Hello. What is the correct way to fail task that have retry logic, but I am sure that even 100 retries will not help?

Exactly, you can raise an ENDRUN signal and this way Prefect won't attempt any additional retries

It will fail whole flow as well, right?

No, it will only end the task run in which you raise the signal. But if this task is a reference task, then yes the effect will be that this flow run will be considered failed

Will I see error message in Prefect UI?

test it out 🙂 you should see exactly the same message that you defined in your Failed state:

from prefect.engine.signals import ENDRUN
from prefect.engine.state import Failed
from prefect import task


@task
def end_with_failure():
    raise ENDRUN(Failed(message="your_message"))

Thanks!