[Dire]

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f you really do believe that his range is primarily bluffs/draws then you have to expect that he's going to be aggressive enough to fire another barrel on the river

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you aren't the first person to say this...

i think its very reasonable for him to semibluff the turn and give up on the river. its also reasonable for him to shove the river.

i dont think it is so cut and dry that if he semis the turn, he absolutely will bluff the river.

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The thing is the odds you're being laid. You're getting 2:1 on the river. He only has to follow through with a bluff 33% of the time to make it a profitable call. In other words, you'd have to be 66% certain he never follows through with a bluff on the river to justify a fold. This alone makes it a must-call on the river, if you do believe he's bluffing/drawing on the turn.