diff --git a/offline-backend-20260512/backend/packages/harness/deerflow/runtime/scheduler/service.py b/offline-backend-20260512/backend/packages/harness/deerflow/runtime/scheduler/service.py
index 1c200f34..7f011ce3 100644
--- a/offline-backend-20260512/backend/packages/harness/deerflow/runtime/scheduler/service.py
+++ b/offline-backend-20260512/backend/packages/harness/deerflow/runtime/scheduler/service.py
@@ -459,12 +459,17 @@ class ScheduledTaskService:
             self._running_task_ids.discard(task["task_id"])
 
     async def _execute_task(self, task: dict[str, Any], *, scheduled_for: datetime, manual: bool) -> None:
+        # Each run uses its own throwaway thread so message history cannot accumulate
+        # across runs and blow past the model's input window. The per-user
+        # task["scheduler_thread_id"] is no longer used to drive the LLM call —
+        # we keep it on the task row only for backwards compatibility.
+        runtime_thread_id = str(uuid.uuid4())
         run_row = await self.store.create_run_once(
             {
                 "id": str(uuid.uuid4()),
                 "task_id": task["task_id"],
                 "user_id": task["user_id"],
-                "scheduler_thread_id": task["scheduler_thread_id"],
+                "scheduler_thread_id": runtime_thread_id,
                 "scheduled_for": scheduled_for,
                 "status": "running",
                 "started_at": datetime.now(UTC),
@@ -478,16 +483,16 @@ class ScheduledTaskService:
             await self.store.update_task(task["task_id"], task["user_id"], {"next_run_at": next_run_at, "last_status": "running"})
 
         try:
-            await self._ensure_thread_meta(task["user_id"], task["scheduler_thread_id"])
+            await self._ensure_thread_meta(task["user_id"], runtime_thread_id)
             async with self._agent_run_lock:
-                output_snapshot = self._snapshot_result_files(task["scheduler_thread_id"], task["user_id"])
-                record = await self._start_agent_run(task, scheduled_for=scheduled_for)
+                output_snapshot = self._snapshot_result_files(runtime_thread_id, task["user_id"])
+                record = await self._start_agent_run(task, scheduled_for=scheduled_for, runtime_thread_id=runtime_thread_id)
                 await self.store.update_run(run_row["id"], {"agent_run_id": record.run_id})
                 if record.task is not None:
                     await record.task
-                result_text = await self._extract_latest_ai_text(task["scheduler_thread_id"])
+                result_text = await self._extract_latest_ai_text(runtime_thread_id)
                 result_files = await self._extract_result_files(
-                    task["scheduler_thread_id"],
+                    runtime_thread_id,
                     task["user_id"],
                     run_id=run_row["id"],
                     output_snapshot=output_snapshot,
@@ -542,7 +547,7 @@ class ScheduledTaskService:
         finally:
             reset_current_user(token)
 
-    async def _start_agent_run(self, task: dict[str, Any], *, scheduled_for: datetime):
+    async def _start_agent_run(self, task: dict[str, Any], *, scheduled_for: datetime, runtime_thread_id: str):
         from app.gateway.routers.thread_runs import RunCreateRequest
         from app.gateway.services import start_run
 
@@ -555,7 +560,7 @@ class ScheduledTaskService:
             execution_context.update(
                 {
                     "is_scheduled_run": True,
-                    "thread_id": task["scheduler_thread_id"],
+                    "thread_id": runtime_thread_id,
                     "thinking_enabled": execution_context.get("thinking_enabled", True),
                     "is_plan_mode": execution_context.get("is_plan_mode", True),
                 }
@@ -572,7 +577,7 @@ class ScheduledTaskService:
                 context=execution_context,
                 multitask_strategy="enqueue" if False else "reject",
             )
-            return await start_run(body, task["scheduler_thread_id"], request)  # type: ignore[arg-type]
+            return await start_run(body, runtime_thread_id, request)  # type: ignore[arg-type]
         finally:
             reset_current_user(token)