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

# Rate limits

> The CBS API usage-limit response headers and what happens when a trial key reaches its cap.

The CBS API reports your usage on every response and enforces a calculation cap on trial keys.

## Limit-status headers

Every API response includes these headers:

| Header                     | Meaning                                                 |
| -------------------------- | ------------------------------------------------------- |
| `X-Pcc-Api-Calc-Limit`     | Calculations allowed for the key (`0` means unlimited). |
| `X-Pcc-Api-Calc-Remaining` | Calculations remaining (`N/A` when unlimited).          |
| `X-Pcc-Api-Exp-Date`       | The key's expiration date.                              |

Monitor `X-Pcc-Api-Calc-Remaining` to avoid interruptions as you approach the cap.

## When you hit the limit

The cap is checked on **calculation requests** (`POST`). When a trial key exceeds its limit, the API returns HTTP **401 Unauthorized** with:

```json theme={null}
{ "message": "API rate limit has been exceeded." }
```

<Note>
  The calculation cap applies to **trial keys**. Standard (paid) keys are not blocked by this limit. The headers still report status, but production usage isn't capped by this mechanism. Confirm your key's limits with Symmetry.
</Note>

For how keys are authenticated, see [Authentication model](/cbs/core-concepts/authentication-model).