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I built a simple validator for custom formats to use with Premodern/Prefire, it uses a JSON file with the list of valid cards as data source and runs as a static website. If you know how to code it should not be too hard to generate a new format based on MTGJSON:
https://github.com/Badaro/validator
https://github.com/Badaro/validatortools
Disclaimer that my design skills are close to zero.
When I try to validate a deck, I only see the message “Loading data, please wait…” and nothing happens. So I’m not sure if it’s a problem with my JSON export, the file path, or the validator itself.
I exported the Standard Penny collection from Moxfield to JSON using a Python script:
import csv
import json
input_csv = 'moxfield_haves_2025-10-21-1123Z.csv'
output_json = 'standard_penny.json'
sets = set()
cards = []
with open(input_csv, newline='', encoding='utf-8') as csvfile:
reader = csv.DictReader(csvfile)
for row in reader:
name = row.get('Name')
edition = row.get('Edition')
if name:
cards.append(name)
if edition:
sets.add(edition.upper())
sets = sorted(list(sets))
output_data = {
"sets": sets,
"cards": cards
}
with open(output_json, 'w', encoding='utf-8') as jsonfile:
json.dump(output_data, jsonfile, indent=2)
print(f"JSON saved to {output_json}")
I saved the JSON file as validator/formats/standardpenny.json and added it to the validator’s config:
{ "name": "Standard Penny", "key": "standardpenny", "datafile":"formats/standardpenny.json" },
Then I tried to validate this deck exported as Plain Text from Moxfield and got the error.
Check the console for errors? Or send me a link of where you got it published and I'll take a look.
I've managed to write another script that seems to work:
import json
import re
def load_legal_cards(json_file):
"""
Load legal cards from a JSON file with structure:
{ "sets": [], "cards": [], "banned": [] }
"""
with open(json_file, 'r', encoding='utf-8') as f:
data = json.load(f)
legal_cards = [card.lower() for card in data.get('cards', [])]
banned_cards = [card.lower() for card in data.get('banned', [])] if 'banned' in data else []
return legal_cards, banned_cards
def clean_line(line):
"""
Remove quantities, set info, markers, and whitespace
Skip lines that are section headers like 'Deck', 'Sideboard'
"""
line = re.sub(r'^\d+\s*x?\s*', '', line) # "2 " or "2x "
line = re.sub(r'\(.*?\)', '', line) # "(SET)"
line = re.sub(r'\*\w+\*', '', line) # "*F*"
line = line.strip()
if re.match(r'^(deck|sideboard)\s*:?\s*$', line, re.IGNORECASE):
return None
return line if line else None
def validate_deck(deck_file, legal_cards, banned_cards):
"""
Returns a list of illegal cards
"""
illegal_cards = []
with open(deck_file, 'r', encoding='utf-8') as f:
lines = f.readlines()
for line in lines:
card_name = clean_line(line)
if not card_name or card_name.startswith("#"):
continue # skip empty or comment lines
card_lower = card_name.lower()
if card_lower in banned_cards or card_lower not in legal_cards:
illegal_cards.append(card_name)
return illegal_cards
def main():
legal_cards_file = 'legal_cards.json' # JSON with "cards" and optional "banned"
decklist_file = 'decklist.txt' # Your decklist input
legal_cards, banned_cards = load_legal_cards(legal_cards_file)
illegal_cards = validate_deck(decklist_file, legal_cards, banned_cards)
if illegal_cards:
print("Illegal cards:")
for card in illegal_cards:
print(card)
if __name__ == "__main__":
main()
Seems simple enough, so I'm going to try and make one myself. Here's the idea I have so far:
I'm not aware of a single tool, but you could ensure the deck is standard legal in any normal deck building tool, then additionally check it against the Penny Dreadful deck checker - if it passes both, it should be legal in your format (assuming I understand what you're doing correctly.)
Edit: Nevermind, I see you're limiting it to $1, not $0.01, despite borrowing the name. Penny Dreadful checker won't work.
I’m not aware of a single tool, but you could ensure the deck is standard legal in any normal deck building tool, then additionally check it against the Penny Dreadful deck checker - if it passes both, it should be legal in your format (assuming I understand what you’re doing correctly.)
Edit: Nevermind, I see you’re limiting it to $1, not $0.01, despite borrowing the name. Penny Dreadful checker won’t work.
Yeah Penny Dreadful uses tix<=0.02 and this uses both tix<=0.1 and usd<=1