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fastmcp.client.tasks

SEP-1686 client Task classes.

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Classes

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TaskNotificationHandler

MessageHandler that routes task status notifications to Task objects. Methods:

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dispatch

Dispatch messages, including task status notifications.

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Task

Abstract base class for MCP background tasks (SEP-1686). Provides a uniform API whether the server accepts background execution or executes synchronously (graceful degradation per SEP-1686). Methods:

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task_id

Get the task ID.

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returned_immediately

Check if server executed the task immediately. Returns:
  • True if server executed synchronously (graceful degradation or no task support)
  • False if server accepted background execution

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on_status_change

Register callback for status change notifications. The callback will be invoked when a notifications/tasks/status is received for this task (optional server feature per SEP-1686 lines 436-444). Supports both sync and async callbacks (auto-detected). Args:
  • callback: Function to call with GetTaskResult when status changes. Can return None (sync) or Awaitable[None] (async).

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status

Get current task status. If server executed immediately, returns synthetic completed status. Otherwise queries the server for current status.

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result

Wait for and return the task result. Must be implemented by subclasses to return the appropriate result type.

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wait

Wait for task to reach a specific state or complete. Uses event-based waiting when notifications are available (fast), with fallback to polling (reliable). Optimally wakes up immediately on status changes when server sends notifications/tasks/status. Args:
  • state: Desired state (‘submitted’, ‘working’, ‘completed’, ‘failed’). If None, waits for any terminal state (completed/failed)
  • timeout: Maximum time to wait in seconds
Returns:
  • Final task status
Raises:
  • TimeoutError: If desired state not reached within timeout

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cancel

Cancel this task, transitioning it to cancelled state. Sends a tasks/cancel protocol request. The server will attempt to halt execution and move the task to cancelled state. Note: If server executed immediately (graceful degradation), this is a no-op as there’s no server-side task to cancel.

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ToolTask

Represents a tool call that may execute in background or immediately. Provides a uniform API whether the server accepts background execution or executes synchronously (graceful degradation per SEP-1686). Methods:

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result

Wait for and return the tool result. If server executed immediately, returns the immediate result. Otherwise waits for background task to complete and retrieves result. Returns:
  • The parsed tool result (same as call_tool returns)

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PromptTask

Represents a prompt call that may execute in background or immediately. Provides a uniform API whether the server accepts background execution or executes synchronously (graceful degradation per SEP-1686). Methods:

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result

Wait for and return the prompt result. If server executed immediately, returns the immediate result. Otherwise waits for background task to complete and retrieves result. Returns:
  • The prompt result with messages and description

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ResourceTask

Represents a resource read that may execute in background or immediately. Provides a uniform API whether the server accepts background execution or executes synchronously (graceful degradation per SEP-1686). Methods:

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result

Wait for and return the resource contents. If server executed immediately, returns the immediate result. Otherwise waits for background task to complete and retrieves result. Returns:
  • list[ReadResourceContents]: The resource contents