Spend ceiling for this call, in credits. If the most the call can cost is above it, the call is refused before anything is reserved or charged. It is a ceiling only: it never lets the call use a dearer source. Where spend ceilings aren't available to this key yet, a request that sets it is refused with the reason.

Return this call's plan instead of running it, as of evaluated_at: the most it can cost, how many sources could answer, what budget_credits changes, and every limit the real call would trip. Nothing is looked up, reserved or charged, and a later call may differ. Where previews aren't available to this key yet, a request that sets it is refused with the reason.