Seal public Protocol reference types to prevent external inheritance

Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

Author: Copilot

src/ModelContextProtocol.Core/Protocol/ContentBlock.cs

@@ -69,7 +69,7 @@ private protected ContentBlock()
     /// Provides a polymorphic converter for the <see cref="ContentBlock"/> class that doesn't  require
     /// setting <see cref="JsonSerializerOptions.AllowOutOfOrderMetadataProperties"/> explicitly.
     [EditorBrowsable(EditorBrowsableState.Never)]
-    public class Converter : JsonConverter<ContentBlock>
+    public sealed class Converter : JsonConverter<ContentBlock>
     {
         /// <inheritdoc/>
         public override ContentBlock? Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)

src/ModelContextProtocol.Core/Protocol/ElicitRequestParams.cs

@@ -103,7 +103,7 @@ public string Mode
     public McpTaskMetadata? Task { get; set; }
 
     /// <summary>Represents a request schema used in a form mode elicitation request.</summary>
-    public class RequestSchema
+    public sealed class RequestSchema
     {
         /// <summary>Gets the type of the schema.</summary>
         /// <remarks>This value is always "object".</remarks>
@@ -161,7 +161,7 @@ protected private PrimitiveSchemaDefinition()
         /// Provides a polymorphic converter for the <see cref="PrimitiveSchemaDefinition"/> class that doesn't require
         /// setting <see cref="JsonSerializerOptions.AllowOutOfOrderMetadataProperties"/> explicitly.
         [EditorBrowsable(EditorBrowsableState.Never)]
-        public class Converter : JsonConverter<PrimitiveSchemaDefinition>
+        public sealed class Converter : JsonConverter<PrimitiveSchemaDefinition>
         {
             /// <inheritdoc/>
             public override PrimitiveSchemaDefinition? Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
@@ -647,6 +647,25 @@ public override void Write(Utf8JsonWriter writer, PrimitiveSchemaDefinition valu
                         }
                         break;
 
+#pragma warning disable MCP9001 // EnumSchema is deprecated but supported for backward compatibility
+                    case EnumSchema enumSchema:
+#pragma warning restore MCP9001
+                        if (enumSchema.Enum is not null)
+                        {
+                            writer.WritePropertyName("enum");
+                            JsonSerializer.Serialize(writer, enumSchema.Enum, McpJsonUtilities.JsonContext.Default.IListString);
+                        }
+                        if (enumSchema.EnumNames is not null)
+                        {
+                            writer.WritePropertyName("enumNames");
+                            JsonSerializer.Serialize(writer, enumSchema.EnumNames, McpJsonUtilities.JsonContext.Default.IListString);
+                        }
+                        if (enumSchema.Default is not null)
+                        {
+                            writer.WriteString("default", enumSchema.Default);
+                        }
+                        break;
+
                     default:
                         throw new JsonException($"Unexpected schema type: {value.GetType().Name}");
                 }
@@ -1002,8 +1021,42 @@ public override string Type
     /// This schema is deprecated in favor of <see cref="TitledSingleSelectEnumSchema"/>.
     /// </remarks>
     [Obsolete(Obsoletions.LegacyTitledEnumSchema_Message, DiagnosticId = Obsoletions.LegacyTitledEnumSchema_DiagnosticId, UrlFormat = Obsoletions.LegacyTitledEnumSchema_Url)]
-    public sealed class EnumSchema : LegacyTitledEnumSchema
+    public sealed class EnumSchema : PrimitiveSchemaDefinition
     {
+        /// <inheritdoc/>
+        [JsonPropertyName("type")]
+        public override string Type
+        {
+            get => "string";
+            set
+            {
+                if (value is not "string")
+                {
+                    throw new ArgumentException("Type must be 'string'.", nameof(value));
+                }
+            }
+        }
+
+        /// <summary>Gets or sets the list of allowed string values for the enum.</summary>
+        [JsonPropertyName("enum")]
+        [field: MaybeNull]
+        public IList<string> Enum
+        {
+            get => field ??= [];
+            set
+            {
+                Throw.IfNull(value);
+                field = value;
+            }
+        }
+
+        /// <summary>Gets or sets optional display names corresponding to the enum values.</summary>
+        [JsonPropertyName("enumNames")]
+        public IList<string>? EnumNames { get; set; }
+
+        /// <summary>Gets or sets the default value for the enum.</summary>
+        [JsonPropertyName("default")]
+        public string? Default { get; set; }
     }
 
     /// <summary>
@@ -1013,7 +1066,7 @@ public sealed class EnumSchema : LegacyTitledEnumSchema
     /// This schema is deprecated in favor of <see cref="TitledSingleSelectEnumSchema"/>.
     /// </remarks>
     [Obsolete(Obsoletions.LegacyTitledEnumSchema_Message, DiagnosticId = Obsoletions.LegacyTitledEnumSchema_DiagnosticId, UrlFormat = Obsoletions.LegacyTitledEnumSchema_Url)]
-    public class LegacyTitledEnumSchema : PrimitiveSchemaDefinition
+    public sealed class LegacyTitledEnumSchema : PrimitiveSchemaDefinition
     {
         /// <inheritdoc/>
         [JsonPropertyName("type")]

src/ModelContextProtocol.Core/Protocol/JsonRpcMessageContext.cs

@@ -12,7 +12,7 @@ namespace ModelContextProtocol.Protocol;
 /// but is not serialized as part of the JSON-RPC payload. This includes transport references, execution context,
 /// and authenticated user information.
 /// </remarks>
-public class JsonRpcMessageContext
+public sealed class JsonRpcMessageContext
 {
     /// <summary>
     /// Gets or sets the transport the <see cref="JsonRpcMessage"/> was received on or should be sent over.

src/ModelContextProtocol.Core/Protocol/ResourceContents.cs

@@ -61,7 +61,7 @@ private protected ResourceContents()
     /// Provides a <see cref="JsonConverter"/> for <see cref="ResourceContents"/>.
     /// </summary>
     [EditorBrowsable(EditorBrowsableState.Never)]
-    public class Converter : JsonConverter<ResourceContents>
+    public sealed class Converter : JsonConverter<ResourceContents>
     {
         /// <inheritdoc/>
         public override ResourceContents? Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)