diff --git a/dotnet/src/Microsoft.Agents.AI/ChatClient/ChatClientAgent.cs b/dotnet/src/Microsoft.Agents.AI/ChatClient/ChatClientAgent.cs
index 1133e10a8a..ff6d27aa7c 100644
--- a/dotnet/src/Microsoft.Agents.AI/ChatClient/ChatClientAgent.cs
+++ b/dotnet/src/Microsoft.Agents.AI/ChatClient/ChatClientAgent.cs
@@ -38,6 +38,8 @@ namespace Microsoft.Agents.AI;
 /// </remarks>
 public sealed partial class ChatClientAgent : AIAgent
 {
+    private const string AGUIProviderName = "ag-ui";
+
     private readonly ChatClientAgentOptions? _agentOptions;
     private readonly HashSet<string> _aiContextProviderStateKeys;
     private readonly AIAgentMetadata _agentMetadata;
@@ -815,7 +817,7 @@ internal void UpdateSessionConversationId(ChatClientAgentSession session, string
 
         if (!string.IsNullOrWhiteSpace(responseConversationId))
         {
-            if (this._agentOptions?.ChatHistoryProvider is not null)
+            if (!IsAGUIProviderName(this._agentMetadata.ProviderName) && this._agentOptions?.ChatHistoryProvider is not null)
             {
                 // The agent has a ChatHistoryProvider configured, but the service returned a conversation id,
                 // meaning the service manages chat history server-side. Both cannot be used simultaneously.
@@ -929,6 +931,9 @@ private bool RequiresPerServiceCallChatHistoryPersistence
         }
     }
 
+    private static bool IsAGUIProviderName(string? providerName) =>
+        string.Equals(providerName, AGUIProviderName, StringComparison.Ordinal);
+
     /// <summary>
     /// Ensures that <see cref="AIAgent.CurrentRunContext"/> contains the resolved session.
     /// </summary>
@@ -976,12 +981,17 @@ private void WarnOnMissingPerServiceCallChatHistoryPersistingChatClient()
 
     private ChatHistoryProvider? ResolveChatHistoryProvider(ChatOptions? chatOptions)
     {
-        ChatHistoryProvider? provider = chatOptions?.ConversationId is null ? this.ChatHistoryProvider : null;
+        ChatHistoryProvider? provider =
+            chatOptions?.ConversationId is null || IsAGUIProviderName(this._agentMetadata.ProviderName)
+            ? this.ChatHistoryProvider
+            : null;
 
         // If someone provided an override ChatHistoryProvider via AdditionalProperties, we should use that instead.
         if (chatOptions?.AdditionalProperties?.TryGetValue(out ChatHistoryProvider? overrideProvider) is true)
         {
-            if (this._agentOptions?.ThrowOnChatHistoryProviderConflict is true && string.IsNullOrWhiteSpace(chatOptions?.ConversationId) is false)
+            if (!IsAGUIProviderName(this._agentMetadata.ProviderName) &&
+                this._agentOptions?.ThrowOnChatHistoryProviderConflict is true &&
+                string.IsNullOrWhiteSpace(chatOptions?.ConversationId) is false)
             {
                 throw new InvalidOperationException(
                     $"Only {nameof(ChatClientAgentSession.ConversationId)} or {nameof(this.ChatHistoryProvider)} may be used, but not both. The current {nameof(ChatClientAgentSession)} has a {nameof(ChatClientAgentSession.ConversationId)} indicating server-side chat history management, but an override {nameof(this.ChatHistoryProvider)} was provided via {nameof(AgentRunOptions.AdditionalProperties)}.");
diff --git a/dotnet/tests/Microsoft.Agents.AI.AGUI.UnitTests/AGUIChatClientTests.cs b/dotnet/tests/Microsoft.Agents.AI.AGUI.UnitTests/AGUIChatClientTests.cs
index ede2c07d37..d5890bb5f2 100644
--- a/dotnet/tests/Microsoft.Agents.AI.AGUI.UnitTests/AGUIChatClientTests.cs
+++ b/dotnet/tests/Microsoft.Agents.AI.AGUI.UnitTests/AGUIChatClientTests.cs
@@ -243,6 +243,46 @@ public async Task RunStreamingAsync_ReturnsStreamingUpdates_AfterCompletionAsync
         Assert.Contains(updates, u => u.Text == "Hello");
     }
 
+    [Fact]
+    public async Task RunStreamingAsync_WithSession_SendsFullHistoryAfterThreadIdIsSetAsync()
+    {
+        // Arrange
+        var captureHandler = new StateCapturingTestDelegatingHandler();
+        captureHandler.AddResponse(
+        [
+            new RunStartedEvent { ThreadId = "thread1", RunId = "run1" },
+            new TextMessageStartEvent { MessageId = "msg1", Role = AGUIRoles.Assistant },
+            new TextMessageContentEvent { MessageId = "msg1", Delta = "First response" },
+            new TextMessageEndEvent { MessageId = "msg1" },
+            new RunFinishedEvent { ThreadId = "thread1", RunId = "run1" }
+        ]);
+        captureHandler.AddResponse(
+        [
+            new RunStartedEvent { ThreadId = "thread1", RunId = "run2" },
+            new TextMessageStartEvent { MessageId = "msg2", Role = AGUIRoles.Assistant },
+            new TextMessageContentEvent { MessageId = "msg2", Delta = "Second response" },
+            new TextMessageEndEvent { MessageId = "msg2" },
+            new RunFinishedEvent { ThreadId = "thread1", RunId = "run2" }
+        ]);
+        using HttpClient httpClient = new(captureHandler);
+
+        var chatClient = new AGUIChatClient(httpClient, "http://localhost/agent", null, AGUIJsonSerializerContext.Default.Options);
+        AIAgent agent = chatClient.AsAIAgent(instructions: null, name: "agent1", description: "Test agent", tools: []);
+        AgentSession session = await agent.CreateSessionAsync();
+
+        // Act
+        await foreach (var _ in agent.RunStreamingAsync([new ChatMessage(ChatRole.User, "First")], session))
+        {
+        }
+
+        await foreach (var _ in agent.RunStreamingAsync([new ChatMessage(ChatRole.User, "Second")], session))
+        {
+        }
+
+        // Assert
+        Assert.Equal([1, 3], captureHandler.CapturedMessageCounts);
+    }
+
     [Fact]
     public async Task DeserializeSession_WithValidState_ReturnsChatClientAgentSessionAsync()
     {
@@ -1686,10 +1726,12 @@ private static HttpResponseMessage CreateResponse(BaseEvent[] events)
 internal sealed class StateCapturingTestDelegatingHandler : DelegatingHandler
 {
     private readonly Queue<Func<HttpRequestMessage, Task<HttpResponseMessage>>> _responseFactories = new();
+    private readonly List<int> _capturedMessageCounts = [];
 
     public bool RequestWasMade { get; private set; }
     public JsonElement? CapturedState { get; private set; }
     public int CapturedMessageCount { get; private set; }
+    public IReadOnlyList<int> CapturedMessageCounts => this._capturedMessageCounts;
 
     public void AddResponse(BaseEvent[] events)
     {
@@ -1714,6 +1756,7 @@ protected override async Task<HttpResponseMessage> SendAsync(HttpRequestMessage
                 this.CapturedState = input.State;
             }
             this.CapturedMessageCount = input.Messages.Count();
+            this._capturedMessageCounts.Add(this.CapturedMessageCount);
         }
 
         if (this._responseFactories.Count == 0)