Seal public Protocol reference types to prevent external inheritance (#1232)

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
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)
@@ -315,11 +315,9 @@ public class Converter : JsonConverter<PrimitiveSchemaDefinition>
                         {
                             if (enumNames is not null)
                             {
-                                // EnumSchema is deprecated but supported for backward compatibility.
-                                // Use the EnumSchema class, which is an alias for LegacyTitledEnumSchema,
-                                // to ensure backward compatibility with existing code relying on that type.
+                                // LegacyTitledEnumSchema is deprecated but supported for backward compatibility.
 #pragma warning disable MCP9001
-                                psd = new EnumSchema
+                                psd = new LegacyTitledEnumSchema
 #pragma warning restore MCP9001
                                 {
                                     Enum = enumValues,
@@ -996,24 +994,12 @@ public override string Type
 
     /// <summary>
     /// Represents a legacy schema for an enum type with enumNames.
-    /// This is a compatibility alias for <see cref="LegacyTitledEnumSchema"/>.
     /// </summary>
     /// <remarks>
     /// 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
-    {
-    }
-
-    /// <summary>
-    /// Represents a legacy schema for an enum type with enumNames.
-    /// </summary>
-    /// <remarks>
-    /// 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)

tests/ModelContextProtocol.Tests/Protocol/EnumSchemaTests.cs

@@ -298,7 +298,7 @@ public void LegacyTitledEnumSchema_Serializes_Correctly()
 
         // Assert
         Assert.NotNull(deserialized);
-        var result = Assert.IsType<ElicitRequestParams.EnumSchema>(deserialized);
+        var result = Assert.IsType<ElicitRequestParams.LegacyTitledEnumSchema>(deserialized);
         Assert.Equal("string", result.Type);
         Assert.Equal("Environment", result.Title);
         Assert.Equal("Deployment environment", result.Description);
@@ -309,10 +309,10 @@ public void LegacyTitledEnumSchema_Serializes_Correctly()
     }
 
     [Fact]
-    public void EnumSchema_Serializes_Correctly()
+    public void LegacyTitledEnumSchema_Direct_Serializes_Correctly()
     {
         // Arrange
-        var schema = new ElicitRequestParams.EnumSchema
+        var schema = new ElicitRequestParams.LegacyTitledEnumSchema
         {
             Title = "Environment",
             Description = "Deployment environment",
@@ -327,7 +327,7 @@ public void EnumSchema_Serializes_Correctly()
 
         // Assert
         Assert.NotNull(deserialized);
-        var result = Assert.IsType<ElicitRequestParams.EnumSchema>(deserialized);
+        var result = Assert.IsType<ElicitRequestParams.LegacyTitledEnumSchema>(deserialized);
         Assert.Equal("string", result.Type);
         Assert.Equal("Environment", result.Title);
         Assert.Equal("Deployment environment", result.Description);
@@ -338,9 +338,9 @@ public void EnumSchema_Serializes_Correctly()
     }
 
     [Fact]
-    public void Enum_WithEnumNames_Deserializes_As_EnumSchema()
+    public void Enum_WithEnumNames_Deserializes_As_LegacyTitledEnumSchema()
     {
-        // Arrange - JSON with enumNames should deserialize as (deprecated) EnumSchema
+        // Arrange - JSON with enumNames should deserialize as (deprecated) LegacyTitledEnumSchema
         string json = """
             {
                 "type": "string",
@@ -356,7 +356,7 @@ public void Enum_WithEnumNames_Deserializes_As_EnumSchema()
 
         // Assert
         Assert.NotNull(deserialized);
-        var result = Assert.IsType<ElicitRequestParams.EnumSchema>(deserialized);
+        var result = Assert.IsType<ElicitRequestParams.LegacyTitledEnumSchema>(deserialized);
         Assert.Equal("string", result.Type);
         Assert.Equal("Environment", result.Title);
         Assert.Equal("Deployment environment", result.Description);

tests/ModelContextProtocol.Tests/Protocol/PrimitiveSchemaDefinitionTests.cs

@@ -283,7 +283,7 @@ public static void LegacyTitledEnumSchema_UnknownProperties_AreIgnored()
             json,
             McpJsonUtilities.DefaultOptions);
         Assert.NotNull(result);
-        var enumSchema = Assert.IsType<ElicitRequestParams.EnumSchema>(result);
+        var enumSchema = Assert.IsType<ElicitRequestParams.LegacyTitledEnumSchema>(result);
         Assert.Equal("string", enumSchema.Type);
         Assert.Equal(2, enumSchema.Enum.Count);
         Assert.Contains("option1", enumSchema.Enum);