It counts every card you enter, so it always knows exactly what is left in the 52-card deck. Every position is rated by a scoring model trained on millions of simulated games: lookup tables for each pile, each pair of piles and your stays, busts and streak, plus a small neural network that corrects the tables using all four piles and the exact cards left.

When you enter a card, it considers every legal move (each pile, with or without a stay, or timing out), then looks ahead: for every card that could come next, and the ones after it, it finds the best reply and weighs it by how many of that card are left. By default it looks three cards ahead; the 3 cards ahead button switches to two, which is faster on slow phones. If three cards would take more than about five seconds, it answers from two and says so. With 10 or fewer cards left it solves the endgame exactly, over every possible order of the remaining cards. The score and bust odds under each pile are exact for the next card.

Time out. Letting the game timer run out ends the game and still pays the 1,000 no-bust bonus if you haven't busted. The coach only says Time out when it is forced: the card in hand would bust every pile and no stay is left, so placing it would throw the bonus away. Otherwise it plays every card to the end of the deck.

Auto-place is on to start with: tapping the card you were dealt puts it where the coach says as soon as it decides, so there is no place button to press. Turn it off to place cards yourself. Undo takes a move back and lets you place that card yourself. On a keyboard, 1 is an ace, 2 to 9 are themselves, 0 is a ten (or J, Q, K) and - is undo. When the game ends, the coach replays the cards you saw with their order now known, shows the best score it can find for them, and lists the moves that cost you points against that line. Voice says each call out loud: "Pile 2", "Stay pile 3", "Bust pile 1" or "Time out".

Looking two cards ahead, it averaged 8,525 points on 587 shared simulated decks against 8,393 for the earlier playout-based coach (+132 ± 98, 95% interval). The third card changes about one move in 22 and is worth +32 ± 25 points per game (each changed move played out 200 times both ways on the same cards, 71 games). With the time-out rule on, it never needed to time out in 300 test games: its endgame play already keeps clear of forced busts. The call is there for games that reach high piles with nothing left that can make 21. Version 5 also ships a neural correction trained on 2 million more self-play games: +36 ± 8 points per game when playing one card at a time (60,000 paired games); with lookahead the difference is too small to measure on 300 test decks.

Rules of thumb

• Protect the no-bust bonus. Simple play (always take a score, otherwise pick the safest pile) averages about 5,300 in simulation and busts in three of every four games. The coach finishes with no busts in about four games out of five.
• Take the score most of the time, not every time. About one time in five it passes on an available score to set up two or three piles that can each finish on the next card, because each scoring card that directly follows another pays a growing bonus: +100, then +200, +300 and +400 for every further clear in the chain.
• Open new piles freely. When nothing scores and a pile is empty, it uses the empty pile about two times in three.
• Tens start piles. Half of the tens that don't score go into an empty pile, and about one in six goes on a lone ten. That 20 waits for an ace, and clearing a 20 is what half of its stays are for.
• Small cards build 5-card hands. It makes about 1.6 five-card hands a game, and most of them add to exactly 21 for 1,000.
• Spend stays before you're forced to. It uses about 1.7 stays a game, four times in five before it is actually forced, usually around the middle of the deck.

Scoring it plays for

Aces count 1 or 11, whichever helps. 10, J, Q and K all count 10, so they share one key. A stay empties a pile (2 per game). Three busts end the game. Letting the timer run out also ends it, and the 1,000 no-bust bonus still counts if you haven't busted, so near the end the coach may tell you to stop placing cards.