.NET: coalesce AG-UI text completions

[he-yufeng]

Fixes #6010.

Summary

• Keep one AG-UI text message open across consecutive assistant text completions, even when the provider assigns a new ChatResponseUpdate.MessageId per completion.
• Continue to split text messages when the chat role changes.
• Add a regression test covering assistant text around tool-call/tool-result events with different completion IDs.

Validation

• dotnet build dotnet\tests\Microsoft.Agents.AI.Hosting.AGUI.AspNetCore.UnitTests\Microsoft.Agents.AI.Hosting.AGUI.AspNetCore.UnitTests.csproj --no-restore --tl:off
• dotnet dotnet\tests\Microsoft.Agents.AI.Hosting.AGUI.AspNetCore.UnitTests\bin\Debug\net10.0\Microsoft.Agents.AI.Hosting.AGUI.AspNetCore.UnitTests.dll --filter-class Microsoft.Agents.AI.Hosting.AGUI.AspNetCore.UnitTests.ChatResponseUpdateAGUIExtensionsTests --timeout 2m
• dotnet format dotnet\agent-framework-dotnet.slnx --verify-no-changes --include dotnet\src\Microsoft.Agents.AI.AGUI\Shared\ChatResponseUpdateAGUIExtensions.cs dotnet\tests\Microsoft.Agents.AI.Hosting.AGUI.AspNetCore.UnitTests\ChatResponseUpdateAGUIExtensionsTests.cs --no-restore --verbosity minimal
• git diff --check

[Copilot]

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[he-yufeng]

Rebased on current upstream/main and resolved the AGUI text-stream conflict by keeping both fixes: the upstream empty-message-id fallback stays local to text events, and this PR still coalesces assistant text completions across adjacent text chunks.

Validated locally:

dotnet build .\tests\Microsoft.Agents.AI.Hosting.AGUI.AspNetCore.UnitTests\Microsoft.Agents.AI.Hosting.AGUI.AspNetCore.UnitTests.csproj -f net10.0 -c Debug --tl:off
dotnet test --project .\tests\Microsoft.Agents.AI.Hosting.AGUI.AspNetCore.UnitTests\Microsoft.Agents.AI.Hosting.AGUI.AspNetCore.UnitTests.csproj -f net10.0 -c Debug --no-build -v Normal --report-xunit-trx --ignore-exit-code 8 -- --filter-class Microsoft.Agents.AI.Hosting.AGUI.AspNetCore.UnitTests.ChatResponseUpdateAGUIExtensionsTests
git diff --check upstream/main...HEAD

Result: the targeted test class passed after the conflict resolution.

[he-yufeng]

Rebased onto current upstream/main and reran the targeted AG-UI validation.

Validation on Windows:

dotnet build .\dotnet\tests\Microsoft.Agents.AI.Hosting.AGUI.AspNetCore.UnitTests\Microsoft.Agents.AI.Hosting.AGUI.AspNetCore.UnitTests.csproj -f net10.0 -c Debug --tl:off
# build succeeded, 0 warnings, 0 errors

cd dotnet
dotnet test --project .\tests\Microsoft.Agents.AI.Hosting.AGUI.AspNetCore.UnitTests\Microsoft.Agents.AI.Hosting.AGUI.AspNetCore.UnitTests.csproj -f net10.0 -c Debug --no-build -v Normal -- --filter-class Microsoft.Agents.AI.Hosting.AGUI.AspNetCore.UnitTests.ChatResponseUpdateAGUIExtensionsTests
# 11 passed

git diff --check upstream/main...HEAD

Note: because this repo's global.json with the Microsoft.Testing.Platform runner lives under dotnet/, the targeted dotnet test command has to be run from that directory.

[he-yufeng]

Updated this to avoid keeping a text message open across tool events.

The converter now closes the current TextMessage* bracket before emitting ToolCall* or ToolCallResult*, then starts a new text message if text continues afterward. That keeps the .NET AG-UI producer aligned with the Python producer shape instead of nesting tool events inside an open text message.

I also changed the regression test to assert the wire order directly:

TEXT_MESSAGE_START completion-1
TEXT_MESSAGE_CONTENT completion-1
TEXT_MESSAGE_END completion-1
TOOL_CALL_*
TOOL_RESULT
TEXT_MESSAGE_START completion-2
TEXT_MESSAGE_CONTENT completion-2
TEXT_MESSAGE_END completion-2

Validation on Windows:

dotnet build .\dotnet\tests\Microsoft.Agents.AI.Hosting.AGUI.AspNetCore.UnitTests\Microsoft.Agents.AI.Hosting.AGUI.AspNetCore.UnitTests.csproj -f net10.0 -c Debug --tl:off
# build succeeded, 0 warnings, 0 errors

dotnet .\dotnet\tests\Microsoft.Agents.AI.Hosting.AGUI.AspNetCore.UnitTests\bin\Debug\net10.0\Microsoft.Agents.AI.Hosting.AGUI.AspNetCore.UnitTests.dll --filter-class Microsoft.Agents.AI.Hosting.AGUI.AspNetCore.UnitTests.ChatResponseUpdateAGUIExtensionsTests --no-progress
# 11 passed

git diff --check

[he-yufeng]

Friendly ping: this has two approvals and the latest CI/merge-gatekeeper run is green. Please let me know if there is any remaining AG-UI concern before merge.

[moonbox3]

Friendly ping: this has two approvals and the latest CI/merge-gatekeeper run is green. Please let me know if there is any remaining AG-UI concern before merge.

There was another test failing during merge, which I think was unrelated. Trying the merge again.

[he-yufeng]

This one has been green and approved for a few weeks now. Anything still blocking the merge, or is it just waiting in the queue?

[he-yufeng]

This has two approvals (@moonbox3, @westey-m) and is mergeable with green CI — could someone merge it when convenient? Happy to rebase if it goes stale. Thanks!

[moonbox3]

@he-yufeng there is an integration test failing:

failed Microsoft.Agents.AI.Hosting.AGUI.AspNetCore.IntegrationTests.BasicStreamingTests.AgentSendsMultipleMessagesInOneTurnAsync (677ms)
  Expected collection to contain more than 1 item(s) because agent should send multiple messages, but found 1: {"8e920ac81a5349919f4481ba0d5236a6"}.
  from /home/runner/work/agent-framework/agent-framework/dotnet/tests/Microsoft.Agents.AI.Hosting.AGUI.AspNetCore.IntegrationTests/bin/Release/net10.0/Microsoft.Agents.AI.Hosting.AGUI.AspNetCore.IntegrationTests.dll (net10.0|x64)
  Expected collection to contain more than 1 item(s) because agent should send multiple messages, but found 1: {"8e920ac81a5349919f4481ba0d5236a6"}.
    at FluentAssertions.Execution.LateBoundTestFramework.Throw(String message)
    at FluentAssertions.Execution.DefaultAssertionStrategy.HandleFailure(String message)
    at FluentAssertions.Execution.AssertionScope.AddPreFormattedFailure(String formattedFailureMessage)
    at FluentAssertions.Execution.AssertionChain.FailWith(Func`1 getFailureReason)
    at FluentAssertions.Execution.AssertionChain.FailWith(Func`1 getFailureReason)
    at FluentAssertions.Execution.AssertionChain.FailWith(String message, Object[] args)
    at FluentAssertions.Execution.GivenSelector`1.FailWith(String message, Object[] args)
    at FluentAssertions.Execution.GivenSelector`1.FailWith(String message, Func`2[] args)
    at FluentAssertions.Collections.GenericCollectionAssertions`3.<>c__DisplayClass48_0.<HaveCountGreaterThan>b__0(AssertionChain chain)
    at FluentAssertions.Execution.AssertionChain.WithExpectation(String message, Action`1 chain, Object[] args)
    at FluentAssertions.Execution.AssertionChain.WithExpectation(String message, Object arg1, Action`1 chain)
    at FluentAssertions.Collections.GenericCollectionAssertions`3.HaveCountGreaterThan(Int32 expected, String because, Object[] becauseArgs)
    at Microsoft.Agents.AI.Hosting.AGUI.AspNetCore.IntegrationTests.BasicStreamingTests.AgentSendsMultipleMessagesInOneTurnAsync() in /home/runner/work/agent-framework/agent-framework/dotnet/tests/Microsoft.Agents.AI.Hosting.AGUI.AspNetCore.IntegrationTests/BasicStreamingTests.cs:189
    --- End of stack trace from previous location ---
/home/runner/work/agent-framework/agent-framework/dotnet/tests/Microsoft.Agents.AI.Hosting.AGUI.AspNetCore.IntegrationTests/bin/Release/net10.0/Microsoft.Agents.AI.Hosting.AGUI.AspNetCore.IntegrationTests.dll (net10.0|x64) failed with 1 error(s) (5s 868ms)
Exit code: 2

[he-yufeng]

Thanks @moonbox3, good catch. I looked into it and the failure is a real consequence of this PR, not flaky.

AgentSendsMultipleMessagesInOneTurnAsync feeds three assistant text completions, each with its own MessageId and nothing between them (no tool call, no role change), then asserts the AG-UI round-trip yields more than one message. This PR coalesces consecutive same-role text across MessageId changes, so those three collapse into one text message and the assertion trips.

The wrinkle is that this is the exact shape #6010 wants coalesced. The issue's own repro (TwoCompletionsWithDifferentMessageIds_ProducesTwoTextMessages) treats "two consecutive text completions, different MessageIds, nothing between them" as the bug, while this integration test treats the same shape as the desired result. Both can't hold: at the update level a fragmented single completion and two deliberately distinct messages are indistinguishable (same role, distinct non-null MessageIds, no separator).

My read is that the integration test encodes the pre-#6010 behavior and should move to the coalesced contract: one text message, content preserved in order, still split whenever the role changes or a tool call/result lands between (the unit test in this PR already covers those boundaries). I'm happy to rebase onto main and push that test update now.

If you'd rather keep multi-message turns distinct on the AG-UI surface, then #6010 needs a narrower fix, probably where the provider hands out a fresh MessageId per completion above AG-UI, and I'll close this in favor of that. Tell me which way you'd like to go and I'll turn it around.