diff --git a/backend/packages/harness/deerflow/tools/builtins/task_tool.py b/backend/packages/harness/deerflow/tools/builtins/task_tool.py
index 71b348c..9f3f13b 100644
--- a/backend/packages/harness/deerflow/tools/builtins/task_tool.py
+++ b/backend/packages/harness/deerflow/tools/builtins/task_tool.py
@@ -1,7 +1,7 @@
 """Task tool for delegating work to subagents."""
 
+import asyncio
 import logging
-import time
 import uuid
 from dataclasses import replace
 from typing import Annotated, Literal
@@ -19,7 +19,7 @@ logger = logging.getLogger(__name__)
 
 
 @tool("task", parse_docstring=True)
-def task_tool(
+async def task_tool(
     runtime: ToolRuntime[ContextT, ThreadState],
     description: str,
     prompt: str,
@@ -129,67 +129,102 @@ def task_tool(
     # Send Task Started message'
     writer({"type": "task_started", "task_id": task_id, "description": description})
 
-    while True:
-        result = get_background_task_result(task_id)
+    try:
+        while True:
+            result = get_background_task_result(task_id)
 
-        if result is None:
-            logger.error(f"[trace={trace_id}] Task {task_id} not found in background tasks")
-            writer({"type": "task_failed", "task_id": task_id, "error": "Task disappeared from background tasks"})
-            cleanup_background_task(task_id)
-            return f"Error: Task {task_id} disappeared from background tasks"
+            if result is None:
+                logger.error(f"[trace={trace_id}] Task {task_id} not found in background tasks")
+                writer({"type": "task_failed", "task_id": task_id, "error": "Task disappeared from background tasks"})
+                cleanup_background_task(task_id)
+                return f"Error: Task {task_id} disappeared from background tasks"
 
-        # Log status changes for debugging
-        if result.status != last_status:
-            logger.info(f"[trace={trace_id}] Task {task_id} status: {result.status.value}")
-            last_status = result.status
+            # Log status changes for debugging
+            if result.status != last_status:
+                logger.info(f"[trace={trace_id}] Task {task_id} status: {result.status.value}")
+                last_status = result.status
 
-        # Check for new AI messages and send task_running events
-        current_message_count = len(result.ai_messages)
-        if current_message_count > last_message_count:
-            # Send task_running event for each new message
-            for i in range(last_message_count, current_message_count):
-                message = result.ai_messages[i]
-                writer(
-                    {
-                        "type": "task_running",
-                        "task_id": task_id,
-                        "message": message,
-                        "message_index": i + 1,  # 1-based index for display
-                        "total_messages": current_message_count,
-                    }
-                )
-                logger.info(f"[trace={trace_id}] Task {task_id} sent message #{i + 1}/{current_message_count}")
-            last_message_count = current_message_count
+            # Check for new AI messages and send task_running events
+            current_message_count = len(result.ai_messages)
+            if current_message_count > last_message_count:
+                # Send task_running event for each new message
+                for i in range(last_message_count, current_message_count):
+                    message = result.ai_messages[i]
+                    writer(
+                        {
+                            "type": "task_running",
+                            "task_id": task_id,
+                            "message": message,
+                            "message_index": i + 1,  # 1-based index for display
+                            "total_messages": current_message_count,
+                        }
+                    )
+                    logger.info(f"[trace={trace_id}] Task {task_id} sent message #{i + 1}/{current_message_count}")
+                last_message_count = current_message_count
 
-        # Check if task completed, failed, or timed out
-        if result.status == SubagentStatus.COMPLETED:
-            writer({"type": "task_completed", "task_id": task_id, "result": result.result})
-            logger.info(f"[trace={trace_id}] Task {task_id} completed after {poll_count} polls")
-            cleanup_background_task(task_id)
-            return f"Task Succeeded. Result: {result.result}"
-        elif result.status == SubagentStatus.FAILED:
-            writer({"type": "task_failed", "task_id": task_id, "error": result.error})
-            logger.error(f"[trace={trace_id}] Task {task_id} failed: {result.error}")
-            cleanup_background_task(task_id)
-            return f"Task failed. Error: {result.error}"
-        elif result.status == SubagentStatus.TIMED_OUT:
-            writer({"type": "task_timed_out", "task_id": task_id, "error": result.error})
-            logger.warning(f"[trace={trace_id}] Task {task_id} timed out: {result.error}")
-            cleanup_background_task(task_id)
-            return f"Task timed out. Error: {result.error}"
+            # Check if task completed, failed, or timed out
+            if result.status == SubagentStatus.COMPLETED:
+                writer({"type": "task_completed", "task_id": task_id, "result": result.result})
+                logger.info(f"[trace={trace_id}] Task {task_id} completed after {poll_count} polls")
+                cleanup_background_task(task_id)
+                return f"Task Succeeded. Result: {result.result}"
+            elif result.status == SubagentStatus.FAILED:
+                writer({"type": "task_failed", "task_id": task_id, "error": result.error})
+                logger.error(f"[trace={trace_id}] Task {task_id} failed: {result.error}")
+                cleanup_background_task(task_id)
+                return f"Task failed. Error: {result.error}"
+            elif result.status == SubagentStatus.TIMED_OUT:
+                writer({"type": "task_timed_out", "task_id": task_id, "error": result.error})
+                logger.warning(f"[trace={trace_id}] Task {task_id} timed out: {result.error}")
+                cleanup_background_task(task_id)
+                return f"Task timed out. Error: {result.error}"
 
-        # Still running, wait before next poll
-        time.sleep(5)  # Poll every 5 seconds
-        poll_count += 1
+            # Still running, wait before next poll
+            await asyncio.sleep(5)
+            poll_count += 1
 
-        # Polling timeout as a safety net (in case thread pool timeout doesn't work)
-        # Set to execution timeout + 60s buffer, in 5s poll intervals
-        # This catches edge cases where the background task gets stuck
-        # Note: We don't call cleanup_background_task here because the task may
-        # still be running in the background. The cleanup will happen when the
-        # executor completes and sets a terminal status.
-        if poll_count > max_poll_count:
-            timeout_minutes = config.timeout_seconds // 60
-            logger.error(f"[trace={trace_id}] Task {task_id} polling timed out after {poll_count} polls (should have been caught by thread pool timeout)")
-            writer({"type": "task_timed_out", "task_id": task_id})
-            return f"Task polling timed out after {timeout_minutes} minutes. This may indicate the background task is stuck. Status: {result.status.value}"
+            # Polling timeout as a safety net (in case thread pool timeout doesn't work)
+            # Set to execution timeout + 60s buffer, in 5s poll intervals
+            # This catches edge cases where the background task gets stuck
+            # Note: We don't call cleanup_background_task here because the task may
+            # still be running in the background. The cleanup will happen when the
+            # executor completes and sets a terminal status.
+            if poll_count > max_poll_count:
+                timeout_minutes = config.timeout_seconds // 60
+                logger.error(f"[trace={trace_id}] Task {task_id} polling timed out after {poll_count} polls (should have been caught by thread pool timeout)")
+                writer({"type": "task_timed_out", "task_id": task_id})
+                return f"Task polling timed out after {timeout_minutes} minutes. This may indicate the background task is stuck. Status: {result.status.value}"
+    except asyncio.CancelledError:
+        async def cleanup_when_done() -> None:
+            max_cleanup_polls = max_poll_count
+            cleanup_poll_count = 0
+
+            while True:
+                result = get_background_task_result(task_id)
+                if result is None:
+                    return
+
+                if result.status in {SubagentStatus.COMPLETED, SubagentStatus.FAILED, SubagentStatus.TIMED_OUT} or getattr(result, "completed_at", None) is not None:
+                    cleanup_background_task(task_id)
+                    return
+
+                if cleanup_poll_count > max_cleanup_polls:
+                    logger.warning(
+                        f"[trace={trace_id}] Deferred cleanup for task {task_id} timed out after {cleanup_poll_count} polls"
+                    )
+                    return
+
+                await asyncio.sleep(5)
+                cleanup_poll_count += 1
+
+        def log_cleanup_failure(cleanup_task: asyncio.Task[None]) -> None:
+            if cleanup_task.cancelled():
+                return
+
+            exc = cleanup_task.exception()
+            if exc is not None:
+                logger.error(f"[trace={trace_id}] Deferred cleanup failed for task {task_id}: {exc}")
+
+        logger.debug(f"[trace={trace_id}] Scheduling deferred cleanup for cancelled task {task_id}")
+        asyncio.create_task(cleanup_when_done()).add_done_callback(log_cleanup_failure)
+        raise
diff --git a/backend/tests/test_task_tool_core_logic.py b/backend/tests/test_task_tool_core_logic.py
index 19d1b82..bae7da1 100644
--- a/backend/tests/test_task_tool_core_logic.py
+++ b/backend/tests/test_task_tool_core_logic.py
@@ -1,10 +1,13 @@
 """Core behavior tests for task tool orchestration."""
 
+import asyncio
 import importlib
 from enum import Enum
 from types import SimpleNamespace
 from unittest.mock import MagicMock
 
+import pytest
+
 from deerflow.subagents.config import SubagentConfig
 
 # Use module import so tests can patch the exact symbols referenced inside task_tool().
@@ -61,10 +64,23 @@ def _make_result(
     )
 
 
+def _run_task_tool(**kwargs):
+    return asyncio.run(task_tool_module.task_tool.coroutine(**kwargs))
+
+
+async def _no_sleep(_: float) -> None:
+    return None
+
+
+class _DummyScheduledTask:
+    def add_done_callback(self, _callback):
+        return None
+
+
 def test_task_tool_returns_error_for_unknown_subagent(monkeypatch):
     monkeypatch.setattr(task_tool_module, "get_subagent_config", lambda _: None)
 
-    result = task_tool_module.task_tool.func(
+    result = _run_task_tool(
         runtime=None,
         description="执行任务",
         prompt="do work",
@@ -109,11 +125,11 @@ def test_task_tool_emits_running_and_completed_events(monkeypatch):
     monkeypatch.setattr(task_tool_module, "get_skills_prompt_section", lambda: "Skills Appendix")
     monkeypatch.setattr(task_tool_module, "get_background_task_result", lambda _: next(responses))
     monkeypatch.setattr(task_tool_module, "get_stream_writer", lambda: events.append)
-    monkeypatch.setattr(task_tool_module.time, "sleep", lambda _: None)
+    monkeypatch.setattr(task_tool_module.asyncio, "sleep", _no_sleep)
     # task_tool lazily imports from deerflow.tools at call time, so patch that module-level function.
     monkeypatch.setattr("deerflow.tools.get_available_tools", get_available_tools)
 
-    output = task_tool_module.task_tool.func(
+    output = _run_task_tool(
         runtime=runtime,
         description="运行子任务",
         prompt="collect diagnostics",
@@ -155,10 +171,10 @@ def test_task_tool_returns_failed_message(monkeypatch):
         lambda _: _make_result(FakeSubagentStatus.FAILED, error="subagent crashed"),
     )
     monkeypatch.setattr(task_tool_module, "get_stream_writer", lambda: events.append)
-    monkeypatch.setattr(task_tool_module.time, "sleep", lambda _: None)
+    monkeypatch.setattr(task_tool_module.asyncio, "sleep", _no_sleep)
     monkeypatch.setattr("deerflow.tools.get_available_tools", lambda **kwargs: [])
 
-    output = task_tool_module.task_tool.func(
+    output = _run_task_tool(
         runtime=_make_runtime(),
         description="执行任务",
         prompt="do fail",
@@ -189,10 +205,10 @@ def test_task_tool_returns_timed_out_message(monkeypatch):
         lambda _: _make_result(FakeSubagentStatus.TIMED_OUT, error="timeout"),
     )
     monkeypatch.setattr(task_tool_module, "get_stream_writer", lambda: events.append)
-    monkeypatch.setattr(task_tool_module.time, "sleep", lambda _: None)
+    monkeypatch.setattr(task_tool_module.asyncio, "sleep", _no_sleep)
     monkeypatch.setattr("deerflow.tools.get_available_tools", lambda **kwargs: [])
 
-    output = task_tool_module.task_tool.func(
+    output = _run_task_tool(
         runtime=_make_runtime(),
         description="执行任务",
         prompt="do timeout",
@@ -225,10 +241,10 @@ def test_task_tool_polling_safety_timeout(monkeypatch):
         lambda _: _make_result(FakeSubagentStatus.RUNNING, ai_messages=[]),
     )
     monkeypatch.setattr(task_tool_module, "get_stream_writer", lambda: events.append)
-    monkeypatch.setattr(task_tool_module.time, "sleep", lambda _: None)
+    monkeypatch.setattr(task_tool_module.asyncio, "sleep", _no_sleep)
     monkeypatch.setattr("deerflow.tools.get_available_tools", lambda **kwargs: [])
 
-    output = task_tool_module.task_tool.func(
+    output = _run_task_tool(
         runtime=_make_runtime(),
         description="执行任务",
         prompt="never finish",
@@ -261,7 +277,7 @@ def test_cleanup_called_on_completed(monkeypatch):
         lambda _: _make_result(FakeSubagentStatus.COMPLETED, result="done"),
     )
     monkeypatch.setattr(task_tool_module, "get_stream_writer", lambda: events.append)
-    monkeypatch.setattr(task_tool_module.time, "sleep", lambda _: None)
+    monkeypatch.setattr(task_tool_module.asyncio, "sleep", _no_sleep)
     monkeypatch.setattr("deerflow.tools.get_available_tools", lambda **kwargs: [])
     monkeypatch.setattr(
         task_tool_module,
@@ -269,7 +285,7 @@ def test_cleanup_called_on_completed(monkeypatch):
         lambda task_id: cleanup_calls.append(task_id),
     )
 
-    output = task_tool_module.task_tool.func(
+    output = _run_task_tool(
         runtime=_make_runtime(),
         description="执行任务",
         prompt="complete task",
@@ -301,7 +317,7 @@ def test_cleanup_called_on_failed(monkeypatch):
         lambda _: _make_result(FakeSubagentStatus.FAILED, error="error"),
     )
     monkeypatch.setattr(task_tool_module, "get_stream_writer", lambda: events.append)
-    monkeypatch.setattr(task_tool_module.time, "sleep", lambda _: None)
+    monkeypatch.setattr(task_tool_module.asyncio, "sleep", _no_sleep)
     monkeypatch.setattr("deerflow.tools.get_available_tools", lambda **kwargs: [])
     monkeypatch.setattr(
         task_tool_module,
@@ -309,7 +325,7 @@ def test_cleanup_called_on_failed(monkeypatch):
         lambda task_id: cleanup_calls.append(task_id),
     )
 
-    output = task_tool_module.task_tool.func(
+    output = _run_task_tool(
         runtime=_make_runtime(),
         description="执行任务",
         prompt="fail task",
@@ -341,7 +357,7 @@ def test_cleanup_called_on_timed_out(monkeypatch):
         lambda _: _make_result(FakeSubagentStatus.TIMED_OUT, error="timeout"),
     )
     monkeypatch.setattr(task_tool_module, "get_stream_writer", lambda: events.append)
-    monkeypatch.setattr(task_tool_module.time, "sleep", lambda _: None)
+    monkeypatch.setattr(task_tool_module.asyncio, "sleep", _no_sleep)
     monkeypatch.setattr("deerflow.tools.get_available_tools", lambda **kwargs: [])
     monkeypatch.setattr(
         task_tool_module,
@@ -349,7 +365,7 @@ def test_cleanup_called_on_timed_out(monkeypatch):
         lambda task_id: cleanup_calls.append(task_id),
     )
 
-    output = task_tool_module.task_tool.func(
+    output = _run_task_tool(
         runtime=_make_runtime(),
         description="执行任务",
         prompt="timeout task",
@@ -388,7 +404,7 @@ def test_cleanup_not_called_on_polling_safety_timeout(monkeypatch):
         lambda _: _make_result(FakeSubagentStatus.RUNNING, ai_messages=[]),
     )
     monkeypatch.setattr(task_tool_module, "get_stream_writer", lambda: events.append)
-    monkeypatch.setattr(task_tool_module.time, "sleep", lambda _: None)
+    monkeypatch.setattr(task_tool_module.asyncio, "sleep", _no_sleep)
     monkeypatch.setattr("deerflow.tools.get_available_tools", lambda **kwargs: [])
     monkeypatch.setattr(
         task_tool_module,
@@ -396,7 +412,7 @@ def test_cleanup_not_called_on_polling_safety_timeout(monkeypatch):
         lambda task_id: cleanup_calls.append(task_id),
     )
 
-    output = task_tool_module.task_tool.func(
+    output = _run_task_tool(
         runtime=_make_runtime(),
         description="执行任务",
         prompt="never finish",
@@ -407,3 +423,117 @@ def test_cleanup_not_called_on_polling_safety_timeout(monkeypatch):
     assert output.startswith("Task polling timed out after 0 minutes")
     # cleanup should NOT be called because the task is still RUNNING
     assert cleanup_calls == []
+
+
+def test_cleanup_scheduled_on_cancellation(monkeypatch):
+    """Verify cancellation schedules deferred cleanup for the background task."""
+    config = _make_subagent_config()
+    events = []
+    cleanup_calls = []
+    scheduled_cleanup_coros = []
+    poll_count = 0
+
+    def get_result(_: str):
+        nonlocal poll_count
+        poll_count += 1
+        if poll_count == 1:
+            return _make_result(FakeSubagentStatus.RUNNING, ai_messages=[])
+        return _make_result(FakeSubagentStatus.COMPLETED, result="done")
+
+    async def cancel_on_first_sleep(_: float) -> None:
+        raise asyncio.CancelledError
+
+    monkeypatch.setattr(task_tool_module, "SubagentStatus", FakeSubagentStatus)
+    monkeypatch.setattr(
+        task_tool_module,
+        "SubagentExecutor",
+        type("DummyExecutor", (), {"__init__": lambda self, **kwargs: None, "execute_async": lambda self, prompt, task_id=None: task_id}),
+    )
+    monkeypatch.setattr(task_tool_module, "get_subagent_config", lambda _: config)
+    monkeypatch.setattr(task_tool_module, "get_skills_prompt_section", lambda: "")
+    monkeypatch.setattr(task_tool_module, "get_background_task_result", get_result)
+    monkeypatch.setattr(task_tool_module, "get_stream_writer", lambda: events.append)
+    monkeypatch.setattr(task_tool_module.asyncio, "sleep", cancel_on_first_sleep)
+    monkeypatch.setattr(
+        task_tool_module.asyncio,
+        "create_task",
+        lambda coro: scheduled_cleanup_coros.append(coro) or _DummyScheduledTask(),
+    )
+    monkeypatch.setattr("deerflow.tools.get_available_tools", lambda **kwargs: [])
+    monkeypatch.setattr(
+        task_tool_module,
+        "cleanup_background_task",
+        lambda task_id: cleanup_calls.append(task_id),
+    )
+
+    with pytest.raises(asyncio.CancelledError):
+        _run_task_tool(
+            runtime=_make_runtime(),
+            description="执行任务",
+            prompt="cancel task",
+            subagent_type="general-purpose",
+            tool_call_id="tc-cancelled-cleanup",
+        )
+
+    assert cleanup_calls == []
+    assert len(scheduled_cleanup_coros) == 1
+
+    asyncio.run(scheduled_cleanup_coros.pop())
+
+    assert cleanup_calls == ["tc-cancelled-cleanup"]
+
+
+def test_cancelled_cleanup_stops_after_timeout(monkeypatch):
+    """Verify deferred cleanup gives up after a bounded number of polls."""
+    config = _make_subagent_config()
+    config.timeout_seconds = 1
+    events = []
+    cleanup_calls = []
+    scheduled_cleanup_coros = []
+
+    async def cancel_on_first_sleep(_: float) -> None:
+        raise asyncio.CancelledError
+
+    monkeypatch.setattr(task_tool_module, "SubagentStatus", FakeSubagentStatus)
+    monkeypatch.setattr(
+        task_tool_module,
+        "SubagentExecutor",
+        type("DummyExecutor", (), {"__init__": lambda self, **kwargs: None, "execute_async": lambda self, prompt, task_id=None: task_id}),
+    )
+    monkeypatch.setattr(task_tool_module, "get_subagent_config", lambda _: config)
+    monkeypatch.setattr(task_tool_module, "get_skills_prompt_section", lambda: "")
+    monkeypatch.setattr(
+        task_tool_module,
+        "get_background_task_result",
+        lambda _: _make_result(FakeSubagentStatus.RUNNING, ai_messages=[]),
+    )
+    monkeypatch.setattr(task_tool_module, "get_stream_writer", lambda: events.append)
+    monkeypatch.setattr(task_tool_module.asyncio, "sleep", cancel_on_first_sleep)
+    monkeypatch.setattr(
+        task_tool_module.asyncio,
+        "create_task",
+        lambda coro: scheduled_cleanup_coros.append(coro) or _DummyScheduledTask(),
+    )
+    monkeypatch.setattr("deerflow.tools.get_available_tools", lambda **kwargs: [])
+    monkeypatch.setattr(
+        task_tool_module,
+        "cleanup_background_task",
+        lambda task_id: cleanup_calls.append(task_id),
+    )
+
+    with pytest.raises(asyncio.CancelledError):
+        _run_task_tool(
+            runtime=_make_runtime(),
+            description="执行任务",
+            prompt="cancel task",
+            subagent_type="general-purpose",
+            tool_call_id="tc-cancelled-timeout",
+        )
+
+    async def bounded_sleep(_seconds: float) -> None:
+        return None
+
+    monkeypatch.setattr(task_tool_module.asyncio, "sleep", bounded_sleep)
+    asyncio.run(scheduled_cleanup_coros.pop())
+
+    assert cleanup_calls == []