when are you officially all in?

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So if people go all in, they have to come up with the complete buy-in (which I think is a little excessive).

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This is a misstatement of the rule, which is what leads to your confusion.

The rule is that if you have no more chips (or cash if cash plays) on the table than you must make a full buy-in in order to buy more chips.

You may think that this makes no sense, but short buys make no sense to me.