How does your opponent verify that you're able to cast a Companion card?

Question

Normally, in Magic the Gathering, your library is a hidden zone; neither you nor your opponent are allowed to look at the cards in it during a game of Magic: the Gathering unless an effect says that you can. However, with the release of Ikoria, there are now Companion cards that allow you to cast them if you've obeyed certain deck building restrictions.

When you go to cast one from outside the game, how does your opponent verify that you meet the deckbuilding requirement for doing so? They can't look at the contents of your library, so it's entirely possible that your deck has one or more cards that violate the Companion's deckbuilding restriction, and you just haven't played them to a public zone yet. I've look at the article on the WotC website explaining the set mechanics of Ikoria, and it doesn't mention this problem at all.

You can choose a Companion that you're incapable of casting (e.g. to trick the opponent into thinking your deck is something it isn't), right? Is the only way for your opponent to verify your ability to cast a Companion to call a Judge to look at your decklist?

Answer 1

Presenting a card as your companion requires you meet the companion condition. Like every keyword in magic, there is a section of the comprehensive rules that cover the ability, and those rules are what really matter. I think what you're doing is reading the reminder text as if it were the entirety of the rules for companion (reminding that you could originally cast a companion from outside the game once, now can add it to your hand for 3 mana) Here are the sections, with my emphasis:

103.1b If a player wishes to reveal a card with a companion ability that they own from outside the game, they may do so after setting aside their sideboard. A player may reveal no more than one card this way, and may do so only if their deck fulfills the condition of that card’s companion ability. (See rule 702.138, “Companion.”)

702.138a Companion is a keyword ability that functions outside the game. It’s written as “Companion—[Condition].” Before the game begins, you may reveal one card you own from outside the game with a companion ability whose condition is fulfilled by your starting deck. (See rule 103.1b.) If you do, once during that game, you may play that card from outside the game. Once during the game, any time you have priority and the stack is empty, but only during a main phase of your turn, you may pay {3} and put that card into your hand. This is a special action that doesn’t use the stack (see rule 116.2g). This is a change from previous rules.

These two connected rules, with one being a restating of part of the other with a different focus (one from the start of game rules section, the other from the companion section) explain how companion really works - that the deck MUST meet the companion restriction for you to reveal this card. The text in brackets on the card (the reminder text) originally told you what you can do with your companion, before the rules change, but the rules themselves say how a card can become your companion. Revealing a companion that your deck does not meet the conditions for makes it an illegal deck.

As for how you verify if a deck is legal under companion rules, this works the same way as any other deck construction rule, that is you assume the deck is legally constructed until you prove otherwise. If you see a 5th copy of any of the cards (other than the 16 that can legally have more than 4 copies of) you see the deck is illegal and act on that. So if you see a 2 drop in a deck with Keruga, the Macrosage as the companion, you know the deck rules have been violated. This is actually pretty hard to hide - You can't play that card, it can't be in your hand when you get hit with something like Thoughtseize, it can't be milled from your deck, and your opponent can't play any card that lets them look at your deck like Surgical Extraction. An invalid companion would be just as hard to hide as a 5th copy of a card to an opponent paying attention.

Any potential psychological advantage fooling your opponent about what your deck is by announcing a companion that couldn't legally be your companion really doesn't exist - now you're playing with a deck that doesn't have 60 useful cards in it, it has say 50 cards that match the restriction and 10 dead ones that show you to be cheating so you could never let your opponent see them. It's also just poor sportsmanship to try and cheat.

Answer 2

Companion-related cheating is much the same as other existing forms of cheating regarding illegal decks and sideboard. Call a judge just like in any other situation where you suspect someone is running an illegal deck.

The updated Infraction Procedure Guide (IPG) has specifically addressed Companion cards. At a high level, Companion deck restrictions fall under other existing deck restrictions and enforcement actions. For example, it was already illegal to run more than 4 copies of any card (other than basic land) or a 59 card constructed deck.

For constructed events, decklists should be used. From the decklist, Judges should be able to verify that an opponent's deck does or doesn't meet the required deckbuilding restriction for companion. Most decklists errors are penalized with a game loss, but the Companion-related errors (if caught early enough) may be remedied instead.

Companions affect what the player intended to play, and may produce a situation in which the deck and decklist match, but violate the restriction on the intended companion. In these situations, it is acceptable to alter the deck and sideboard configuration to meet the restriction.

If there is no problem with the decklist, but you suspect that a player has improperly sideboarded, this fall under a Deck Problem (section 3.5). By default, these errors (which are assumed to be honest mistakes) carry only a warning, but a judge may upgrade these sideboarding errors as needed. New guidance regarding Companion-related Deck Problems includes:

If the error caused a violation of a companion restriction and it is a post-sideboard game (or a pre-sideboard game and the player has elected to continue with the deck they registered that does not match the revealed companion condition), choose at random from cards in the sideboard that meet the companion restriction; the opponent decides where each chosen card goes. If making the deck match the companion condition is impossible, upgrade the penalty to a Game Loss.

And possible upgrades to a game loss regarding companion:

Upgrade: If an error resulted in more copies of a main deck card being played than were registered or allowed by companion restriction and this was discovered after the game had begun, the penalty is a Game Loss unless all copies of the card are still in the random portion of the library. For example if the decklist has two copies of Shock in the main deck and two in the sideboard, but a search finds two copies of Shock in the library with another already in the graveyard, the penalty is upgraded.

Answer 3

A player may declare a companion only if that player's starting deck conforms to that companion's deckbuilding rule. You cannot directly check your opponent's deck for being legal for that companion; only a judge may perform a deck check. You assume the opponent follows all rules until you see they don't; in that case, you call a judge. That being said, there is no way for players to gain an unfair advantage from misconstructing a deck under the companion rules or otherwise, since all such attempts would become public the moment they would reveal the illegal card(s).

By declaring a card as a companion, a player claims that their current starting deck conforms to the additional deckbuilding rule imposed by that companion. If that player's current starting deck would not in fact be legal for that companion, the player is not allowed to declare that companion in the first place, let alone to cast it from outside the game. Of course, as stated above, you have no way of checking that at the start of a game, but the "contract" is that you believe the opponent until you see contrary evidence.

Declaring a companion happens before each game starts and is public knowledge, because the declared companion card is visible to all players while it's still in the sideboard:

702.138a Companion is a keyword ability that functions outside the game. It’s written as “Companion—[Condition].” Before the game begins, you may reveal one card you own from outside the game with a companion ability whose condition is fulfilled by your starting deck. (See rule 103.1b.) If you do, once during that game, you may play that card from outside the game.

Note that, unlike e.g. the Commander in Commander games, declaring a companion is neither mandatory nor fixed. If you choose to not declare a companion for a game, then there are no additional deckbuilding rules for that game. Sideboarding between games changes your starting deck and therefore might affect the legality of a certain companion. You can even declare a different companion for each game, as long as you sideboarded accordingly:

Starting Deck

After a player has set aside their sideboard, their remaining deck becomes their starting deck. See rule 103.1.

If, at any point during the game, a card that does violate the current companion's rule is revealed, you call a jduge who will then perform a deck check. I'm no judge, but intuitively, I would apply a harsh penalty (game loss) for illegally declaring a companion, even by mistake - declaring a companion can completely change the way their opponent would play the game. Even if the companion, or none of the illegal cards, are played, their implied potential can cause drastic strategic changes, such as withholding counters or removal.

If the illegal companion was declared intentionally, it would be cheating and cause for disqualification. Determining which applies is up to the judge.