Add initial task extension conformance coverage (#1734)

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 7c99928c-a943-49f4-9a63-8b04a0e4d18b

src/ModelContextProtocol.Core/Client/StreamableHttpClientSessionTransport.cs

@@ -592,7 +592,8 @@ internal sealed partial class StreamableHttpClientSessionTransport : TransportBa
         headers.Add(McpHttpHeaders.Method, method);
 
         // Add Mcp-Name header for methods that target a specific named resource
-        string? name = message switch
+#pragma warning disable MCPEXP002
+        string? name = message.Context?.RoutingName ?? message switch
         {
             JsonRpcRequest { Method: RequestMethods.ToolsCall or RequestMethods.PromptsGet } request
                 => GetParamsStringProperty(request.Params, "name"),
@@ -600,6 +601,7 @@ internal sealed partial class StreamableHttpClientSessionTransport : TransportBa
                 => GetParamsStringProperty(request.Params, "uri"),
             _ => null,
         };
+#pragma warning restore MCPEXP002
 
         if (name is not null)
         {

src/ModelContextProtocol.Core/Protocol/JsonRpcMessageContext.cs

@@ -1,4 +1,5 @@
 using ModelContextProtocol.Server;
+using System.Diagnostics.CodeAnalysis;
 using System.Security.Claims;
 using System.Text.Json.Serialization;
 
@@ -75,6 +76,17 @@ public sealed class JsonRpcMessageContext
     /// </remarks>
     public IDictionary<string, object?>? Items { get; set; }
 
+    /// <summary>
+    /// Gets or sets the routing name for this message.
+    /// </summary>
+    /// <remarks>
+    /// 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)]
+    [JsonIgnore]
+    public string? RoutingName { get; set; }
+
     /// <summary>
     /// Gets or sets the protocol version from the transport-level header (e.g. <c>Mcp-Protocol-Version</c>)
     /// that accompanied this JSON-RPC message.

src/ModelContextProtocol.Core/Server/McpServerImpl.cs

@@ -1011,6 +1011,12 @@ internal sealed partial class McpServerImpl : McpServer
                     $"A custom request handler registered through {nameof(McpServerOptions)}.{nameof(McpServerOptions.RequestHandlers)} has a null or empty {nameof(McpServerRequestHandler.Method)}.");
             }
 
+            if (entry.RoutingNameParameter is not null && string.IsNullOrWhiteSpace(entry.RoutingNameParameter))
+            {
+                throw new InvalidOperationException(
+                    $"A custom request handler registered through {nameof(McpServerOptions)}.{nameof(McpServerOptions.RequestHandlers)} has an empty {nameof(McpServerRequestHandler.RoutingNameParameter)}.");
+            }
+
             // Custom handlers are registered after all built-in handlers, so a method already present
             // belongs to a built-in method (e.g. initialize, tools/call) or an earlier custom handler.
             // Silently overwriting it would bypass the built-in handler's filters and protocol gating,

src/ModelContextProtocol.Core/Server/McpServerRequestHandler.cs

@@ -24,6 +24,17 @@ public sealed class McpServerRequestHandler
     /// </summary>
     public required string Method { get; init; }
 
+    /// <summary>
+    /// Gets the name of the top-level request parameter whose value is mirrored in the
+    /// <c>Mcp-Name</c> HTTP routing header.
+    /// </summary>
+    /// <remarks>
+    /// 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)]
+    public string? RoutingNameParameter { get; init; }
+
     /// <summary>
     /// Gets the handler function that processes incoming requests for the specified method.
     /// </summary>