Make enums serialize as strings if using the reflection-based serializer.

src/ModelContextProtocol.Core/CustomizableJsonStringEnumConverter.cs

@@ -5,8 +5,8 @@
 using System.Diagnostics.CodeAnalysis;
 #if !NET9_0_OR_GREATER
 using System.Reflection;
-using System.Text.Json;
 #endif
+using System.Text.Json;
 using System.Text.Json.Serialization;
 #if !NET9_0_OR_GREATER
 using ModelContextProtocol;
@@ -66,6 +66,31 @@ public override string ConvertName(string name) =>
         }
 #endif
     }
+
+    /// <summary>
+    /// A JSON converter for enums that allows customizing the serialized string value of enum members
+    /// using the <see cref="JsonStringEnumMemberNameAttribute"/>.
+    /// </summary>
+    /// <remarks>
+    /// This is a temporary workaround for lack of System.Text.Json's JsonStringEnumConverter&lt;T&gt;
+    /// 9.x support for custom enum member naming. It will be replaced by the built-in functionality
+    /// once .NET 9 is fully adopted.
+    /// </remarks>
+    [EditorBrowsable(EditorBrowsableState.Never)]
+    [RequiresUnreferencedCode("Requires unreferenced code to instantiate the generic enum converter.")]
+    [RequiresDynamicCode("Requires dynamic code to instantiate the generic enum converter.")]
+    public sealed class CustomizableJsonStringEnumConverter : JsonConverterFactory
+    {
+        /// <inheritdoc/>
+        public override bool CanConvert(Type typeToConvert) => typeToConvert.IsEnum;
+        /// <inheritdoc/>
+        public override JsonConverter? CreateConverter(Type typeToConvert, JsonSerializerOptions options)
+        {
+            Type converterType = typeof(CustomizableJsonStringEnumConverter<>).MakeGenericType(typeToConvert)!;
+            var factory = (JsonConverterFactory)Activator.CreateInstance(converterType)!;
+            return factory.CreateConverter(typeToConvert, options);
+        }
+    }
 }
 
 #if !NET9_0_OR_GREATER

src/ModelContextProtocol.Core/McpJsonUtilities.cs

@@ -2,6 +2,7 @@
 using ModelContextProtocol.Protocol;
 using ModelContextProtocol.Server;
 using System.Diagnostics.CodeAnalysis;
+using System.Reflection;
 using System.Text.Json;
 using System.Text.Json.Serialization;
 using System.Text.Json.Serialization.Metadata;
@@ -34,14 +35,22 @@ public static partial class McpJsonUtilities
     /// Creates default options to use for MCP-related serialization.
     /// </summary>
     /// <returns>The configured options.</returns>
+    [UnconditionalSuppressMessage("ReflectionAnalysis", "IL3050:RequiresDynamicCode", Justification = "Converter is guarded by IsReflectionEnabledByDefault check.")]
+    [UnconditionalSuppressMessage("Trimming", "IL2026:Members annotated with 'RequiresUnreferencedCodeAttribute' require dynamic access", Justification = "Converter is guarded by IsReflectionEnabledByDefault check.")]
     private static JsonSerializerOptions CreateDefaultOptions()
     {
         // Copy the configuration from the source generated context.
         JsonSerializerOptions options = new(JsonContext.Default.Options);
 
-        // Chain with all supported types from MEAI
+        // Chain with all supported types from MEAI.
         options.TypeInfoResolverChain.Add(AIJsonUtilities.DefaultOptions.TypeInfoResolver!);
 
+        // Add a converter for user-defined enums, if reflection is enabled by default.
+        if (JsonSerializer.IsReflectionEnabledByDefault)
+        {
+            options.Converters.Add(new CustomizableJsonStringEnumConverter());
+        }
+
         options.MakeReadOnly();
         return options;
     }

tests/ModelContextProtocol.Tests/McpJsonUtilitiesTests.cs

@@ -1,4 +1,6 @@
 using System.Text.Json;
+using System.Text.Json.Serialization;
+using System.Text.Json.Serialization.Metadata;
 
 namespace ModelContextProtocol.Tests;
 
@@ -22,4 +24,27 @@ public static void DefaultOptions_UseReflectionWhenEnabled()
 
         Assert.Equal(JsonSerializer.IsReflectionEnabledByDefault, options.TryGetTypeInfo(anonType, out _));
     }
+
+    [Fact]
+    public static void DefaultOptions_UnknownEnumHandling()
+    {
+        var options = McpJsonUtilities.DefaultOptions;
+
+        if (JsonSerializer.IsReflectionEnabledByDefault)
+        {
+            Assert.Equal("\"A\"", JsonSerializer.Serialize(EnumWithoutAnnotation.A, options));
+            Assert.Equal("\"A\"", JsonSerializer.Serialize(EnumWithAnnotation.A, options));
+        }
+        else
+        {
+            options = new(options) { TypeInfoResolver = new DefaultJsonTypeInfoResolver() };
+            Assert.Equal("1", JsonSerializer.Serialize(EnumWithoutAnnotation.A, options));
+            Assert.Equal("\"A\"", JsonSerializer.Serialize(EnumWithAnnotation.A, options));
+        }
+    }
+
+    public enum EnumWithoutAnnotation { A = 1, B = 2, C = 3 }
+
+    [JsonConverter(typeof(JsonStringEnumConverter<EnumWithAnnotation>))]
+    public enum EnumWithAnnotation { A = 1, B = 2, C = 3 }
 }