.NET: Fix AG-UI "Unknown chat role: reasoning" on follow-up turns

dotnet/src/Microsoft.Agents.AI.Hosting.AGUI.AspNetCore/AGUIEndpointRouteBuilderExtensions.cs

@@ -134,6 +134,7 @@ public static IEndpointConventionBuilder MapAGUIServer(
             var streamOptions = context.GetEndpoint()?.Metadata.GetMetadata<AGUIStreamOptions>()
                 ?? context.RequestServices.GetService<IOptions<AGUIStreamOptions>>()?.Value;
 
+            RemoveReasoningMessages(input);
             var ctx = input.ToChatRequestContext(jsonSerializerOptions, streamOptions);
 
             // AG-UI continuation is keyed by thread id. When the client does not supply one, generate a
@@ -179,6 +180,22 @@ private static void MarkFeatureUsed()
 #pragma warning restore MAAI001
     }
 
+    /// <summary>
+    /// Removes <see cref="AGUIReasoningMessage"/> entries from the request history. Clients such as CopilotKit
+    /// echo reasoning back on follow-up turns, but the AG-UI SDK cannot map the <c>reasoning</c> role to a
+    /// <see cref="ChatMessage"/> and throws.
+    /// </summary>
+    internal static void RemoveReasoningMessages(RunAgentInput input)
+    {
+        for (int i = input.Messages.Count - 1; i >= 0; i--)
+        {
+            if (input.Messages[i] is AGUIReasoningMessage)
+            {
+                input.Messages.RemoveAt(i);
+            }
+        }
+    }
+
     private static async IAsyncEnumerable<BaseEvent> SaveSessionAfterStreamingAsync(
         IAsyncEnumerable<BaseEvent> events,
         AIHostAgent hostAgent,

dotnet/tests/Microsoft.Agents.AI.Hosting.AGUI.AspNetCore.UnitTests/AGUIEndpointRouteBuilderExtensionsTests.cs

@@ -2,9 +2,12 @@
 
 using System;
 using System.Collections.Generic;
+using System.Linq;
 using System.Text.Json;
 using System.Threading;
 using System.Threading.Tasks;
+using AGUI.Abstractions;
+using AGUI.Server;
 using Microsoft.AspNetCore.Builder;
 using Microsoft.AspNetCore.Routing;
 using Microsoft.Extensions.AI;
@@ -223,6 +226,35 @@ public void MapAGUIServer_WithNullAgentBuilder_ThrowsArgumentNullException()
             endpointsMock.Object.MapAGUIServer((IHostedAgentBuilder)null!, "/api/agent"));
     }
 
+    [Fact]
+    public void RemoveReasoningMessages_FollowUpTurnWithReasoning_AllowsChatRequestConversion()
+    {
+        // Arrange
+        Microsoft.AspNetCore.Http.Json.JsonOptions jsonOptions = new();
+        new ConfigureAGUIJsonOptions().Configure(jsonOptions);
+        const string Body = """
+            {
+              "threadId": "t1", "runId": "r2", "state": {}, "tools": [], "context": [], "forwardedProps": {},
+              "messages": [
+                { "id": "u1", "role": "user", "content": "hi" },
+                { "id": "rs1", "role": "reasoning", "content": "thinking..." },
+                { "id": "a1", "role": "assistant", "content": "hello" },
+                { "id": "u2", "role": "user", "content": "follow up" }
+              ]
+            }
+            """;
+        RunAgentInput input = JsonSerializer.Deserialize<RunAgentInput>(Body, jsonOptions.SerializerOptions)!;
+        Assert.Throws<InvalidOperationException>(() => input.ToChatRequestContext(jsonOptions.SerializerOptions, null));
+
+        // Act
+        AGUIEndpointRouteBuilderExtensions.RemoveReasoningMessages(input);
+        var ctx = input.ToChatRequestContext(jsonOptions.SerializerOptions, null);
+
+        // Assert
+        Assert.Equal(["u1", "a1", "u2"], input.Messages.Select(m => m.Id));
+        Assert.Equal(3, ctx.Messages.Count);
+    }
+
     private sealed class TestAgent : AIAgent
     {
         protected override Task<AgentResponse> RunCoreAsync(IEnumerable<ChatMessage> messages, AgentSession? session = null, AgentRunOptions? options = null, CancellationToken cancellationToken = default) => throw new NotImplementedException();