--- title: Tasks description: Run long-running tool invocations asynchronously with status polling and input requests. uid: tasks --- ## Tasks Tasks let an MCP server run a request asynchronously and report its result to the client later. The primary use case today is long-running tool invocations: the tool is offloaded to a background task, and the client polls for status, optionally exchanging additional input along the way. > **Status**: Experimental — diagnostic ID `MCPEXP001`. The implementation tracks > [SEP-2663 (Tasks Extension)](https://github.com/modelcontextprotocol/modelcontextprotocol/blob/main/seps/2663-tasks-extension.md). > See [Experimental APIs](xref:experimental) for how to opt in. ### Overview A client opts into tasks on a per-request basis by including the `io.modelcontextprotocol/tasks` extension key in the request's `_meta`. When that opt-in is present, the server **may** respond with a instead of the standard result (e.g., ). The client then polls `tasks/get` until the task reaches a terminal state. Per the SEP, the server **must not** return `CreateTaskResult` for a request that did not include the extension opt-in. The SDK enforces this on the server side. #### Task lifecycle ```text ┌─────────────────────────┐ ▼ │ (start) → Working ──→ InputRequired │ │ │ │ │ └──────────────┘ (client responds via tasks/update) │ ├──→ Completed (terminal — includes tool results with isError: true) ├──→ Cancelled (terminal) └──→ Failed (terminal — JSON-RPC errors only) ``` wire values are serialized in snake_case: `working`, `input_required`, `completed`, `cancelled`, `failed`. The discriminator field on the response payload is `"task"` for and `"complete"` for ordinary results. ### Server configuration #### Using the task store Tasks support lives in the `ModelContextProtocol.Extensions.Tasks` package. The easiest way to enable tasks is to call on the server builder, passing an . The SDK ships for development and tests: ```csharp #pragma warning disable MCPEXP001 using ModelContextProtocol.Extensions.Tasks; builder.Services.AddMcpServer() .WithTools() .WithTasks(new InMemoryMcpTaskStore()); ``` When tasks are enabled with `WithTasks` the SDK automatically: - Wires the `tasks/get`, `tasks/update`, and `tasks/cancel` handlers from the store. - Advertises the `io.modelcontextprotocol/tasks` extension in . - Wraps each `[McpServerTool]` invocation so that, when the client opts in to the extension, the tool is offloaded to a background task tracked by the store. - Establishes a task scope so that , , and called from inside the tool surface as entries in the task's `inputRequests` instead of as direct JSON-RPC requests. - Plumbs a `CancellationToken` through to the tool that fires when the client invokes `tasks/cancel`, so cancellation propagates cooperatively. For production scenarios that need durability, session isolation, multi-process routing, or TTL-based cleanup, implement yourself (see [Implementing a custom task store](#implementing-a-custom-task-store) below). #### Returning a task from a tool handler For full control without the store's auto-wrapping, set . It returns a , so each invocation can choose between an immediate result and an alternate result such as a : ```csharp using ModelContextProtocol.Extensions.Tasks; options.Handlers.CallToolWithAlternateHandler = async (context, ct) => { if (ShouldRunInline(context.Params!)) { return new CallToolResult { Content = [/* … */] }; } var taskId = await StartBackgroundWorkAsync(context.Params!, ct); var created = new CreateTaskResult { TaskId = taskId, Status = McpTaskStatus.Working, CreatedAt = DateTimeOffset.UtcNow, LastUpdatedAt = DateTimeOffset.UtcNow, PollIntervalMs = 1000, }; return new ResultOrAlternate(created, McpTasksJsonContext.Default.CreateTaskResult); }; ``` > This low-level handler is mutually exclusive with `WithTasks`. When a store is configured, the > SDK does the wrapping for you and throws `InvalidOperationException` if the alternate handler also > returns an alternate. Use one mechanism or the other. When you return a task this way you are also > responsible for serving `tasks/get`, `tasks/update`, and `tasks/cancel`, which the store provides > automatically. > > and > are mutually exclusive. Setting one while the other is already non-null throws > `InvalidOperationException` at the property setter. #### Task scope for server-initiated requests When you start background work from a custom (rather than the SDK's auto-wrapping), use to route elicitation, sampling, and `roots/list` calls through the task store as input requests instead of direct JSON-RPC messages: ```csharp using ModelContextProtocol.Extensions.Tasks; using (server.CreateMcpTaskScope(taskId, taskStore)) { // ElicitAsync/SampleAsync/RequestRootsAsync calls in here are surfaced as // entries in the task's inputRequests, then await client responses via tasks/update. var elicit = await server.ElicitAsync(elicitParams, ct); } ``` `CreateMcpTaskScope` returns an `IDisposable` that restores the prior ambient context on `Dispose`. The scope is established automatically for `[McpServerTool]` methods that run via `WithTasks`, so this API is only needed for custom handlers. ### Client usage #### Automatic polling handles the full task lifecycle automatically: - Injects the `io.modelcontextprotocol/tasks` extension capability into the request's `_meta`. - Polls `tasks/get` at the cadence the server suggests via `pollIntervalMs`. - Dispatches input requests through the client's registered handlers ( and ). - Deduplicates already-resolved input request keys across polls so each request is handled at most once. - Returns the final when the task completes, or throws on `Failed`/`Cancelled`. ```csharp using ModelContextProtocol.Extensions.Tasks; var result = await client.CallToolWithPollingAsync( new CallToolRequestParams { Name = "long-running-tool", Arguments = arguments }, cancellationToken: cancellationToken); ``` #### Manual control Use to receive the raw without auto-polling, then drive the lifecycle yourself using , , and : ```csharp using ModelContextProtocol.Extensions.Tasks; var raw = await client.CallToolAsTaskAsync(requestParams, cancellationToken); if (raw.IsTask) { var taskId = raw.TaskCreated!.TaskId; while (true) { await Task.Delay(TimeSpan.FromMilliseconds(raw.TaskCreated.PollIntervalMs ?? 1000), cancellationToken); var state = await client.GetTaskAsync(taskId, cancellationToken); // Handle InputRequiredTaskResult by calling UpdateTaskAsync, // CompletedTaskResult by deserializing its Result property, etc. } } ``` #### Stuck-task detector `CallToolWithPollingAsync` includes a safety net for misbehaving servers: if the task stays in across many consecutive polls without exposing any new input request keys (i.e. every previously requested input has already been resolved by the client and yet the server keeps returning `InputRequired`), the client gives up, issues a best-effort `tasks/cancel`, and throws . This guards against a server that never transitions out of `InputRequired` and prevents an unbounded poll loop. The threshold defaults to `60` consecutive stuck polls and is configurable via the `maxConsecutiveStuckPolls` parameter on `CallToolWithPollingAsync`. The effective wall-clock timeout is roughly `maxConsecutiveStuckPolls * pollIntervalMs`, so tune the value with the server-side poll cadence in mind. Setting it too low risks false positives for servers that are slow to surface follow-up input requests; setting it too high can mask misbehaving servers. ### Input requests (multi-round-trip) When a task needs additional input from the client, the server transitions it to and returns the outstanding requests in . Each entry is an arbitrary key paired with a `{ method, params }` envelope representing an equivalent standalone server-to-client request. The client provides answers via , keyed by the same identifiers. Supported input request methods: | Method | Dispatched to the client handler | | --- | --- | | `elicitation/create` | | | `sampling/createMessage` | | Per SEP-2663: - Each input request key **must** be unique over the lifetime of the task. - Clients **should** deduplicate keys across polls so a request is only presented to the user or model once. `CallToolWithPollingAsync` does this automatically. - Servers **should** ignore `inputResponses` entries whose key does not currently correspond to an outstanding request, including responses for terminal-state tasks. follows this rule. ### Implementing a custom task store Implement for production scenarios. Key requirements drawn from the SEP and the SDK contract: 1. **Thread safety** — every method may be called concurrently. 2. **Idempotent terminal transitions** — , , and must be no-ops on a task that is already in a terminal state so a late cancellation cannot overwrite a result. 3. **`InputResponseReceived` event** — after persisting an input response inside , raise for each resolved entry. This is the only mechanism that wakes a pending `server.ElicitAsync`/`server.SampleAsync` call waiting inside a task scope. In distributed deployments where a different server instance receives the `tasks/update`, the event must be propagated to the originating server (for example via Redis pub/sub, SignalR, or a custom transport). 4. **Strong-consistency on `CreateTaskAsync`** — must not return until the task is durably persisted, so that a subsequent with the returned task ID resolves immediately — even from a different process or node. Stores backed by eventually-consistent storage must wait for the write to become visible (quorum acknowledgement, write-through, etc.) before returning. Required by SEP-2663 §306. 5. **Singleton under stateless HTTP** — when the server runs in stateless mode (each request spins up a fresh server instance), the same `IMcpTaskStore` instance must be shared across requests — either by registering it as a singleton in DI, or by backing it with external storage that every instance can reach. Otherwise `tasks/get` polls from subsequent requests will see an empty in-memory store and never find the task. ```csharp public sealed class MyTaskStore : IMcpTaskStore { public event Action? InputResponseReceived; public async Task ResolveInputRequestsAsync( string taskId, IDictionary inputResponses, CancellationToken cancellationToken = default) { // 1. Atomically persist the resolved requests, ignoring keys that are no longer // outstanding or that target a terminal task. await PersistResolvedResponsesAsync(taskId, inputResponses, cancellationToken); // 2. Then notify subscribers so any awaiting server.ElicitAsync/SampleAsync resumes. foreach (var kvp in inputResponses) { InputResponseReceived?.Invoke(new InputResponseReceivedEventArgs { TaskId = taskId, RequestId = kvp.Key, Response = kvp.Value, }); } } // … other IMcpTaskStore members } ``` ### Status semantics is the terminal status whenever the underlying request produced its standard result, *including a with `IsError = true`*. Per SEP-2663, tool-level error results are not promoted to `Failed`. is reserved for JSON-RPC protocol-level errors during execution — for example, a malformed request, or an unhandled exception in a custom handler that the SDK converts to a JSON-RPC error. Use for domain-level errors the model should see. ### Cancellation semantics Per SEP-2663, `tasks/cancel` is **eventually consistent and cooperative**: the server acknowledges the request immediately, but is not required to actually stop the work or to transition to `Cancelled`. The notifications-cancelled mechanism (used for plain JSON-RPC requests) is not used for task cancellation; clients must use `tasks/cancel`. In the built-in SDK pipeline, when a task is wrapped by a configured `TaskStore`: 1. The store's `SetCancelledAsync` transitions the task to `Cancelled` (a no-op if the task is already terminal). 2. The associated `CancellationTokenSource` is signaled, propagating cancellation to the tool's `CancellationToken` so cooperative cleanup can run. 3. Whichever side (the cancel handler or the background runner's `finally` block) wins `TryRemove` on the cancellation source owns disposal, avoiding `ObjectDisposedException`. ### Architecture notes #### Immutable store design uses immutable record snapshots with compare-and-swap updates for lock-free thread safety. `InputRequests` and `InputResponses` are exposed as `ImmutableDictionary<,>` so observers cannot mutate internal state. #### Capability bypass inside a task scope When `server.ElicitAsync`/`server.SampleAsync`/`server.RequestRootsAsync` execute inside a task scope, the SDK intentionally skips the normal client-capability negotiation checks (`ThrowIfElicitationUnsupported`, etc.). The tasks extension itself is the negotiated capability: the client opted in by including the extension marker in the originating request, so it is responsible for handling — or rejecting — the input requests surfaced through `tasks/get`. ### Known limitations - **Server-push task status notifications (SEP-2575)**: not yet implemented. Clients rely on polling exclusively. - **Lazy task creation**: when a tool runs through the task store, the store's is invoked eagerly before the inner handler runs, so tools that complete inline still incur a store write. There is currently no built-in deferral. - **Mid-execution promotion to task**: an `[McpServerTool]` method cannot start executing synchronously and then transition its remaining work to a background task. Use a custom if you need that pattern. - **`roots/list` as an input request**: the server SDK routes `RequestRootsAsync` through the task channel when called from inside a task scope, but the client SDK does not currently dispatch a handler for that method. Avoid calling `server.RequestRootsAsync` from within a task scope until client-side support is added. - **`ServerCapabilities.Extensions` round-trip**: the dictionary is typed as `IDictionary` so its values cannot be deserialized by the source generator. The negotiated extension surfaces correctly at the wire level, but round-tripping arbitrary extension payloads in-process is not supported.