Fix handling of multiple responses messages

src/Libraries/Microsoft.Extensions.AI.OpenAI/OpenAIResponseChatClient.cs

@@ -111,10 +111,15 @@ public async Task<ChatResponse> GetResponseAsync(
                 switch (outputItem)
                 {
                     case MessageResponseItem messageItem:
+                        if (message.MessageId is not null && message.MessageId != messageItem.Id)
+                        {
+                            message = new ChatMessage();
+                            response.Messages.Add(message);
+                        }
+
                         message.MessageId = messageItem.Id;
                         message.RawRepresentation = messageItem;
                         message.Role = ToChatRole(messageItem.Role);
-                        (message.AdditionalProperties ??= []).Add(nameof(messageItem.Id), messageItem.Id);
                         ((List<AIContent>)message.Contents).AddRange(ToAIContents(messageItem.Content));
                         break;
 

test/Libraries/Microsoft.Extensions.AI.OpenAI.Tests/OpenAIResponseClientTests.cs

@@ -479,6 +479,122 @@ public async Task ChatOptions_DoNotOverwrite_NotNullPropertiesInRawRepresentatio
         Assert.Equal("Hello! How can I assist you today?", response.Text);
     }
 
+    [Fact]
+    public async Task MultipleOutputItems_NonStreaming()
+    {
+        const string Input = """
+            {
+                "temperature":0.5,
+                "model":"gpt-4o-mini",
+                "input": [{
+                    "type":"message",
+                    "role":"user",
+                    "content":[{"type":"input_text","text":"hello"}]
+                }],
+                "max_output_tokens":20
+            }
+            """;
+
+        const string Output = """
+            {
+              "id": "resp_67d327649b288191aeb46a824e49dc40058a5e08c46a181d",
+              "object": "response",
+              "created_at": 1741891428,
+              "status": "completed",
+              "error": null,
+              "incomplete_details": null,
+              "instructions": null,
+              "max_output_tokens": 20,
+              "model": "gpt-4o-mini-2024-07-18",
+              "output": [
+                {
+                  "type": "message",
+                  "id": "msg_67d32764fcdc8191bcf2e444d4088804058a5e08c46a181d",
+                  "status": "completed",
+                  "role": "assistant",
+                  "content": [
+                    {
+                      "type": "output_text",
+                      "text": "Hello!",
+                      "annotations": []
+                    }
+                  ]
+                },
+                {
+                  "type": "message",
+                  "id": "msg_67d32764fcdc8191bcf2e444d4088804058a5e08c46a182e",
+                  "status": "completed",
+                  "role": "assistant",
+                  "content": [
+                    {
+                      "type": "output_text",
+                      "text": " How can I assist you today?",
+                      "annotations": []
+                    }
+                  ]
+                }
+              ],
+              "parallel_tool_calls": true,
+              "previous_response_id": null,
+              "reasoning": {
+                "effort": null,
+                "generate_summary": null
+              },
+              "store": true,
+              "temperature": 0.5,
+              "text": {
+                "format": {
+                  "type": "text"
+                }
+              },
+              "tool_choice": "auto",
+              "tools": [],
+              "top_p": 1.0,
+              "usage": {
+                "input_tokens": 26,
+                "input_tokens_details": {
+                  "cached_tokens": 0
+                },
+                "output_tokens": 10,
+                "output_tokens_details": {
+                  "reasoning_tokens": 0
+                },
+                "total_tokens": 36
+              },
+              "user": null,
+              "metadata": {}
+            }
+            """;
+
+        using VerbatimHttpHandler handler = new(Input, Output);
+        using HttpClient httpClient = new(handler);
+        using IChatClient client = CreateResponseClient(httpClient, "gpt-4o-mini");
+
+        var response = await client.GetResponseAsync("hello", new()
+        {
+            MaxOutputTokens = 20,
+            Temperature = 0.5f,
+        });
+        Assert.NotNull(response);
+
+        Assert.Equal("resp_67d327649b288191aeb46a824e49dc40058a5e08c46a181d", response.ResponseId);
+        Assert.Equal("resp_67d327649b288191aeb46a824e49dc40058a5e08c46a181d", response.ConversationId);
+        Assert.Equal("gpt-4o-mini-2024-07-18", response.ModelId);
+        Assert.Equal(DateTimeOffset.FromUnixTimeSeconds(1_741_891_428), response.CreatedAt);
+        Assert.Null(response.FinishReason);
+
+        Assert.Equal(2, response.Messages.Count);
+        Assert.Equal(ChatRole.Assistant, response.Messages[0].Role);
+        Assert.Equal("Hello!", response.Messages[0].Text);
+        Assert.Equal(ChatRole.Assistant, response.Messages[1].Role);
+        Assert.Equal(" How can I assist you today?", response.Messages[1].Text);
+
+        Assert.NotNull(response.Usage);
+        Assert.Equal(26, response.Usage.InputTokenCount);
+        Assert.Equal(10, response.Usage.OutputTokenCount);
+        Assert.Equal(36, response.Usage.TotalTokenCount);
+    }
+
     /// <summary>Converts an Extensions function to an OpenAI response chat tool.</summary>
     private static ResponseTool ToOpenAIResponseChatTool(AIFunction aiFunction)
     {