401 and 403 — carries these headers:
When you’re over the limit
A429 response adds a Retry-After header (seconds to wait) and the standard error body:
Backing off
X-RateLimit-Remaining on every response and slowing down before you hit 0 is friendlier than waiting for a 429 — and every response gives you that number for free.
Repeated authentication failures from the same address can also trigger
429, independently of your per-key quota.