.NET: Refactor Workflows MessageMerger to preserve message order and structure

[marcominerva]

Summary

Refactored MessageMerger to delegate update grouping and merging to M.E.AI, preserving the correct order and structure of assistant messages, especially for reasoning content without message IDs. Removed per-message bucketing and CreatedAt-based sorting. Added tests to verify message order and correct merging of reasoning and text updates.

Refactoring and Simplification:

• The ResponseMergeState class now maintains a single ordered list of updates (_updates) instead of bucketing updates by message ID and separately tracking dangling updates. Grouping and coalescing of updates into messages is now handled by M.E.AI's ToAgentResponse method, ensuring correct merging of reasoning and text, as well as preserving message order.
• The custom merge logic for responses and the manual ordering by CreatedAt timestamp have been removed, eliminating previous issues where reasoning content without a timestamp could be reordered incorrectly. [1] [2] [3]

Bug Fixes and Behavioral Improvements:

• The merger now preserves the order of reasoning and text messages, even when reasoning messages lack a CreatedAt timestamp, and correctly merges id-less reasoning updates with subsequent text into a single message when appropriate.

Testing:

• Added two new unit tests to verify that (1) message order is preserved when reasoning lacks a CreatedAt timestamp, and (2) reasoning and text are merged into a single message when reasoning updates lack a message ID, matching the behavior of directly-invoked agents.

Closes #6329

[Copilot]

Pull request overview

This PR refactors the .NET MessageMerger in Microsoft.Agents.AI.Workflows to preserve assistant message order/structure by delegating update grouping and coalescing to Microsoft.Extensions.AI’s ToAgentResponse, addressing the reasoning-vs-text ordering/structure discrepancy reported in #6329.

Changes:

• Refactored MessageMerger to maintain a single ordered update list per response and rely on ToAgentResponse for message reconstruction (removing per-message bucketing and CreatedAt-based sorting).
• Simplified response merging logic and handling of id-less (“dangling”) updates.
• Added unit tests to validate ordering when reasoning lacks CreatedAt, and merging reasoning + text into a single assistant message when reasoning updates lack MessageId.

Reviewed changes

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

File	Description
dotnet/src/Microsoft.Agents.AI.Workflows/MessageMerger.cs	Refactors merge state to preserve streaming/arrival order and delegate message grouping to ToAgentResponse.
dotnet/tests/Microsoft.Agents.AI.Workflows.UnitTests/MessageMergerTests.cs	Adds tests covering ordering and reasoning/text merge behavior for id-less reasoning updates.

[marcominerva]

Hi @rogerbarreto!

I'm just asking to understand the situation, I definitely don't want to rush you. Is there anything I need to do on the branch so you can continue with the review?

I noticed there's one failing test, but it seems to be related to an XunitException in OpenTelemetryAgentTests.Ctor_NullOrWhitespaceSourceName_AutoWiredChatClientUsesDefaultSource_Async:
https://github.com/microsoft/agent-framework/actions/runs/28799163307/job/85397632168#step:9:230

[peibekwe]

@marcominerva could you please resolve the conflict with main? We recently made changes to MessageMerger as well to address #5720.

[marcominerva]

@peibekwe MessageMerger.cs has had several changes. I believe I've reconciled everything, but please let me know if I missed anything.

[peibekwe]

@marcominerva - Thanks for the quick update to the PR branch! Please take a look at the few comments and we can get the review finalized.

[marcominerva]

@peibekwe done 😉