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The experimental-ext-tasks repository contains the full specification and documentation for MCP Tasks.

modelcontextprotocol/experimental-ext-tasks

Full specification and documentation for MCP Tasks.
Not every tool call returns instantly. Some operations — CI pipelines, batch processing, human approvals — take seconds, minutes, or longer. MCP Tasks let servers return a durable handle instead of blocking, so clients can poll for progress, provide input when needed, and retrieve the final result after reconnecting.

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Why not just block?

You could hold the connection open until the work finishes. Tasks solve problems that blocking cannot:
  • No long-lived connections. Blocking ties up a connection for the duration of the operation. Many clients and transport intermediaries impose timeouts that make this impractical beyond a few seconds.
  • Crash resilience. A task ID is a durable handle. If the client disconnects or restarts, it can resume polling with the same ID.
  • Progress visibility. Tasks carry status metadata (working, input_required, completed, failed, cancelled) and optional status messages, giving clients visibility into progress.
  • Mid-flight interaction. When a task needs input (e.g., an elicitation for user confirmation), it moves to input_required and surfaces the request. The client responds via tasks/update — no second connection or unsolicited server-to-client messages required.
  • Server-directed. The server decides per-request whether to create a task. Clients opt in once via the extension capability and handle whichever result shape arrives. No per-tool warmup or per-request flag.

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How Tasks work

Tasks extend the standard request flow. When a server decides a request will be long-running, it returns a task handle instead of the final result. The client polls for completion.
  1. Capability negotiation. The client includes io.modelcontextprotocol/tasks in its per-request capabilities. The server advertises the same extension in its own server/discover capabilities.
  2. Task creation. In response to a supported request, the server returns a CreateTaskResult (identified by resultType: "task") containing a taskId, initial status, TTL, and suggested polling interval. The task is durably created before the response is sent.
  3. Polling. The client calls tasks/get with the taskId. The response carries the current status and, for terminal states, the final result or error.
  4. Mid-flight input. If the task moves to input_required, the tasks/get response includes an inputRequests map with elicitations or other server requests. The client fulfills these via tasks/update.
  5. Completion. When the status reaches completed, the result field contains what the original request would have returned synchronously. If the status is failed, the error field contains the JSON-RPC error.
  6. Cancellation. The client can send tasks/cancel at any time. Cancellation is cooperative — the server acknowledges the intent but is not obligated to stop the work.

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When to use Tasks

Tasks are a good fit when your use case involves: Long-running operations. CI pipelines, batch data processing, or model training jobs that take minutes or hours. Human-in-the-loop workflows. Approval gates, review steps, or any operation that pauses for user confirmation. The task moves to input_required and the client presents the request. External job systems. If your server wraps an API that already uses job IDs (cloud deployments, async APIs, queued work), return a task when you create the job and resolve it when the job completes. Unreliable connections. Mobile clients, intermittent networks, or environments where connections drop. Task IDs survive disconnects. Batch processing. Operations that process many items (bulk imports, mass updates) where partial progress is meaningful. Status messages report progress.

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Task lifecycle

StatusMeaning
workingThe operation is in progress.
input_requiredThe server needs client input before continuing. See inputRequests.
completedThe operation finished. The result field contains the final output.
failedA JSON-RPC error occurred during execution. The error field has details.
cancelledThe operation was cancelled (not always honored).
completed, failed, and cancelled are terminal — once reached, the task’s state does not change.

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Notifications

Servers can push status updates via notifications/tasks/status. Clients opt into these through the subscriptions/listen mechanism. Each notification carries the full task state, eliminating the need for an extra tasks/get round-trip. Polling is the default. If a server supports notifications, clients can rely on them instead of polling.

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Implementation guide

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For MCP clients

To consume task-augmented responses, your client must:
1

Declare support

Include the extension in per-request capabilities:
{
  "params": {
    "_meta": {
      "io.modelcontextprotocol/clientCapabilities": {
        "extensions": {
          "io.modelcontextprotocol/tasks": {}
        }
      }
    }
  }
}
2

Handle polymorphic results

When issuing a supported request (e.g., tools/call), be prepared to receive either the standard result or a CreateTaskResult with resultType: "task".
3

Poll for completion

Call tasks/get with the returned taskId, respecting the pollIntervalMs value. Continue polling until the task reaches a terminal status (completed, failed, or cancelled).
4

Handle input requests

If the task status is input_required, read the inputRequests map, present the requests to the user or model, and submit responses via tasks/update.
5

Persist task IDs

Store task IDs durably so polling can resume after a client crash or restart.

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For MCP servers

To return tasks from your server:
1

Advertise support

Include the extension in your server/discover capabilities:
{
  "capabilities": {
    "extensions": {
      "io.modelcontextprotocol/tasks": {}
    }
  }
}
2

Check client capabilities

Before returning a CreateTaskResult, verify that the client included the extension in its per-request capabilities. Never return a task to a client that did not declare support.
3

Return CreateTaskResult

When a request will be long-running, respond with resultType: "task" and a Task object containing a unique taskId, initial status, ttlMs, and pollIntervalMs. The task must be durably created before sending the response.
4

Serve tasks/get

Return the current task state on each poll. For terminal states, include the result (on completed) or error (on failed) field.
5

Handle tasks/update

Accept inputResponses keyed to outstanding inputRequests. Acknowledge with an empty result. Ignore responses for unknown or already-satisfied keys.
6

Handle tasks/cancel

Acknowledge cancellation requests with an empty result. Honor them when possible, but cancellation is cooperative — the task may still reach a non-cancelled terminal status.

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Client support

MCP Tasks is an extension to the core MCP specification. Host support varies by client.
See the client matrix for extension support across clients. Task support requires explicit opt-in from both client and server.

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Specification

The Tasks extension is specified in the experimental-ext-tasks repository. It uses the standard MCP extension negotiation mechanism: clients and servers declare support in the extensions field of their capabilities during initialization.