diff --git a/dotnet/src/Microsoft.Agents.AI.AGUI/Shared/ChatResponseUpdateAGUIExtensions.cs b/dotnet/src/Microsoft.Agents.AI.AGUI/Shared/ChatResponseUpdateAGUIExtensions.cs
index f5fb103bd4..b11475ddfc 100644
--- a/dotnet/src/Microsoft.Agents.AI.AGUI/Shared/ChatResponseUpdateAGUIExtensions.cs
+++ b/dotnet/src/Microsoft.Agents.AI.AGUI/Shared/ChatResponseUpdateAGUIExtensions.cs
@@ -341,8 +341,17 @@ public static async IAsyncEnumerable<BaseEvent> AsAGUIEventStreamAsync(
         };
 
         string? currentMessageId = null;
+        string? streamingMessageId = null;
         await foreach (var chatResponse in updates.WithCancellation(cancellationToken).ConfigureAwait(false))
         {
+            // Generate a fallback MessageId when the provider doesn't supply one.
+            // This ensures all AGUI events have a valid messageId regardless of agent type.
+            if (string.IsNullOrWhiteSpace(chatResponse.MessageId))
+            {
+                streamingMessageId ??= Guid.NewGuid().ToString("N");
+                chatResponse.MessageId = streamingMessageId;
+            }
+
             if (chatResponse is { Contents.Count: > 0 } &&
                 chatResponse.Contents[0] is TextContent &&
                 !string.Equals(currentMessageId, chatResponse.MessageId, StringComparison.Ordinal))
diff --git a/dotnet/tests/Microsoft.Agents.AI.AGUI.UnitTests/AGUIStreamingMessageIdTests.cs b/dotnet/tests/Microsoft.Agents.AI.AGUI.UnitTests/AGUIStreamingMessageIdTests.cs
new file mode 100644
index 0000000000..5c55408ff8
--- /dev/null
+++ b/dotnet/tests/Microsoft.Agents.AI.AGUI.UnitTests/AGUIStreamingMessageIdTests.cs
@@ -0,0 +1,228 @@
+// Copyright (c) Microsoft. All rights reserved.
+
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Runtime.CompilerServices;
+using System.Threading;
+using System.Threading.Tasks;
+using Microsoft.Agents.AI.AGUI.Shared;
+using Microsoft.Extensions.AI;
+
+namespace Microsoft.Agents.AI.AGUI.UnitTests;
+
+/// <summary>
+/// Tests for AGUI streaming behavior when MessageId is null or missing from
+/// ChatResponseUpdate objects (e.g., providers like Google GenAI/Vertex AI
+/// that don't supply MessageId on streaming chunks).
+/// </summary>
+public sealed class AGUIStreamingMessageIdTests
+{
+    /// <summary>
+    /// When ChatResponseUpdate objects with null MessageId are fed directly to
+    /// AsAGUIEventStreamAsync, the AGUI layer generates a fallback MessageId so
+    /// that events are valid regardless of agent type or provider.
+    /// </summary>
+    [Fact]
+    public async Task TextStreaming_NullMessageId_GeneratesFallbackInAGUILayerAsync()
+    {
+        // Arrange - Simulate a provider that does NOT set MessageId
+        List<ChatResponseUpdate> providerUpdates =
+        [
+            new ChatResponseUpdate(ChatRole.Assistant, "Hello"),
+            new ChatResponseUpdate(ChatRole.Assistant, " world"),
+            new ChatResponseUpdate(ChatRole.Assistant, "!")
+        ];
+
+        // Act
+        List<BaseEvent> aguiEvents = [];
+        await foreach (BaseEvent evt in providerUpdates.ToAsyncEnumerableAsync()
+            .AsAGUIEventStreamAsync("thread-1", "run-1", AGUIJsonSerializerContext.Default.Options))
+        {
+            aguiEvents.Add(evt);
+        }
+
+        // Assert - AGUI layer should generate a fallback MessageId
+        List<TextMessageStartEvent> startEvents = aguiEvents.OfType<TextMessageStartEvent>().ToList();
+        List<TextMessageContentEvent> contentEvents = aguiEvents.OfType<TextMessageContentEvent>().ToList();
+
+        Assert.Single(startEvents);
+        Assert.False(string.IsNullOrEmpty(startEvents[0].MessageId));
+
+        Assert.Equal(3, contentEvents.Count);
+        Assert.All(contentEvents, e => Assert.False(string.IsNullOrEmpty(e.MessageId)));
+
+        // All events should share the same generated MessageId
+        string?[] distinctIds = contentEvents.Select(e => e.MessageId).Distinct().ToArray();
+        Assert.Single(distinctIds);
+        Assert.Equal(startEvents[0].MessageId, distinctIds[0]);
+    }
+
+    /// <summary>
+    /// Full pipeline: ChatClientAgent → AsChatResponseUpdatesAsync → AsAGUIEventStreamAsync
+    /// with a provider that returns null MessageId. Verifies that fallback MessageId
+    /// generation ensures valid AGUI events.
+    /// </summary>
+    [Fact]
+    public async Task FullPipeline_NullProviderMessageId_ProducesValidAGUIEventsAsync()
+    {
+        // Arrange - ChatClientAgent with a mock client that omits MessageId
+        IChatClient mockChatClient = new NullMessageIdChatClient();
+        ChatClientAgent agent = new(mockChatClient, name: "test-agent");
+
+        ChatMessage userMessage = new(ChatRole.User, "tell me about agents");
+
+        // Act - Run the full pipeline exactly as MapAGUI does
+        List<BaseEvent> aguiEvents = [];
+        await foreach (BaseEvent evt in agent
+            .RunStreamingAsync([userMessage])
+            .AsChatResponseUpdatesAsync()
+            .AsAGUIEventStreamAsync("thread-1", "run-1", AGUIJsonSerializerContext.Default.Options))
+        {
+            aguiEvents.Add(evt);
+        }
+
+        // Assert — The pipeline should produce AGUI events with valid messageId
+        List<TextMessageStartEvent> startEvents = aguiEvents.OfType<TextMessageStartEvent>().ToList();
+        List<TextMessageContentEvent> contentEvents = aguiEvents.OfType<TextMessageContentEvent>().ToList();
+
+        Assert.NotEmpty(startEvents);
+        Assert.NotEmpty(contentEvents);
+
+        foreach (TextMessageStartEvent startEvent in startEvents)
+        {
+            Assert.False(
+                string.IsNullOrEmpty(startEvent.MessageId),
+                "TextMessageStartEvent.MessageId should not be null/empty when provider omits it");
+        }
+
+        foreach (TextMessageContentEvent contentEvent in contentEvents)
+        {
+            Assert.False(
+                string.IsNullOrEmpty(contentEvent.MessageId),
+                "TextMessageContentEvent.MessageId should not be null/empty when provider omits it");
+        }
+
+        // All content events should share the same messageId
+        string?[] distinctMessageIds = contentEvents.Select(e => e.MessageId).Distinct().ToArray();
+        Assert.Single(distinctMessageIds);
+    }
+
+    /// <summary>
+    /// When ChatResponseUpdate has empty string MessageId, the AGUI layer generates
+    /// a fallback so ToolCallStartEvent.ParentMessageId is valid.
+    /// </summary>
+    [Fact]
+    public async Task ToolCalls_EmptyMessageId_GeneratesFallbackParentMessageIdAsync()
+    {
+        // Arrange - ChatResponseUpdate with a tool call but empty MessageId
+        FunctionCallContent functionCall = new("call_abc123", "GetWeather")
+        {
+            Arguments = new Dictionary<string, object?> { ["location"] = "San Francisco" }
+        };
+
+        List<ChatResponseUpdate> providerUpdates =
+        [
+            new ChatResponseUpdate
+            {
+                Role = ChatRole.Assistant,
+                MessageId = "",
+                Contents = [functionCall]
+            }
+        ];
+
+        // Act
+        List<BaseEvent> aguiEvents = [];
+        await foreach (BaseEvent evt in providerUpdates.ToAsyncEnumerableAsync()
+            .AsAGUIEventStreamAsync("thread-1", "run-1", AGUIJsonSerializerContext.Default.Options))
+        {
+            aguiEvents.Add(evt);
+        }
+
+        // Assert — ParentMessageId should have a generated fallback
+        ToolCallStartEvent? toolCallStart = aguiEvents.OfType<ToolCallStartEvent>().FirstOrDefault();
+        Assert.NotNull(toolCallStart);
+        Assert.Equal("call_abc123", toolCallStart.ToolCallId);
+        Assert.Equal("GetWeather", toolCallStart.ToolCallName);
+        Assert.False(
+            string.IsNullOrEmpty(toolCallStart.ParentMessageId),
+            "ParentMessageId should have a generated fallback for empty provider MessageId");
+    }
+
+    /// <summary>
+    /// When a provider properly sets MessageId (e.g., OpenAI), the AGUI pipeline
+    /// produces valid events with correct messageId values.
+    /// </summary>
+    [Fact]
+    public async Task TextStreaming_WithProviderMessageId_ProducesValidAGUIEventsAsync()
+    {
+        // Arrange — Provider that properly sets MessageId
+        List<ChatResponseUpdate> providerUpdates =
+        [
+            new ChatResponseUpdate(ChatRole.Assistant, "Hello")
+            {
+                MessageId = "chatcmpl-abc123"
+            },
+            new ChatResponseUpdate(ChatRole.Assistant, " world")
+            {
+                MessageId = "chatcmpl-abc123"
+            }
+        ];
+
+        // Act
+        List<BaseEvent> aguiEvents = [];
+        await foreach (BaseEvent evt in providerUpdates.ToAsyncEnumerableAsync()
+            .AsAGUIEventStreamAsync("thread-1", "run-1", AGUIJsonSerializerContext.Default.Options))
+        {
+            aguiEvents.Add(evt);
+        }
+
+        // Assert
+        List<TextMessageStartEvent> startEvents = aguiEvents.OfType<TextMessageStartEvent>().ToList();
+        List<TextMessageContentEvent> contentEvents = aguiEvents.OfType<TextMessageContentEvent>().ToList();
+
+        Assert.Single(startEvents);
+        Assert.Equal("chatcmpl-abc123", startEvents[0].MessageId);
+
+        Assert.Equal(2, contentEvents.Count);
+        Assert.All(contentEvents, e => Assert.Equal("chatcmpl-abc123", e.MessageId));
+    }
+}
+
+/// <summary>
+/// Mock IChatClient that simulates a provider not setting MessageId on streaming chunks
+/// (e.g., Google GenAI / Vertex AI).
+/// </summary>
+internal sealed class NullMessageIdChatClient : IChatClient
+{
+    public void Dispose()
+    {
+    }
+
+    public object? GetService(Type serviceType, object? serviceKey = null) => null;
+
+    public Task<ChatResponse> GetResponseAsync(
+        IEnumerable<ChatMessage> messages,
+        ChatOptions? options = null,
+        CancellationToken cancellationToken = default)
+    {
+        return Task.FromResult(new ChatResponse([new(ChatRole.Assistant, "response")]));
+    }
+
+    public async IAsyncEnumerable<ChatResponseUpdate> GetStreamingResponseAsync(
+        IEnumerable<ChatMessage> messages,
+        ChatOptions? options = null,
+        [EnumeratorCancellation] CancellationToken cancellationToken = default)
+    {
+        foreach (string chunk in (string[])["Agents", " are", " autonomous", " programs."])
+        {
+            yield return new ChatResponseUpdate
+            {
+                Role = ChatRole.Assistant,
+                Contents = [new TextContent(chunk)]
+            };
+
+            await Task.Yield();
+        }
+    }
+}
diff --git a/dotnet/tests/Microsoft.Agents.AI.Abstractions.UnitTests/AgentResponseUpdateExtensionsTests.cs b/dotnet/tests/Microsoft.Agents.AI.Abstractions.UnitTests/AgentResponseUpdateExtensionsTests.cs
index 89cff04de8..5cf0b75eef 100644
--- a/dotnet/tests/Microsoft.Agents.AI.Abstractions.UnitTests/AgentResponseUpdateExtensionsTests.cs
+++ b/dotnet/tests/Microsoft.Agents.AI.Abstractions.UnitTests/AgentResponseUpdateExtensionsTests.cs
@@ -384,6 +384,45 @@ public void AsChatResponseUpdate_WithRawRepresentationAsChatResponseUpdate_Retur
         Assert.Same(originalChatResponseUpdate, result);
     }
 
+    [Fact]
+    public void AsChatResponseUpdate_WithRawRepresentationNullMessageId_ReturnsRawDirectly()
+    {
+        // Arrange - RawRepresentation has null MessageId
+        ChatResponseUpdate originalChatResponseUpdate = new()
+        {
+            ResponseId = "original-update",
+            Contents = [new TextContent("Hello")]
+        };
+        AgentResponseUpdate agentResponseUpdate = new(originalChatResponseUpdate);
+
+        // Act
+        ChatResponseUpdate result = agentResponseUpdate.AsChatResponseUpdate();
+
+        // Assert - Returns the raw representation directly without mutation
+        Assert.Same(originalChatResponseUpdate, result);
+        Assert.Null(result.MessageId);
+    }
+
+    [Fact]
+    public void AsChatResponseUpdate_WithRawRepresentationExistingMessageId_PreservesOriginal()
+    {
+        // Arrange - RawRepresentation already has MessageId set by provider
+        ChatResponseUpdate originalChatResponseUpdate = new()
+        {
+            ResponseId = "original-update",
+            MessageId = "provider-message-id",
+            Contents = [new TextContent("Hello")]
+        };
+        AgentResponseUpdate agentResponseUpdate = new(originalChatResponseUpdate);
+
+        // Act
+        ChatResponseUpdate result = agentResponseUpdate.AsChatResponseUpdate();
+
+        // Assert - Provider's original MessageId should be preserved
+        Assert.Same(originalChatResponseUpdate, result);
+        Assert.Equal("provider-message-id", result.MessageId);
+    }
+
     [Fact]
     public void AsChatResponseUpdate_WithoutRawRepresentation_CreatesNewChatResponseUpdate()
     {
diff --git a/dotnet/tests/Microsoft.Agents.AI.UnitTests/ChatClient/ChatClientAgentTests.cs b/dotnet/tests/Microsoft.Agents.AI.UnitTests/ChatClient/ChatClientAgentTests.cs
index 7241ca763e..42b115c8bb 100644
--- a/dotnet/tests/Microsoft.Agents.AI.UnitTests/ChatClient/ChatClientAgentTests.cs
+++ b/dotnet/tests/Microsoft.Agents.AI.UnitTests/ChatClient/ChatClientAgentTests.cs
@@ -1793,6 +1793,75 @@ public async Task VerifyChatClientAgentStreamingAsync()
             Times.Once);
     }
 
+    /// <summary>
+    /// Verify that RunStreamingAsync passes through null MessageId from provider without modification.
+    /// MessageId generation is handled by downstream consumers (e.g., AGUI layer), not ChatClientAgent.
+    /// </summary>
+    [Fact]
+    public async Task RunStreamingAsync_WithNullMessageId_PassesThroughNullAsync()
+    {
+        // Arrange - Provider returns updates WITHOUT MessageId
+        ChatResponseUpdate[] returnUpdates =
+            [
+                new ChatResponseUpdate(role: ChatRole.Assistant, content: "Hello"),
+                new ChatResponseUpdate(role: ChatRole.Assistant, content: " world"),
+            ];
+
+        Mock<IChatClient> mockService = new();
+        mockService.Setup(
+            s => s.GetStreamingResponseAsync(
+                It.IsAny<IEnumerable<ChatMessage>>(),
+                It.IsAny<ChatOptions>(),
+                It.IsAny<CancellationToken>())).Returns(ToAsyncEnumerableAsync(returnUpdates));
+
+        ChatClientAgent agent = new(mockService.Object);
+
+        // Act
+        List<AgentResponseUpdate> result = [];
+        await foreach (var update in agent.RunStreamingAsync([new ChatMessage(ChatRole.User, "Hi")]))
+        {
+            result.Add(update);
+        }
+
+        // Assert - MessageId should be null (ChatClientAgent does not generate fallback IDs)
+        Assert.Equal(2, result.Count);
+        Assert.All(result, u => Assert.Null(u.MessageId));
+    }
+
+    /// <summary>
+    /// Verify that RunStreamingAsync preserves provider-supplied MessageId when present.
+    /// </summary>
+    [Fact]
+    public async Task RunStreamingAsync_WithProviderMessageId_PreservesItAsync()
+    {
+        // Arrange - Provider returns updates WITH MessageId (like OpenAI)
+        ChatResponseUpdate[] returnUpdates =
+            [
+                new ChatResponseUpdate(role: ChatRole.Assistant, content: "Hello") { MessageId = "chatcmpl-abc123" },
+                new ChatResponseUpdate(role: ChatRole.Assistant, content: " world") { MessageId = "chatcmpl-abc123" },
+            ];
+
+        Mock<IChatClient> mockService = new();
+        mockService.Setup(
+            s => s.GetStreamingResponseAsync(
+                It.IsAny<IEnumerable<ChatMessage>>(),
+                It.IsAny<ChatOptions>(),
+                It.IsAny<CancellationToken>())).Returns(ToAsyncEnumerableAsync(returnUpdates));
+
+        ChatClientAgent agent = new(mockService.Object);
+
+        // Act
+        List<AgentResponseUpdate> result = [];
+        await foreach (var update in agent.RunStreamingAsync([new ChatMessage(ChatRole.User, "Hi")]))
+        {
+            result.Add(update);
+        }
+
+        // Assert - Provider's MessageId should be preserved, not overwritten
+        Assert.Equal(2, result.Count);
+        Assert.All(result, u => Assert.Equal("chatcmpl-abc123", u.MessageId));
+    }
+
     /// <summary>
     /// Verify that RunStreamingAsync uses the ChatHistoryProvider factory when the chat client returns no conversation id.
     /// </summary>