this post was submitted on 21 Oct 2025
8 points (100.0% liked)

MTG

2357 readers
1 users here now

Magic: the Gathering discussion

General discussion, questions, and media related to Magic: the Gathering that doesn't fit within a more specific community. Our equivalent of /r/magicTCG!

Type [[Card name]] in your posts and comments and CardBot will reply with a link to the card! More info here.

founded 2 years ago
MODERATORS
andrew@mtgzone.com
mike@mtgzone.com
8
MTG Deck Legality Checker for custom formats! (git.disroot.org)
submitted 2 months ago* (last edited 1 month ago) by counterspell@mtgzone.com to c/mtg@mtgzone.com
3 comments fedilink hide all child comments
 

Hey everyone!

I just finished building a web tool to help validate Magic: The Gathering decklists in any custom format. It fetches card data from Scryfall, checks for banned or illegal cards, and gives instant feedback right in your browser.

It’s perfect for experimenting with niche formats, casual play, or even building decks for some existing formats like Standard Penny Dreadful, 2015 Modern, Classic Legacy, Pre Fire Modern, Pre Horizons Modern, Pre Modern. It can also integrate with Moxfield for deck building.

Check it out here: https://git.disroot.org/hirrolot19/mtg-legality-checker

I’d love to hear your thoughts, suggestions, or ideas for improving it. Hope it helps deckbuilders and casual players alike!

you are viewing a single comment's thread
view the rest of the comments
[–] counterspell@mtgzone.com 1 points 2 months ago* (last edited 2 months ago)

MTG Deck Legality Web Checker

A simple web tool for validating Magic: The Gathering decklists in a custom format.

Inspiration

This project was inspired by several sources:

  • Badaro's Validator GitHub, a simple web tool for checking card lists in "nostalgia" Magic the Gathering formats: https://badaro.github.io/validator
  • Penny Dreadful, an unofficial, budget-friendly MTG Constructed format where only cards costing 0.02 tix or less are legal. Decks are 60 cards with a 15-card sideboard, promoting creative play with inexpensive cards.
  • Heirloom Constructed, a fan-created format combining Legacy-style interactions with price caps. Cards are legal based on current prices, with different caps for mythics, rares, uncommons, and commons. The format rotates weekly after new Standard set releases, providing a dynamic but affordable competitive environment.

Features

  • Automatic setup: Downloads Oracle bulk-data from Scryfall and builds legality files on first run.
  • Custom format validation: Checks decks for banned or out-of-format cards.
  • Browser interface: Paste a decklist, click Validate, and view results instantly.

Installation

It is recommended to use a virtual environment to keep dependencies isolated.

1. Clone the Repository

git clone https://git.disroot.org/hirrolot19/mtg-legality-checker.git
cd mtg-legality-checker

2. Create and Activate a Virtual Environment

python -m venv venv
source venv/bin/activate

3. Install Dependencies

pip install -r requirements.txt

Running the App

From the project root (with the virtual environment activated):

python app.py

Then open your browser and navigate to:

http://127.0.0.1:5000/

First Run Behavior

On first launch, the app will:

  1. Download Scryfall’s Oracle card data.
  2. Filter legal cards for the custom format based on a Scryfall query. Default query is f:standard usd<=1 tix<=0.1
  3. Convert the filtered data into a validation JSON file.

This process may take a few minutes.
Once complete, cached files are stored persistently for future sessions.

Using the Web Checker

  1. Paste your decklist into the text box.
  2. Click Validate.
  3. The app displays any cards that are banned or not legal in the format.

Decklist Rules

  • One card per line.
  • Quantities accepted (4 Lightning Bolt, 2x Opt).
  • Comments start with #.
  • “Deck” or “Sideboard” headers ignored.

Advanced Usage

For detailed information about the supporting scripts and command-line tools, see tools/README.md.