Default draft protocol support: sessionless + handshake-less (SEP-2575 + SEP-2567) (#1610)

src/ModelContextProtocol.Core/Protocol/MetaKeys.cs

@@ -0,0 +1,76 @@
+namespace ModelContextProtocol.Protocol;
+
+/// <summary>
+/// Provides constants for well-known <c>_meta</c> field keys defined by the MCP protocol and its extensions.
+/// </summary>
+public static class MetaKeys
+{
+    /// <summary>
+    /// The metadata key used to carry the MCP protocol version in a request's <c>_meta</c> field.
+    /// </summary>
+    /// <remarks>
+    /// Introduced by the draft protocol revision (SEP-2575). For HTTP transports, the value MUST
+    /// match the <c>MCP-Protocol-Version</c> header. Servers reject mismatched versions with
+    /// <see cref="McpErrorCode.UnsupportedProtocolVersion"/>.
+    /// </remarks>
+    public const string ProtocolVersion = "io.modelcontextprotocol/protocolVersion";
+
+    /// <summary>
+    /// The metadata key used to identify the client software in a request's <c>_meta</c> field.
+    /// </summary>
+    /// <remarks>
+    /// Introduced by the draft protocol revision (SEP-2575). Carries an <see cref="Protocol.Implementation"/>
+    /// describing the client; replaces the <c>clientInfo</c> previously sent only with <c>initialize</c>.
+    /// </remarks>
+    public const string ClientInfo = "io.modelcontextprotocol/clientInfo";
+
+    /// <summary>
+    /// The metadata key used to declare client capabilities in a request's <c>_meta</c> field.
+    /// </summary>
+    /// <remarks>
+    /// Introduced by the draft protocol revision (SEP-2575). Carries a <see cref="Protocol.ClientCapabilities"/>
+    /// describing what optional features the client supports for this specific request. Servers MUST NOT
+    /// infer capabilities from previous requests.
+    /// </remarks>
+    public const string ClientCapabilities = "io.modelcontextprotocol/clientCapabilities";
+
+    /// <summary>
+    /// The metadata key used to specify the desired log level for a request's resulting log notifications.
+    /// </summary>
+    /// <remarks>
+    /// Introduced by the draft protocol revision (SEP-2575). Carries a <see cref="Protocol.LoggingLevel"/>.
+    /// Replaces the legacy <see cref="RequestMethods.LoggingSetLevel"/> RPC. When absent, the server
+    /// MUST NOT send log notifications for the request.
+    /// </remarks>
+    public const string LogLevel = "io.modelcontextprotocol/logLevel";
+
+    /// <summary>
+    /// The metadata key used to associate a notification with the request ID of an active
+    /// <see cref="RequestMethods.SubscriptionsListen"/> subscription.
+    /// </summary>
+    /// <remarks>
+    /// Introduced by the draft protocol revision (SEP-2575). Allows clients to demultiplex notifications
+    /// belonging to different subscriptions on a shared channel (especially STDIO).
+    /// </remarks>
+    public const string SubscriptionId = "io.modelcontextprotocol/subscriptionId";
+
+    /// <summary>
+    /// The metadata key used to associate requests, responses, and notifications with a task.
+    /// </summary>
+    /// <remarks>
+    /// <para>
+    /// This constant defines the key <c>"io.modelcontextprotocol/related-task"</c> used in the
+    /// <c>_meta</c> field to associate messages with their originating task across the entire
+    /// request lifecycle.
+    /// </para>
+    /// <para>
+    /// For example, an elicitation that a task-augmented tool call depends on must share the
+    /// same related task ID with that tool call's task.
+    /// </para>
+    /// <para>
+    /// For <c>tasks/get</c>, <c>tasks/list</c>, and <c>tasks/cancel</c> operations, this
+    /// metadata should not be included as the taskId is already present in the message structure.
+    /// </para>
+    /// </remarks>
+    public const string RelatedTask = "io.modelcontextprotocol/related-task";
+}