Add Extensions support to ClientCapabilities and ServerCapabilities (#1317)

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: stephentoub <2642209+stephentoub@users.noreply.github.com>

src/Common/Experimentals.cs

@@ -35,4 +35,25 @@ internal static class Experimentals
     /// URL for the experimental MCP Tasks feature.
     /// </summary>
     public const string Tasks_Url = "https://github.com/modelcontextprotocol/csharp-sdk/blob/main/docs/list-of-diagnostics.md#mcpexp001";
+
+    /// <summary>
+    /// Diagnostic ID for the experimental MCP Extensions feature.
+    /// </summary>
+    /// <remarks>
+    /// This uses the same diagnostic ID as <see cref="Tasks_DiagnosticId"/> because both
+    /// Tasks and Extensions are covered by the same MCPEXP001 diagnostic for experimental
+    /// MCP features. Having separate constants improves code clarity while maintaining a
+    /// single diagnostic suppression point.
+    /// </remarks>
+    public const string Extensions_DiagnosticId = "MCPEXP001";
+
+    /// <summary>
+    /// Message for the experimental MCP Extensions feature.
+    /// </summary>
+    public const string Extensions_Message = "The Extensions feature is part of a future MCP specification version that has not yet been ratified and is subject to change.";
+
+    /// <summary>
+    /// URL for the experimental MCP Extensions feature.
+    /// </summary>
+    public const string Extensions_Url = "https://github.com/modelcontextprotocol/csharp-sdk/blob/main/docs/list-of-diagnostics.md#mcpexp001";
 }

src/ModelContextProtocol.Core/Protocol/ClientCapabilities.cs

@@ -93,4 +93,31 @@ public sealed class ClientCapabilities
     [JsonInclude]
     [JsonPropertyName("tasks")]
     internal McpTasksCapability? TasksCore { 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. Clients advertise extension support via this field during the initialization handshake.
+    /// </para>
+    /// </remarks>
+    [Experimental(Experimentals.Extensions_DiagnosticId, UrlFormat = Experimentals.Extensions_Url)]
+    [JsonIgnore]
+    public IDictionary<string, object>? Extensions
+    {
+        get => ExtensionsCore;
+        set => ExtensionsCore = value;
+    }
+
+    // See ExperimentalInternalPropertyTests.cs before modifying this property.
+    [JsonInclude]
+    [JsonPropertyName("extensions")]
+    internal IDictionary<string, object>? ExtensionsCore { get; set; }
 }

src/ModelContextProtocol.Core/Protocol/ServerCapabilities.cs

@@ -92,4 +92,31 @@ public sealed class ServerCapabilities
     [JsonInclude]
     [JsonPropertyName("tasks")]
     internal McpTasksCapability? TasksCore { 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. Servers advertise extension support via this field during the initialization handshake.
+    /// </para>
+    /// </remarks>
+    [Experimental(Experimentals.Extensions_DiagnosticId, UrlFormat = Experimentals.Extensions_Url)]
+    [JsonIgnore]
+    public IDictionary<string, object>? Extensions
+    {
+        get => ExtensionsCore;
+        set => ExtensionsCore = value;
+    }
+
+    // See ExperimentalInternalPropertyTests.cs before modifying this property.
+    [JsonInclude]
+    [JsonPropertyName("extensions")]
+    internal IDictionary<string, object>? ExtensionsCore { get; set; }
 }