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What is the correct way RAW or RAI, (not DM Fiat, or Homebrew), That a negative Dex modifier should be applied to armor class for Light, Medium, and Heavy Armor.  I was looking at D&D Beyond and it applies the total negative Dex modifier to AC for light and medium armor, but does not apply it at all to Heavy Armor.

The rule for calculating AC in the Player's Handbook is:

• Armor Class
• A creature’s Armor Class represents how well the creature avoids being wounded in combat. The AC of a character is determined at character creation (see chapter 2), whereas the AC of a monster appears in its stat block.
• Calculating AC. All creatures start with the same base AC calculation:
• Base AC = 10 + the creature’s Dexterity modifier
• A creature’s AC can then be modified by armor, magic items, spells, and more.

Furthermore, the Armor itself limits how Dexterity can be applied to calculating AC as Light Armor (+ Dex Modifier), Medium Armor (+ Dex Modifier (max 2)), and Heavy Armor (Nothing listed - I can only presume an oversight)

I can accept that the intention of the Heavy Armor is intended to be something like +Dex Modifier (max 0).  My question is why would you apply a negative Dex modifier to Light and Medium Armors, but not Heavy Armor.  If we use Medium armor as an example, its says the max we can add is +2.  It doesn't say anything about a minimum Dex modifier to apply. That would be consistent with how other stats work.

Is this a situation that WoTC needs to errata?  I am missing something?

The Dexterity modifier not being applied to heavy armor at all is not an oversight; it's very intentional, and it has always been that way in 5e D&D.

(Except that it doesn't say that)

Also D&D Beyond applies any negative Dex modifier to AC, not just -2

I recommend just reading the rules as they are and not assuming something is missing.  5e has been around more than 10 years, it's fully vetted out.  What you are missing is that it's a rule set, not a combat simulator.

The base rule, as I posted, says to add your Dex Bonus to your ac calculation.  It also says that armor may modify this calculation.  I can accept you dont get to add dex to Heavy Armor due to the way that light and medium is written.  Nothing in any of them says you should or shouldnt have a min modifier.  Given that the armor is silent on the how much dex you can subtract, then you refer to the base rule.

That is what Rules as Written means.  Not the way it was in some previous version.  Not the way you think it should be done.

If there isnt a minimum for one, then there isnt a minimum for any of them.  If you are saying there is no limit on light, because the max is not limited, then why would there be no limit on medium that has a max of +2?  You logic fails.  You need to apply the same logic to all of the armors, not however you decide makes sense to you.  Its either unlimited light, -2 to +2 for medium,  -0 to +0 for Heavy  Or -any for all, any + for light, +2 for medium, and +0 for Heavy.  Without further explanation from the authors, there is no other way to interpret it. 

Also the rules for ac is for characters and monsters,

 Either Medium armor should be capped at -2, or Heavy armor is capped at any minimum - you cant have it both ways

But the base rule already said it is added.  If something doesnt change that, then it is still added.  Effectively, as it is specifically written, there is no change from the base rules for heavy armor.

Here is my point clearer.

In D&D 2014 the following text is present in the rules

“Heavy armor doesn’t let you add your Dexterity modifier to your Armor Class, but it also doesn’t penalize you if your Dexterity modifier is negative.”

That statement is why in 2014 your interpretation of the Armor Chart would have been correct.

That statement does not exist in 2024, so technically, RAW, Heavy Armor is modified by base rule for calculating AC, there is no limit to the plus or minus.

That is RAW.  Clearly not RAI, but absolutely RAW.  This needs an errata.

Quote from Hamarion >>

Here is my point clearer.

In D&D 2014 the following text is present in the rules

“Heavy armor doesn’t let you add your Dexterity modifier to your Armor Class, but it also doesn’t penalize you if your Dexterity modifier is negative.”

That statement is why in 2014 your interpretation of the Armor Chart would have been correct.

That statement does not exist in 2024, so technically, RAW, Heavy Armor is modified by base rule for calculating AC, there is no limit to the plus or minus.

That is RAW.  Clearly not RAI, but absolutely RAW.  This needs an errata.

Let's see if we can get this in one go.

First some basics before we get into Rules Quotes.

• The Combat section tells you how the default way to calculate Armor Class.
• Rhythm of Play tells you that "Exceptions Supersede General Rules".
• The Armor section gives the method of calculating the base AC for each piece of armor. Magic versions of these sometimes modifies these further (these are exceptions superseding the general rule).

First the Base Armor Class:

A creature’s Armor Class represents how well the creature avoids being wounded in combat. The AC of a character is determined at character creation (see chapter 2), whereas the AC of a monster appears in its stat block.

Calculating AC. All creatures start with the same base AC calculation:

Base AC = 10 + the creature’s Dexterity modifier

A creature’s AC can then be modified by armor, magic items, spells, and more.

Only One Base AC. Some spells and class features give characters a different way to calculate their AC. A character with multiple features that give different ways to calculate AC must choose which one to use; only one base calculation can be in effect for a creature.

So we establish that a Base AC of 10 + your Dexterity Modifier is your default. It is not explicit but this option is available to you even if you have other Base AC options. It is unlikely that you have a worse option though. However, a Fighter can reach a 30 Dexterity with Epic Boons after level 20 and could therefore prefer it to an AC of 10 + 10 (20) over the base Full Plate  (18) option. Only One Base AC says that if you have multiple options you can choose.

Now, let's look at Rhythm of Play for exceptions supersede general rules:

Exceptions Supersede General Rules

General rules govern each part of the game. For example, the combat rules tell you that melee attacks use Strength and ranged attacks use Dexterity. That’s a general rule, and a general rule is in effect as long as something in the game doesn’t explicitly say otherwise.

The game also includes elements—class features, feats, weapon properties, spells, magic items, monster abilities, and the like—that sometimes contradict a general rule. When an exception and a general rule disagree, the exception wins. For example, if a feature says you can make melee attacks using your Charisma, you can do so, even though that statement disagrees with the general rule.

Because of this, anything that changes your Base AC calculation, by RAW, the old formula no longer applies (or is modified by the specific rule). Arguably because of Only One Base AC, if you have multiple rules for calculating your AC, you choose which one you want to use.

Now we go to the Armor Section of Equipment:

Armor Class (AC). The table’s Armor Class column tells you what your base AC is when you wear a type of armor. For example, if you wear Leather Armor, your base AC is 11 plus your Dexterity modifier, whereas your AC is 16 in Chain Mail.

This specific rule has now overridden the Base AC calculation and tells us that the new Base AC formula is what we see in the table.

This means for example:

• Studded Leather is 12 + your Dexterity Modifier.
• A Breastplate is 14 + your Dexterity Modifier (Maximum of 2).
• Full Plate is 18.
• Shields are +2 that can be added onto most Base AC calculations but not all.

Note: The numbers in the AC column are a new formula and not added to your Base AC from Above. Nothing says if you are wearing Studded Leather use a 12 instead of a 10. It's not 10 + 12 + Dexterity. It's just fully a new formula to replace 10 + Dexterity. The same with Full Plate, the formula just becomes Base AC = 18.

Some examples of features that give an alternative AC calculation formula:

• Mage Armor
• Monk Unarmored Defense (no Shields)
• Barbarian Unarmored Defense
• Being an Autognome
• Being a Lizardfolk
• Bing a Loxodon (The natural armor is completely Constitution based)
• Being a Beasthide Shifter
• Being a Simic Hybrid
• Being a Thri-Kreen
• Being a Tortle

Being a Warforged modifies your AC but doesn't give a new formula per se.

If the formula you are choose to use does not include Dexterity, it does not apply.

My Point was that silence on behalf of the Heavy armor category does not create a new formula.  You can imply that, and that makes it Rules As Intended, but it doesn't make it Rules As Written.  Thus, according to RAW, the silence in the Heavy Armor category relating to Dexterity results only in a modification to the base formula as it relates to your base armor class and the portion regarding adding your dexterity is unchanged.

The lack of the clarifying statement from the 2014 version effectively modifies the way that Heavy Armor Dex calculations has been done for 50+ years.

I am not proposing that everyone start using Dex modifiers + or - in their plate armor.  This is clearly not the rule as it was intended.  All I was doing was calling out a deficiency in the 2024 rules relating to the rules as they are written and asking that the developers errata this error.

Except you are ignoring that the armor section of the rulebook literally tells you that it's a new "Base AC" calculation. As in, the old one no longer applies, nor is it a modification to the old one. It's a replacement. Some armors explicitly tell you that DEX is part of the new calculation, others tell you it's part of it up to a maximum, and yet others do not mention DEX because it's explicitly NOT part of the calculation.

I am absolutely amazed with the lack of understanding of the difference between Rules as Written and Rules as Intended.  I have never said that the developers intention was not exactly what everyone has stated.

I ran this scenario through several AI sources and everyone of them once they reviewed the applicable issues agreed that the RAW was that Heavy Armor should include Dex modifiers.  One of them even went so far to comment that it was amazing that it was written this way since for over 50 years Heavy Armor has always been written to exclude Dex modifiers of any kind and that it would be interesting to see if an errata was forthcoming or if the developers intended something different.

In short, RAW is exactly as it is written in the books.  Not what it was before, not what it should be, not what the developers intended.  In this case, exactly as it is written, there is no modification to the dexterity rules for heavy armor.

Sorry, that literally made me laugh out loud. LLMs will tell you Genghis Khan is the current president of the US if you argue with it long enough (and tell you how you're such an intelligent person to finally uncover the truth).

Exactly how are you parsing the following and NOT coming to the conclusion everyone else is?

Armor Class (AC). The table’s Armor Class column tells you what your base AC is when you wear a type of armor. For example, if you wear Leather Armor, your base AC is 11 plus your Dexterity modifier, whereas your AC is 16 in Chain Mail.

It literally gives you an example of heavy armor not including DEX mod alongside a light armor that does.

Let me put it a different way:
Let's assume the Base AC calculation (not wearing armor) did not include DEX mod. Would you use your DEX mod on those armors that had it in the calculation (like light and medium)?

Thank you , that is exactly the citation I was looking for.  Something to clarify what the other section meant.

... that was the 4th time it was cited in this thread alone...

I just went back and looked.

It was posted in the very long response by SmiteMakesRight_3_5 and I missed it.  It wasn't posted 4 times.  Exaggerations and opinions were not what I was asking for.  I was asking for specific citations that impacted what I was seeing.

I appreciate both Smite and you posting that citation.  You don't need to act like a jerk about me asking a question on a message board that is for asking questions.

I apologize. You are correct.  I missed the significance of the quote at the time.