Who called what, when
The API key's account, the model that served the call, and the time.
What a receipt proves
A log says what happened. A receipt lets someone else check.
The content, without the content
A salted hash of the prompt and the answer. You keep the text; the log keeps a commitment to it.
A chain with no gaps
Each receipt points at the one before it, so a missing call is detectable rather than merely absent.
Verified without us
Fetch the Merkle proof and check it client-side. The ledger is not trusted to vouch for its own log.