SRD, page 102 - Spell Targeting:
To target something, you must have a clear path to it, so it can’t be behind total cover.
Sending
Range: Unlimited
The spell doesn't seem to make an exception for cover. It only specifies distance, but the Range does that as well. Can Sending send a message to a creature behind Total Cover from you?
To be clear, this is purely a matter of technicality. Under no circumstances should anyone rule that sending requires a clear path to the recipient—that would defeat the purpose of the spell utterly, as other answers have stated.
But that doesn’t change the fact that the rules, as written, don’t really spell that out. We’re left to infer it, on the basis that this limitation would make the spell too weak, and/or would make the unlimited range and cross-planar properties of the spell nearly meaningless. Which we should do, again, we should realize that all those things are true and rule accordingly, but the rules shouldn’t force us to make inferences like that. The rules state that a clear path is required unless something says otherwise, and in the case of sending, nothing quite says otherwise. It should.¹
So it’s an oversight. They forgot to say something because of the “obvious” purpose of the spell, or because of the assumed familiarity with the spell and its long history,² or because of any other editing error. It’s very difficult to notice what’s missing from the description of something you’re already very familiar with: your brain automatically fills in the blanks for you. On some level, part of the value Wizards of the Coast purports to provide you is having made the effort to counteract that tendency and actually thought through things. But on the other hand, the rules text is very large, and we don’t see a lot of questions about discrepancies like this. Mistakes are inevitable—they aren’t a reason to damage your game by implementing a poor rule.
To be totally clear, by “should” here, I mean “as written, the rules need a statement that sending does not require an unobstructed path as other spells do.” This is not merely “it would be clearer if they spelled it out,” this is “it is wrong the way it’s written.” “Specific beats general” cannot resolve this, because nothing in sending specifically addresses obstructions. Having “Range: Unlimited” or “across any distance” is not a “more specific” statement than “requires an unobstructed path,” they’re orthogonal statements about two separate requirements. Likewise, “even to other planes of existence” doesn’t tell us to ignore other obstructions. It is difficult to leverage these properties while needing an unobstructed path, but not impossible. Sending could be an unlimited-range spell, that can cross over into other planes, and yet still require an unobstructed path. It just isn’t, because that would make the spell worthless.
Sending is over 30 years old, and despite appearing in the core Player’s Handbook for every edition of D&D since 1989, it has never clearly stated that it ignores cover, even though this is the entire purpose of the spell and always has been. D&D 4e mentioned it could target a creature “anywhere on the same plane” (4e sending didn’t work across planes), but that was only as a result of a DC 40 Arcana check; lower check results give a range but don’t mention cover.
The spell explicitly says it works across any distance:
You can send the message across any distance and even to other planes of existence....
and furthermore has a range of "Unlimited". Clearly, you could not send to creatures across any distance or on other planes of existence if the spell were restricted to creatures not behind total cover.
The only other spell I am aware of with a range of "Unlimited", the 8th level spell telepathy, has similar wording:
The creature can be anywhere on the same plane of existence as you.
In both cases the specifics of the range of "Unlimited" beats the general of the cover rule. However, as KRyan pointed out, it would have been useful and simple for the spell authors to state in the spell description that total cover is ignored.
In our games sending has been quite an important spell, and as the party's level has increased, using sending to communicate over long distances has been significant. It would be a dramatic change to limit sending (and telepathy) to line of sight.
If we decide that the total cover rule for spell targeting applies, then much of the spell’s obvious function ceases to work. The unlimited range would not work on spherical planets, as even if the surface were perfectly uniform, the surface would obstruct your straight line path eventually. Ally walks to the other side of a hill? Sending doesn’t work. Your message recipient is indoors? Sending doesn’t work. Your target is on another plane? Sending doesn’t work, even though it says it does. Your friend is in the next room over at the motel? Sending doesn’t work, but the cantrip message does. These are all cases where it’s abundantly obvious the spell should work. The spell says:
You send a short message of twenty-five words or less to a creature with which you are familiar. The creature hears the message in its mind
But if the total cover rule applies, this actually almost never works, and is worse than the message cantrip.
The reason that total cover normally prevents you from targeting a creature is because targeting requires a clear path the the target (also known as "line of effect"), and there is no clear path to a target that is behind total cover, as described in the rules for spell targeting:
To target something, you must have a clear path to it, so it can't be behind total cover.
However, Sending makes an exception to the normal requirement for line of effect:
You can send the message across any distance and even to other planes of existence, [...]
If Sending required line of effect from the caster to the target, it would not be able to contact creatures on other planes (except in specific cases). From this, we can conclude, even though it doesn't say so directly, that Sending is able to contact creatures even when you don't have direct line of effect to them for any reason, including total cover.
(Could it be worded more clearly? Definitely. But I believe that the existing wording is sufficient to establish that Sending bypasses the requirement for line of effect entirely.)
The beginning of the targeting rule state:
A typical spell requires you to pick one or more targets to be affected by the spell's magic. A spell's description tells you whether the spell targets creatures, objects, or a point of origin for an area of effect (described below).
So yes, a spell might require you to see the target (or point, or area) so full cover would be a factor.
But when we read the Sending spell description:
You send a short message of twenty-five words or less to a creature with which you are familiar. [...] You can send the message across any distance and even to other planes of existence...
It lists that it merely needs a creature with which you are familiar. They can even be on a different plane of existence. So in this case, per the spell's description, you don't need to see them, just know that they exist somewhere.
