Rename MCPEXP002 Subclassing_* constants to Extensibility_* (#1739)

Co-authored-by: arimu1 <19286898+arimu1@users.noreply.github.com>

src/Common/Experimentals.cs

@@ -13,9 +13,10 @@ namespace ModelContextProtocol;
 /// These APIs may change as the specification evolves.
 /// </description></item>
 /// <item><description>
-/// <c>MCPEXP002</c> covers experimental SDK APIs that are unrelated to the MCP specification,
-/// such as subclassing internal types or SDK-specific extensibility hooks. These APIs may
-/// change or be removed based on SDK design feedback.
+/// <c>MCPEXP002</c> covers SDK extensibility APIs that enable features to be implemented
+/// in standalone packages without requiring Core to understand those features. The Tasks
+/// package is one such consumer. These APIs remain experimental until additional
+/// extensibility scenarios validate the design.
 /// </description></item>
 /// </list>
 /// <para>
@@ -65,31 +66,31 @@ internal static class Experimentals
     public const string Apps_Url = "https://github.com/modelcontextprotocol/csharp-sdk/blob/main/docs/list-of-diagnostics.md#mcpexp003";
 
     /// <summary>
-    /// Diagnostic ID for experimental SDK APIs unrelated to the MCP specification,
-    /// such as subclassing <c>McpClient</c>/<c>McpServer</c> or referencing <c>RunSessionHandler</c>.
+    /// Diagnostic ID for SDK extensibility APIs that enable independently packaged features,
+    /// such as Tasks, without requiring Core awareness of those features.
     /// </summary>
     /// <remarks>
     /// This diagnostic ID covers experimental SDK-level extensibility APIs. All constants
     /// in this group share the same diagnostic ID so users need only one suppression point
     /// for SDK design preview features.
     /// </remarks>
-    public const string Subclassing_DiagnosticId = "MCPEXP002";
+    public const string Extensibility_DiagnosticId = "MCPEXP002";
 
     /// <summary>
-    /// Message for experimental subclassing of McpClient and McpServer.
+    /// Message for experimental extensibility points in the C# SDK implementation.
     /// </summary>
-    public const string Subclassing_Message = "Subclassing McpClient and McpServer is experimental and subject to change.";
+    public const string Extensibility_Message = "This C# SDK extensibility API is experimental and subject to change.";
 
     /// <summary>
-    /// URL for experimental subclassing of McpClient and McpServer.
+    /// URL for experimental extensibility points in the C# SDK implementation.
     /// </summary>
-    public const string Subclassing_Url = "https://github.com/modelcontextprotocol/csharp-sdk/blob/main/docs/list-of-diagnostics.md#mcpexp002";
+    public const string Extensibility_Url = "https://github.com/modelcontextprotocol/csharp-sdk/blob/main/docs/list-of-diagnostics.md#mcpexp002";
 
     /// <summary>
     /// Diagnostic ID for the experimental <c>RunSessionHandler</c> API.
     /// </summary>
     /// <remarks>
-    /// This uses the same diagnostic ID as <see cref="Subclassing_DiagnosticId"/> because
+    /// This uses the same diagnostic ID as <see cref="Extensibility_DiagnosticId"/> because
     /// both are experimental SDK APIs unrelated to the MCP specification.
     /// </remarks>
     public const string RunSessionHandler_DiagnosticId = "MCPEXP002";

src/ModelContextProtocol.Core/Client/McpClient.cs

@@ -12,7 +12,7 @@ public abstract partial class McpClient : McpSession
     /// <summary>
     /// Initializes a new instance of the <see cref="McpClient"/> class.
     /// </summary>
-    [Experimental(Experimentals.Subclassing_DiagnosticId, UrlFormat = Experimentals.Subclassing_Url)]
+    [Experimental(Experimentals.Extensibility_DiagnosticId, UrlFormat = Experimentals.Extensibility_Url)]
     protected McpClient()
     {
     }

src/ModelContextProtocol.Core/Protocol/JsonRpcMessageContext.cs

@@ -83,7 +83,7 @@ public sealed class JsonRpcMessageContext
     /// Streamable HTTP transports emit this value in the <c>Mcp-Name</c> header. This enables
     /// extension methods to identify the named resource targeted by a request.
     /// </remarks>
-    [Experimental(Experimentals.Subclassing_DiagnosticId, UrlFormat = Experimentals.Subclassing_Url)]
+    [Experimental(Experimentals.Extensibility_DiagnosticId, UrlFormat = Experimentals.Extensibility_Url)]
     [JsonIgnore]
     public string? RoutingName { get; set; }
 

src/ModelContextProtocol.Core/Protocol/ResultOrAlternate.cs

@@ -20,7 +20,7 @@ namespace ModelContextProtocol.Protocol;
 /// <see cref="Result"/> for the immediate result or <see cref="Alternate"/> for the alternate.
 /// </para>
 /// </remarks>
-[Experimental(Experimentals.Subclassing_DiagnosticId, UrlFormat = Experimentals.Subclassing_Url)]
+[Experimental(Experimentals.Extensibility_DiagnosticId, UrlFormat = Experimentals.Extensibility_Url)]
 public class ResultOrAlternate<TResult> where TResult : Result
 {
     private readonly TResult? _result;

src/ModelContextProtocol.Core/Server/McpRequestFilters.cs

@@ -84,7 +84,7 @@ public sealed class McpRequestFilters
     /// asynchronously, filters registered after it execute as part of that asynchronous operation.
     /// </para>
     /// </remarks>
-    [Experimental(Experimentals.Subclassing_DiagnosticId, UrlFormat = Experimentals.Subclassing_Url)]
+    [Experimental(Experimentals.Extensibility_DiagnosticId, UrlFormat = Experimentals.Extensibility_Url)]
     public IList<McpRequestInvocationFilter<CallToolRequestParams, ResultOrAlternate<CallToolResult>>> CallToolWithAlternateFilters
     {
         get => field ??= [];

src/ModelContextProtocol.Core/Server/McpRequestInvocationFilter.cs

@@ -11,7 +11,7 @@ namespace ModelContextProtocol.Server;
 /// <param name="next">The next request handler in the pipeline for this invocation.</param>
 /// <param name="cancellationToken">The cancellation token for the current request.</param>
 /// <returns>The result of the filtered request invocation.</returns>
-[Experimental(Experimentals.Subclassing_DiagnosticId, UrlFormat = Experimentals.Subclassing_Url)]
+[Experimental(Experimentals.Extensibility_DiagnosticId, UrlFormat = Experimentals.Extensibility_Url)]
 public delegate ValueTask<TResult> McpRequestInvocationFilter<TParams, TResult>(
     RequestContext<TParams> context,
     McpRequestHandler<TParams, TResult> next,

src/ModelContextProtocol.Core/Server/McpServer.Methods.cs

@@ -39,7 +39,7 @@ public abstract partial class McpServer : McpSession
     /// On the returned facade, redirected methods skip their client-capability checks,
     /// because the alternate channel is responsible for delivering the request to the client.
     /// </remarks>
-    [Experimental(Experimentals.Subclassing_DiagnosticId, UrlFormat = Experimentals.Subclassing_Url)]
+    [Experimental(Experimentals.Extensibility_DiagnosticId, UrlFormat = Experimentals.Extensibility_Url)]
     public McpServer WithOutgoingRequestInterceptor(Func<string, JsonNode?, CancellationToken, ValueTask<JsonNode?>> interceptor)
     {
         Throw.IfNull(interceptor);

src/ModelContextProtocol.Core/Server/McpServer.cs

@@ -11,7 +11,7 @@ public abstract partial class McpServer : McpSession
     /// <summary>
     /// Initializes a new instance of the <see cref="McpServer"/> class.
     /// </summary>
-    [Experimental(Experimentals.Subclassing_DiagnosticId, UrlFormat = Experimentals.Subclassing_Url)]
+    [Experimental(Experimentals.Extensibility_DiagnosticId, UrlFormat = Experimentals.Extensibility_Url)]
     protected McpServer()
     {
     }

src/ModelContextProtocol.Core/Server/McpServerHandlers.cs

@@ -78,7 +78,7 @@ public sealed class McpServerHandlers
     /// </para>
     /// </remarks>
     /// <exception cref="InvalidOperationException"><see cref="CallToolHandler"/> is already set.</exception>
-    [Experimental(Experimentals.Subclassing_DiagnosticId, UrlFormat = Experimentals.Subclassing_Url)]
+    [Experimental(Experimentals.Extensibility_DiagnosticId, UrlFormat = Experimentals.Extensibility_Url)]
     public McpRequestHandler<CallToolRequestParams, ResultOrAlternate<CallToolResult>>? CallToolWithAlternateHandler
     {
         get;

src/ModelContextProtocol.Core/Server/McpServerOptions.cs

@@ -217,6 +217,6 @@ public sealed class McpServerOptions
     /// Handlers registered here take precedence over built-in handlers for the same method.
     /// </para>
     /// </remarks>
-    [Experimental(Experimentals.Subclassing_DiagnosticId, UrlFormat = Experimentals.Subclassing_Url)]
+    [Experimental(Experimentals.Extensibility_DiagnosticId, UrlFormat = Experimentals.Extensibility_Url)]
     public IList<McpServerRequestHandler>? RequestHandlers { get; set; }
 }

src/ModelContextProtocol.Core/Server/McpServerRequestHandler.cs

@@ -16,7 +16,7 @@ namespace ModelContextProtocol.Server;
 /// <see cref="JsonNode"/> response, giving extensions full control over request/response serialization.
 /// </para>
 /// </remarks>
-[Experimental(Experimentals.Subclassing_DiagnosticId, UrlFormat = Experimentals.Subclassing_Url)]
+[Experimental(Experimentals.Extensibility_DiagnosticId, UrlFormat = Experimentals.Extensibility_Url)]
 public sealed class McpServerRequestHandler
 {
     /// <summary>
@@ -32,7 +32,7 @@ public sealed class McpServerRequestHandler
     /// When set, Streamable HTTP servers require the request to include an <c>Mcp-Name</c>
     /// header whose decoded value matches the string value of this parameter.
     /// </remarks>
-    [Experimental(Experimentals.Subclassing_DiagnosticId, UrlFormat = Experimentals.Subclassing_Url)]
+    [Experimental(Experimentals.Extensibility_DiagnosticId, UrlFormat = Experimentals.Extensibility_Url)]
     public string? RoutingNameParameter { get; init; }
 
     /// <summary>