Halo Forager didn't allow selection of Connive//Concoct
After playing Halo Forager and paying 4 I couldn't select Connive from my graveyard. According to the errata of Connive//Concoct:
"If an effect allows you to cast a spell with certain characteristics, consider only the characteristics of the half you're casting. For example, if an effect allows you to cast a sorcery spell with mana value 2 or less from among cards in your graveyard, you could cast Assault this way, but not Battery."
So selecting just the half with a cost of 4 mana should have been possible.

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Rezzahan#77802 commented
The trigger targets a card with mana value X. The mana value of a split card in the graveyard is the sum of both halves. To get either half of Conive // Concoct you have to pay X=9. The resulting spell will then have a mana value of the half you've cast. The issue here is the targeting restriction, which looks at the card, not the spell it later becomes.