diff --git a/backend/packages/harness/deerflow/sandbox/sandbox_provider.py b/backend/packages/harness/deerflow/sandbox/sandbox_provider.py
index 8111629fd..60fcb53db 100644
--- a/backend/packages/harness/deerflow/sandbox/sandbox_provider.py
+++ b/backend/packages/harness/deerflow/sandbox/sandbox_provider.py
@@ -5,6 +5,7 @@ from typing import TYPE_CHECKING
 
 from deerflow.config import get_app_config
 from deerflow.reflection import resolve_class
+from deerflow.sandbox.lease import run_sync_lifecycle_operation
 from deerflow.sandbox.sandbox import Sandbox
 
 if TYPE_CHECKING:
@@ -62,8 +63,8 @@ class SandboxProvider(ABC):
         user_id: str,
         projection: "SkillProjectionPaths",
     ) -> None:
-        """Async wrapper for upload-based skill synchronization."""
-        await asyncio.to_thread(
+        """Async wrapper that keeps lifecycle ownership until sync finishes."""
+        await run_sync_lifecycle_operation(
             self.sync_agent_skills,
             sandbox_id,
             thread_id=thread_id,
diff --git a/backend/tests/test_sandbox_skill_sync_cancellation.py b/backend/tests/test_sandbox_skill_sync_cancellation.py
new file mode 100644
index 000000000..a30e1eacc
--- /dev/null
+++ b/backend/tests/test_sandbox_skill_sync_cancellation.py
@@ -0,0 +1,166 @@
+from __future__ import annotations
+
+import asyncio
+import threading
+
+import pytest
+from langgraph.runtime import Runtime
+
+from deerflow.sandbox.middleware import SandboxMiddleware
+from deerflow.sandbox.sandbox import Sandbox
+from deerflow.sandbox.sandbox_provider import SandboxProvider, reset_sandbox_provider, set_sandbox_provider
+from deerflow.sandbox.search import GrepMatch
+
+
+class _SandboxStub(Sandbox):
+    def execute_command(
+        self,
+        command: str,
+        env: dict[str, str] | None = None,
+        timeout: float | None = None,
+    ) -> str:
+        del command, env, timeout
+        return "OK"
+
+    def read_file(
+        self,
+        path: str,
+        start_line: int | None = None,
+        end_line: int | None = None,
+    ) -> str:
+        del path, start_line, end_line
+        return "content"
+
+    def download_file(self, path: str) -> bytes:
+        del path
+        return b"content"
+
+    def list_dir(self, path: str, max_depth: int = 2) -> list[str]:
+        del path, max_depth
+        return []
+
+    def write_file(self, path: str, content: str, append: bool = False) -> None:
+        del path, content, append
+
+    def glob(
+        self,
+        path: str,
+        pattern: str,
+        *,
+        include_dirs: bool = False,
+        max_results: int = 200,
+    ) -> tuple[list[str], bool]:
+        del path, pattern, include_dirs, max_results
+        return [], False
+
+    def grep(
+        self,
+        path: str,
+        pattern: str,
+        *,
+        glob: str | None = None,
+        literal: bool = False,
+        case_sensitive: bool = False,
+        max_results: int = 100,
+    ) -> tuple[list[GrepMatch], bool]:
+        del path, pattern, glob, literal, case_sensitive, max_results
+        return [], False
+
+    def update_file(self, path: str, content: bytes) -> None:
+        del path, content
+
+
+class _BlockingSkillSyncProvider(SandboxProvider):
+    supports_agent_skill_isolation = True
+
+    def __init__(self) -> None:
+        self.sandbox = _SandboxStub("skill-sync-sandbox")
+        self.sync_started = threading.Event()
+        self.allow_sync_finish = threading.Event()
+        self.sync_finished = threading.Event()
+        self.release_calls: list[str] = []
+
+    def acquire(self, thread_id: str | None = None, *, user_id: str | None = None) -> str:
+        del thread_id, user_id
+        return self.sandbox.id
+
+    def get(self, sandbox_id: str) -> Sandbox | None:
+        return self.sandbox if sandbox_id == self.sandbox.id else None
+
+    def release(self, sandbox_id: str) -> None:
+        self.release_calls.append(sandbox_id)
+
+    def sync_agent_skills(
+        self,
+        sandbox_id: str,
+        *,
+        thread_id: str,
+        user_id: str,
+        projection,
+    ) -> None:
+        del sandbox_id, thread_id, user_id, projection
+        self.sync_started.set()
+        assert self.allow_sync_finish.wait(timeout=5), "test did not unblock skill sync"
+        self.sync_finished.set()
+
+
+@pytest.mark.anyio
+async def test_cancelled_policy_sync_keeps_lease_until_sync_worker_finishes(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    provider = _BlockingSkillSyncProvider()
+    middleware = SandboxMiddleware(lazy_init=True, available_skills={"allowed"})
+    projection = object()
+    release_entry_sync_finished: list[bool] = []
+    original_release_sandbox_async = middleware._release_sandbox_async
+
+    async def _record_release_entry(sandbox_id: str, *, owner_id: str | None) -> None:
+        release_entry_sync_finished.append(provider.sync_finished.is_set())
+        await original_release_sandbox_async(sandbox_id, owner_id=owner_id)
+
+    monkeypatch.setattr(
+        middleware,
+        "_prepare_agent_skill_projection",
+        lambda *_args, **_kwargs: projection,
+    )
+    monkeypatch.setattr(middleware, "_release_sandbox_async", _record_release_entry)
+    set_sandbox_provider(provider)
+    task = asyncio.create_task(
+        middleware.abefore_agent(
+            {},
+            Runtime(context={"thread_id": "thread-policy", "user_id": "user-policy"}),
+        )
+    )
+    try:
+        assert await asyncio.to_thread(provider.sync_started.wait, 2), "skill sync did not start"
+
+        task.cancel()
+        await asyncio.sleep(0)
+
+        assert not task.done()
+        assert release_entry_sync_finished == []
+        assert provider.release_calls == []
+        assert not provider.sync_finished.is_set()
+
+        task.cancel()
+        await asyncio.sleep(0)
+
+        assert not task.done()
+        assert release_entry_sync_finished == []
+        assert provider.release_calls == []
+        assert not provider.sync_finished.is_set()
+
+        provider.allow_sync_finish.set()
+        with pytest.raises(asyncio.CancelledError):
+            await task
+
+        assert provider.sync_finished.is_set()
+        assert release_entry_sync_finished == [True]
+        assert provider.release_calls == [provider.sandbox.id]
+    finally:
+        provider.allow_sync_finish.set()
+        if not task.done():
+            task.cancel()
+            with pytest.raises(asyncio.CancelledError):
+                await task
+        reset_sandbox_provider()