[EarlPurple]

My table:

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Another table where the contract made:

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First it's rather unfair that we don't all play the same hands. Some hands are just totally flat.
Secondly, it's unfair if you have to pick a random sequence to make the bot mess up the defence.

So we all played in 4 spades doubled and when my LHO got in with the ace of spades he played ace of hearts, heart to the king and got a ruff. Absolutely nothing you can do about it, except pick a random sequence so East ducks the second heart instead of going up with the king to give his partner a ruff.

I pulled out in disgust 2 hands later after more nonsense but primarily because I was playing from my phone and my finger accidentally touched the screen and it played a card I had no intention of playing. I want to be able to pull the cards in to play them not just touch them.

[smerriman]

There's a huge difference between the two scenarios. In the second case, North has one trump left in dummy.

If declarer has the J, clearly playing the K is best.

But if declarer doesn't have the J (no assumptions are made about what card West will return from what holding), clearly playing low is better, as West can knock out the last trump and prevent declarer from ruffing what would likely be a diamond loser.

While GIB will still play the K more often than not, giving it a losing option sometimes works. This is a useful strategy for stealing extra tricks, like when you have an open suit, cashing a side ace may cause GIB to think that suit is equally likely to be open too, and guess wrong.

[pescetom]

Please don't use 800|600 which is exaggerated when viewing on a PC and difficult when viewing on a phone.
400|300 is more than enough.

[EarlPurple]

 smerriman, on 2022-July-24, 14:46, said:

There's a huge difference between the two scenarios. In the second case, North has one trump left in dummy.

If declarer has the J, clearly playing the K is best.

But if declarer doesn't have the J (no assumptions are made about what card West will return from what holding), clearly playing low is better, as West can knock out the last trump and prevent declarer from ruffing what would likely be a diamond loser.

While GIB will still play the K more often than not, giving it a losing option sometimes works. This is a useful strategy for stealing extra tricks, like when you have an open suit, cashing a side ace may cause GIB to think that suit is equally likely to be open too, and guess wrong.

Only because West also ducked the first trump on the lower one and not on the upper one (my table). If declarer has the jack trebleton, partner can ruff the 3rd round. If jack doubleton you want to play the king. So King seems obvious in both cases.

[smerriman]

 EarlPurple, on 2022-July-24, 16:27, said:

If declarer has the jack trebleton, partner can ruff the 3rd round. If jack doubleton you want to play the king. So King seems obvious in both cases.

Like I said, if declarer has J (doubleton or trebleton), the K is right. But if West has the J, low is right. Here's a copy of your diagram, with a slight tweak, and the same play up to that point:

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Click through to the same trick and you'll see East has to play low.

GIB takes both cases into account (no matter whether someone might have played differently earlier with that holding). If its simulated deals involve enough cases when West has the J, it will play low.

K is clearly the right play for humans, primarily due to inferences based on earlier play - like why declarer didn't cover with the 9. But GIB doesn't ask those questions, and when you give it a losing option, its simulations may stumble upon too many of the failing cases to flip the balance.