How to allow cantrips to be changed

The wizard in my game has requested the ability to change out his cantrips similarly to his higher level spells. His justification is that every other wizard spell is memorizable, so having fixed spells seems antithetical. I kind of agree with him.

I know that there is no mechanism for changing cantrips, but I’m trying to come up with a fair means of doing so.

The two extremes, as I see it, are:

  1. Cantrips can be swapped out just like preparing any other spell.
  2. Retrain a the cantrip, taking 250 days of Downtime, as if learning a new skill.

I’m interested in having a good middle ground option, patterned on other rules, if possible.

Here’s the house rules I have tentatively decided on. Critiques welcomed.

Cantrip House Rules

For all following rules, when using a cantrip that you don’t currently “know”, for classes that use a spell book, the cantrip must be in the spell book. Cantrips count as 1st level spells for purposes of price and scribing.

  • When you gain a level, you may swap one cantrip for another that you have available.

  • As a Downtime Activity, a cantrip can be swapped for another that you have available after 10 days of practice and the expenditure of 20gp.

  • Cantrips can be prepared and cast as 1st level spells. If the cantrip effects improve with level, the spell slot used to cast it must be 2 levels higher per step of improvement.

  • Cantrips can be cast as rituals. The cantrip improves with level as normal.

As with any house rule, I reserve the right to change it if it breaks.

Be warned! My DMing style is quite liberal. If you use this, wizards, druids, & clerics in your group may become unbalanced.

Has anyone tested the effect of allowing cantrips to be repeatedly cast between battles?

Short Version: Some characters have cantrips (or unlimited-used abilities) that they would like to have in combat, like Shillelagh. Has anyone tested or experienced methods of allowing these to be ready in advance?

Long Version: I have a player who wants to build an Arcana Cleric who uses Shillelagh, a non-concentration cantrip with a 1-minute duration, for melee combat. She would like to have it ready when combat begins for action economy (to do such other things as casting Spiritual Weapon on the first turn). She figures she can simply cast it continuously while walking around between fights.

I was considering allowing this, with a few caveats. My thoughts were:

  • Constantly casting a cantrip will give Disadvantage to be aware of surroundings, giving a -5 to Passive Perception/Investigation.
  • Since it has a Verbal component, she will be constantly emitting noise, possibly giving enemies some advance warning of the party’s approach.
  • Originally I was going to have her roll a d10 at the start of any battle to see how much time was left on the 1-minute cantrip (since it’s conveniently 10 rounds of duration), but technically she could recast it every 6 seconds to restart the timer, RAW, rather than waiting until it expired.

There are other potential factors, such as aggravating any NPCs that travel with them (imagine someone repeating the same 6 seconds of magic words over and over. ALL. DAY. LONG.), and not being able to renew it while her focus was required elsewhere (like solving a tricky puzzle, or while hiding, or while having a conversation).

Here are my questions:

  • Are there any other factors that need to be weighed, mechanically-speaking?
  • Is there any problem with my interpretation of the ability to recast it every 6 seconds?
  • Has anyone used or witnessed their own method of handling a PC constantly re-using a similar spell or ability?

Can the blade created by Shadow Blade be used with the Booming Blade or Green Flame Blade cantrips?

The spell shadow blade allows a spellcaster to produce a discrete weapon that can then be used to make attacks:

You weave together threads of shadow to create a sword of solidified gloom in your hand. This magic sword lasts until the spell ends. It counts as a simple melee weapon with which you are proficient. It deals 2d8 psychic damage on a hit and has the finesse, light, and thrown properties (range 20/60). In addition, when you use the sword to attack a target that is in dim light or darkness, you make the attack roll with advantage.

Shadow Blade, Xanathar’s Guide to Everything, pg. 164

As part of recent errata to the green-flame blade and booming blade cantrips, their material components have been amended to "a melee weapon worth at least 1 sp" (Sword Coast Adventurer’s Guide, 142).

Can shadow blade still produce a weapon that can be used with these cantrips?

What are the repercussions of the Sword Coast Adventurers Guide errata to the cantrips Green-Flame Blade and Booming Blade?

In the most recent update to the Sage Advice article, Green-Flame Blade and Booming Blade have received errata to their range and spell text, being altered from

Range: 5 feet


Range: Self (5 foot radius)

Furthermore, the text of GFB and BB have been altered from

As part of the action used to cast this spell, you must make a melee attack with a weapon against one creature within the spell’s range, otherwise the spell fails.

to the phrase

You brandish the weapon used in the spell’s casting and make a melee attack with it against one creature within 5 feet of you.

How do these changes alter the way that these spells are used, and how they interact with features or feats, such as Spell Snipers effect of doubling a spells range.

Can an Echo Knight with the War Caster feat use cantrips for opportunity attacks made by their Echo?

The Echo Knight has the following ability:

When a creature that you can see within 5 feet of your echo moves at least 5 feet away from it, you can use your reaction to make an opportunity attack against that creature as if you were in the echo’s space.

The Warcaster feat has the following benefit (emphasis mine):

When a hostile creature’s movement provokes an opportunity attack from you, you can use your reaction to cast a spell at the creature, rather than making an opportunity attack.

If an Echo Knight has the War Caster feat and knows a cantrip like, say, Booming Blade, can they use the cantrip for opportunity attacks made from the position of the Echo?

Can a spellbook contain cantrips?

Is it possible for cantrips to be written in a spellbook? Their description suggests not, as well as the fact that the Wizard’s cantrips are ‘known’ rather than being in their spellbook.

For example, if a Wizard found a scroll with a cantrip on it (can cantrips even be on scrolls?), could they copy it into their spellbook and ‘know’ it henceforth?

The reason for the question isn’t that I’m hoping to know more cantrips than the rules allow, it’s for a character’s backstory I’m working on. The character will only know the allowed number of cantrips, but it doesn’t make any sense for the character to know any cantrips unless they were in a spellbook. (The character is a Rogue who stole a spellbook and started learning magic from it.)

(It’s fine if this can’t be answered now, I’m happy to wait for the DMG release if there’s no answer beyond ‘who cares, just make it up’.)

Artificer Armorer Guardian attack and cantrips like Booming blade

So the UA specialist for the Artificer, the Armorer, has an interesting ability for one of the models you can take.

Thunder Gauntlets. Your armored fists each count as a simple melee weapon, and each deals 1d8 thunder damage on a hit. A creature hit by the gauntlet has disadvantage on attack rolls against targets other than you until the start of your next turn, as the armor magically emits a distracting pulse when the creature attacks someone else.

Should you end up with a cantrip like Booming Blade through some means, could you use Thunder Gauntlets to perform it? Would you then get the unique effect of Thunder Gauntlet alongside the normal effects of such a cantrip?

Cantrips and wearbeast PC’s

If a cantrip is an at will spell, why can’t a wear-raven or anything in the wear family cast it in their animal form? The write up on cantrips says it’s just like any other spell, except it doesn’t take up a spell slot, yet it also doesn’t say you have to cast it exactly like a normal spell. It says it’s an at will action. So wouldn’t a wear beast be able to cast it? Also I specify wear-raven because raven’s have mimicry. They can literally continue to speak a language after transformed, it’s just not as eloquent. So shouldn’t a wear-raven be able to cast a single word cantrip?