House Rules - Spellcasting

Between editions, D&D (and Pathfinder) has gone back and forth on so many rules that a few hours of research has given me a distinct case of whiplash.

Things already true but not always clearly worded in the books:

• Just about any character in the world recognizes spellcasting when it is done in front of them; a verbal component consists of mystical words (explained below) and a somatic component is weird moving around of arms/hands/fingers. However, unless they know something about magic, they won't have any idea what spell has just been cast, unless there are clear effects.
• A character does not automatically know they are under the effect of a spell they failed to save against if there are no perceptible effects. After the spell ends they won't automatically know they were under a spell but their might be reasons for them to suspect so.
• A spellcaster doesn't automatically know whether a spell's target succeeded on a saving throw against the spell, but with most spells, the effects are perceivable on the target. (Jeremy Crawford) [Note that in 3.5 the official rule was that you did know for targeted spells.]
• If you're concentrating on a spell, you know when that spell ends, whether you end your concentration willingly, your concentration is broken, or the spell ends in some other way.

Spellcasting Changes

This is a quick listing of the changes. The next section explains why I came to these changes.

• A character knows they have been the target of a spell, if they succeed on the save imposed by the spell. However, unless they have anything else to go on, they might not know the source of the magic. Also note that this is a general rule; if an ability, feat, item, or spell description says the target won't notice anything even on a successful save, that supersedes this rule.
• Characters can voluntarily fail a saving throw but only when warned about the save beforehand and not under a magical compulsion.

Background

Knowing a spell has been cast on you.

This is definitely a tricky one. It will also lead to inevitable meta gaming when the spell is cast on one or more pc's. When the DM tells the players to roll a save, they fail and the DM ask them to just continue what they were doing cause 'nothing happened', the actual result will still be that even the best roleplayers will consider those failed saves when planning further moves.

Clearly the 'problem' is mostly with a few enchantment spells; the effect of most other spells is usually quite evident. But when you cast Enthrall it is certainly relevant that when the creature makes its save, if it knows you just cast Enthrall on it. Looking at all the versions of the rules, internal logic both within the rules and the setting, I think the fairest interpretation is this:

A character saving against a spell (such as Enthrall) will know a spell has just been cast on it, and that they resisted it. However, unless they know anything about magic (by being a spellcaster themselves, having the Arcana skill and rolling well, etc.), they won't know what spell was cast. If they did not see anyone cast the spell (because the caster was behind them or invisible, and there was no verbal component, etc.) then they also won't know the source of the spell. If they did see someone cast a spell right in front of them, then they will assume this was a unfriendly/hostile action, unless warned ahead of time ("I'm going to cast a spell now").

Do you always know when you’re under the effect of a spell? (Sage Advice)

You’re aware that a spell is affecting you if it has a perceptible effect or if its text says you’re aware of it (see PHB , under “Targets”). Most spells are obvious. For example, fireball burns you, cure wounds heals you, and command forces you to suddenly do something you didn’t intend. Certain spells are more subtle, yet you become aware of the spell at a time specified in the spell’s description. Charm person and detect thoughts are examples of such spells.

Some spells are so subtle that you might not know you were ever under their effects. A prime example of that sort of spell is suggestion. Assuming you failed to notice the spellcaster casting the spell, you might simply remember the caster saying, “The treasure you’re looking for isn’t here. Go look for it in the room at the top of the next tower.” You failed your saving throw, and off you went to the other tower, thinking it was your idea to go there. You and your companions might deduce that you were beguiled if evidence of the spell is found. It’s ultimately up to the DM whether you discover the presence of inconspicuous spells. Discovery usually comes through the use of skills like Arcana, Investigation, Insight, and Perception or through spells like detect magic.

Succeeding on a Saving Throw (Pathfinder SRD)

A creature that successfully saves against a spell that has no obvious physical effects feels a hostile force or a tingle, but cannot deduce the exact nature of the attack.

Failing a save willingly

Normally a save is a split second reaction to an unexpected event; jumping away from the suddenly appearing fireball (or car). When it comes to the three mental abilities we have no real world examples. The 5E rules don't mention you can voluntarily forgo a save but earlier editions of D&D and Pathfinder do. An on Twitter two designers have said they would allow it in most cases.

Voluntarily Giving up a Saving Throw (Pathfinder SRD)

A creature can voluntarily forgo a saving throw and willingly accept a spell’s result. Even a character with a special resistance to magic can suppress this quality.

Twitter Sources

Jeremy Crawford: No rule lets you opt to fail a save. As DM, I might allow it, assuming you aren't incapacitated or dominated.

Mike Mearls: I'd allow it only if the creature is not under any magical compulsions like charm person.

Command, Suggestion, etc

Is the sentence of suggestion in the suggestion spell the verbal component, or is the verbal component separate? (Sage Advice)

Verbal components are mystic words, not normal speech. The spell’s suggestion is an intelligible utterance that is separate from the verbal component. The command spell is the simplest example of this principle. The utterance of the verbal component is separate from, and precedes, any verbal utterance that would bring about the spell’s effect.