Right okay so here's a question I'm hoping someone can provide clarity to under the D&D 3.5 rule set. I've used the PHB, DMG and the 3.0 Rules of the Game Animals Part 4 to research an answer though I'm no closer to a solution.
From the ranger or druid class ability animal companion, spells can be shared with the companion via the share spells ability. This is also true of magic items, from Rules of the Game Animals Part 4:
You and your animal companion can share effects from magic items only if the effect is a spell that you can target on yourself. For example, you and your animal companion can share a barkskin spell cast from a wand, but cannot share the armor bonus from bracers of armor.
How does this apply to magic items with continuous ongoing effects? Say for example the character is wearing a Mantle of Spell Resistance granting the wearer spell resistance 21, the wearer approaches and stays within 5ft of his animal companion choosing to share his spells with the animal companion. Does the animal companion also receive spell resistance 21 provided the wearer stays within 5ft of his animal companion?
Or is the understanding that sharing spells is only applicable if the character casts a spell or produces some effect at the time or after the character approaches and remains within 5ft. For example the character is riding along on his animal companion heavy horse suddenly the ground beneath them gives and both the animal companion and the character plunge down a 30ft pit. But luckily the character is wearing a ring of feather falling and therefore the feather fall spell is activated and BOTH the character and the animal companion drift slowly and safely to the bottom?
Last example using the character and heavy horse animal companion again from above, the character and animal companion are riding along the trail when suddenly an orc shaman jumps out of the bushes and casts magic missile, the character is wearing a brooch of shielding which protects against magic missile however the orc shaman casts the spell at the horse. Does the shield spell effect of the brooch of shielding protect the horse?
All of the above examples use reactive spells or effects, the character hasn't actively cast a spell or produced an effect to share.
Hoping someone can shed some light on this.
Best Answer
Typically only a spell from a spell completion or spell trigger magic item can be shared because share spells requires casting the shared spell
The description of the share spells extraordinary ability says, in part, says, "At the druid's [or ranger's] option, she may have any spell (but not any spell-like ability) she casts upon herself also affect her animal companion" (Player's Handbook 36). Skip Williams's Rules of the Game Web column "Animals (Part 4)" doesn't change this when it says
The column's previous paragraph says, "To share a spell, you must cast the spell and you must select yourself as its target. The spell must have a target entry; you cannot share effect and area spells," and that's really important.
Neither brooch of shielding (Dungeon Master's Guide 250) (1,500 gp; 0 lbs.), mantle of spell resistance (DMG 261) (90,000 gp; 0 lbs.) nor ring of feather falling (DMG 232) (2,200 gp; 0 lbs.) sees the animal companion's master actually cast a spell, so a master can't share those magic items' effects with an animal companion… much like how Williams says that the master can't share the effects of the bracers of armor (DMG 250) (1,000+ gp; 1 lb.).
This isn't a contradiction. In much the same way that slapping onto his wrists the bracers of armor is not the same as the wearer casting the 1st-level Sor/Wiz spell mage armor [conj] (Player's Handbook 249), when the ring of feather falling activates, the wearer is not himself casting the 1st-level Sor/Wiz spell the spell feather fall [trans] (PH 229). Instead, when the ring activates, the wearer does nothing and gains automatically the feather fall spell effect from the ring. Similarly, when a creature dons a mantle of spell resistance, this isn't the same as the creature casting upon itself the 5th-level cleric spell spell resistance [abjur] (PH 282), and when the creature pins to that mantle a brooch of shielding, this isn't the same as the creature casting upon itself the 1st-level Sor/Wiz spell shield [abjur] (PH 278).
However, when an animal companion's master activates a wand of barkskin [trans] (PH 203) (2nd-level spell at caster level 3) (90 gp/charge), the master is activating that wand to cast the spell barkskin, and that barkskin spell effect can be shared with the animal companion. (The Dungeon Master's Guide on Wands actually says, "Wands use the spell trigger activation method, so casting a spell from a wand is usually a standard action that doesn’t provoke attacks of opportunity" (245 and emphasis mine).)
Spell completion items like scrolls and spell trigger items like staffs and wands are the usual magic items that literally allow their users to cast spells so that they can be shared with an animal companion, but some rare magic items that aren't spell completion or spell trigger magic items also allow their bearers to cast spells. For example, the shield of the sun (Dungeon Master's Guide 282 as shield of Prator) (major artifact; 15 lbs.) allows the bearer to cast spells like a level 20 paladin.
However, most other magic items—for example, all but the most obscure potions, the vast majority of magic armors and weapons, almost all wondrous items—are either use activated or command word activated and don't see their bearers casting spells, despite spell effects sometimes taking place when their command words are spoken or the items used.