Google Calendar API Intermittent 401 Errors Despite Valid Access Token

[jayanta04tou]

Hello,

We're facing a recurring issue while integrating the Google Calendar API with our Solution.

Issue Summary

Even though we are correctly refreshing the access token using the stored refresh token via the standard OAuth 2.0 flow, the Google Calendar API intermittently returns a 401 Unauthorized error:

{
"error": {
"code": 401,
"message": "Request had invalid authentication credentials.",
"status": "UNAUTHENTICATED"
}
}
This issue reappears every few days—even when using a freshly refreshed and previously working access token.

Sample API Request

GET https://www.googleapis.com/calendar/v3/calendars/CALENDAR_ID/events?timeMin=2025-04-28T10:00:00Z
Headers:
Authorization: Bearer ACCESS_TOKEN
Content-Type: application/json
Response:
{
"error": {
"code": 401,
"message": "Request had invalid authentication credentials.",
"status": "UNAUTHENTICATED"
}
}

We are using the following Google API endpoints:

GOOGLE_OAUTH_URI = 'https://accounts.google.com/o/oauth2/';
GOOGLE_TOKEN_URI = 'https://oauth2.googleapis.com/';
GOOGLE_CAL_URI = 'https://www.googleapis.com/calendar/v3';
GOOGLE_API_SCOPE = 'https://www.googleapis.com/auth/calendar';

• The access token is saved with an expiration time and refreshed correctly.

• The refresh token is not revoked or changed.

• No manual changes are made to the OAuth credentials or project settings in Google Cloud Console.

• We are applying the new access token immediately after receiving it.

Questions for the Community or Google Support

1. Why might a freshly refreshed, valid access token intermittently return 401 errors?

2. Are there any token invalidation rules, hidden revocation triggers, or quota limits that could cause this?

3. Could this behavior be account-specific or related to internal API caching or security policies?

4. Is there a way to validate access token health before making API calls?

This issue is impacting our production systems, and any guidance or insights from the community or Google team would be greatly appreciated.

Thank you!

[AlexET]

Hello @jayanta04tou!

Thanks for posting. This forum is mainly for questions about Google Cloud Application Integration.

If you are experiencing this Calendar API issue while using Application Integration in your workflows, please let us know the details, and we'd be happy to connect and explore potential solutions with you. 😊

Otherwise, you might find more specific help for general Calendar API authentication issues in other forums - https://stackoverflow.com/questions/tagged/google-calendar-api 

Hope you get it sorted out!

[jayanta04tou]

Hello,

Thank you for the quick response and for clarifying the scope of this forum.

Our current issue is not related to Google Cloud Application Integration workflows, but rather to direct integration with the Google Calendar API as part of a custom solution we're building.

We understand that this forum may not be the best fit for this particular issue, and we appreciate your guidance.
Thanks again for your time and support!

[AlexET]

Got it, thanks for clarifying @jayanta04tou!

If you ever get curious about streamlining your integrations, we have some great resources! You can check out recordings of our past office hours and tech talks here.

We also post updates about upcoming free sessions here so you can stay in the loop: https://rsvp.withgoogle.com/events/apigee-emea-office-hours-2024 

Good luck with your project! 😊