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Know Your Enemy (PHB, p. 73-74) is a Battle Master fighter feature that lets you determine whether a creature's Armor Class is superior, equal to, or inferior to yours.

What happens if a character's Armor Class changes during the 1 minute you observe them for this feature (such as by donning armor)? Does it take the creature's Armor Class at the beginning or end of the minute?

Does it consider temporary effects that are added such as shield of faith, mage armor, or a shield being wielded, or just the creature's base AC?

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It's not explicit, but it's likely the end of the minute.

It's not explicitly stated in the text of the feature, but it makes sense for you to know the creature's AC at the end of that minute because you have to be observing or interacting with the creature for the entire minute.

It would probably be obvious to you that the creature had put on armor or picked up a shield (in fact, it takes at least 1 minute to put on most kinds of armor), since you would be watching them do it. Shield of faith is also explicitly visible, so you would know that at least something is up. Mage armor explicitly changes your base AC, which means that you don't have a "temporary AC" and a "real AC" during the spell--you just have one AC that has changed.

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I feel this is completely a DMs call. Except if it is you donning a new armor for a minute, you are not observing or interacting with your target so the feature fails. If your target is, then you know what kind of armor he is wearing now as you watched him don it, you should get information about the end AC.

For the spells or other temporary affects that start or end during your interaction (as unlikely as they are to suddenly decide to cast mage armor out of combat) there is no go to rule. DM's call through and through.

The DM could simply rule you are to choose another characteristics to get information on as the constantly changing AC is making it difficult to tell or because there is magic involved. Or just tell you the end AC or the start AC. Only your DM knows the correct answer to this.

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  • \$\begingroup\$ What if you are the one helping him don the armor while talking to him? There are ways he could don armor during the minute. \$\endgroup\$
    – David Coffron
    Apr 25, 2018 at 12:19
  • \$\begingroup\$ "you know what kind of armor he is wearing now as you watched him don it, you should get information about the end AC." \$\endgroup\$
    – FenrirG
    Apr 25, 2018 at 12:27
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Cut the minute up in parts and narrate the evolution back to your player.

RAW I agree with Icyfire's answer; however, this is not how I would rule it when running the game.

Since the Battle Master is observing the target actively and knows about these things, they should be able to assess that the target is getting better and better defended. Only giving the player the information at the end of the minute seems to be doing them a disservice when they should be rewarded for using the feature in a smart way. As such, give them time to act during the change in AC:

  • Player: I observe my opponent and try to assess his weaknesses using my class feature
  • DM: You catch your opponent in the midst of his preparation for the upcoming fight, he has already put on his bracers, but his breastplate still lies on the floor at his feet.
  • Smart Player: I take advantage of this and attack, focusing my attacks on his obviously unprotected chest.

Result: the AC counted is as it was at the beginning of the minute (you could go further than this by having an actual gradual climb in the AC over time, but I feel like that would take you too far).

If the player decides not to act in that moment and lets the target get ready for the fight, follow Icyfire's answer and give them the assessment as given by the feature at the end of the minute.

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  • \$\begingroup\$ I feel the feature doesn't change anything in your example, anyone should be able to see the enemy is still in the process of donning his armor... While a good kind of encounter it seems unrelated to the feature in question... \$\endgroup\$
    – FenrirG
    Apr 25, 2018 at 14:32
  • \$\begingroup\$ @FenrirG In my eyes they're related since I wouldn't generally allow players to interfere at that point and just have them battle the fully prepared opponent. The fact that the battlemaster is trained at recognizing armor weaknesses is what would prompt me to put this in when they try to assess their opponent's defensive capabilities. \$\endgroup\$
    – DonFusili
    Apr 25, 2018 at 14:37
  • \$\begingroup\$ The scenario you've described in your answer seems like an excellent example to use when discussing when to apply advantage rather than when to state a feature tells you the current AC of a creature. \$\endgroup\$
    – Lino Frank Ciaralli
    Apr 28, 2018 at 4:43
  • \$\begingroup\$ 5E doesn't have called shots like that, so the so-called "smart player" isn't actually doing anything special. \$\endgroup\$
    – T.J.L.
    Feb 17, 2020 at 16:14

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