Because exiling a card with foretell from your hand is a special action, you can do so any time you have priority during your turn, including in response to spells and abilities. Once you announce you’re taking the action, no other player can respond by trying to remove the card from your hand.
Cosmos Charger’s third ability applies only if it’s on the battlefield. Notably, it doesn’t affect when you can foretell Cosmos Charger itself or how much you pay to do so.
Casting a foretold card from exile follows the timing rules for that card. If you foretell an instant card, you can cast it as soon as the next player’s turn. In most cases, if you foretell a card that isn’t an instant (or doesn’t have flash), you’ll have to wait until your next turn to cast it.
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While you control Cosmos Charger, you can foretell cards any time you have priority.
If you’re casting a foretold card from exile for its foretell cost, you can’t choose to cast it for any other alternative costs. You can, however, pay additional costs, such as kicker costs. If the card has any mandatory additional costs, those must be paid to cast the spell.
The cost reduction refers to the cost you pay as you exile the card from your hand face down, not the cost you pay as you cast the spell from exile.
If you control two Cosmos Chargers, you can foretell cards from your hand by paying {0}. Additional Cosmos Chargers won’t affect this cost further.
