What Is a Deck Builder and How Do You Use One for TCGs?
A deck builder is the part of a card-game platform where you search a card database and assemble a deck list. On Untap.in it sits next to a virtual table, so the same tool that lets you build also lets you import an existing list, share it by link, and immediately play-test it against live opponents or an empty seat. Use it when you want to construct, store, or iterate on a deck before or between matches; it is not a rules engine, so it will not stop you from building something illegal.
Deck builder vs. deck tester vs. play-testing table
These three get used interchangeably, but they do different jobs:
| Tool | What it does | What it does not do |
|---|---|---|
| Deck builder | Search card databases, add and cut cards, save a list | Enforce format legality or rules |
| Deck tester / goldfish | Draw opening hands, mulligan, run your lines against no opponent | Simulate an opponent's decisions |
| Play-testing table | Play a full game against a live opponent | Auto-resolve rules or triggers |
Untap.in combines all three. You build in the deck builder, goldfish solo to check opening hands and mulligans, then open a table or join from the public lobby for a real match. Because the platform does not enforce rules, you and your opponent track the game state the way you would across a physical table.
How to build a deck
- Pick the game you are building for. Untap.in hosts full card databases for 40+ games, including Magic: The Gathering, Pokémon TCG, Yu-Gi-Oh!, One Piece Card Game, Disney Lorcana, Flesh and Blood, Star Wars: Unlimited, Digimon Card Game, Dragon Ball Fusion World, Final Fantasy TCG, Cardfight!! Vanguard, and Android: Netrunner.
- Search the card database for the cards you want and add them to the list.
- Save the deck. You can share it with a link so others can view or copy it.
- Move to testing: goldfish it solo, or take it into a live game.
If you already have a list, skip the searching. Paste a plain-text decklist, or import one by URL from a popular deck-building site, and start play-testing straight away.
Building for a custom or homebrew game
Untap.in is not limited to published games. You can upload your own cards — art, text, and whatever fields your game needs — and build decks from them exactly like a published game. Since no rules are enforced, a homebrew set behaves the same as anything else on the table.
For fleshing out a prototype, solo play is the fastest route: goldfish a deck against an empty seat, or use Deck vs. Deck (a supporter mode) to sit on both sides and play two prototypes against each other.
Draft, cube, and sealed
The same platform covers limited formats. You can run booster drafts with friends, and bots fill any empty seats so a pod is always ready. That makes it usable for testing a cube or a sealed pool without coordinating a full group.
What to check before you commit
- Rules enforcement: none. If you need the platform to catch illegal plays or auto-resolve triggers, this is not that kind of tool.
- Cost: Untap.in states it is free to play-test — building, goldfishing, drafting, and live multiplayer at no cost — with optional supporter perks. Deck vs. Deck is listed as a supporter mode.
- Install: none required. It runs in a browser on desktop, tablet, and phone, and can install as an app from the browser.
- Affiliation: Untap.in describes itself as an independent, fan-built play-testing platform, not affiliated with or endorsed by the game publishers.
If your goal is to get a list out of your head, onto a table, and in front of an opponent quickly, the build-import-share-test loop is the whole workflow. If your goal is a rules-accurate simulation, look for a platform that enforces game rules instead.