Charging per AI image generation: Stripe metered billing or my own credits system?

Given your worry, prepaid credits. You can't lose money on usage you already got paid for.

For the race condition, do the decrement atomically in the database:

update credit_balances
set balance = balance - 1
where user_id = $1 and balance >= 1
returning balance;

No row returned = not enough credits, don't generate. Two parallel requests can't both spend the last credit. Also write every change to a credit_ledger table so you can explain a balance to an angry user.

Credits come in through a checkout.session.completed webhook for a one-time payment.

I'd lean the other way for B2B customers: meters (Billing > Meters, report with stripe.billing.meterEvents.create) plus a spending threshold is nice UX, because nobody has to top up mid-work. You can also bill mid-month when usage passes an amount, which limits the "5,000 images and walk away" risk.

For consumers, agree: prepaid credits.

Thanks both. Mostly small shops, so somewhere in between... starting with credits since it's simpler, will revisit.