Add Core seams for Tasks extension

- Add IMcpServerRawHandlerRegistry seam (McpRawRequestHandler delegate, registry over request handlers, IsDraftProtocolRequest) and McpServerOptions.RawRequestHandlerConfigurators
- Add McpServer.CurrentOutgoingRequestInterceptor AsyncLocal seam and McpOutgoingRequestInterceptor delegate; route SampleAsync/ElicitAsync/RequestRootsAsync through it
- Make McpSession.IsDraftProtocol public and McpClient.ResolveInputRequestsAsync public abstract (MCPEXP002)
- Remove Tasks coupling from Core: ConfigureTasks, task-store tools/call wrapper, SetTaskAugmented, CallToolWithTaskHandler/Filters, task lifecycle handlers, MaxConsecutiveStuckPolls, task method/notification/meta consts, task JsonSerializable entries
- Rewrite client CallToolAsync to a plain tools/call returning CallToolResult
- Relocate Tasks DTOs and store types into ModelContextProtocol.Extensions.Tasks; drop McpTaskExecutionContext
- Add ModelContextProtocol.Extensions.Tasks project and TasksJsonContext

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

Author: jeffhandley

src/ModelContextProtocol.Core/Client/McpClient.Methods.cs

@@ -73,7 +73,7 @@ protected McpClient()
     public abstract Task<ClientCompletionDetails> Completion { get; }
 
     /// <summary>
-    /// Resolves input requests embedded in an <see cref="InputRequiredTaskResult"/> by dispatching
+    /// Resolves input requests embedded in a task that requires input by dispatching
     /// each request to the appropriate registered handler.
     /// </summary>
     /// <param name="inputRequests">
@@ -82,16 +82,10 @@ protected McpClient()
     /// </param>
     /// <param name="cancellationToken">The <see cref="CancellationToken"/> to monitor for cancellation requests.</param>
     /// <returns>A dictionary of responses keyed by the same identifiers as the input requests.</returns>
-    private protected abstract ValueTask<IDictionary<string, InputResponse>> ResolveInputRequestsAsync(
+    [System.Diagnostics.CodeAnalysis.Experimental(Experimentals.Subclassing_DiagnosticId, UrlFormat = Experimentals.Subclassing_Url)]
+    public abstract ValueTask<IDictionary<string, InputResponse>> ResolveInputRequestsAsync(
         IDictionary<string, InputRequest> inputRequests, CancellationToken cancellationToken);
 
-    /// <summary>
-    /// Gets the maximum number of consecutive stuck-in-<see cref="McpTaskStatus.InputRequired"/> polls
-    /// allowed by <see cref="PollTaskToCompletionAsync"/> before the client cancels and throws.
-    /// Sourced from <see cref="McpClientOptions.MaxConsecutiveStuckPolls"/>.
-    /// </summary>
-    private protected abstract int MaxConsecutiveStuckPolls { get; }
-
     /// <summary>
     /// Registers one or more tool definitions in the client's tool cache, enabling the transport
     /// to send <c>Mcp-Param-*</c> headers for those tools without requiring a prior <see cref="McpClient.ListToolsAsync(RequestOptions?, CancellationToken)"/> call.

src/ModelContextProtocol.Core/Client/McpClient.cs

@@ -41,7 +41,6 @@ internal sealed partial class McpClientImpl : McpClient
     /// <param name="options">Options for the client, defining protocol version and capabilities.</param>
     /// <param name="loggerFactory">The logger factory.</param>
     internal McpClientImpl(ITransport transport, string endpointName, McpClientOptions? options, ILoggerFactory? loggerFactory)
-#pragma warning restore MCPEXP002
     {
         options ??= new();
 
@@ -178,10 +177,7 @@ private void RegisterHandlers(McpClientOptions options, NotificationHandlers not
     public override Task<ClientCompletionDetails> Completion => _sessionHandler.CompletionTask;
 
     /// <inheritdoc/>
-    private protected override int MaxConsecutiveStuckPolls => _options.MaxConsecutiveStuckPolls;
-
-    /// <inheritdoc/>
-    private protected override async ValueTask<IDictionary<string, InputResponse>> ResolveInputRequestsAsync(
+    public override async ValueTask<IDictionary<string, InputResponse>> ResolveInputRequestsAsync(
         IDictionary<string, InputRequest> inputRequests,
         CancellationToken cancellationToken)
     {

src/ModelContextProtocol.Core/Client/McpClientImpl.cs

@@ -146,42 +146,4 @@ public McpClientHandlers Handlers
             field = value;
         }
     }
-
-    /// <summary>
-    /// Gets or sets the maximum number of consecutive task polls during which a task may report
-    /// <see cref="McpTaskStatus.InputRequired"/> without publishing any new input requests, before
-    /// the client treats the task as stuck, issues a best-effort <c>tasks/cancel</c>, and throws
-    /// an <see cref="McpException"/>.
-    /// </summary>
-    /// <value>
-    /// The maximum number of consecutive stuck polls allowed. The default value is <c>60</c>.
-    /// </value>
-    /// <remarks>
-    /// <para>
-    /// This guard prevents an unbounded poll loop when the server keeps a task in
-    /// <see cref="McpTaskStatus.InputRequired"/> but never publishes new input requests after the
-    /// client has already responded to every previously surfaced request. It only affects the
-    /// long-poll path used by <see cref="McpClient.CallToolAsync(CallToolRequestParams, CancellationToken)"/>;
-    /// it does not affect direct <see cref="McpClient.GetTaskAsync(string, CancellationToken)"/> calls.
-    /// </para>
-    /// <para>
-    /// Callers should size this value with the configured server-side poll interval in mind: the
-    /// effective wall-clock timeout is roughly <c>MaxConsecutiveStuckPolls * pollIntervalMs</c>.
-    /// Setting this to a very small value can cause false positives for servers that are slow to
-    /// surface follow-up input requests; setting it too large can mask misbehaving servers.
-    /// </para>
-    /// </remarks>
-    /// <exception cref="ArgumentOutOfRangeException">The value is less than <c>1</c>.</exception>
-    public int MaxConsecutiveStuckPolls
-    {
-        get;
-        set
-        {
-            if (value < 1)
-            {
-                throw new ArgumentOutOfRangeException(nameof(value), value, "must be greater than or equal to 1.");
-            }
-            field = value;
-        }
-    } = 60;
 }

src/ModelContextProtocol.Core/Client/McpClientOptions.cs

@@ -120,12 +120,6 @@ internal static bool IsValidToolOutputSchema(JsonElement element) =>
     [JsonSerializable(typeof(ResourceUpdatedNotificationParams))]
     [JsonSerializable(typeof(RootsListChangedNotificationParams))]
     [JsonSerializable(typeof(ToolListChangedNotificationParams))]
-    [JsonSerializable(typeof(TaskStatusNotificationParams))]
-    [JsonSerializable(typeof(WorkingTaskNotificationParams))]
-    [JsonSerializable(typeof(CompletedTaskNotificationParams))]
-    [JsonSerializable(typeof(FailedTaskNotificationParams))]
-    [JsonSerializable(typeof(CancelledTaskNotificationParams))]
-    [JsonSerializable(typeof(InputRequiredTaskNotificationParams))]
 
     // MCP Request Params / Results
     [JsonSerializable(typeof(CallToolRequestParams))]
@@ -174,19 +168,6 @@ internal static bool IsValidToolOutputSchema(JsonElement element) =>
     [JsonSerializable(typeof(IDictionary<string, InputRequest>))]
     [JsonSerializable(typeof(IDictionary<string, InputResponse>))]
 
-    [JsonSerializable(typeof(GetTaskRequestParams))]
-    [JsonSerializable(typeof(GetTaskResult))]
-    [JsonSerializable(typeof(WorkingTaskResult))]
-    [JsonSerializable(typeof(CompletedTaskResult))]
-    [JsonSerializable(typeof(FailedTaskResult))]
-    [JsonSerializable(typeof(CancelledTaskResult))]
-    [JsonSerializable(typeof(InputRequiredTaskResult))]
-    [JsonSerializable(typeof(UpdateTaskRequestParams))]
-    [JsonSerializable(typeof(UpdateTaskResult))]
-    [JsonSerializable(typeof(CancelTaskRequestParams))]
-    [JsonSerializable(typeof(CancelTaskResult))]
-    [JsonSerializable(typeof(CreateTaskResult))]
-
     // MCP Content
     [JsonSerializable(typeof(ContentBlock))]
     [JsonSerializable(typeof(TextContentBlock))]

src/ModelContextProtocol.Core/McpJsonUtilities.cs

@@ -57,6 +57,16 @@ public abstract partial class McpSession : IAsyncDisposable
     internal bool IsJuly2026OrLaterProtocol() =>
         McpHttpHeaders.IsJuly2026OrLaterProtocolVersion(NegotiatedProtocolVersion);
 
+    /// <summary>
+    /// Gets a value indicating whether the negotiated protocol version supports the draft protocol
+    /// features (the <c>2026-07-28</c> revision or later).
+    /// </summary>
+    /// <remarks>
+    /// This is a public seam used by bolt-on extensions (such as the Tasks extension) to gate
+    /// functionality that requires the <c>2026-07-28</c> or later protocol revision.
+    /// </remarks>
+    public bool IsDraftProtocol() => IsJuly2026OrLaterProtocol();
+
     /// <summary>
     /// Sends a JSON-RPC request to the connected session and waits for a response.
     /// </summary>

src/ModelContextProtocol.Core/McpSession.cs

@@ -53,24 +53,4 @@ public static class MetaKeys
     /// 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";
 }

src/ModelContextProtocol.Core/Protocol/MetaKeys.cs

@@ -144,16 +144,6 @@ public static class NotificationMethods
     /// </remarks>
     public const string CancelledNotification = "notifications/cancelled";
 
-    /// <summary>
-    /// The name of the notification sent by the server to push task status updates to subscribed clients.
-    /// </summary>
-    /// <remarks>
-    /// Part of the <c>io.modelcontextprotocol/tasks</c> extension.
-    /// Each notification carries a complete task state for the current status, identical to what
-    /// <c>tasks/get</c> would have returned at that moment.
-    /// </remarks>
-    public const string TaskStatusNotification = "notifications/tasks/status";
-
     /// <summary>
     /// The name of the notification sent first on a <see cref="RequestMethods.SubscriptionsListen"/>
     /// response stream to indicate which notification types the server agreed to deliver.

src/ModelContextProtocol.Core/Protocol/NotificationMethods.cs

@@ -125,33 +125,6 @@ public static class RequestMethods
     /// </remarks>
     public const string Initialize = "initialize";
 
-    /// <summary>
-    /// The name of the request method sent from the client to poll for task completion.
-    /// </summary>
-    /// <remarks>
-    /// Part of the <c>io.modelcontextprotocol/tasks</c> extension.
-    /// Clients poll for task status by sending this request with the task ID.
-    /// </remarks>
-    public const string TasksGet = "tasks/get";
-
-    /// <summary>
-    /// The name of the request method sent from the client to provide input responses to a task.
-    /// </summary>
-    /// <remarks>
-    /// Part of the <c>io.modelcontextprotocol/tasks</c> extension.
-    /// Used when a task has <c>input_required</c> status and the client needs to fulfill outstanding requests.
-    /// </remarks>
-    public const string TasksUpdate = "tasks/update";
-
-    /// <summary>
-    /// The name of the request method sent from the client to signal intent to cancel a task.
-    /// </summary>
-    /// <remarks>
-    /// Part of the <c>io.modelcontextprotocol/tasks</c> extension.
-    /// Cancellation is cooperative — the server decides whether and when to honor it.
-    /// </remarks>
-    public const string TasksCancel = "tasks/cancel";
-
     /// <summary>
     /// The name of the request method sent from the client to discover the server's protocol versions,
     /// capabilities, and metadata.

src/ModelContextProtocol.Core/Protocol/RequestMethods.cs

@@ -30,8 +30,8 @@ private protected Result()
     /// When absent or set to <c>"complete"</c>, the result is a normal completed response.
     /// When set to <c>"input_required"</c>, the result is an <see cref="InputRequiredResult"/> indicating
     /// that additional input is needed before the request can be completed.
-    /// When set to <c>"task"</c>, the result is a <see cref="CreateTaskResult"/> indicating that the server
-    /// has created a long-running task in lieu of returning the result directly.
+    /// When set to <c>"task"</c>, the server has created a long-running task in lieu of returning the
+    /// result directly.
     /// </para>
     /// </remarks>
     /// <value>Defaults to <see langword="null"/>, which is equivalent to <c>"complete"</c>.</value>