Overview

MCP Tasks is an extension to the Model Context Protocol. Servers return a durable task handle instead of blocking on a long-running operation; clients poll for progress and retrieve the result when ready.

Extension Identifier: io.modelcontextprotocol/tasks

Why Tasks?

Tool calls block until work finishes. Tasks solve the cases where blocking is impractical:

• CI pipelines and batch jobs — minutes or hours of execution
• Human-in-the-loop workflows — approval gates that block until a person responds
• External job systems — cloud deployments, queued work, async APIs with their own job IDs
• Unreliable connections — mobile clients, intermittent networks, transport intermediary timeouts

A task ID is a durable handle that survives disconnects and carries status metadata, without requiring long-lived connections or unsolicited server-to-client messages.

Progressive Enhancement

Servers only return task handles to clients that declared the extension in their per-request capabilities — otherwise, the server either blocks for a regular result as usual or returns a capability error.

A server that requires task support from the client for a given request returns a -32003 (Missing Required Client Capability) error, like so:

{
  "jsonrpc": "2.0",
  "id": 1,
  "error": {
    // MISSING_REQUIRED_CLIENT_CAPABILITY
    "code": -32003,
    "message": "Missing required client capability",
    "data": {
      "requiredCapabilities": {
        "extensions": {
          "io.modelcontextprotocol/tasks": {}
        }
      }
    }
  }
}

Architecture

• Client — Drives all interaction: issues tool calls, polls for completion, and fulfills input requests.
• Server — Decides per-request whether to create a task and manages task state durably.
• Task Store — Durable state reachable by tasks/get even if the worker or connection has died. A CreateTaskResult is not returned until the task is findable here.
• Worker — The computation backing the task. Updates the task store as it progresses and writes the final result or error on completion.

Lifecycle

1. Capability negotiation — The client includes io.modelcontextprotocol/tasks in _meta.io.modelcontextprotocol/clientCapabilities.extensions. The server advertises the same in server/discover. No per-tool warmup or per-request flag.

2. Task creation — The server returns a CreateTaskResult with resultType: "task", containing a taskId, initial status, TTL, and polling interval. The task is durably created before the response is sent.

3. Polling — The client calls tasks/get with the taskId, respecting pollIntervalMs, until the task reaches a terminal status (which includes the final result or error).

4. Mid-flight input — If the task moves to input_required, tasks/get includes an inputRequests map that the client fulfills via tasks/update, after which the task transitions back to working.

5. Completion — completed: result contains what the original request would have returned synchronously. failed: error contains the JSON-RPC error.

Task Status

Status	Meaning
working	The operation is in progress.
input_required	The server needs client input before continuing. See inputRequests.
completed	The operation finished. result contains the final output.
failed	A JSON-RPC error occurred during execution. error has details.
cancelled	The operation was cancelled (not always honored).

completed, failed, and cancelled are terminal.

Each task also carries:

• statusMessage — Optional description of current state
• createdAt / lastUpdatedAt — ISO 8601 timestamps
• ttlMs — Time-to-live from creation in milliseconds; may change over lifetime; null for unlimited
• pollIntervalMs — Suggested polling interval; may change over lifetime

Mid-flight Input

When a task needs client input, it transitions to input_required and the tasks/get response includes an inputRequests map. The client fulfills these via tasks/update, which returns an empty ack:

{
  "status": "input_required",
  "inputRequests": {
    "approval": {
      "method": "elicitation/create",
      "params": {
        "mode": "form",
        "message": "Approve deployment to production?",
        "requestedSchema": {
          "type": "object",
          "properties": { "approved": { "type": "boolean" } },
          "required": ["approved"]
        }
      }
    }
  }
}

Each key in inputRequests is unique over the lifetime of a task. The server may accept partial responses; the task remains input_required until all arrive. Reads (tasks/get) and writes (tasks/update) are separate to keep reads idempotent and cacheable.

See the specification for the full tasks/update request shape and consistency semantics.

Cancellation and Notifications

Clients send tasks/cancel to signal cancellation intent. The server acks with an empty result — cancellation is cooperative, and the task may still reach a non-cancelled terminal status.

Servers may also push status updates via notifications/tasks, which clients opt into through subscriptions/listen. Each notification carries the full task state, identical to a tasks/get response.

See the specification for details on both mechanisms.

Security

• Task ID unguessability. Task IDs are generated with sufficient entropy to prevent enumeration, and may serve as bearer tokens for stored state.
• No task enumeration. There is no tasks/list, so one caller's tasks are not visible to another.
• Input-request trust model. inputRequests carry elicitation/sampling payloads from server to client. Hosts apply the same trust model as for standard elicitation/sampling requests.

Supported Methods

Task-augmented execution is currently supported for:

• tools/call

Learn More

• Specification — Full protocol specification
• SEP-2663 — The proposal defining this extension
• Schema — TypeScript types and generated JSON Schema