Errors & rate limits - Sync2Books

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

Error shape

{
  "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.
`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.

Because most writes are asynchronous (you receive a syncBatchId rather than waiting for the external system), you rarely need aggressive polling. Prefer webhooks over tight retry loops.