Add MCP Extensions support to ClientCapabilities and ServerCapabilities

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

Author: Copilot

docs/list-of-diagnostics.md

@@ -23,4 +23,4 @@ If you use experimental APIs, you will get one of the diagnostics shown below. T
 
 | Diagnostic ID | Description |
 | :------------ | :---------- |
-| `MCPEXP001` | MCP task-related APIs are experimental. Tasks provide a mechanism for asynchronous long-running operations that can be polled for status and results. See [MCP Tasks specification](https://modelcontextprotocol.io/specification/draft/basic/utilities/tasks) for details. |
+| `MCPEXP001` | MCP experimental APIs including Tasks and Extensions. Tasks provide a mechanism for asynchronous long-running operations that can be polled for status and results (see [MCP Tasks specification](https://modelcontextprotocol.io/specification/draft/basic/utilities/tasks)). Extensions provide a framework for extending the Model Context Protocol while maintaining interoperability (see [SEP-2133](https://github.com/modelcontextprotocol/modelcontextprotocol/pull/2133)). |

src/ModelContextProtocol.Core/Protocol/ClientCapabilities.cs

@@ -1,4 +1,5 @@
 using System.ComponentModel;
+using System.Diagnostics.CodeAnalysis;
 using System.Text.Json.Serialization;
 using ModelContextProtocol.Client;
 using ModelContextProtocol.Server;
@@ -82,4 +83,23 @@ public sealed class ClientCapabilities
     /// </remarks>
     [JsonPropertyName("tasks")]
     public McpTasksCapability? Tasks { get; set; }
+
+    /// <summary>
+    /// Gets or sets optional MCP extensions that the client supports.
+    /// </summary>
+    /// <remarks>
+    /// <para>
+    /// Keys are extension identifiers in reverse domain notation with an extension name
+    /// (e.g., <c>"io.modelcontextprotocol/oauth-client-credentials"</c>), and values are
+    /// per-extension settings objects. An empty object indicates support with no additional settings.
+    /// </para>
+    /// <para>
+    /// Extensions provide a framework for extending the Model Context Protocol while maintaining
+    /// interoperability. Both clients and servers advertise extension support via this field during
+    /// the initialization handshake.
+    /// </para>
+    /// </remarks>
+    [JsonPropertyName("extensions")]
+    [Experimental(Experimentals.Tasks_DiagnosticId, UrlFormat = Experimentals.Tasks_Url)] // SEP-2133: https://github.com/modelcontextprotocol/modelcontextprotocol/pull/2133
+    public IDictionary<string, object>? Extensions { get; set; }
 }

src/ModelContextProtocol.Core/Protocol/ServerCapabilities.cs

@@ -1,4 +1,5 @@
 using System.ComponentModel;
+using System.Diagnostics.CodeAnalysis;
 using System.Text.Json.Serialization;
 using ModelContextProtocol.Server;
 
@@ -81,4 +82,23 @@ public sealed class ServerCapabilities
     /// </remarks>
     [JsonPropertyName("tasks")]
     public McpTasksCapability? Tasks { get; set; }
+
+    /// <summary>
+    /// Gets or sets optional MCP extensions that the server supports.
+    /// </summary>
+    /// <remarks>
+    /// <para>
+    /// Keys are extension identifiers in reverse domain notation with an extension name
+    /// (e.g., <c>"io.modelcontextprotocol/apps"</c>), and values are per-extension settings
+    /// objects. An empty object indicates support with no additional settings.
+    /// </para>
+    /// <para>
+    /// Extensions provide a framework for extending the Model Context Protocol while maintaining
+    /// interoperability. Both clients and servers advertise extension support via this field during
+    /// the initialization handshake.
+    /// </para>
+    /// </remarks>
+    [JsonPropertyName("extensions")]
+    [Experimental(Experimentals.Tasks_DiagnosticId, UrlFormat = Experimentals.Tasks_Url)] // SEP-2133: https://github.com/modelcontextprotocol/modelcontextprotocol/pull/2133
+    public IDictionary<string, object>? Extensions { get; set; }
 }