API calls fail with HTTP 429 error: "Resource has been exhausted (e.g. check quota)" or "Quota exceeded for quota metric 'Write requests' and limit 'Write requests per minute per user' of service 'sheets.googleapis.com'". Requests succeed sporadically but fail under load.

Exceeding per-minute rate quotas: e.g., 300 read requests/minute/project, 60 write requests/minute/user. Quotas refill every minute; concurrent or looped requests in AI agents/scripts often hit these without backoff, triggering 429 errors even if daily totals are low.[Google Sheets API Limits](https://developers.google.com/sheets/api/limits)