diff --git a/backend/packages/harness/deerflow/sandbox/lease.py b/backend/packages/harness/deerflow/sandbox/lease.py
index 2fc3d6ff3..b0878cbe8 100644
--- a/backend/packages/harness/deerflow/sandbox/lease.py
+++ b/backend/packages/harness/deerflow/sandbox/lease.py
@@ -343,7 +343,7 @@ class SandboxLeaseManager:
                 release_on_last=release_on_last,
             )
         if release_previous and previous is not None:
-            await asyncio.to_thread(
+            await run_sync_lifecycle_operation(
                 self._provider.release,
                 previous.sandbox_id,
             )
@@ -396,7 +396,7 @@ class SandboxLeaseManager:
                     release_on_last=release_on_last,
                 )
             if release_previous and previous is not None:
-                await asyncio.to_thread(
+                await run_sync_lifecycle_operation(
                     self._provider.release,
                     previous.sandbox_id,
                 )
@@ -529,7 +529,7 @@ class SandboxLeaseManager:
                         release_on_last=release_on_last,
                     )
                 if release_previous and previous is not None:
-                    await asyncio.to_thread(
+                    await run_sync_lifecycle_operation(
                         self._provider.release,
                         previous.sandbox_id,
                     )
diff --git a/backend/tests/test_sandbox_lease_rebind_cancellation.py b/backend/tests/test_sandbox_lease_rebind_cancellation.py
new file mode 100644
index 000000000..2e9d9bbb9
--- /dev/null
+++ b/backend/tests/test_sandbox_lease_rebind_cancellation.py
@@ -0,0 +1,107 @@
+from __future__ import annotations
+
+import asyncio
+import threading
+
+import pytest
+
+from deerflow.sandbox.lease import SandboxLeaseManager
+from deerflow.sandbox.sandbox_provider import SandboxProvider
+
+
+class _BlockingReleaseProvider(SandboxProvider):
+    def __init__(self) -> None:
+        self._sandboxes: dict[str, object] = {
+            "old": object(),
+            "new": object(),
+        }
+        self.release_started = threading.Event()
+        self.allow_release = threading.Event()
+        self.release_finished = threading.Event()
+
+    def acquire(self, thread_id=None, *, user_id=None) -> str:
+        return "new"
+
+    def get(self, sandbox_id):
+        return self._sandboxes.get(sandbox_id)
+
+    def release(self, sandbox_id) -> None:
+        if sandbox_id == "old":
+            self.release_started.set()
+            assert self.allow_release.wait(timeout=1)
+        self._sandboxes.pop(sandbox_id, None)
+        self.release_finished.set()
+
+
+@pytest.mark.anyio
+async def test_cancelled_async_rebind_holds_serializer_until_previous_release_finishes() -> None:
+    provider = _BlockingReleaseProvider()
+    manager = SandboxLeaseManager(provider)
+    manager.retain(
+        "owner",
+        "old",
+        thread_id="thread-1",
+        user_id="user-1",
+    )
+
+    rebind_task = asyncio.create_task(
+        manager.retain_async(
+            "owner",
+            "new",
+            thread_id="thread-1",
+            user_id="user-1",
+        )
+    )
+    contender: threading.Thread | None = None
+
+    try:
+        assert await asyncio.to_thread(provider.release_started.wait, 1)
+
+        rebind_task.cancel("first cancellation")
+        await asyncio.sleep(0)
+        rebind_task.cancel("second cancellation")
+        await asyncio.sleep(0)
+
+        # Cancelling the awaiter must not release the per-thread serializer
+        # while the blocking provider release is still running in its worker.
+        assert not rebind_task.done()
+
+        contender_done = threading.Event()
+
+        def retain_next_owner() -> None:
+            manager.retain(
+                "next",
+                "new",
+                thread_id="thread-1",
+                user_id="user-1",
+            )
+            contender_done.set()
+
+        contender = threading.Thread(target=retain_next_owner)
+        contender.start()
+        contender.join(timeout=0.05)
+        assert contender.is_alive()
+        assert not contender_done.is_set()
+
+        provider.allow_release.set()
+        with pytest.raises(asyncio.CancelledError) as exc_info:
+            await rebind_task
+        assert exc_info.value.args == ("first cancellation",)
+
+        contender.join(timeout=1)
+        assert not contender.is_alive()
+        assert contender_done.is_set()
+        assert provider.release_finished.is_set()
+        assert manager.binding_for("owner") == "new"
+        assert manager.binding_for("next") == "new"
+    finally:
+        provider.allow_release.set()
+        if contender is not None:
+            contender.join(timeout=1)
+        if not rebind_task.done():
+            rebind_task.cancel()
+            try:
+                await rebind_task
+            except asyncio.CancelledError:
+                pass
+        manager.close()