Calendar MCP Permission Errors: A Troubleshooting Checklist

A failed calendar write does not always mean Google is down. With multiple accounts and per-calendar grants, the first thing to check is which calendar the agent targeted and what that API key is allowed to do there.

Start with discovery

Call list_calendars with the same key used by the failing agent. It returns the account, calendar ID, enabled state, Read and Write grants, and whether the connection uses OAuth or a service account. Compare that output with the calendar ID in the failed tool call.

Four common mismatches

1. The key has Read but not Write access to the target calendar.
2. The calendar is disabled or belongs to a different connected account.
3. The agent omitted calendarId and selected the wrong default write target.
4. The requested operation changes attendees on a service-account connection.

CalendarMCP rejects writes without a Write grant. It also rejects attendee changes on service-account-connected calendars; use an OAuth connection for that workflow. Do not broaden every calendar grant just to make one request succeed. Identify the intended account and change only the necessary grant.

After the fix

Re-run list_calendars, then try a read on the explicit target calendar. If a write is still required, preview the event, attendees, and notifications before retrying it. This sequence separates authentication, account selection, and write permission without creating test events.

Source: CalendarMCP tool documentation.