Credits meter the work. One request consumes them in proportion to what it actually did.
Why metered this way
A question that reads your model and answers costs a fraction of a request that rebuilds your organisation model, checks every stage and produces twenty artefacts. Charging per request would make one of those a bargain and the other a rip-off.
What drives the cost
Writing back
Some of what you spend goes on writing what a run learned into the organisation model. It shows up separately in usage. It is the part that makes later tasks cheaper and better informed, so it is worth understanding rather than minimising.
Running out
Work stops. Nothing degrades quietly into worse output.
Controlling spend
Choose the right scope. Most requests do not need the widest one, and an Only this that turns out to need more can be re-run.