diff --git a/src/api/tasks.py b/src/api/tasks.py
index de4bf505..92779d09 100644
--- a/src/api/tasks.py
+++ b/src/api/tasks.py
@@ -26,7 +26,7 @@ async def cancel_task(request: Request, task_service, session_manager):
     task_id = request.path_params.get("task_id")
     user = request.state.user
 
-    success = task_service.cancel_task(user.user_id, task_id)
+    success = await task_service.cancel_task(user.user_id, task_id)
     if not success:
         return JSONResponse(
             {"error": "Task not found or cannot be cancelled"}, status_code=400
diff --git a/src/services/task_service.py b/src/services/task_service.py
index 3052ba4f..be5312a0 100644
--- a/src/services/task_service.py
+++ b/src/services/task_service.py
@@ -352,7 +352,7 @@ class TaskService:
         tasks.sort(key=lambda x: x["created_at"], reverse=True)
         return tasks
 
-    def cancel_task(self, user_id: str, task_id: str) -> bool:
+    async def cancel_task(self, user_id: str, task_id: str) -> bool:
         """Cancel a task if it exists and is not already completed.
 
         Supports cancellation of shared default tasks stored under the anonymous user.
@@ -384,18 +384,28 @@ class TaskService:
             and not upload_task.background_task.done()
         ):
             upload_task.background_task.cancel()
+            # Wait for the background task to actually stop to avoid race conditions
+            try:
+                await upload_task.background_task
+            except asyncio.CancelledError:
+                pass  # Expected when we cancel the task
+            except Exception:
+                pass  # Ignore other errors during cancellation
 
         # Mark task as failed (cancelled)
         upload_task.status = TaskStatus.FAILED
         upload_task.updated_at = time.time()
 
-        # Mark all pending file tasks as failed
+        # Mark all pending and running file tasks as failed
         for file_task in upload_task.file_tasks.values():
-            if file_task.status == TaskStatus.PENDING:
+            if file_task.status in [TaskStatus.PENDING, TaskStatus.RUNNING]:
+                # Increment failed_files counter for both pending and running
+                # (running files haven't been counted yet in either counter)
+                upload_task.failed_files += 1
+
                 file_task.status = TaskStatus.FAILED
                 file_task.error = "Task cancelled by user"
                 file_task.updated_at = time.time()
-                upload_task.failed_files += 1
 
         return True