[lycier]

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After 1♠,GIB pass,however this is a unacceptable final contract with trump 4-2 fit.

[1eyedjack]

The problem may be linked to GIB’s other "agreement", inferior in my opinion, that with a balanced hand you rebid 1-suit in preference to 1N. So that 1S rebid is quite likely to have fewer than 5xC.
GIB makes a courtesy response on 1H because there is scope for 4H game. That scope disappears after the 1S rebid, so as the saying goes, once you realise that you are in a whole, best to stop digging early.

I wonder if GIB would have rebid 2C sitting South? I think it has some merit, at IMP, in the context of the rest of its system. Provided that the bid might systemically conceal 4xS there remains room to find a Spade game. You miss out on some Spade partials when responder is weak with Spade fit, but gain when there is no Spade fit.

On the other hand I would not want to rebid 2C at MP, and while it is fine for GIB to take the scoring into account in matters of judgement, it is asking a lot to have fundamentally two systems.

Perhaps you recommend 1N rebid by responder? That could work. Trade off between these types of hands and those where opener then comes out of the bushes with a strong bid 3rd time around.