel/RequestExternalInputExecutor.cs
index 239b178415..172348b37e 100644
--- a/dotnet/src/Microsoft.Agents.AI.Workflows.Declarative/ObjectModel/RequestExternalInputExecutor.cs
+++ b/dotnet/src/Microsoft.Agents.AI.Workflows.Declarative/ObjectModel/RequestExternalInputExecutor.cs
@@ -45,7 +45,11 @@ internal sealed class RequestExternalInputExecutor(RequestExternalInput model, R
                 await agentProvider.CreateMessageAsync(workflowConversationId, inputMessage, cancellationToken).ConfigureAwait(false);
             }
         }
-        await context.SetLastMessageAsync(response.Messages.Last()).ConfigureAwait(false);
+        ChatMessage? lastMessage = response.Messages.LastOrDefault();
+        if (lastMessage is not null)
+        {
+            await context.SetLastMessageAsync(lastMessage).ConfigureAwait(false);
+        }
         await this.AssignAsync(this.Model.Variable?.Path, response.Messages.ToFormula(), context).ConfigureAwait(false);
 
         await context.RaiseCompletionEventAsync(this.Model, cancellationToken).ConfigureAwait(false);
diff --git a/dotnet/tests/Microsoft.Agents.AI.Workflows.Declarative.UnitTests/ObjectModel/RequestExternalInputExecutorTest.cs b/dotnet/tests/Microsoft.Agents.AI.Workflows.Declarative.UnitTests/ObjectModel/RequestExternalInputExecutorTest.cs
index 8eda895b15..bc1bc10284 100644
--- a/dotnet/tests/Microsoft.Agents.AI.Workflows.Declarative.UnitTests/ObjectModel/RequestExternalInputExecutorTest.cs
+++ b/dotnet/tests/Microsoft.Agents.AI.Workflows.Declarative.UnitTests/ObjectModel/RequestExternalInputExecutorTest.cs
@@ -85,6 +85,29 @@ public sealed class RequestExternalInputExecutorTest(ITestOutputHelper output) :
             expectMessagesCreated: true);
     }
 
+    [Fact]
+    public async Task CaptureResponseWithEmptyMessagesAsync()
+    {
+        await this.CaptureResponseTestAsync(
+            displayName: nameof(CaptureResponseWithEmptyMessagesAsync),
+            variableName: "TestVariable",
+            messageCount: 0);
+    }
+
+    [Fact]
+    public async Task CaptureResponseWithEmptyMessagesAndWorkflowConversationAsync()
+    {
+        // Arrange
+        this.State.Set(SystemScope.Names.ConversationId, FormulaValue.New("WorkflowConversationId"), VariableScopeNames.System);
+
+        // Act & Assert
+        await this.CaptureResponseTestAsync(
+            displayName: nameof(CaptureResponseWithEmptyMessagesAndWorkflowConversationAsync),
+            variableName: "TestVariable",
+            messageCount: 0,
+            expectMessagesCreated: false);
+    }
+
     private async Task ExecuteTestAsync(
         string displayName,
         string variableName)