.Net: Use empty string instead of null content in assistant message i…

…n Azure/OpenAI connectors (#9150)

### Motivation and Context

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Resolves: #8072
Resolves: #8488

Azure Chat Completion with data service returns `400 (Bad Request)
Invalid chat message detected: message content must be string` response
when assistant message in chat history doesn't contain `content`
property or when this property is an array of content items. This case
is applicable for function calling, where AI model returns set of tools
to call instead of message content.

This PR contains a change to set `content` property as an empty string
only for cases when content doesn't exist (`null`).

### Description

<!-- Describe your changes, the overall approach, the underlying design.
These notes will help understanding how your code works. Thanks! -->

### Contribution Checklist

<!-- Before submitting this PR, please make sure: -->

- [x] The code builds clean without any errors or warnings
- [x] The PR follows the [SK Contribution
Guidelines](https:/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
and the [pre-submission formatting
script](https:/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md#development-scripts)
raises no violations
- [x] All unit tests pass, and I have added new tests where possible
- [x] I didn't break anyone 😄

dotnet/src/Connectors/Connectors.AzureOpenAI.UnitTests/Services/AzureOpenAIChatCompletionServiceTests.cs

@@ -1373,6 +1373,51 @@ public async Task ItDoesNotChangeDefaultsForToolsAndChoiceIfNeitherOfFunctionCal
  Assert.False(optionsJson.TryGetProperty("tool_choice", out var _));
  }
 
+ [Fact]
+ public async Task ItSendsEmptyStringWhenAssistantMessageContentIsNull()
+ {
+ // Arrange
+ var sut = new AzureOpenAIChatCompletionService("deployment", "https://endpoint", "api-key", "model-id", this._httpClient);
+
+ using var responseMessage = new HttpResponseMessage(HttpStatusCode.OK)
+ {
+ Content = new StringContent(AzureOpenAITestHelper.GetTestResponse("chat_completion_test_response.json"))
+ };
+ this._messageHandlerStub.ResponsesToReturn.Add(responseMessage);
+
+ List<ChatToolCall> assistantToolCalls = [ChatToolCall.CreateFunctionToolCall("id", "name", BinaryData.FromString("args"))];
+
+ var chatHistory = new ChatHistory()
+ {
+ new ChatMessageContent(role: AuthorRole.User, content: "User content", modelId: "any"),
+ new ChatMessageContent(role: AuthorRole.Assistant, content: null, modelId: "any", metadata: new Dictionary<string, object?>
+ {
+ ["ChatResponseMessage.FunctionToolCalls"] = assistantToolCalls
+ }),
+ new ChatMessageContent(role: AuthorRole.Tool, content: null, modelId: "any")
+ {
+ Items = [new FunctionResultContent("FunctionName", "PluginName", "CallId", "Function result")]
+ },
+ };
+
+ var executionSettings = new AzureOpenAIPromptExecutionSettings();
+
+ // Act
+ await sut.GetChatMessageContentsAsync(chatHistory, executionSettings);
+
+ // Assert
+ var actualRequestContent = Encoding.UTF8.GetString(this._messageHandlerStub.RequestContents[0]!);
+ Assert.NotNull(actualRequestContent);
+
+ var requestContent = JsonSerializer.Deserialize<JsonElement>(actualRequestContent);
+ var messages = requestContent.GetProperty("messages").EnumerateArray().ToList();
+
+ var assistantMessage = messages.First(message => message.GetProperty("role").GetString() == "assistant");
+ var assistantMessageContent = assistantMessage.GetProperty("content").GetString();
+
+ Assert.Equal(string.Empty, assistantMessageContent);
+ }
+
  public void Dispose()
  {
  this._httpClient.Dispose();

dotnet/src/Connectors/Connectors.OpenAI.UnitTests/Services/OpenAIChatCompletionServiceTests.cs

@@ -1359,6 +1359,47 @@ public async Task ItDoesNotChangeDefaultsForToolsAndChoiceIfNeitherOfFunctionCal
  Assert.False(optionsJson.TryGetProperty("tool_choice", out var _));
  }
 
+ [Fact]
+ public async Task ItSendsEmptyStringWhenAssistantMessageContentIsNull()
+ {
+ // Arrange
+ var chatCompletion = new OpenAIChatCompletionService(modelId: "any", apiKey: "NOKEY", httpClient: this._httpClient);
+ this._messageHandlerStub.ResponseToReturn = new HttpResponseMessage(System.Net.HttpStatusCode.OK)
+ {
+ Content = new StringContent(ChatCompletionResponse)
+ };
+
+ List<ChatToolCall> assistantToolCalls = [ChatToolCall.CreateFunctionToolCall("id", "name", BinaryData.FromString("args"))];
+
+ var chatHistory = new ChatHistory()
+ {
+ new ChatMessageContent(role: AuthorRole.User, content: "User content", modelId: "any"),
+ new ChatMessageContent(role: AuthorRole.Assistant, content: null, modelId: "any", metadata: new Dictionary<string, object?>
+ {
+ ["ChatResponseMessage.FunctionToolCalls"] = assistantToolCalls
+ }),
+ new ChatMessageContent(role: AuthorRole.Tool, content: null, modelId: "any")
+ {
+ Items = [new FunctionResultContent("FunctionName", "PluginName", "CallId", "Function result")]
+ },
+ };
+
+ // Act
+ await chatCompletion.GetChatMessageContentsAsync(chatHistory, this._executionSettings);
+
+ // Assert
+ var actualRequestContent = Encoding.UTF8.GetString(this._messageHandlerStub.RequestContent!);
+ Assert.NotNull(actualRequestContent);
+
+ var requestContent = JsonSerializer.Deserialize<JsonElement>(actualRequestContent);
+ var messages = requestContent.GetProperty("messages").EnumerateArray().ToList();
+
+ var assistantMessage = messages.First(message => message.GetProperty("role").GetString() == "assistant");
+ var assistantMessageContent = assistantMessage.GetProperty("content").GetString();
+
+ Assert.Equal(string.Empty, assistantMessageContent);
+ }
+
  public void Dispose()
  {
  this._httpClient.Dispose();

dotnet/src/Connectors/Connectors.OpenAI/Core/ClientCore.ChatCompletion.cs

@@ -758,10 +758,10 @@ private static List<ChatMessage> CreateRequestMessages(ChatMessageContent messag
  }
 
  var assistantMessage = new AssistantChatMessage(toolCalls) { ParticipantName = message.AuthorName };
- if (message.Content is { } content)
- {
-  assistantMessage.Content.Add(content);
- }
+
+ // If message content is null, adding it as empty string,
+ // because chat message content must be string.
+ assistantMessage.Content.Add(message.Content ?? string.Empty);
 
  return [assistantMessage];
  }