.NET: [Bug]: AG-UI host generates as many text messages as completions

[tlecomte]

Description

When using the AG-UI hosting package, and an OpenAI model like GPT 5.4 mini, the AG-UI event stream contains as many as TEXT_MESSAGE_START/_CONTENT/_END as completions, causing an AG-UI client to display completions one at a time, each on its own line.

Code Sample

I believe this can be reproduced with https://github.com/microsoft/agent-framework/tree/main/dotnet/samples/05-end-to-end/AGUIWebChat

Error Messages / Stack Traces

Package Versions

Microsoft.Agents.AI.Hosting.AGUI.AspNetCore 1.6.1-preview.260514.1

.NET Version

.NET 10

Additional Context

No response

[github-actions]

🤖 Automated triage reproduction notes (agent-authored — trust but verify)

Agent analysis

Repro: ChatResponseUpdateAGUIExtensions.AsAGUIEventStreamAsync() in dotnet/src/Microsoft.Agents.AI.AGUI/Shared/ChatResponseUpdateAGUIExtensions.cs (lines 466-506) emits a new TEXT_MESSAGE_START/END block every time ChatResponseUpdate.MessageId changes. When OpenAI assigns different MessageIds per completion in a tool-calling loop (FunctionInvokingChatClient), each completion's text becomes a separate AG-UI text message. Minimal repro: feed AsAGUIEventStreamAsync a stream of ChatResponseUpdate objects with TextContent where consecutive groups have different MessageIds (simulating multiple LM completions), and observe that multiple TextMessageStartEvent/TextMessageEndEvent pairs are emitted instead of one.

• Failing test: dotnet/tests/Microsoft.Agents.AI.Hosting.AGUI.AspNetCore.UnitTests/Issue6010_MultipleTextMessagesPerCompletionTests.cs
• Files examined: dotnet/src/Microsoft.Agents.AI.AGUI/Shared/ChatResponseUpdateAGUIExtensions.cs, dotnet/src/Microsoft.Agents.AI.Hosting.AGUI.AspNetCore/AGUIEndpointRouteBuilderExtensions.cs, dotnet/src/Microsoft.Agents.AI.AGUI/AGUIChatClient.cs, dotnet/src/Microsoft.Agents.AI.AGUI/AGUIHttpService.cs, dotnet/src/Microsoft.Agents.AI/ChatClient/ChatClientAgent.cs, dotnet/tests/Microsoft.Agents.AI.Hosting.AGUI.AspNetCore.UnitTests/ChatResponseUpdateAGUIExtensionsTests.cs, dotnet/tests/Microsoft.Agents.AI.Hosting.AGUI.AspNetCore.IntegrationTests/BasicStreamingTests.cs
• Tests run: Issue6010_MultipleTextMessagesPerCompletionTests.AsAGUIEventStreamAsync_MultipleCompletions_WithToolCallsBetween_ProducesTooManyTextMessages, Issue6010_MultipleTextMessagesPerCompletionTests.AsAGUIEventStreamAsync_TwoCompletionsWithDifferentMessageIds_ProducesTwoTextMessages
• Reported version: 1.6.1-preview.260514.1