# Receipts

Every call can produce a signed receipt on the public Zanii ledger.

Set a key's receipt mode when you create it:

- `off`: no receipt.
- `on`: a receipt per call, best effort.
- `required`: receipt or nothing. If a receipt cannot be signed, the call is refunded and refused.

## What is published

The prompt and the answer are **not** published. `payload_hash` is a salted hash: the salt stays
with us, so a short prompt cannot be guessed from the public log, and you can still prove later that
a particular text produced that receipt.

`model_id` is covered by the signature. That is what lets you prove you were served the model you
paid for, and not something cheaper.

## Verifying

Fetch the proof from the ledger and check it client-side with `zanii` in Python or `@zanii/core` in
JavaScript. It is a Merkle inclusion proof against a signed tree head, so the ledger is not trusted
to vouch for its own log.

- [The ledger](https://ledger.zanii.agency)
- [Verify a receipt](https://ledger.zanii.agency/verify/)