.NET: [Bug]: Replying in a CopilotKit webchat with reasoning results in "System.InvalidOperationException: Unknown chat role: reasoning"

Description

Setup:

• Set up Harness Agent with reasoning
• CopilotKit as webchat

Reproduction:

• User sends first message --> Agent responds with reasoning and output
• User sends followup message --> 500 status code from agent service

Code Sample

Program.cs

#pragma warning disable OPENAI001 // Type is for evaluation purposes only and is subject to change or removal in future updates. Suppress this diagnostic to proceed.
    // The Responses API variant is needed for the Reasoning field to work without a 400 Bad Request from the LLM.
    builder.Services.AddKeyedSingleton<IChatClient>("responses-chatclient", (sp, _) =>
{
    var projectClient = new AIProjectClient(foundryUri, new DefaultAzureCredential());
    var openAIClient = projectClient.GetProjectOpenAIClient();
    return openAIClient.GetResponsesClient().AsIChatClient(azureOpenAIDeploymentName);
});
#pragma warning restore OPENAI001 // Type is for evaluation purposes only and is subject to change or removal in future updates. Suppress this diagnostic to proceed.

Agent:

return chatClient.AsHarnessAgent(new HarnessAgentOptions
        {
            // FileAccessStore is left unset, which disables file access; leaving DisableFileMemory/DisableTodoProvider/
            // DisableAgentModeProvider false keeps their defaults, including writing to ./agent-file-memory on disk.
            Name = name,
            HarnessInstructions = Instructions,
            ChatOptions = new ChatOptions { Tools = tools, Reasoning = new() { Effort = ReasoningEffort.Medium, Output = ReasoningOutput.Summary} },
            AIContextProviders = contextProviders,
            DisableWebSearch = true,
            OpenTelemetrySourceName = GenAiSourceNames.Agents,
        });

Error Messages / Stack Traces

HTTP 500: System.InvalidOperationException: Unknown chat role: reasoning
   at AGUI.Abstractions.AGUIChatMessageExtensions.MapChatRole(String role)
   at AGUI.Abstractions.AGUIChatMessageExtensions.AsChatMessages(IEnumerable`1 aguiMessages)+MoveNext()
   at System.Collections.Generic.List`1..ctor(IEnumerable`1 collection)
   at System.Linq.Enumerable.ToList[TSource](IEnumerable`1 source)
   at AGUI.Server.RunAgentInputExtensions.ToChatRequestContext(RunAgentInput input, JsonSerializerOptions jsonSerializerOptions, AGUIStreamOptions streamOptions)
   at Microsoft.Agents.AI.Hosting.AGUI.AspNetCore.AGUIEndpointRouteBuilderExtensions.<>c__DisplayClass2_0.<<MapAGUIServer>b__0>d.MoveNext()
--- End of stack trace from previous location ---
   at Microsoft.AspNetCore.Http.Generated.<GeneratedRouteBuilderExtensions_g>FB87DEE35F409427C0993BF3A8E11207C78AB7E4167B350548AEB87B222E3F58E__GeneratedRouteBuilderExtensionsCore.<>c__DisplayClass2_0.<<MapPost0>g__RequestHandler|5>d.MoveNext()
--- End of stack trace from previous location ---
   at Microsoft.AspNetCore.Routing.EndpointMiddleware.<Invoke>g__AwaitRequestTask|7_0(Endpoint endpoint, Task requestTask, ILogger logger)
   at Microsoft.AspNetCore.Authorization.AuthorizationMiddleware.Invoke(HttpContext context)
   at Microsoft.AspNetCore.Authentication.AuthenticationMiddleware.Invoke(HttpContext context)
   at Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddlewareImpl.Invoke(HttpContext context)

Package Versions

Microsoft.Agents.AI.Hosting.AGUI.AspNetCore: 1.21.0-preview.260911.1
Microsoft.Agents.AI.Harness: 1.21.0
@copilotkit/react-core: ^1.71.1

.NET Version

.NET 10

Additional Context

No response