What's the difference between a 429 error and a 503 Service Unavailable error?

A 429 'Too Many Requests' error is client-side; you're making too many requests. A 503 'Service Unavailable' error is server-side; the server is temporarily unable to handle the request, often due to overload or maintenance. While both result in temporary unavailability, the cause and solution differ. For 429, you need to slow down your requests. For 503, you typically need to wait and retry later.