Why is AIRFLOW__CELERY__TASK_ACKS_LATE True by default?

[sdaberdaku]

Hello all,

I have a question regarding the AIRFLOW__CELERY__TASK_ACKS_LATE configuration. According to the documentation:
https://airflow.apache.org/docs/apache-airflow-providers-celery/stable/configurations-ref.html#task-acks-late

If an Airflow task’s execution time exceeds the visibility_timeout, Celery will re-assign the task to a Celery worker, even if the original task is still running successfully. The new task instance then runs concurrently with the original task and the Airflow UI and logs only show an error message: ‘Task Instance Not Running’ FAILED: Task is in the running state’ Setting task_acks_late to True will force Celery to wait until a task is finished before a new task instance is assigned. This effectively overrides the visibility timeout.

This description makes it seem like setting this variable to True will effectively force Celery to wait until a task is finished before a new task instance is assigned, which is the opposite of what the celery documentation states.
https://docs.celeryq.dev/en/stable/reference/celery.app.task.html#celery.app.task.Task.acks_late

When enabled messages for this task will be acknowledged after the task has been executed, and not right before (the default behavior).

Please note that this means the task may be executed twice if the worker crashes mid execution.

If the visibility_timeout is exceeded, and the task is not acknowledged yet, it will be picked up by another celery worker, regardless of whether the original run has completed or not. I think the default value of this configuration should be set to False instead, since Airflow has its own retry mechanism of detecting failed/zombie tasks and rescheduling them.

[dylannz]

I had this problem on Airflow 3.1.7 as well, and it manifested in the logs as:

[2026-03-10 08:55:46] ERROR - Server indicated the task shouldn't be running anymore. Terminating process detail={"detail":{"reason":"not_running","message":"TI is no longer in the running state and task should terminate","current_state":"failed"}}
[2026-03-10 08:55:51] ERROR - Task killed!

Took me a very long time to debug it, but setting AIRFLOW__CELERY__TASK_ACKS_LATE=False fixed it. Leaving this comment here so that anyone searching for the above error will hopefully find this issue...

I am using Redis as a broker, which I believe is an important detail in this instance as well. By default, anyone using this setup will not be able to run jobs longer than 1 hour (as the default visibility_timeout is 3600).