.NET: Prevent streamed updates loss when resuming streaming

dotnet/src/Microsoft.Agents.AI/ChatClient/ChatClientAgent.cs

@@ -204,6 +204,8 @@ public override async IAsyncEnumerable<AgentRunResponseUpdate> RunStreamingAsync
         (ChatClientAgentThread safeThread, ChatOptions? chatOptions, List<ChatMessage> inputMessagesForChatClient, IList<ChatMessage>? aiContextProviderMessages) =
             await this.PrepareThreadAndMessagesAsync(thread, inputMessages, options, cancellationToken).ConfigureAwait(false);
 
+        ValidateStreamResumptionAllowed(chatOptions?.ContinuationToken, safeThread);
+
         var chatClient = this.ChatClient;
 
         chatClient = ApplyRunOptionsTransformations(options, chatClient);
@@ -621,6 +623,12 @@ await thread.AIContextProvider.InvokedAsync(new(inputMessages, aiContextProvider
         {
             throw new InvalidOperationException("Input messages are not allowed when continuing a background response using a continuation token.");
         }
+
+        if (chatOptions?.ContinuationToken is not null && typedThread.ConversationId is null && typedThread.MessageStore is null)
+        {
+            throw new InvalidOperationException("Continuation tokens are not allowed to be used for initial runs.");
+        }
+
         List<ChatMessage> inputMessagesForChatClient = [];
         IList<ChatMessage>? aiContextProviderMessages = null;
 
@@ -731,6 +739,28 @@ private static Task NotifyMessageStoreOfNewMessagesAsync(ChatClientAgentThread t
         return Task.CompletedTask;
     }
 
+    private static void ValidateStreamResumptionAllowed(ResponseContinuationToken? continuationToken, ChatClientAgentThread safeThread)
+    {
+        if (continuationToken is null)
+        {
+            return;
+        }
+
+        // Streaming resumption is only supported with chat history managed by the agent service because, currently, there's no good solution
+        // to collect updates received in failed runs and pass them to the last successful run so it can store them to the message store.
+        if (safeThread.ConversationId is null)
+        {
+            throw new NotSupportedException("Streaming resumption is only supported when chat history is stored and managed by the underlying AI service.");
+        }
+
+        // Similarly, streaming resumption is not supported when a context provider is used because, currently, there's no good solution
+        // to collect updates received in failed runs and pass them to the last successful run so it can notify the context provider of the updates.
+        if (safeThread.AIContextProvider is not null)
+        {
+            throw new NotSupportedException("Using context provider with streaming resumption is not supported.");
+        }
+    }
+
     private string GetLoggingAgentName() => this.Name ?? "UnnamedAgent";
     #endregion
 }