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Can someone explain GIBs bidding to me? I bid what I thought was a perfectly normal unusual 2N. GIBs cue was described as forcing with 3 diamonds, forcing to 4♥!!. HOW can a cuebid of spades imply diamonds and show hearts? Thinking GIB has something slammish, I cue-bid 4♣ (lowest ace). GIB bids 5♣ which is a horrible contract. If this sequence is some mystery agreement, please let me know.
Let's start with the part that looked the least like a bug, but is actually a big one - the 2NT bid. You hover over the bid, check the description, and then bid it. How many total points do you think GIB will interpret 2NT as showing?
If you answered 12+, you'd be.. incorrect.
There are very few places in the database where GIB uses the vulnerability, but 2NT overcalls are one of them. When non-vulnerable, 2NT promises 12+ total points. When vulnerable, 2NT promises 15+ total points. GIB alerts this accordingly. Indeed, if you place GIB in South for this hand, it will overcall 2NT and describe it as 15+. Yet when a human is vulnerable in South, it will give the wrong description.
This appears to be because the code which looks up what each possible human bid means to provide the hovers completely ignores the vulnerability. I have a feeling this worked years ago, so either something was broken somewhat recently, or it was always broken and I just never noticed. Who knows.
OK, so from this point on, we're assuming 2NT was correctly alerted as 15+ total points - which is fine, because we do actually have that.
GIB's 3♠ cue is alerted as showing 10+ total points, and 3+ diamonds. It does not say forcing to 4♥ as per the OP (that comes later). We can now see where the 10+ comes from: 10 + our secretly promised 15 adds to its magic number of 25 to force to game. But why does this promise 3 diamonds?
A suggestion on Facebook was that 2NT forced the minors, so it's a good idea to play 3♥ as showing clubs and 3♠ as showing diamonds. But not being able to bid hearts naturally seems odd to me, and besides - GIB alerts both 3♥ and 4♥ as natural, so that's definitely not the case. They're probably mixing that up with UVU.
In fact, if you were non-vulnerable, and GIB really did treat 2NT as 12+, it will jump to 4♥ with this hand. Why doesn't it bid 4♥ when vulnerable? Because your minimum 15 points + its 14 total points (2 for each singleton, minus 1 for an honor in a short suit) is 29, which is its magic number for having enough strength for the 5 level. With a point less, it will bid 4♥, but here that rule is invalidated for being too strong.
OK, but why does 3♠ show diamonds? At this point it has run out of specific rules for how to handle unusual 2NT, so hunts around for other more general bidding rules. There's a default rule, completely unrelated to U2NT, that says with enough strength to force to game, cuebidding the opponent's suit shows support for partner's suit. And it just so happens the code for "partner's suit" in this case is "the highest rank suit that partner has shown length in", in order to be unique - thus, diamonds.
Give GIB exactly the same hand but with diamonds and clubs reversed, and that rule doesn't match.. it ends up falling back to another default rule, which happens to tell it to just bid 3♣. Yep, the same bid it'd make with 0 points.
But OK, GIB was lucky to find a rule that kept the bidding going, and has forced to game with diamond support.
After that, control bidding towards slam is always impossible with GIB. But you try anyway; 4♣ is described as showing 18+ total points, which isn't too far off, right? Oops, we're back to the original bug; since all of the ranges are 3 points off, this bid is actually described as 21+ total points, and just mislabelled for humans. Its 14 points + your minimum 21 = 35, which is another magic number that means it should try for grand.
And there's a rule which says if you have a minimum of 35 total points between you, you can try for grand by making a 5 level control bid, a higher priority rule than the ones about cuebidding at the 4 level. But the description part of the rule still says it denies lower controls, because it doesn't understand its own priority system.
Why is 4♣ (not 3♠) described as forcing to 4♥? No particular reason, and nothing to do with hearts; the rule for the cuebid is to force to 4trump+1, which seems equivalent to forcing to 5trump to me.
And then you passed a cuebid.
Funnily enough, if you correct to the agreed trump suit of 5♦, it will raise you to 6♦ and you'll make it.. well done GIB for not having the foggiest of what it was doing and still getting to the right contract by accident.
