How do targeting and line of sight for spells work in regards to allies?

Question

I've been wondering on how spells interact with allies:

• Does your line of sight gets blocked by allies?

• Can allies stand in the way of your spell?

I cannot really find evidence on how the rules work, even though there are some examples and a Sage advice.

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Example: Poison Spray

Poison Spray states: (emphasis mine)

You extend your hand toward a creature you can see within range and project a puff of noxious gas from your palm. The creature must succeed on a Constitution saving throw or take 1d12 poison damage.

If an ally is in front of me, and behind him is an enemy, can I:

1. Target the creature behind my ally? Ally blocks the line of sight?

2. Hit the creature behind my ally? Ally blocks the clear path to the target?

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Example: Sacred Flame (ignores cover)

Sacred flame states:

Flame-like radiance descends on a creature that you can see within range. The target must succeed on a Dexterity saving throw or take 1d8 radiant damage. The target gains no benefit from cover for this saving throw.

If an ally is in front of me, and behind him is an enemy, can I:

3. Target the creature behind my ally? Ally blocks line of sight?

4. Hit the creature behind my ally? Ally blocks the clear path to the target, but the creature does not benefit from cover: so ally doesn't block the spell?

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Conclusion (questions)

Am I correct to assume that:

5. An ally provides half cover for an enemy behind him?

6. An ally blocks a clear path to the enemy?

Answer 1

It works the same way for foes and allies

Regarding to line of sight, being an ally or an enemy is irrelevant.

Any creature between you and your target:

• Gives half cover to the target
• Does not block your vision

See Player's Handbook, "Cover" (page 196):

A target with half cover has a +2 bonus to AC and Dexterity saving throws. A target has half cover if an obstacle blocks at least half of its body. The obstacle might be a low wall. a large piece of furniture, a narrow tree trunk, or a creature, whether that creature is an enemy or a friend.

Only targets with total cover can not be seen:

A target has total cover if it is completely concealed by an obstacle.

You can't target with a spell something you can't see (PHB, p. 204):

To target something, you must have a clear path to it, so it can’t be behind total cover.

So yes, you can cast a spell "through" a creature, but the target gets +2 AC against any spell attack and +2 to dexterity saving throws.

Also, your DM might use the "Hitting Cover" variant rule (DMG p. 272):

First, determine whether the attack roll would have hit the protected target without the cover. If the attack roll falls within a range low enough to miss the target but high enough to strike the target if there had been no cover, the object used for cover is struck. If a creature is providing cover for the missed creature and the attack roll exceeds the AC of the covering creature, the covering creature is hit

This rule works only when a caster makes an attack roll, not when a spell requires a save.