Visible AC

[akr71]

As others have said, it becomes apparent what an opponent's AC is after a few rounds of combat. The second or third time the party fights the same opponent type, I will usually start saying "roll to hit AC 12." I do try to give clues or describe the opponent's armor where appropriate - "this unusually large orc is wearing mis-matched plate armor."

 

[Tony Vargas]

What do you think about playing D&D where you know your opponent's AC and saves, and maybe even HP? I think it could save the DM a step. I think it would avoid players wasting resources on weak threats. It would let you know when it was safe to power attack, or when a reckless strike would be beneficial. Knowing HP would convey a sense of progress in a long fight. What are your thoughts?

Wouldn't be my first choice for 5e or traditional (2e or earlier) D&D. Could conceivably work in 3.x, and I've run 4e 'above board' to almost that degree with good results.

In the 1e DMG, EGG advised DMs to know the game better than their players and essentially keep them in the dark. It was like a Sith master/apprentice relationship, that way. ;P But, it was good advice for a system like AD&D that required a lot of DM intervention. 5e's "ruling not rules" philosophy is similar, if more sophisticated and more positive in tone. IMHO the 5e DM should keep his options open, to take full advantage of his 'Empowerment' and simultaneously preserve his players' sense of verisimilitude, fairness, & 'agency' by taking as much of the mechanics and resolution behind the screen as he's comfortable with (and with keeping track of).

OTOH, it's worth noting that players can figure out a monster's AC by paying attention to their hit vs missed rolls, and can keep track of the hps they've inflicted, so it took them 53 hps of damage to drop a certain monster, and they meet another, they have an idea of how much work's ahead of them.

 

[Satyrn]

I openly track monster HP, counting down so the players who want to know can easily see how the opponents are doing. This was especially fun for me as DM when I was recently granting demons some ad hoc regenerative powers, such as the one that regained but points when standing in the fires of Hell. Well, what was most fun about that was their reaction to the demon submerging under the flames and me adding 100 hp back onto his total.