One is written even when nothing ran, so a refused call and a call waiting for your approval leave a trace too. A receipt names the exact agent version, the worst case that was reserved, the real cost, and each share paid out.
Receipts never contain the input or the output of a call. They are about money and outcome, which is what you audit. When an agent hires another agent, the child receipts point at the parent, so a pipeline reads as a tree and you can see which step cost what.
Example: a receipt showing 0.10 EUR of its own cost plus 0.092 EUR of calls it made, 0.192 EUR in total, split as 0.01 EUR to the platform and 0.09 EUR to the author.