Cooperative Spell

[Cheiromancer]

While the two of you are adjacent, you and another spellcaster with the Cooperative Spell feat can simultaneously cast the same spell at the same time in the round. Add +2 to the save DC of cooperatively cast spells and +1 to caster level checks to beat the target's spell resistance (if any), using the higher base DC and level check of either caster. A cooperative spell uses up a spell slot of the same level as the spell's actual level.

The feat from Complete Arcane (p. 76) is often interpreted to mean that only one spell goes off, but with certain benefits. I don't think this interpretation is correct. There is no reference to how the decisions regarding the spell are made; where it is targeted, how variable effects are chosen (for spells like polymorph and summon monster), nor are there rules for level dependent effects like range and duration. Caster level checks are made using the higher caster level of the two casters (with a +1 bonus) but it does not say what the range, duration etc. are based on.

In other words, if there is a single combined spell results from the actions of multiple spellcasters, the feat does not provide the information needed to determine its effects.

If a separate spell is cast by each caster, information that is not specified is provided by the individual caster for his or her own spell. They can target different individuals, have different durations etc. as per the normal rules of spellcasting.

The usual objection to the 'separate effects' interpretation is that it would be over-powered if a lot of spellcasters had the feat. And of course WotC would never print an over-powered feat. Practically speaking, however, it is hard to optimize in a party. You would have to have an unusual degree of coordination in builds, and probably would need to abuse the Leadership feat as well. It is more likely that a DM will use the feat to power up the cabal of evil spellcasters- which is at should be.

tl;dr Cooperative spell is not playable if the final result is only one combined spell per cooperative casting. Therefore the spells that are cast cooperatively nonetheless have separate effects.

[Maginomicon]

Cooperative spell as I interpret it enhances one spell. This is backed-up by the contents of the Special section of the feat:

For each additional caster with this feat casting the same cooperative spell simultaneously, the spell’s save DC and the bonus on the caster level check both increase by 1. When more than two spellcasters cooperatively cast a spell, each must be adjacent to at least two other casters involved in the casting. For example, two wizards and two sorcerers standing in a circle all have Cooperative Spell. The first three in the initiative order ready an action to cast fireball, casting the spell when the fourth does. The base DC of the spell’s save is equal to the highest save DC among the cooperative casters (as determined by relevant ability scores, other feats, special abilities, or items) +4 (+2 for the first cooperative caster and +1 for each of the other two). As well, whoever has the highest caster level determines the base caster level check, which gains a +3 bonus (+1 for each cooperative caster)

However, I agree that it's essentially unplayable. A lot of metamagic is, which is why in my main game I enhance sorcerers with Metamagic Access (http://www.giantitp.com/forums/showthread.php?t=284296).

The purpose of cooperative spell is to cast a single LA+0 spell that's harder to resist (in DC), beats SR easier (in a bonus to CL checks), and has great flavor to boot.

[Cheiromancer]

Yeah, but 'same spell' is ambiguous. If two wizards cast fireball simultaneously, neither possessing cooperative spell, they are casting the same spell. Separate instances, but the same spell. If they each possess cooperative spell they get a benefit when they cast fireball simultaneously, but the spells are still separate.

I think there is a feat or class ability which gives a bonus if you cast the same spell in two consecutive rounds. That means you cast (say) a fireball in round one and another fireball in round two. It is the same spell, but separate instances. That is the kind of 'same' that I mean. Same in kind, but not numerically the same.

Sometimes 'same' will mean numerically the same. If you have to attack the same target twice in a round to get a benefit, it won't work if you only hit the same kind of target. Hitting Bugbear A and then Bugbear B is hitting the same kind of target, but its sameness in kind, not numerical sameness.

I resolve the ambiguity by noting that on one interpretation the feat is unplayable. If you can't determine the duration, range or target of the spell under the 'combined effect' interpretation, but you can on the 'separate effect' interpretation, you should go with the interpretation that works.

[Chronos]

This interpretation looks justified by the reading to me: Ordinarily, if two casters cast the same spell simultaneously, it goes off twice, and nothing in the feat says that doesn't happen here. Nor does the effect seem overpowered: You need to spend at least two feat slots on it (one for each caster), plus jump through hoops like having to coordinate the build of multiple characters and delaying initiative. +2 to caster level checks and +1 to penetrate spell resistance isn't all that huge a benefit, for that cost.

[Ranting Fool]

I've always assumed one person loses a spell to buff another and thought that it was really very weak (apart from the joys of low level NPC's channeling one massive hold person/Zone of truth or something)