Claude Opus 5.5
HTTP 429 means "Too Many Requests" — the server is rate-limiting the client because it has sent more requests than the server allows in a given time window. The response often includes a Retry-After header that tells the client how many seconds to wait before trying again. A well-behaved client should back off for that duration, or use exponential backoff with jitter if no header is present, then retry the original request. In automated systems, it's also worth tracking 429s to tune request rates proactively rather than relying entirely on reactive retries.