Ask a Magic question, get an answer written in everyday language, and see the exact rule it came from.
“Can I put five copies of the same card in my deck?”
Only basic lands can go past four. Every other card is capped at four copies with the same English name, and a constructed deck needs at least 60 cards.
Comprehensive Rules 100.2a
Built for Arena players who are still learning the game, not for tournament grinders.
Paste a decklist and find out whether it is legal for Standard or Historic, which cards are the problem, and why. It runs on a rules engine, so the same deck always gets the same answer.
Search the Comprehensive Rules and get an answer written for a person, with the passage it was drawn from.
Import a decklist in the format Arena already gives you, keep it here, and export it back out when you want to play it.
Add your collection by pasting in a card list, then see which cards a deck needs that you do not have yet.
Decks and collections are matched against a full card catalogue synced from Scryfall, so a name in your list resolves to a real card.
Deck legality runs on a rules engine rather than a language model, so two identical decks always get the identical answer. Rules answers quote the Comprehensive Rules and show you the passage, so you never have to take our word for it.