Houseruling Intelligence and Wisdom checks as reactions?

Question

My high level players often face problems in combat. Enemy resists damage, something causes an effect, enemy's abilities are unknown. They often desire information about what has happened, i.e., "Why did I do no damage? Is he immune to Fire or is it something else?"

As I DM, I enjoy when players find out enemy information and respond accordingly. This falls under Improvised Actions, in the PHB.

Your character can do things not covered by the actions in this chapter, such as breaking down doors, intimidating enemies, sensing weaknesses in magical defenses, or calling for a parley with a foe. The only limits to the actions you can attempt are your imagination and your character’s ability scores.

However, if this information check is a full action, then PCs usually won't waste said action. They would rather do damage, or cast powerful spells, or anything else. From a player's point of view, I understand this approach. If the enemy goes down before the next round, all problems are solved, so this tiny bit of information seems quite irrelevant.

However, this tiny bit of information can be sometimes be crucial, i.e., "You notice a a crystal shining in the back wall, whose light seems to be shielding the boss and making him immune to damage!", and players need to deactivate this magical aura before engaging the boss. So, I homebrewed a rule,

Intellect and Wisdom checks (i.e., mental checks) can be used as a Reaction to an event that just occurred.

This has a cost for players (no longer able to counterspell, opportunity attack, etc), and allows them to gain information on what just happened. I would keep the information gained from these checks mostly limited, not the same as a full action spent on the check. So, for example, when the boss makes a necrotic attack, the players can use their Reaction to find out "Heh, is this an Undead?", and their Action to find "What are the weaknesses of this enemy?", using for example, a Nature roll (the PC might have found this creature in its past, and is trying to recollect information about it!).

So, bottomline, is this house-ruled mechanic balanced? Are there any classes that actually get a class feature to do such a thing, and therefore, it is not really fair for everyone to be able to do this for free? What are other options to allow players to get relevant information in combat without wasting actions?

@HellSaint You're right. I didn't notice that it was actually mentioned in the question. My apology. I thought you had only inferred it. I'll clean up my comments, and you can feel free to do the same if you'd like. — Bloodcinder, Jun 02 '18 at 18:02
@BlueMoon93 Typically it's considered the DM's job to describe superficial things that are apparent to the PCs. Would this rule replace that aspect of the game, or can the PCs still get some information without these extra checks? — MikeQ, Jun 02 '18 at 18:27
Which rules are you citing to say that INT/WIS checks during combat require an action? Based on this question, there don't seem to be any rules requiring that. — Icyfire, Jun 02 '18 at 18:29
@Icyfire I've also replied to that question, and I've added the relevant rules in here. — BlueMoon93, Jun 02 '18 at 19:12
@MikeQ What is obvious will remain obvious, yes. This is for less than obvious information, only something that would require a skill check (like maybe noticing that discrete shining light, or something) — BlueMoon93, Jun 02 '18 at 19:13
@BlueMoon93 can you exemplify with non-obvious scenario? The beginning of your question is about damage resistances and immunities, which are supposed to be obvious. — HellSaint, Jun 02 '18 at 21:49
@HellSaint why do you claim they are obvious? Hunters get a class feature to make them obvious, so I wouldnt allow other players to know such details without at least some check — BlueMoon93, Jun 03 '18 at 02:17
@BlueMoon93 Bad wording from me, change it for intuitive instead of obvious. And I say that because "assign a vulnerability, resistance, or immunity to a monster only when it's intuitive." - from DMG on how to create a monster. — HellSaint, Jun 03 '18 at 04:56
@BlueMoon93 While the Hunter's Sense is indeed a feature for identifying these stats from the monster, I would say a) it's a bad feature b) it's mostly for being "sure" or for very uncommon cases. But I would rather spending my turn casting something useful in that Fire Elemental rather than checking if it is actually immune to fire. — HellSaint, Jun 03 '18 at 04:59

score 8 · Answer 1 · edited Jun 03 '18 at 19:16

Passive Perception/Investigation is a built-in resource-free check

There is already a system in place to allow characters to notice things that happen around them without using their actions (or reactions): their passive perception and/or investigation. Note that the Observant feat on page 168 of the PHB mentions:

... your passive Wisdom (Perception) and passive Intelligence (Investigation) scores.

These scores are meant to represent a character's constant attention to their environment, which they can exert without special effort. If you want to make sure that your characters can pay attention without losing their action, the mechanic is already there to do so.

The problem with your homebrew rule is not as much a problem with how it will interact with your players, but rather with your monsters. There are several player classes that rely (strongly) on the fact that monsters must take an action to investigate their surroundings. Stealth and Illusions both become practically pointless mechanics if every monster you face can use a reaction to undo them.

score 5 · Answer 2 · edited Jun 16 '20 at 10:23

The Ranger's Monster Slayer archetype lets you learn immunities/resistances/vulnerabilities

Xanathar's Guide to Everything included the Monster Slayer subclass for the ranger, which gets the Hunter's Sense feature at level 3:

At 3rd level, you gain the ability to peer at a creature and magically discern how best to hurt it. As an action, choose one creature you can see within 60 feet of you. You immediately learn whether the creature has any damage immunities, resistances, or vulnerabilities and what they are. If the creature is hidden from divination magic, you sense that it has no damage immunities, resistances, or vulnerabilities.

You can use this feature a number of times equal to your Wisdom modifier (minimum of once). You regain all expended uses of it when you finish a long rest.

Battle Master Fighters and Mastermind Rogues can learn other information

The Battle Master's 7th level feature, Know Your Enemy, gives them some other info about an enemy:

Starting at 7th level, if you spend at least 1 minute observing or interacting with another creature outside combat, you can learn certain information about its capabilities compared to your own. The DM tells you if the creature is your equal, superior, or inferior in regard to two of the following characteristics of your choice:

Strength score

Dexterity score

Constitution score

Armor Class

Current hit points

Total class levels (if any)

Fighter class levels (if any)

The Mastermind's (also from Xanathar's) 9th-level feature, Insightful Manipulator, works similarly:

Starting at 9th level, if you spend at least 1 minute observing or interacting with another creature outside combat, you can learn certain information about its capabilities compared to your own. The DM tells you if the creature is your equal, superior, or inferior in regard to two of the following characteristics of your choice:

Intelligence score

Wisdom score

Charisma score

Class levels (if any)

At the DM’s option, you might also realize you know a piece of the creature’s history or one of its personality traits, if it has any.

How would the characters know this?

If the player characters are fighting a creature that's not common and well-known in the world, how would they innately be able to tell what the creature's immunities/resistances are, or what the source of a temporary immunity is? The player characters should be able to observe the outcome of their actions, and can conclude things based on that, but unless they have some special ability I don't see how they'd just instantly realize that "a magical aura in the air [...] is making [the enemy] immune to damage". It should be clearly justified how they're gaining this information.

Sometimes it's okay

This doesn't mean player characters can never know anything about enemies. In my experience, DMs will often let players make a Nature or Religion or History check for free to recall certain general information about a creature if it's the sort of thing that's well-known in the world. This information comes in the form of rumors and general knowledge, not specific facts that they know to be true; sometimes it may not all be true.

In addition, as far as I know, there are no official mechanics for whether it takes an action or bonus action or reaction (or no cost at all) to make such a check. As such, the entire mechanic of using any action/reaction at all to make a knowledge-related check is itself a house-rule.