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I am wondering whether I could cast a cantrip, for example Thaumaturgy, using a spell slot to get more than a single effect of the cantrip (i.e. both ominous sounds and alter appearance).

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No you cannot (With a potential loophole for Warlocks)

Each class spellcasting description specifically describes what its slots can be used to cast.

The Wizard, Bard, Druid, Cleric, Eldritch Knight, Arcane Trickster, Sorcerer and Ranger classes all say:

The [class] table shows how many spell slots you have to cast your spells of 1st level and higher.

In these cases, the class feature actively prevents you from casting anything other than 1st level spells or higher with your slots. Since cantrips are 0th level, you can't use one of these slots to upcast a cantrip because these class feature rules are more specific than the general rules on spellcasting.

But what about a multiclasses character?

The multiclass rules say:

Each spell you know and prepare is associated with one of your classes, and you use the spellcasting ability of that class when you cast the spell.

meaning that even when you multiclass and cast a spell, you are still limited by the spellcasting feature as defined by the class that it's associated with, with the exception of the number of spell slots you have.

Okay, so how about paladins?

The Paladin spellcasting section and Warlock pact magic feature say:

To cast one of your [class] spells of 1st level or higher, you must expend a slot of the spell’s level or higher

Paladins don't have cantrips normally, so the only way to get one RAW would involve multiclassing (which we know wouldn't work), or the magic initiate feat. However, the magic initiate feat associates the learned cantrips with a different class. And since the paladin's spellcasting feature explicitly only gives you slots to cast paladin spells, you couldn't use them on a cantrip.

What about Warlocks then?

Technically the pact magic feature doesn't explicitly say that you can't cast a warlock cantrip at a higher level. But there really isn't a point to it. No cantrip to date explicitly says that it has any extra effects what cast with a spell slot, and it is a spell's definition that tells you how the spell reacts to higher level casting. Additionally, the concept of allowing a certain action because there is no rule specifically against it might not sit well with some DMs, as it can open up some doors to some...unintended consequences.

Additionally, Jeremy Crawford, the lead rules designer, has shed some light on this topic saying:

No, since cantrips don't use spell slots.

Even if you don't think JC's word is law, it does provide some insight on designer intent. So, even though a DM might technically, with a liberal ruling, allow cantrips for warlocks to be cast using spell slots, doing so is counter to designer intent, doesn't provide any defined benefit, and isn't a sure thing since it relies on a method of reading rules that actively exploits loopholes.

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No, you cannot. Slots can only be used on Level 1+ spells.

like most spells can a known cantrip be cast at a higher spell slot lvl. Aka sacred flame lvl 1 for 2d8 radiant dmg

No, since cantrips don't use spell slots. — Jeremy Crawford, Lead Rules Designer D&D

SOURCE: https://twitter.com/JeremyECrawford/status/635832879012941824

PHB Support: "The Bard table shows how many spell slots you have to cast your spells of 1st level and higher." (PHB, 59)

UPDATE:

Can you please clarify something silly? You said that cantrips can't be cast using slots, but the rules seem to imply otherwise. Do you stand by this? Cantrips are spells and spell can be upcast so I don't see the issue.

The first sentence of the rule on cantrips says they don't use spell slots. The rule isn't kidding. #DnD — Jeremy Crawford

SOURCE: https://twitter.com/JeremyECrawford/status/949353794680180736

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Technically cantrips are level 0 spells, so even though you do not need to, you could use a slot to cast them, but there is no point.

Some spells, such as magic missile and cure wounds, have more powerful effects when cast at a higher level, as detailed in a spell's description. (PHB 201, emphasis mine)

If a spell has a different effect when cast with a higher level slot, it is always called out in its description. No cantrip to date has such.

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  • \$\begingroup\$ Comments are not for extended discussion; this conversation has been moved to chat. \$\endgroup\$ – doppelgreener Jan 19 '18 at 18:36
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Just to address the Thaumaturgy example used: it is a poor example. The spell description states:

If you cast this spell multiple times, you can have up to three of its 1 minute effects active at a time, and you can dismiss such an effect as an action.

So you can have multiple effects active without needing to resort to rule tricks or DM overrides.

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Cantrips are cast without using a spell slot and are thus unable to be cast using a spell slot at any level.

Cantrips cannot use spell slots

A cantrip is a spell that can be cast at will, without using a spell slot and without being prepared in advance. Repeated practice has fixed the spell in the caster's mind and infused the caster with the magic needed to produce the effect over and over. A cantrip's spell level is 0.

Jeremy Crawford has confirmed multiple times that cantrups cannot be used with spell slots:

The first sentence of the rule on cantrips says they don't use spell slots. The rule isn't kidding. (source)

and

Q: Like most spells can a known cantrip be cast at a higher spell slot lvl. Aka sacred flame lvl 1 for 2d8 radiant dmg

A: No, since cantrips don't use spell slots. (source)

So, according to rules and the official rules clarifications, cantrips do not use spell slots.

Spells that do not use spell slots must be cast at their lowest level

According to Sage Advice.

What level is a spell if you cast it without a spell slot?

Such a spell is cast at its lowest possible level, which is the level that appears near the top of its description.

That means that cantrips, as spell that do not use slots, must be cast at their lowest level (level 0) and cannot be cast higher.

And, once again, Jeremy Crawford clarifies and agrees with this interpretation:

A spell must be cast with a spell slot for you to be able to cast it with a higher-level spell slot. This means you can't use a higher-level slot with a cantrip or a spell you cast from a magic item without a slot. (source)

Even if you could, the spell would not have any greater effect

If a spell has different effects at different levels, it will say so in its description. No current cantrip has any such wording including thaumaturgy. Thus, even if you could cast a cantrip using a spell slot, it would do you no good.

For thaumaturgy in particular, you can achieve the effect you are going for by simply casting it multiple times, which the spell allows in its description.

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Normally, there would be no point since increasing the level does not do anything per se — level scaling is on a per-spell basis in that spell's description, for spells which scale.

And, it turns out that most damaging cantrips have their own different scaling mechanism: damage increases with character level rather than spell level. For example, from Fire Bolt:

This spell's damage increases by 1d10 when you reach 5th level (2d10), 11th level (3d10), and 17th level (4d10).

Unfortunately, Thaumaturgy, Prestidigitation, and Druidcraft have no such scaling. I think a house rule allowing them to do a little more at each of the thresholds where other cantrips scale up in damage would be completely reasonable, though ­— as long as it doesn't intrude into the power levels of non-cantrip spells. I'm thinking things like a 17th-level wizard's Prestidigitation doing all of the party's dirty laundry, or 11th-level druid predicting the weather more than 24 hours in advance.

Other than increasing damage or other effect, there's another reason one might cast a cantrip at a higher level: to make it harder to Counterspell. Despite Jeremy Crawford's comments about cantrips not using spell slots, the rules say:

A cantrip is a spell that can be cast at will, without using a spell slot and without being prepared in advance. Repeated practice has fixed the spell in the caster's mind and infused the caster with the magic needed to produce the effect over and over. A cantrip's spell level is 0.

Emphasis mine — note that it does not say "must" or "can only". And the rule on casting spells at higher levels does not say anything about not being a cantrip. That rule is:

When a spellcaster casts a spell using a slot that is of a higher level than the spell, the spell assumes the higher level for that casting. For instance, if Umara casts magic missile using one of her 2nd-level slots, that magic missile is 2nd level.

Effectively, the spell expands to fill the slot it is put into.

That is, a spell which is normally level 0, when cast using a 2nd-level slot, is a 2nd-level spell.

So, if a player wanted to use their spell slots to cast a cantrip at a higher level to make counterspelling harder (or for some similar reason), I'd allow it. There's plenty of room for rules lawyering and arguing over commas and digging up tweets and forum posts and whatever, but the key thing is the consequences of allowing this are very small. The player has to use up a resource that could be used for a non-cantrip spell, in exchange for a very small and situational benefit. There's no way this breaks anything in the game. At my table, this case would be "sure, that's fine" and we'd move right along. But I also wouldn't give a more potent effect — there's absolutely no support for that in any reading — without having the above house rule for the "flavor" cantrips in place already.

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This is one of those "How you interpret the rules makes a big difference" situations.

Cantrips are 0-level spells, and take up 0-level spell slots, and tables don't show them because they'd just make the table bulky with something that can be more easily addressed in class description. However, many view that 0-level doesn't get the "I cast it with Moar Powah" ability other spells do.

That said, various feats can up the slot requirements of spells (including cantrips). For example, if you empower a cantrip, and then you can cast it as a higher level spell regardless.

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