Can spellcasters try to make it difficult to identify what spell they're casting?

Question

How to identify a spell being cast? discusses how to identify a spell.

This question seeks to discover whether there are steps a spellcaster could take during casting to make this identification more difficult. The question is not about hiding the casting (such as casting the spell round a corner, then releasing it with a reaction) - the person trying to identify the spell has full view of the spellcaster casting it.

Such steps might include:

• Putting some irrelevant words around the actual mystic words of the verbal component
• Waving your hands as if performing a somatic component, despite the spell not having one

Is this sort of thing possible while casting a spell (to be clear, without any special capability such as the Sorcerer's Subtle Spell), and if so what effects does doing so have on spell identification?

Assume for the purpose of the question that the rules from Xanathar's Guide on spell identification are in use.

Answer 1

RAW, no.

For a caster who is required to use all components to a spell (Somatic, Verbal, or Material), there is no written mechanic to attempt to hide what is being cast.
In Xanathar's Guide to Everything p.85, rules for identifying a spell say "If a character can perceive the casting, the spells effect, or both, the character can make an Intelligence (Arcana) check with [their] reaction or action. The DC equals 15 + the spell's level." This should apply as a PC watching an NPC cast a spell, or as an NPC watching a PC cast a spell.

Rules designer Jeremy Crawford states on his twitter:

In Xanathar's, Identifying a spell requires an action or a reaction because it involves focused deduction; it's not automatic. Moreover, I didn't want combat to devolve into people identifying every spell.

By what he has written, a spell is already hard to identify, but a DM may homebrew this rule to instead mean a creature must spend a reaction or action to identify a spell due to all casters attempting to hide the spell being cast through some form of obfuscation, as you've mentioned.

Answer 2

There is no specific mechanic for this, but...

As Jon Aristotle says, there is no specific mechanic for attempting to hinder the recognition of spell identity. However, what you are describing fits within the general rule of the GM assessing whether environmental "circumstances" merit rolling an ability check with advantage or disadvantage (PHB 173). Thus, the rule that applies is the GM deciding whether the caster's attempts to obfuscate their spell identity are significant enough to merit disadvantage on the roll of the creature attempting to identify the spell.

A few considerations based on the action economy

What you don't want to have is a caster being permitted to impose disadvantage on spell identification automatically or 'for free'. If it was that easy to do, you would assume that all casters did it all the time. Doing something confounding enough to impose disadvantage must be exceptional, and that means it must either cost something, or be extremely situational, or both. In terms of the cost, it bears mentioning that the creature attempting identification has to spend its reaction to do so. Frustrating their effort should have at least an equivalent cost.

However, the caster is already spending their action on casting the spell. If they are going to cast the spell and in addition disguise it, they must be able to spend something in addition to their action (and see "Other Activity on Your Turn", PHB 190). Possibilities (in my opinion) could include:

Their reaction. "As you begin your spell, you notice that the enemy mage is paying close attention to your gestures." "I would like to use my reaction to turn away from them provided I can still see the spell's target." "Okay, you have obscured your gestures and it is harder for them to hear your voice - they will have disadvantage on their attempt to recognize the spell as you cast it although someone in another position will not be affected."

Their object interaction. In addition to their spell-casting action, the caster might be doing other things with their hands (cf). However, if they aren't, for example if they are casting one of the sixty-some spells that have verbal components only, they could be allowed to spend their object interaction making meaningless gestures so as to throw someone off.

The "brief utterances" they can make on their turn. Especially if they are casting one of the twenty-some spells that don't have a Verbal component, they could fill their turn with nonsense words or even words from other, actual spells so as to confuse the issue.