Yaarel
🇮🇱 🇺🇦 He-Mage
The DM determines if an effort is an autosuccess or an autofailure.We are told that the GM only asks for a D20 test when the outcome is in doubt (ie, there is chance of success and a chance of failure). We are also told that a natural 1 is always a failure, and a natural 20 is always a success.
So, under what situation would a nat 20 be a success without the "autosuccess rule" AND the GM asked for a roll anyway? If say, he determines that a character needs to roll 27 on the D20 to succeed, he's supposed to not ask for a roll. So no possibility of rolling 20. If there is a possibility of success, 20 will already include a success.
Same, in reverse, with 1.
EDIT: changed the title to show that my question wasn't related to the case where the DM disallows a roll altogether (it was a side thought) but to WHEN (and in that case HOW OFTEN) does the "auto-success on 20" change the outcome of a task.
Only if the situation really might go either way, does the DM ask for a d20 roll.
Combat mechanics are the most tested − and most routine − aspect of D&D. But even during a combat, the players might try to do something innovative, which requires the DM to determine how plausible and possible it is in the current situation.