> ## Documentation Index
> Fetch the complete documentation index at: https://docs.zettaai.co/llms.txt
> Use this file to discover all available pages before exploring further.

# Luca Ledger

> Track inference usage and spend.

Zetta can track inference usage for agents.

This is useful for agents that pay for model calls or inference providers.

## What the ledger tracks

* provider
* model
* request type
* input tokens
* output tokens
* total tokens
* latency
* cost
* cost source

## Cost source labels

| Label       | Meaning                                         |
| ----------- | ----------------------------------------------- |
| `actual`    | Provider gave real billing data                 |
| `estimated` | Zetta estimated cost from model and token usage |
| `missing`   | Cost data is unavailable                        |

## Rule

Never show missing cost as real spend.

If cost data is missing, say it clearly.

## Related

* [What Luca Does](/luca/what-luca-does)
* [Surplus Pilot](/luca/surplus-pilot)