Implement SEP-2663 Tasks Extension (#1579)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/ModelContextProtocol.Core/Protocol/CreateTaskResult.cs

@@ -1,28 +1,68 @@
-using System.Diagnostics.CodeAnalysis;
+using System.Text.Json;
+using System.Text.Json.Nodes;
 using System.Text.Json.Serialization;
 
 namespace ModelContextProtocol.Protocol;
 
 /// <summary>
-/// Represents the response to a task-augmented request.
+/// Represents the result returned by a server when it creates a task in lieu of a standard result.
 /// </summary>
 /// <remarks>
 /// <para>
-/// When a client sends a request with a <c>task</c> parameter, the server immediately returns
-/// a <see cref="CreateTaskResult"/> containing the created task information instead of the
-/// normal result type. The actual result can be retrieved later via <c>tasks/result</c>.
+/// A server returns <see cref="CreateTaskResult"/> instead of the standard result shape (e.g., <see cref="CallToolResult"/>)
+/// to indicate that the request will be processed asynchronously. The client then uses
+/// <see cref="TaskId"/> for subsequent <c>tasks/get</c>, <c>tasks/update</c>, and <c>tasks/cancel</c> calls.
 /// </para>
 /// <para>
-/// This type is returned for any task-augmented request including <c>tools/call</c>,
-/// <c>sampling/createMessage</c>, and <c>elicitation/create</c>.
+/// A server must not return <see cref="CreateTaskResult"/> to a client that did not include the
+/// <c>io.modelcontextprotocol/tasks</c> extension capability on its request.
+/// </para>
+/// <para>
+/// See the <see href="https://github.com/modelcontextprotocol/modelcontextprotocol/blob/main/seps/2663-tasks-extension.md">SEP-2663</see>
+/// specification for details.
 /// </para>
 /// </remarks>
-[Experimental(Experimentals.Tasks_DiagnosticId, UrlFormat = Experimentals.Tasks_Url)]
 public sealed class CreateTaskResult : Result
 {
     /// <summary>
-    /// Gets or sets the task data for the newly created task.
+    /// Gets or sets the stable identifier for this task.
     /// </summary>
-    [JsonPropertyName("task")]
-    public McpTask Task { get; set; } = null!;
+    [JsonPropertyName("taskId")]
+    public required string TaskId { get; set; }
+
+    /// <summary>
+    /// Gets or sets the current task status.
+    /// </summary>
+    [JsonPropertyName("status")]
+    public required McpTaskStatus Status { get; set; }
+
+    /// <summary>
+    /// Gets or sets an optional message describing the current task state.
+    /// </summary>
+    [JsonPropertyName("statusMessage")]
+    public string? StatusMessage { get; set; }
+
+    /// <summary>
+    /// Gets or sets the ISO 8601 timestamp when the task was created.
+    /// </summary>
+    [JsonPropertyName("createdAt")]
+    public required DateTimeOffset CreatedAt { get; set; }
+
+    /// <summary>
+    /// Gets or sets the ISO 8601 timestamp when the task was last updated.
+    /// </summary>
+    [JsonPropertyName("lastUpdatedAt")]
+    public required DateTimeOffset LastUpdatedAt { get; set; }
+
+    /// <summary>
+    /// Gets or sets the time-to-live duration from creation in milliseconds, or <see langword="null"/> for unlimited.
+    /// </summary>
+    [JsonPropertyName("ttlMs")]
+    public long? TtlMs { get; set; }
+
+    /// <summary>
+    /// Gets or sets the suggested polling interval in milliseconds.
+    /// </summary>
+    [JsonPropertyName("pollIntervalMs")]
+    public long? PollIntervalMs { get; set; }
 }