Can you cast a spell behind Wall of Force?

Quick setup:

my players attempted to fight a Tarrasque, and the last player standing, decided to create a Wall of Force with the bubble shape, the very next turn he wanted to use Synaptic Static

Now, I told him that he couldn't as the area he was targeting was under full cover; but the discussion is keep going

So... can you or can't you cast across a wall of force?

The spell doesn't actually say it provides Total Cover; it says "nothing can physically pass through the wall". To me, that means that spells that launch some kind of physical projectile like Ice Knife would not be able to penetrate it, but something like Synaptic Static that has a purely mental effect could.

The wall is described as invisible, so it's not going to stop you from targeting things on the other side the way that, say, a Wall of Fire would.

Quote from Nyal_the_fox >>

Quick setup:

my players attempted to fight a Tarrasque, and the last player standing, decided to create a Wall of Force with the bubble shape, the very next turn he wanted to use Synaptic Static

Now, I told him that he couldn't as the area he was targeting was under full cover; but the discussion is keep going

So... can you or can't you cast across a wall of force?

Some people (me included) rule that Wall of Force blocks the requirement for a clear path. Below is my answer from another thread, along with some threads about the topic.

A Clear Path to the Target. To target something with a spell, a caster must have a clear path to it, so it can’t be behind Total Cover.

EDIT: following this ruling, spells like Flame Strike, Hold Monster, Hold Person, Insect Plague, Moonbeam, Summon Greater Demon, Synaptic Static or even Counterspell won't work because you can't target something behind Total Cover.

Quote from TarodNet >>

@Frankiefivefingers, regarding Wall of Force blocking a clear path, it's something that's been discussed around the D&D world. I agree with the answers given in this thread, but there are also some unofficial rulings from the Devs if you want to follow them. For example:

@DerynDraconis Is a glass window considered a total cover for the purpose of targeting a creature with Hold Person spell?
@JeremyECrawford A solid obstacle, regardless of material, can provide total cover. A closed window counts.

 

@Dan_Dillon_1 Targeting spells/clear path: "Unoccupied space you can see" Does this imply targeting? Conjure fey across Wall of Force?
@JeremyECrawford Unless a spell says otherwise, you can't cast it at someone or something behind total cover.

 

@crathjen Wall of Force is invisible...so it doesn't provide cover does it? Blocks physical passage though.
@JeremyECrawford Cover is a physical obstruction, not necessarily a visual one.

Also, here are some more or less recent threads/posts related to Wall of Force, in case they're helpful to you:

• What spells can you cast through glass? 
• Wall of Force and Flame Strike 
• Wall of Force and Casting 
• Wall of Force / Limitations
• Can you teleport through Wall of Force?
• What defeats a Wall of Force?

Line of sight: If you can throw a pebble straight at a place without being blocked by anything, you can cast a spell there.

Can see: What you can literally see, is countered by Blindness, invisibility & heavy obscurement.

Quote from SunIsGettingRealLow96589>>

[...] RULES: "A target with total cover can't be targeted directly by an attack or a spell, although some spells can reach such a target by including it in an area of effect. A target has total cover if it is completely concealed by an obstacle."

You see the mistake they made right there???  "A target has total cover if it is completely concealed by an obstacle."  [...]

 Sorry for snipping your post, but it's too long I just wanted to address this part.

 That rule is from 5e. In 5.5e, Cover rules don't mention "conceal" or "concealed". Those words are only found in the Hide action and the Invisible condition.

What about Sacred Flame?

Quote from Wysperra >>

What about Sacred Flame?

 The same, IMO. That spell doesn't bypass Total Cover. There was some debate with 5e Sacred Flame, though:

Quote from TarodNet >>

Quote from Plaguescarred >>

Quote from Thezzaruz >>

Sure but total cover affects the targeting of spells, it has no stated effect on saving throws. Don't get me wrong, I know that it is a popular interpretation of the spell and that Crawford has said multiple times that Sacred Flame works even against a target that has total cover. If that is their intention it would be helpful if they made an errata to the spell.

 

Quote from up2ng >>

 The rule for total cover doesn't make any reference to providing any benefits to Dexterity saving throws because spells cannot target anything that's behind total cover in the first place.

But Sacred Flame doesn't say the target gains no benefit from cover to his saving throw. It says the target gains no benefit from cover for this saving throw.

So for this saving throw, the target gains no benefit from cover as described in half cover, three-quarters cover and total cover and which include among other things that you can't be targeted directly by an attack or a spell.

At least that is the popular interpretation as you said, shared by Devs and to this date received no errata to better clarify wording.

 While reading the new 2024 spells, I remembered this answer from you. It seems now Sacred Flame has a better wording:

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 Half Cover or Three-Quarters Cover for this save.

Quote from Thezzaruz >>

Quote from TarodNet >>

I know this is debatable, but Sacred Flame says the target "gains no benefit from cover for this saving throw", but in my opinion this doesn't overrule the general rule PHB, p. 204: "To target something, you must have a clear path to it, so it can't be behind total cover"

Also in PHB, p.196: "A target with total cover can't be targeted directly by an attack or a spell"

It seems Jeremy Crawford wrote a tweet or something about RAI (see post from @quindraco) and I hope PHB 2024 gives us some updates for this spell.

It did. And they went for the rules correct version instead of what JC tweeted some years ago.

 

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 Half Cover or Three-Quarters Cover for this save.

Cantrip Upgrade. The damage increases by 1d8 when you reach levels 5 (2d8), 11 (3d8), and 17 (4d8).

Quote from TarodNet >>

Quote from BalthorTheDefiant >>

[...] A similar follow-up question: would you allow the use of sacred flame on the demilich inside the sphere, following the same logic used to justify using sacred flame through a wall of force?

 Just to say Sacred Flame through a Wall of Force or through a transparent panel was under discussion before. However, the 2024 rules for the spell are now unambiguous:

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 Half Cover or Three-Quarters Cover for this save.

PS. Personally, even with the 2014, I was ruling the same way the 2024 spell is stating now.

In the specific case of synaptic static, I would argue that "choosing a point within range", without even needing to see it, does not constitute "targeting". 

The description literally says that you can cast the spell at any location that is within range.

The same is said for Fireball, which makes no sense for it to phase through walls. If you’re using the 5e version, why would Fireball spread around corners if it's aoe can just phase through it? The usage of different damage types or saving throws doesn’t have any innate effects or it would have stated so.

If you rule it differently due to preference I can see that, but otherwise RAW and RAI, it doesn’t say anything about psychic spells going through obstacles.

Putting aside the debate about Wall of Force, if you cast Fireball, Synaptic Static or Cloud of Daggers, good luck if you discover there's an unexpected obstruction and the spells detonate near you.

From Casting Cloud of Dagger into Darkness:

Quote from TarodNet >>

The title says 2024, but the rules are essentially the same as in 2014 in terms of targeting. Spells that say "that you can see" or similar wording require the caster to be able to see the target or the point of origin. Not the case for Cloud of Daggers.

Darkness creates an Heavily Obscured area and, as you said, you have the Blinded condition while trying to see something in that space. But Darkness is not creating Total Cover. So in this context, "clear path" does not mean a path that is see-through; it means a path that is physically uninterrupted.

A Clear Path to the Target. To target something with a spell, a caster must have a clear path to it, so it can’t be behind Total Cover.

But if, for some reason, it's interrupted (e.g., you didn’t know there was a wall in the way, or for the reason R3sistance gave, etc.), then:

Area of Effect
[...] An area of effect has a point of origin, a location from which the effect’s energy erupts. The rules for each shape specify how to position its point of origin. If all straight lines extending from the point of origin to a location in the area of effect are blocked, that location isn’t included in the area of effect. To block a line, an obstruction must provide Total Cover. See also “Cover.”

If the creator of an area of effect places it at an unseen point and an obstruction—such as a wall—is between the creator and that point, the point of origin comes into being on the near side of the obstruction.