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

# Prompt caching

> Providers that cache a repeated prompt prefix report cached tokens in usage, and RedPill bills them at the model's cache rate.

Prompt caching happens at the provider and needs nothing in the request. When a provider serves
part of a prompt from its cache, the response reports it in `usage`, and the cached tokens are billed
at `pricing.input_cache_read` when the model lists one, otherwise at the normal prompt rate.

```json theme={null}
"usage": {
  "prompt_tokens": 1905,
  "prompt_tokens_details": {"cached_tokens": 1792},
  "completion_tokens": 64
}
```

To benefit, keep the stable part of a prompt, such as the system message and reference material,
at the start, and vary only the end. OpenAI models cache prefixes of 1,024 tokens or more on
repeated requests. Which models expose a cache rate is visible in the catalog: look for
`input_cache_read` in `pricing`.

## Related

<CardGroup cols={2}>
  <Card title="Billing" icon="credit-card" href="/get-started/billing" />

  <Card title="Models" icon="layer-group" href="/api-reference/models" />
</CardGroup>