Ovinomancer
No flips for you!
I took you to be saying adding hitpoints during a combat. If you mean rolling hitpoints, or selecting a different value from the average (and including this choice in encounter design), then sure, that's within the set of choices the GM is allowed to make under the rules.Lets put social contract aside for the sake of conversation.
The hit points listed in the MM are the average, therefore one could technically increase them.
Depends, were such changes included in encounter design? Did the GM follow the rules for making changes to or creating new monsters? The GM has many things they can do under the rules, but there's still rules for those.If you (as a player) had not read the MM, and fought a creature, and the DM played it differently (additional Legendary Action) and you would only discover this while reading the stats of the creature post game, would you then accuse the DM of cheating or breaking the rules?
You seem to be on a kick of pointing out things there are rules for as a defense for not following the rules.Do you never amend monsters as DM?
There seems to be quite a lot of leeway in AL games as well. 5e is a poorly constructed ruleset for providing clear rules of play, even with AL guidelines, and quite often just plugs the GM in as the rules. To be clear, this is within the rules -- how stealth works is up to your GM, per the rules. The GM being a total jerk about stealth and denying it in most all cases is by the rules of the game. That's a different problem. Here we're talking about if the GM is actually beholden to the rules or if they're special with regard to not having any rules apply to them.To be clear I'm not talking about Adventure League games. In that constructed game format you play the rules down to the T.