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If you are in combat and an effect increases or decreases your Dexterity enough to change the modifier, does your AC also immediately change to use the new modifier?

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    \$\begingroup\$ Did you have a specific reason in mind why it wouldn't? Like you're not used to being that agile, and don't know how to use it to dodge until you've been that dextrous for some time? I think a dex-increasing item that worked that way would take time before your dex score increased, because your dex score is an instantaneous measure of how good you currently are at all things dex-related. \$\endgroup\$ – Peter Cordes Jan 29 '18 at 12:26
  • \$\begingroup\$ There are not many immediate Dex-altering effects in 5E. Could you maybe give an example of the effect you are concerned about? It might be something from a 3E sourcebook, and worth looking for the equivalent 5E version (which probably just alters your speed, AC or initiative etc). \$\endgroup\$ – Neil Slater Jan 29 '18 at 16:48
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Yes.

Your armor class would certainly respond dynamically to changes in any value that is used to calculate AC.

Your armor falls off? AC changes. Drop your shield? AC changes.

Sprain your foot and lose Dexterity? Yup, your AC is recalculated from your new, lower Dex modifier.

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  • \$\begingroup\$ Ok thank you. I did not see it in the PHB like they have for Constitution. \$\endgroup\$ – Noctum Jan 29 '18 at 1:06
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    \$\begingroup\$ In general, effects that change ability scores immediately affect any associated modifiers that are calculated based on that ability score. \$\endgroup\$ – V2Blast Jan 29 '18 at 8:47
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    \$\begingroup\$ Constitution is a special case because changes to it retroactively changes the number of max HP you got each time you previously levelled up, so it deserves being called out explicitly. \$\endgroup\$ – Quentin Jan 29 '18 at 9:53
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    \$\begingroup\$ @Quentin What really sucks is that if you ended a previous encounter with low HP. A retroactive reduction in HP means you actually died in that encounter, which means you become some kind of undead immediately. And then you have to track if a cleric used turn undead since that encounter. It is a big headache. Wait a second... \$\endgroup\$ – Yakk Jan 29 '18 at 14:13
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    \$\begingroup\$ @Yakk It means retroactively only for calculating current HP. If you lose 4 CON on todays adventure, you still had that CON in the past. You lose 2 HP for ever level you have right now even though you earned the levels in past adventures. Compare this to a game like Oblivion (video game) where increases in HP granting stats only grant extra HP from that level onward. \$\endgroup\$ – Lawtonfogle Jan 29 '18 at 15:02
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Maybe

AC is calculated on a case by case basis, with the player having the choice to take the best result as applicable to them.

For example, if your Dex is 17 and benefiting from both Mage Armor and Barkskin, your AC would be 16. This is because of the following:

  • Mage Armor calculates AC as 13+Dex Mod (+3 here), so your AC is 16.
  • Barkskin calculates AC as 16 and it cannot be lower.

If an effect were to increase your Dex to 18, then your Dex mod would change and you would use the AC calculation for Mage Armor, which will provide you the more beneficial AC of 17.

Conversely, were your Dex lowered due to some effect, you would use the AC calculation for Barkskin, which sets the AC at a static 16.

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