Move Tasks feature into ModelContextProtocol.Extensions.Tasks

- Add TaskMethods/TaskMetaKeys consts (tasks/get|update|cancel, notification, related-task)
- Add McpServerTasksExtensions: WithTasks(McpServerOptions/IMcpServerBuilder), SendTaskStatusNotificationAsync, store-backed tasks/* raw handlers gated to draft, and a tools/call wrapper that runs tasks in the background via the outgoing-request interceptor seam
- Add McpClientTasksExtensions: CallToolAsTaskAsync, CallToolRawAsync, GetTaskAsync/UpdateTaskAsync/CancelTaskAsync, PollTaskToCompletionAsync
- Add JsonNode to TasksJsonContext; replace internal Throw with inline null checks in ResultOrCreatedTask
- Include only the attribute polyfills the Tasks DTOs need (required/init) on netstandard2.0
- Relax Result/RequestParams/NotificationParams base ctors to protected so the moved DTOs can derive across the assembly boundary
- Register the new project in ModelContextProtocol.slnx

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

Author: jeffhandley

ModelContextProtocol.slnx

@@ -69,6 +69,7 @@
     <Project Path="src/ModelContextProtocol.AspNetCore/ModelContextProtocol.AspNetCore.csproj" />
     <Project Path="src/ModelContextProtocol.Core/ModelContextProtocol.Core.csproj" />
     <Project Path="src/ModelContextProtocol.Extensions.Apps/ModelContextProtocol.Extensions.Apps.csproj" />
+    <Project Path="src/ModelContextProtocol.Extensions.Tasks/ModelContextProtocol.Extensions.Tasks.csproj" />
     <Project Path="src/ModelContextProtocol/ModelContextProtocol.csproj" />
   </Folder>
   <Folder Name="/tests/">

src/ModelContextProtocol.Core/Protocol/NotificationParams.cs

@@ -8,8 +8,8 @@ namespace ModelContextProtocol.Protocol;
 /// </summary>
 public abstract class NotificationParams
 {
-    /// <summary>Prevent external derivations.</summary>
-    private protected NotificationParams()
+    /// <summary>Allow derivations only within the SDK and its extension packages.</summary>
+    protected NotificationParams()
     {
     }
 

src/ModelContextProtocol.Core/Protocol/RequestParams.cs

@@ -11,8 +11,8 @@ namespace ModelContextProtocol.Protocol;
 /// </remarks>
 public abstract class RequestParams
 {
-    /// <summary>Prevent external derivations.</summary>
-    private protected RequestParams()
+    /// <summary>Allow derivations only within the SDK and its extension packages.</summary>
+    protected RequestParams()
     {
     }
 

src/ModelContextProtocol.Core/Protocol/Result.cs

@@ -8,8 +8,8 @@ namespace ModelContextProtocol.Protocol;
 /// </summary>
 public abstract class Result
 {
-    /// <summary>Prevent external derivations.</summary>
-    private protected Result()
+    /// <summary>Allow derivations only within the SDK and its extension packages.</summary>
+    protected Result()
     {
     }
 

src/ModelContextProtocol.Extensions.Tasks/Client/McpClientTasksExtensions.cs

@@ -26,11 +26,21 @@
     <Compile Include="..\Common\Experimentals.cs" Link="Experimentals.cs" />
   </ItemGroup>
 
-  <!-- Exclude polyfills inherited from Directory.Build.props; this package only needs the ExperimentalAttribute polyfill. -->
+  <!-- Exclude polyfills inherited from Directory.Build.props; include only the attribute polyfills these DTOs need. -->
   <ItemGroup>
     <Compile Remove="..\Common\Polyfills\**\*.cs" />
     <Compile Include="..\Common\Polyfills\System\Diagnostics\CodeAnalysis\ExperimentalAttribute.cs"
              Link="Polyfills\ExperimentalAttribute.cs" />
+    <Compile Include="..\Common\Polyfills\System\Diagnostics\CodeAnalysis\NullableAttributes.cs"
+             Link="Polyfills\NullableAttributes.cs" />
+    <Compile Include="..\Common\Polyfills\System\Diagnostics\CodeAnalysis\SetsRequiredMembersAttribute.cs"
+             Link="Polyfills\SetsRequiredMembersAttribute.cs" />
+    <Compile Include="..\Common\Polyfills\System\Runtime\CompilerServices\CompilerFeatureRequiredAttribute.cs"
+             Link="Polyfills\CompilerFeatureRequiredAttribute.cs" />
+    <Compile Include="..\Common\Polyfills\System\Runtime\CompilerServices\RequiredMemberAttribute.cs"
+             Link="Polyfills\RequiredMemberAttribute.cs" />
+    <Compile Include="..\Common\Polyfills\System\Runtime\CompilerServices\IsExternalInit.cs"
+             Link="Polyfills\IsExternalInit.cs" />
   </ItemGroup>
 
   <ItemGroup>

src/ModelContextProtocol.Extensions.Tasks/ModelContextProtocol.Extensions.Tasks.csproj

@@ -31,8 +31,7 @@ public class ResultOrCreatedTask<TResult> where TResult : Result
     /// <param name="result">The standard result returned by the server.</param>
     public ResultOrCreatedTask(TResult result)
     {
-        Throw.IfNull(result);
-        _result = result;
+        _result = result ?? throw new ArgumentNullException(nameof(result));
     }
 
     /// <summary>
@@ -41,8 +40,7 @@ public ResultOrCreatedTask(TResult result)
     /// <param name="taskCreated">The task creation result returned by the server.</param>
     public ResultOrCreatedTask(CreateTaskResult taskCreated)
     {
-        Throw.IfNull(taskCreated);
-        _taskCreated = taskCreated;
+        _taskCreated = taskCreated ?? throw new ArgumentNullException(nameof(taskCreated));
     }
 
     /// <summary>

src/ModelContextProtocol.Extensions.Tasks/Protocol/ResultOrCreatedTask.cs

@@ -36,6 +36,7 @@ namespace ModelContextProtocol.Extensions.Tasks;
 [JsonSerializable(typeof(IDictionary<string, InputRequest>))]
 [JsonSerializable(typeof(IDictionary<string, InputResponse>))]
 [JsonSerializable(typeof(JsonObject))]
+[JsonSerializable(typeof(JsonNode))]
 [JsonSerializable(typeof(JsonElement))]
 [JsonSerializable(typeof(JsonRpcErrorDetail))]
 internal sealed partial class TasksJsonContext : JsonSerializerContext

src/ModelContextProtocol.Extensions.Tasks/Server/McpServerTasksExtensions.cs

@@ -0,0 +1,28 @@
+namespace ModelContextProtocol.Extensions.Tasks;
+
+/// <summary>
+/// Provides constants for the JSON-RPC methods defined by the MCP Tasks extension (SEP-2663).
+/// </summary>
+public static class TaskMethods
+{
+    /// <summary>The <c>tasks/get</c> request method, used to retrieve the current state of a task.</summary>
+    public const string Get = "tasks/get";
+
+    /// <summary>The <c>tasks/update</c> request method, used to provide input responses to a task.</summary>
+    public const string Update = "tasks/update";
+
+    /// <summary>The <c>tasks/cancel</c> request method, used to request cancellation of a task.</summary>
+    public const string Cancel = "tasks/cancel";
+
+    /// <summary>The <c>notifications/tasks/status</c> notification method, used to push task status updates.</summary>
+    public const string StatusNotification = "notifications/tasks/status";
+}
+
+/// <summary>
+/// Provides constants for the <c>_meta</c> keys defined by the MCP Tasks extension (SEP-2663).
+/// </summary>
+public static class TaskMetaKeys
+{
+    /// <summary>The <c>_meta</c> key carrying the related task identifier.</summary>
+    public const string RelatedTask = "io.modelcontextprotocol/related-task";
+}