The safest course of action would be to take your cat to the vet right away if coffee beans were eaten.
Coffee is very bad for cats (and dogs as well), and severe caffeine poisoning can be life-threatening. Just as with humans, a little caffeine will make a pet hyper or 'buzzed', a lot can cause a racing heart, even more can be dangerous. In addition to being much smaller, cats and dogs are more sensitive to caffeine than humans, and it's hard to quantify the exact dose at which it starts to be harmful. Most sources I can find say that 'a taste' of coffee or tea may not be harmful, but that caffeine is toxic to pets even with a relatively small amount consumed.
Monitor your cat for symptoms of caffeine poisoning, which include elevated heart rate, rapid breathing, restlessness/hyperactivity, vomiting and seizures. If anything seems amiss, see the vet right away.
For the future, even if your kitten shows no adverse effects from the coffee beans, be sure to keep coffee (including decaf), tea, hot cocoa and all caffeinated products out of reach. Not only is the caffeine harmful, but other compounds found in coffee, such as theobromine, can also cause adverse effects in cats and dogs.
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