Do unused monthly credits carry over to the next cycle?

Monthly credits don't carry over. Whatever you don't use within the billing cycle expires at renewal, and a fresh allocation replaces it.

If you want credits that never expire, buy PAYG instead — those stay in your account forever. Plenty of users combine the two: a monthly subscription for the predictable cadence + occasional PAYG top-ups when a big project pushes them past the allocation.

If your monthly bucket is consistently empty before the cycle ends, your plan size is probably too small. If it's consistently mostly unused, the plan is probably too big — consider downgrading.