Fix ChatMessage.CreatedAt being always overwritten by the latest timestamp.

src/Libraries/Microsoft.Extensions.AI.Abstractions/ChatCompletion/ChatResponseExtensions.cs

@@ -346,7 +346,7 @@ private static void ProcessUpdate(ChatResponseUpdate update, ChatResponse respon
             message.AuthorName = update.AuthorName;
         }
 
-        if (update.CreatedAt is not null)
+        if (message.CreatedAt is null || (update.CreatedAt is not null && update.CreatedAt > message.CreatedAt))
         {
             message.CreatedAt = update.CreatedAt;
         }
@@ -391,7 +391,7 @@ private static void ProcessUpdate(ChatResponseUpdate update, ChatResponse respon
             response.ConversationId = update.ConversationId;
         }
 
-        if (update.CreatedAt is not null)
+        if (response.CreatedAt is null || (update.CreatedAt is not null && update.CreatedAt > response.CreatedAt))
         {
             response.CreatedAt = update.CreatedAt;
         }

test/Libraries/Microsoft.Extensions.AI.Abstractions.Tests/ChatCompletion/ChatResponseUpdateExtensionsTests.cs

@@ -359,6 +359,51 @@
         Assert.Equal("Hello, world!", Assert.IsType<TextContent>(Assert.Single(Assert.Single(response.Messages).Contents)).Text);
     }
 
+    [Theory]
+    [InlineData(false)]
+    [InlineData(true)]
+    public async Task ToChatResponse_AlternativeTimestamps(bool useAsync)
+    {
+        DateTimeOffset early = new(2024, 1, 1, 10, 0, 0, TimeSpan.Zero);
+        DateTimeOffset middle = new(2024, 1, 1, 11, 0, 0, TimeSpan.Zero);
+        DateTimeOffset late = new(2024, 1, 1, 12, 0, 0, TimeSpan.Zero);
+        DateTimeOffset unixEpoch = new(1970, 1, 1, 0, 0, 0, TimeSpan.Zero);
+
+        ChatResponseUpdate[] updates =
+        [
+            // Start with an early timestamp
+            new(ChatRole.Tool, "a") { MessageId = "4", CreatedAt = early },
+
+            // Unix epoch (as "null") should not overwrite
+            new(null, "b") { CreatedAt = unixEpoch },
+
+            // Newer timestamp should overwrite
+            new(null, "c") { CreatedAt = middle },
+
+            // Older timestamp should not overwrite
+            new(null, "d") { CreatedAt = early },
+
+            // Even newer timestamp should overwrite
+            new(null, "e") { CreatedAt = late },
+
+            // Unix epoch should not overwrite again
+            new(null, "f") { CreatedAt = unixEpoch },
+
+            // null should not overwrite
+            new(null, "g") { CreatedAt = null },
+        ];
+
+        ChatResponse response = useAsync ?
+            updates.ToChatResponse() :
+            await YieldAsync(updates).ToChatResponseAsync();
+        Assert.Single(response.Messages);
+
+        Assert.Equal("abcdefg", response.Messages[0].Text);
+        Assert.Equal(ChatRole.Tool, response.Messages[0].Role);
+        Assert.Equal(late, response.Messages[0].CreatedAt);
+        Assert.Equal(late, response.CreatedAt);
+    }
+
     private static async IAsyncEnumerable<ChatResponseUpdate> YieldAsync(IEnumerable<ChatResponseUpdate> updates)
     {
         foreach (ChatResponseUpdate update in updates)