decks.json has traditional cards + sideboard structure, with commanders reusing sideboard.
decks_v2.json has cards + sideboard + commander structure. You should use this one.
Data format
Data file i a JSON array, with every element representing one deck.
Fields for each deck:
name - deck name
type - deck type
set_code - mtgjson set code
set_name - set name
release_date - deck release date (many decks are released much after their set)
cards - list of cards in the deck’s mainboard
sideboard - list of cards in the deck’s sideboard
commander - any commanders deck has (can be multiple for partners)
Each card is:
name - card name
set_code - mtgjson set card is from (decks often have cards from multiple sets)
number - card collector number
foil - is this a foil version
count - how many of given card
mtgjson_uuid - mtgjson uuid
multiverseid - Gatherer multiverseid of card if cards is on Gatherer (optional field)
Data Limitations
All precons ever released by Wizards of the Coast should be present, and decklists should always contain right cards, with correct number and foiling, and mainboard/sideboard/commander status.
Source decklists generally do not say which printing (set and card number) each card is from, so we need to use heuristics to figure that out.
We use a script to infer most likely set for each card based on some heuristics, and as far as I know, it always matches perfectly.
That just leaves situation where there are multiple printings of same card in same set.
If some of the printings are special (full art basics, Jumpstart basics, showcase frames etc.), these have been manually chosen to match every product.
If you see any errors for anything mentioned above, please report them, so they can be fixed.
That just leaves the case of multiple non-special printings of same card in same set - most commonly basic lands. In such case one of them is chosen arbitrarily, even though in reality a core set deck with 16 Forests would likely have 4 of each Forest in that core set, not 16 of one of them.
Feel free to create issue with data on exact priting if you want, but realistically we’ll never get them all, and it’s not something most people care about much.