[AI Abstractions]: ChatMessage.CreatedAt overwritten by "null" timestamps in chat response updates

[philliphoff]

Description

In Microsoft.Extensions.AI.ChatResponseExtensions.ToChatResponse(), a ChatMessage is built from a series of ChatResponseUpdate items. Each ChatResponseUpdate may have a CreatedAt timestamp and the last non-null timestamp of the updates becomes the timestamp for the resulting ChatMessage.

However, it appears that some LLM calls stream updates with timestamps set to the Unix epoch (i.e. 1/1/1970 12AM) as an equivalent for a "null" value. In those cases, because the deserialized value is a real DateTimeOffset, the resulting ChatMessage will end up with CreatedAt timestamp set to that "null" value.

It may be better to update the ChatMessage timestamp upon the next update only if that update's timestamp is later than the current timestamp. (Or, alternatively, treat a timestamp set to the UNIX epoch as equivalent to null.)

Reproduction Steps

Adding this test to the ChatResponseUpdateExtensionsTests.cs in the Microsoft.Extensions.AI.Abstractions.Tests exposes the issue.

    [Theory]
    [InlineData(false)]
    [InlineData(true)]
    public async Task ToChatResponse_AlternativeTimestamps(bool useAsync)
    {
        DateTimeOffset now = DateTimeOffset.UtcNow;

        ChatResponseUpdate[] updates =
        [
            new(ChatRole.Tool, "f") { MessageId = "4", CreatedAt = now },
            new(null, "g") { CreatedAt = DateTimeOffset.UnixEpoch },
        ];

        ChatResponse response = useAsync ?
            updates.ToChatResponse() :
            await YieldAsync(updates).ToChatResponseAsync();
        Assert.Single(response.Messages);

        Assert.Equal("fg", response.Messages[0].Text);
        Assert.Equal(ChatRole.Tool, response.Messages[0].Role);
        Assert.Equal(now, response.Messages[0].CreatedAt);
    }

Expected behavior

The first update's timestamp becomes the timestamp (i.e. CreatedAt) for the ChatMessage.

Actual behavior

The CreatedAt property of the ChatMessage is set to DateTimeOffset.UnixEpoc.

Regression?

Unknown.

Known Workarounds

None.

Configuration

.NET 9 with Microsoft.Extensions.AI.Abstractions version 9.9.0

Other information

No response

[stephentoub]

It may be better to update the ChatMessage timestamp upon the next update only if that update's timestamp is later than the current timestamp.

This sounds reasonable.