Our bots: how the Jass bots on wyys.ch play

When you play against bots

On wyys.ch the bots aren’t a stopgap but a proper part of the game. In a solo game you sit at the table with three bots. In Molotow, which is played by three to six players, it’s two to five. The whole game then runs in your browser, bots included, and is saved there so that you can carry on later.

At a table with friends the bots play every seat that is still empty. So the host can start the game even when only two people are there, and friends who join later take over a bot’s seat. If someone loses their connection during the game, the table waits 20 seconds. After that a bot plays on for that person until they’re back, and then hands the seat straight back. So no round gets stuck because a phone lost its signal.

Easy, Normal and Strong

When you open a table or a solo game, you choose how strong the bots are. There are three strengths, and of course all three follow every rule. They follow suit when they must, don’t undertrump when it’s forbidden, and declare their Weis.

Easy

A bot at the Easy strength plays by a smaller set of rules of thumb. About every third card, it deliberately plays something other than the card it thinks is best. So it makes mistakes, the kind that happen at a real table too. This strength suits you if you’re just learning Jass or simply want a relaxed game.

Normal

The Normal strength plays solidly and knows the usual conventions. The bot remembers every card played, and it also remembers who could no longer follow a suit. It weighs up whether to call trump or say Schieben, and it plays to help its partner. Many of its decisions depend on weights we didn’t set by hand. They were learnt over a great many games in which the bots played against each other and the better setting was kept each time.

In Schieber and Coiffeur two bots on the same team also signal to each other which suits they hold. You choose at the table how they do it: with the Discard signal, the Come-on signal or not at all. The setting doesn’t affect human players. It only decides what the bots mean by the cards they throw off and how they read them. Where everyone plays for themselves, as in Differenzler and Molotow, there is no partner and so no signals. There the bot keeps track of the points still in play.

Strong

The Strong strength can do everything Normal can, and on top of that it thinks ahead with every card. Like you, a bot knows only what it holds and what has already been on the table. It doesn’t know how the remaining cards are distributed. So it plays through many possible distributions in its head. Each of them fits everything that has happened so far. If an opponent didn’t follow a suit, for example, they get no card of that suit in any of these distributions. If someone showed four in a row when declaring a Weis, those four cards are with them in every distribution.

For every card it may play, the bot plays the round through to the end many times in these distributions and looks at which card brings its team the most on average. Experts call this Information Set Monte Carlo Tree Search. Put simply: the bot considers what the others might hold and picks the card that turns out well in most of those cases. In Differenzler it doesn’t aim for as many points as possible but for the number it predicted. In Molotow it aims for as few as possible.

Is Strong really stronger?

We only promise what we measure. Our tests include a series of games in which the strengths compete against each other in Schieber. Every deal is played twice, once with the seats swapped, so that the luck of the cards cancels out and only the bots’ play counts. Over many hundreds of such rounds, the tests require each strength to win more rounds than the strength below it and to score more points on average. A change to the bots that no longer manages that fails these tests.

We haven’t measured how strong a bot is compared with a human, so we make no claims about it. Good Jass players will keep surprising a bot with a clever card. The Strong strength is meant to be a serious opponent for them all the same.

A bot never sees your cards

A bot gets to see exactly what a human in its seat would see: its own cards, the cards on the table, the Weis that were shown and the score. It never gets the cards of the other seats. That isn’t just a good intention. It is how the game is built: the bot receives the same view of the table as a human player does, and our tests check that no other player’s hand is in that view. So when a bot plays exactly the right card, it worked it out rather than cheated.

Where the bots do their thinking

In a solo game the bots think in your own browser, in a background process, so the page stays smooth. At a table with friends it’s a bit different, and we want to explain that openly. The server the tables run on has very little computing time for each step. That is enough for an ordinary card, but not for the Strong strength’s thinking ahead. So the server sends that task to the browser of one player at the same table. That browser works in the background and sends back only the chosen card.

For this task the browser gets the cards of the one seat the bot is playing, and no others. It doesn’t show them anywhere, and they are sent in masked form. If the bot is playing for someone who lost their connection, those are that seat’s cards. The task goes first to the browser of the partner who is on the same side as the bot. The server checks every card sent back against the rules like any other move. That’s exactly how it is described in our privacy notice.

If a browser takes too long, the table waits and after a few seconds gives the task to another browser as well. Quietly playing a weaker card is out of the question. A bot always plays as strongly as its strength promises. Only when truly nobody can do the thinking does it play the simple card, as an exception, so that the game doesn’t grind to a halt.

Which strength suits you

• Easy suits you if you’re just learning the Schieber rules or playing with children.
• Normal suits a typical evening game. The bots play cleanly and keep to the usual signals.
• Strong suits you if you want a challenge. These bots think ahead with every card before they play.

The strength applies to all bots at the table, and in a solo game you can change it before every new game. How the whole site works is in the article How playing Jass on wyys.ch works, and if you want to play right away, take the shortcut via Jass against the computer.