> ## Documentation Index
> Fetch the complete documentation index at: https://docs.sync2books.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Errors & rate limits

> The consistent error shape, common HTTP status codes, and rate-limit headers used across the Sync2Books API.

The Sync2Books API uses conventional HTTP status codes and a single, consistent
error shape across every product.

## Error shape

```json theme={null}
{
  "statusCode": 400,
  "message": "Validation failed",
  "error": "Bad Request"
}
```

`message` may be a string or an array of validation messages. Always read the
HTTP status first, then `message` for the human-readable detail.

## Common status codes

| Status | Meaning                                                                                                       |
| ------ | ------------------------------------------------------------------------------------------------------------- |
| `200`  | Success.                                                                                                      |
| `201`  | Created.                                                                                                      |
| `400`  | Bad request — validation error.                                                                               |
| `401`  | Unauthorized — missing/invalid API key, or invalid signature. See [Authentication](/concepts/authentication). |
| `403`  | Authenticated, but not allowed for this resource.                                                             |
| `404`  | Not found.                                                                                                    |
| `409`  | Conflict — e.g. connecting a company that is already connected.                                               |
| `429`  | Too many requests — rate limit exceeded.                                                                      |
| `500`  | Internal server error.                                                                                        |

## Rate limits

Limits are applied **per application, per minute**. Every response includes the
current state in headers:

| Header                  | Meaning                                   |
| ----------------------- | ----------------------------------------- |
| `X-RateLimit-Limit`     | Maximum requests per window.              |
| `X-RateLimit-Remaining` | Requests remaining in the current window. |
| `X-RateLimit-Reset`     | Unix timestamp when the window resets.    |

When you receive a `429`, back off until the time in `X-RateLimit-Reset` and retry.

<Tip>
  Because most writes are asynchronous (you receive a `syncBatchId` rather than
  waiting for the external system), you rarely need aggressive polling. Prefer
  [webhooks](/concepts/sync-model) over tight retry loops.
</Tip>