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I've asked around among friends and also Reddit but we've not reached a consensus: How does Companion work after sideboarding?

I can find the following relevant rule snippets.

Your companion's deckbuilding rule applies only to your starting deck, which is the deck you begin play with each game. It won't consider any cards in your sideboard, so those cards don't have to follow the deck-building rule.

(Source: Ikoria: Lair of Behemoths mechanics)

In Magic: The Gathering, a match is a multiplayer game or a two-player series of games (usually best-two-of-three) played in a tournament.

(Source: https://mtg.gamepedia.com/Match, referencing the official rules)

I cannot find the concept of a "starting deck" being mentioned anywhere in the rules.

Reading the above snippets makes me believe that in games following sideboarding a Companion card cannot be chosen as the Companion for that game if any of the cards in the new deck obtained after sideboarding violate the Companion requirements.

Is this correct?

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    I'm not completely clear on what the question is... is it whether "starting deck" means the deck you started each match with; vs the deck you started each game with? It seems clear to me that what matters is that your deck for each individual game is legal; so if you have a Companion in at particular game; your deck must follow the restrictions given by the Companion. But of course we don't have comprehensive rules to quote yet. – GendoIkari Apr 14 at 20:41
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    Actually I just noticed that the quote you give already specifies this.. "which is the deck you begin play with each game". So what's the alternative interpretation? – GendoIkari Apr 14 at 20:47
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    Note that the comprehensive rules for Ikoria are not yet actually released. The current version is from THB so we don't have stuff about starting decks yet. – doppelgreener Apr 14 at 21:31
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Your interpretation seems valid.

The Ikoria: Lair of Behemoths mechanics rule snippet you quote addresses this directly.

Your companion's deckbuilding rule applies only to your starting deck, which is the deck you begin play with each game.

The comprehensive rules currently don't directly address any issue of a "starting deck". However, the "deck" referred to in rule 103.1 is a pretty solid candidate. 103.1. At the start of a game, each player shuffles their deck so that the cards are in a random order. Each player may then shuffle or cut their opponents’ decks. The players’ decks become their libraries.

The wording is probably intended to prevent confusion due to card interactions from the other part of your quote.

It won't consider any cards in your sideboard, so those cards don't have to follow the deck-building rule.

You choose your companion before starting the game, as per the same article.

Just before the game begins, reveal your chosen companion to all players. Once during the game, you may cast your chosen companion from your sideboard.

So if your starting deck contains only cards of CMC 3 or greater, you can choose Keruga, the Macrosage as your companion. After the game begins and your deck becomes your library, if you cast the Research half of Research//Developement (legally over CMC 3 as per rule 202.3d) and shuffle some 1 or 2 CMC cards from your sideboard into your library, it shouldn't affect the legality of your chosen companion. Similarly, some companions could make your library not match the criteria even if your deck did. A copy of Lutri, the Spellchaser in a deck and one in your sideboard with all other cards of different names would be perfectly legal.

However, newer players may not realize "deck" does not equal "library" and the additional wording "starting deck" may be intended to mitigate confusion.

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    The comprehensive rules do currently define "starting life total" and "starting hand size". It's possible that the next revision of the comprehensive rules will update wording regarding the 103 rules. – InternetHobo Apr 14 at 23:04
  • 702.138b If a companion ability refers to your starting deck, it refers to your deck after you’ve set aside any sideboard cards. In a Commander game, this is also before you’ve set aside your commander. The rules now do explicitly define the starting deck as far as companions are concerned. – Andrew Apr 20 at 8:50
  • "After the game begins and your deck becomes your library, if you cast the Research half of Research//Developement (legally over CMC 3 as per rule 202.3d) and shuffle some 1 or 2 CMC cards from your sideboard into your library, it shouldn't affect the legality of your chosen companion" I can confirm: Fae of Wishes lets you do this on Arena (well, not for Keruga of course). – Karl Knechtel Apr 20 at 19:51
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Your interpretation is correct, Companion requirements apply to decks after sideboarding is complete.

Now that the rules have been updated we have a definition for starting deck

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

And the new companion rule inserted before removing your commander from the starting deck.

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.”)

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