Cantrip

Cantrip is the common colloquial term for any Dominion Action card which offers +1 Action and +1 Card. It is essentially self-replacing (not taking up space in your hand or taking up an action) and will usually offer some side benefit, such as the topdecking effect Harbinger offers or the additional Favor of Underling.

In the narrowest definition, a cantrip offers exactly +1 Card and +1 Action, and no extra amount of +Card, +Actions, or +. In a broader sense, the term cantrips encompasses all those cards that also fall into another category, such as non-terminal draw (if the card draws more than one card), village (if it provides more than +1 Action), or Peddler variant (if it also offers +).

Strategy

In most cases, cantrips are seen as cards that cannot harm the deck, since you receive a benefit by playing them and doing so does not prevent or delay you from drawing other cards in your deck. There are a few exceptions.

Discard attacks like Militia or Margrave which require the player to make a choice of which cards to discard are more powerful against hands containing a few cantrips. This is because the player discarding does not know which cards are going to be drawn by the cantrips, so the chance of making a sub-optimal choice is greater.

Similarly, although cantrips don't "take up space" in your deck on your turn since you can draw right past them, there may be cards you want to have in your hand between turns—Reactions to protect you from an opponent's Attack, Province to block an opponent's Tournament, etc. Filling your deck with cantrips will make it less likely that you have in hand the card you need when it's not your turn. (Cards other than cantrips have this property as well, of course; but it's easier to lose sight of the fact with cantrips since they don't have this property on your turn.)

Like any other Action card, a cantrip can be drawn dead—i.e., drawn by a terminal Action card without sufficient +Actions to be able to play the cantrip after you draw it—and thus can harm a Big Money deck that depends on terminal draw without villages.

Cantrips may also be less useful with draw-up-to-X cards like Library, Watchtower, and Jack of All Trades. Although cantrips do not outright harm the deck in these cases, they under-perform other cards which do not draw cards (and which typically offer more benefits in compensation for the lack of draw).

List of cantrip cards

Any village that draws a card, non-terminal draw, and Peddler variant can be considered a cantrip but are not listed here. See their respective pages for lists of those cards. Cards in italics have been removed.

Simple cantrips

Dominion: Harbinger, Sentry, Spy
Intrigue: Great Hall, Mill, Pawn
Seaside: Pearl Diver
Alchemy: Familiar
Cornucopia & Guilds: Farrier, Shop
Hinterlands: Highway, Scheme, Trail, Wheelwright
Dark Ages: Cartographer, Market Square, Sir Bailey, Urchin
Adventures: Guide, Page, Ratcatcher
Empires: Groundskeeper, Settlers
Nocturne: Pixie, Zombie Spy
Menagerie: Sanctuary
Allies: Blacksmith, Hill Fort, Innkeeper, Old Map, Sorcerer, Sorceress, Swap, Town Crier, Underling
Rising Sun: Alley. Daimyo, Gold Mine, Snake Witch

Cantrips this turn

These cards are non-terminal draw, village, or Peddler variants but play over multiple turns and only act as cantrips on the turn they are played.

Conditionally non-terminal draw, village, or peddler variant

These cards can act as non-terminal draw, as a village, or as a Peddler variant under specific circumstances, but act as a simple cantrip otherwise.

Dominion: Merchant
Intrigue: Wishing Well
Seaside: Sea Chart
Alchemy: Apothecary
Cornucopia & Guilds: Hamlet, Herald, Menagerie
Dark Ages: Ironmonger, Vagrant
Adventures: Magpie, Storyteller
Empires: Patrician
Nocturne: Will-o'-Wisp
Renaissance: Seer
Allies: Battle Plan
Rising Sun: Poet
Promos: Sauna

Handsize-decreasing cantrips

These cards provide +1 Card, +1 Action, but mandatorily decrease handsize by trashing, discarding, or setting aside a card from the hand.

Alley — when played from hand.
Haven
Modify — can be a cantrip depending on player choice.
Rats — handsize is not decreased if the player has only Rats in hand.
Upgrade

Sifting cantrips

These cards give +1 Action and put a card into your hand, but offer some sifting rather than just drawing the top card of your deck.

Making other cards become cantrips

Enchantress causes the first Action each other player plays on their turn to become a cantrip.
Enlightenment allows all Treasure cards to be cantrips.
Way of the Pig allows any Action card to be played as a cantrip.

Fringe cases

Ironworks becomes a cantrip if it is used to gain an Action–Victory card
Groom becomes a cantrip if it gains any Victory card.
Spice Merchant is a Copper trasher with a net effect resembling a cantrip: after you trash a Copper from your hand, it can give you +2 Cards and +1 Action, leaving you with the same handsize and action count you had before you played it.
Ferryman, Forum, Fugitive, and Secret Passage draw more than 1 card, but maintain handsize. The same also applies to Tide Pools if its entire effect over both turns is considered.
Raze maintains handsize if it trashes itself. Otherwise, if trashing a card costing more than , it becomes a handsize-decreasing cantrip.
Apprentice, Artist, and Goatherd give +1 Action and draw a varying number of cards which could result in them being played as cantrips.
Scrap can give +1 Card and +1 Action if it trashes a card costing or more.
Royal Galley draws and doesn't give +Action, but does itself play an Action from your hand.
Wish gains a card to your hand instead of drawing which also maintains handsize.
Recruiter and Sculptor can also maintain handsize and act as a cantrips if immediately spending the gained Villager.
Teacher, Lost Arts, and Pathfinding can use the +1 Action and +1 Card tokens to make terminal draw or village cards into cantrips.
All non-terminal draw cards will play as cantrips if the player only has a single card left to draw from the deck and discard.