.Net: fix: fall back to ToString() when logging function results with unregistered types

dotnet/src/SemanticKernel.Abstractions/Functions/KernelFunctionLogMessages.cs

@@ -161,7 +161,17 @@ private static void LogFunctionResultValueInternal(this ILogger logger, string?
             }
             catch (NotSupportedException ex)
             {
-                s_logFunctionResultValue(logger, pluginName, functionName, "Failed to log function result value", ex);
+                // Fall back to ToString() when JSON serialization isn't supported for this type
+                // (e.g. Microsoft.Extensions.AI.TextContent is not registered in AbstractionsJsonContext)
+                try
+                {
+                    var toStringValue = resultValue?.Value?.ToString() ?? string.Empty;
+                    s_logFunctionResultValue(logger, pluginName, functionName, toStringValue, null);
+                }
+                catch
+                {
+                    s_logFunctionResultValue(logger, pluginName, functionName, "Failed to log function result value", ex);
+                }
             }
         }
     }

dotnet/src/SemanticKernel.UnitTests/Functions/KernelFunctionLogMessagesTests.cs

@@ -1,6 +1,7 @@
 // Copyright (c) Microsoft. All rights reserved.
 
 using System;
+using System.Collections.Generic;
 using System.Text.Json.Serialization;
 using Microsoft.Extensions.Logging;
 using Microsoft.SemanticKernel;
@@ -48,9 +49,46 @@ public void ItShouldLogFunctionResultOfAnyType(Type resultType)
             It.IsAny<Func<It.IsAnyType, Exception?, string>>()));
     }
 
+    [Fact]
+    public void ItShouldFallBackToToStringWhenJsonSerializationIsNotSupported()
+    {
+        // Arrange
+        var logger = new Mock<ILogger>();
+        logger.Setup(l => l.IsEnabled(It.IsAny<LogLevel>())).Returns(true);
+
+        // TypeNotInJsonContext cannot be cast to string and is not registered in the restricted JSON context
+        var unserializableValue = new TypeNotInJsonContext();
+        var functionResult = new FunctionResult(KernelFunctionFactory.CreateFromMethod(() => { }), unserializableValue);
+
+        // Use a restricted JsonSerializerOptions that knows about object but not TypeNotInJsonContext,
+        // simulating the AOT scenario where AbstractionsJsonContext is used and an unregistered
+        // MEAItype (e.g. Microsoft.Extensions.AI.TextContent) is returned from an MCP tool.
+        var restrictedOptions = RestrictedJsonContext.Default.Options;
+
+        // Act
+        logger.Object.LogFunctionResultValue("p1", "f1", functionResult, restrictedOptions);
+
+        // Assert - ToString() fallback should have been used, not the error message
+        logger.Verify(l => l.Log(
+            LogLevel.Trace,
+            0,
+            It.Is<It.IsAnyType>((o, _) => o.ToString() == "Function p1-f1 result: TypeNotInJsonContext()"),
+            null,
+            It.IsAny<Func<It.IsAnyType, Exception?, string>>()));
+    }
+
     private sealed class User
     {
         [JsonPropertyName("name")]
         public string? Name { get; set; }
     }
+
+    private sealed class TypeNotInJsonContext
+    {
+        public override string ToString() => "TypeNotInJsonContext()";
+    }
+
+    [JsonSerializable(typeof(IDictionary<string, object?>))]
+    [JsonSerializable(typeof(object))]
+    private sealed partial class RestrictedJsonContext : JsonSerializerContext { }
 }