.Net: Fix chat history reducers crashing when target count is one and a system message is present

dotnet/src/SemanticKernel.Core/AI/ChatCompletion/ChatHistoryReducerExtensions.cs

@@ -104,7 +104,8 @@ public static int LocateSafeReductionIndex(
         int offsetCount = 0,
         bool hasSystemMessage = false)
     {
-        targetCount -= hasSystemMessage ? 1 : 0;
+        // The system message counts toward the target, but at least one other message is always retained
+        targetCount = Math.Max(1, targetCount - (hasSystemMessage ? 1 : 0));
 
         // Compute the index of the truncation threshold
         int thresholdIndex = chatHistory.Count - (thresholdCount ?? 0) - targetCount;

dotnet/src/SemanticKernel.UnitTests/AI/ChatCompletion/ChatHistorySummarizationReducerTests.cs

@@ -245,6 +245,27 @@ public async Task VerifySystemMessageIsNotReducedAsync()
         Assert.Contains(messages, m => m.Role == AuthorRole.System);
     }
 
+    /// <summary>
+    /// Validate history summarized and the latest message retained when the target count is one and a system message is present.
+    /// </summary>
+    [Fact]
+    public async Task VerifySystemMessageWithSingleTargetCountAsync()
+    {
+        // Arrange
+        Mock<IChatCompletionService> mockCompletionService = this.CreateMockCompletionService();
+        IReadOnlyList<ChatMessageContent> sourceHistory = MockChatHistoryGenerator.CreateSimpleHistory(20, includeSystemMessage: true).ToArray();
+        ChatHistorySummarizationReducer reducer = new(mockCompletionService.Object, 1);
+
+        // Act
+        IEnumerable<ChatMessageContent>? reducedHistory = await reducer.ReduceAsync(sourceHistory);
+
+        // Assert
+        ChatMessageContent[] messages = VerifyReducedHistory(reducedHistory, 3);
+        Assert.Equal(AuthorRole.System, messages[0].Role);
+        VerifySummarization(messages[1]);
+        Assert.Same(sourceHistory[^1], messages[2]);
+    }
+
     private static ChatMessageContent[] VerifyReducedHistory(IEnumerable<ChatMessageContent>? reducedHistory, int expectedCount)
     {
         Assert.NotNull(reducedHistory);

dotnet/src/SemanticKernel.UnitTests/AI/ChatCompletion/ChatHistoryTruncationReducerTests.cs

@@ -155,6 +155,25 @@ public async Task VerifySystemMessageIsNotReducedAsync()
         Assert.Contains(reducedHistory!, m => m.Role == AuthorRole.System);
     }
 
+    /// <summary>
+    /// Validate history reduced to the system message and the latest message when the target count is one.
+    /// </summary>
+    [Fact]
+    public async Task VerifySystemMessageWithSingleTargetCountAsync()
+    {
+        // Arrange
+        IReadOnlyList<ChatMessageContent> sourceHistory = MockChatHistoryGenerator.CreateSimpleHistory(20, includeSystemMessage: true).ToArray();
+        ChatHistoryTruncationReducer reducer = new(1);
+
+        // Act
+        IEnumerable<ChatMessageContent>? reducedHistory = await reducer.ReduceAsync(sourceHistory);
+
+        // Assert
+        VerifyReducedHistory(reducedHistory, 2);
+        Assert.Equal(AuthorRole.System, reducedHistory!.First().Role);
+        Assert.Same(sourceHistory[^1], reducedHistory!.Last());
+    }
+
     private static void VerifyReducedHistory(IEnumerable<ChatMessageContent>? reducedHistory, int expectedCount)
     {
         Assert.NotNull(reducedHistory);