Flat pricing — "10 credits per message" — is easy to understand and quietly unfair.
A one-line question and a forty-turn debugging session are not the same amount of work. Under flat pricing, the person asking quick questions subsidises the person running long sessions on a large model.
What we do instead
Every completion reports its prompt and completion token counts. We multiply the total by a per-model rate and round up to at least one credit.
Every deduction is written to an append-only ledger with the model, provider and token counts attached. Your balance is a cached number; the ledger is the truth, and it can always be replayed to recompute it.