Fix ToChatResponse to use first appropriate CreatedAt instead of last by Copilot · Pull Request #7193 · dotnet/extensions

Copilot · 2026-01-05T14:48:58Z

Understand the issue: ToChatResponse{Async} should use the first update's CreatedAt instead of the last
Identify the code to change: ChatResponseExtensions.cs lines 521-524 (message) and 566-569 (response)
Implement the fix: Change CreatedAt logic to prefer first non-null, non-epoch value
Update tests: Modify tests that expect "last wins" behavior to expect "first wins"
Build and test: All 108 ChatResponseUpdateExtensionsTests pass on net8.0 and net9.0
Address PR review feedback:
- Updated comments to clarify CreatedAt uses first valid value
- Used DateTimeOffset.UnixEpoch under #if NET instead of custom field
- Simplified IsValidCreatedAt to use direct nullable comparison
- Added tests for datetime values less than Unix epoch

Summary

Changed the ToChatResponse{Async} implementation so that the CreatedAt property reflects the first appropriate update's CreatedAt property rather than the last. Values that are less than or equal to the Unix epoch are treated as null (invalid). This ensures that when streaming responses from providers like Mistral, the CreatedAt accurately reflects when the message content was generated, not when the usage update was received.

Original prompt

This section details on the original issue you should resolve

<issue_title>Inaccurate CreatedAt returned by ToChatResponse</issue_title>
<issue_description>```csharp
ChatResponseUpdate[] updates =
[
new()
{
Role = ChatRole.Assistant,
Contents = [new TextContent("Hi")],
CreatedAt = DateTimeOffset.UtcNow,
},
new()
{
Role = ChatRole.Assistant,
Contents = [new UsageContent { Details = new UsageDetails { TotalTokenCount = 1 } }],
CreatedAt = DateTimeOffset.UtcNow + TimeSpan.FromMinutes(11),
},
];
Console.WriteLine(updates.ToChatResponse().Messages[0].CreatedAt);
In this example `Messages[0].CreatedAt` will get the value of the second update. I think it would make more sense to get the value of the first update as the second update is not really part of the message.

In the mistral API, I noticed significant delay between the usage update and the last text update so I would like it to be reflected in the ChatResponse.</issue_description>

<agent_instructions>Please update the ToChatResponse{Async} implementation so that the CreatedAt property reflects the first appropriate update's CreatedAt property rather than the last (it should treat any value that's less than or equal to the Unix epoch as being the same as null). Update existing tests accordingly.</agent_instructions>

## Comments on the Issue (you are @copilot in this section)

<comments>
<comment_new><author>@stephentoub</author><body>
If you do non-streaming with a really large response, is the creation date you get back closer to the time of the request or the time of completion? </body></comment_new>
<comment_new><author>@stephentoub</author><body>
Ok. I played around with some other providers, and it does look like CreatedAt should reflect the first rather than last update.</body></comment_new>
</comments>

Fixes Inaccurate CreatedAt returned by ToChatResponse #7186

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Microsoft Reviewers: Open in CodeFlow

…f last Changed the logic in ProcessUpdate to prefer the first valid CreatedAt value (non-null and after Unix epoch) rather than the latest one. This fixes the issue where UsageContent updates with later timestamps were incorrectly overwriting the message CreatedAt with a value that doesn't reflect when the actual message content was generated. Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

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

…ixEpoch under #if NET - Updated comments in ProcessUpdate to clarify that CreatedAt prefers first valid value - Used DateTimeOffset.UnixEpoch under #if NET instead of custom _unixEpoch field Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

Copilot

Pull request overview

This PR fixes a bug where ToChatResponse{Async} was incorrectly using the timestamp from the last update rather than the first update when constructing a ChatResponse. This caused inaccurate CreatedAt values when usage updates arrive significantly after content updates (e.g., in the Mistral API where usage updates can be delayed by several minutes).

Key Changes:

Modified timestamp handling to use "first valid value wins" behavior instead of "last value wins"
Added IsValidCreatedAt helper that treats values ≤ Unix epoch (1970-01-01) as null/invalid
Updated all affected tests to reflect the new first-wins behavior and use valid timestamps (years ≥ 1970)

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
`src/Libraries/Microsoft.Extensions.AI.Abstractions/ChatCompletion/ChatResponseExtensions.cs`	Changed `ProcessUpdate` to set `CreatedAt` only when null (first-wins), added `IsValidCreatedAt` helper and Unix epoch constant
`test/Libraries/Microsoft.Extensions.AI.Abstractions.Tests/ChatCompletion/ChatResponseUpdateExtensionsTests.cs`	Updated test timestamps from invalid years (1, 2) to valid ones (2024, 2025), updated expected values to match first-wins behavior, and corrected comments to reflect new semantics

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

...crosoft.Extensions.AI.Abstractions.Tests/ChatCompletion/ChatResponseUpdateExtensionsTests.cs

Simplified the IsValidCreatedAt helper to use direct comparison (createdAt > UnixEpoch) instead of pattern matching first. The comparison returns false when the nullable value is null. Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

Added test cases to validate that datetime values less than Unix epoch (not just equal to it) are treated the same as null: - Added test cases to ToChatResponse_TimestampFolding_MemberData for 1969 and 1960 dates - Updated ToChatResponse_AlternativeTimestamps to include a before-epoch timestamp Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

Initial plan

2fdc6d0

Copilot AI assigned Copilot and stephentoub Jan 5, 2026

Copilot started work on behalf of stephentoub January 5, 2026 14:49 View session

Copilot AI and others added 3 commits January 5, 2026 15:03

Complete code review and security analysis

53639eb

Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

Remove accidentally committed nuget.exe

9e6fe0f

Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

Copilot AI changed the title ~~[WIP] Fix inaccurate CreatedAt returned by ToChatResponse~~ Fix ToChatResponse to use first appropriate CreatedAt instead of last Jan 5, 2026

Copilot AI requested a review from stephentoub January 5, 2026 15:09

Copilot finished work on behalf of stephentoub January 5, 2026 15:09

stephentoub marked this pull request as ready for review January 5, 2026 15:11

stephentoub requested a review from a team as a code owner January 5, 2026 15:11

Copilot AI review requested due to automatic review settings January 5, 2026 15:11

stephentoub reviewed Jan 5, 2026

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Copilot AI requested a review from stephentoub January 5, 2026 15:19

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Copilot AI reviewed Jan 5, 2026

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src/Libraries/Microsoft.Extensions.AI.Abstractions/ChatCompletion/ChatResponseExtensions.cs Show resolved Hide resolved

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...crosoft.Extensions.AI.Abstractions.Tests/ChatCompletion/ChatResponseUpdateExtensionsTests.cs Show resolved Hide resolved

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Fix ToChatResponse to use first appropriate CreatedAt instead of last#7193

Fix ToChatResponse to use first appropriate CreatedAt instead of last#7193
stephentoub merged 7 commits intomainfrom
copilot/fix-created-at-value

Copilot AI commented Jan 5, 2026 •

edited

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Copilot AI commented Jan 5, 2026 •

edited

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