Use an explicit reflection-based converter for enums.

Author: eiriktsarpalis

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;
@@ -34,16 +35,17 @@ 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.")]
     private static JsonSerializerOptions CreateDefaultOptions()
     {
         // Copy the configuration from the source generated context.
         JsonSerializerOptions options = new(JsonContext.Default.Options);
 
         // Chain with all supported types and converters from MEAI
         options.TypeInfoResolverChain.Add(AIJsonUtilities.DefaultOptions.TypeInfoResolver!);
-        foreach (JsonConverter converter in AIJsonUtilities.DefaultOptions.Converters)
+        if (JsonSerializer.IsReflectionEnabledByDefault)
         {
-            options.Converters.Add(converter);
+            options.Converters.Add(new UserDefinedJsonStringEnumConverter());
         }
 
         options.MakeReadOnly();
@@ -104,6 +106,18 @@ internal static bool IsValidMcpToolSchema(JsonElement element)
         return AIJsonUtilities.CreateJsonSchema(returnType, serializerOptions: function.JsonSerializerOptions, inferenceOptions: schemaCreateOptions);
     }
 
+
+    // Defines a JsonStringEnumConverter that only applies to enums outside of the current assembly.
+    // This is to ensure that we're not overriding the CustomizableJsonStringEnumConverter that our
+    // built-in enums are relying on.
+    [RequiresDynamicCode("Depends on JsonStringEnumConverter which requires dynamic code.")]
+    private sealed class UserDefinedJsonStringEnumConverter : JsonConverterFactory
+    {
+        private readonly JsonStringEnumConverter _converter = new();
+        public override bool CanConvert(Type typeToConvert) => _converter.CanConvert(typeToConvert) && typeToConvert.Assembly != Assembly.GetExecutingAssembly();
+        public override JsonConverter? CreateConverter(Type typeToConvert, JsonSerializerOptions options) => _converter.CreateConverter(typeToConvert, options);
+    }
+
     // Keep in sync with CreateDefaultOptions above.
     [JsonSourceGenerationOptions(JsonSerializerDefaults.Web,
         DefaultIgnoreCondition = JsonIgnoreCondition.WhenWritingNull,