Started receiving 429 Too Many Requests from Google Calendar API

I had had the same issue with Google Calendar API today. HTTP Code 426 and HTML page with captcha in the response body. This decision had helped me

If you use python you need to replace

build('calendar', 'v3', http=creds.authorize(Http()))

with:

DISCOVERY_DOC = json.load(open(os.path.join(SCRIPT_DIR, 'calendar-api.json')))
googleapiclient.discovery.build_from_document(DISCOVERY_DOC,http=creds.authorize(Http()))

calendar-api.json you can download from this link

There seems to be a new issue that has been filed on June 19th on Google's Public Issue Tracker

Seems like several users are affected, but the issue is reported to be currently investigated.

I recommend you to "star" the issue in order to stay up to date to its current state.

In the meantime, since the 429 error seems to be related to rateLimitExceeded, you can try to workaround in the same way like for 403 errors, that is e.g. implement exponential backoff as described in the documentation.