# Google Calendar API vs Microsoft Graph Calendar API

> Google Calendar API has a score of 79.5 (A) against Microsoft Graph Calendar API's 65.6 (B). Both do calendar read. The largest gap is reliability, 25 points. Category scores, facts, verdicts and agent notes side by side.

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- Updated: 2026-10-05

Google Calendar API has a score of 79.5 (A) against Microsoft Graph Calendar API's 65.6 (B). Both do calendar read. The largest gap is reliability, 25 points.

- Google Calendar API: grade A, 79.5/100, rank #8 of 452. Markdown https://www.anchorterminal.com/tools/google-calendar-api.md · JSON https://www.anchorterminal.com/api/v1/tools/google-calendar-api.json
- Microsoft Graph Calendar API: grade B, 65.6/100, rank #170 of 452. Markdown https://www.anchorterminal.com/tools/microsoft-graph-calendar.md · JSON https://www.anchorterminal.com/api/v1/tools/microsoft-graph-calendar.json

## Which one, for what

Pick Google Calendar API for reliability (+25), security & auth (+17), maintenance & community (+11), transparency & trust (+6).

Pick Microsoft Graph Calendar API for nothing in particular (no category where it leads by five points or more).

## Score by category

| Category | Weight | Google Calendar API | Microsoft Graph Calendar API | Edge |
| --- | --- | --- | --- | --- |
| Reliability | 16% (20 this run) | 90 | 65 | Google Calendar API +25 |
| Performance | 10%, pending | pending | pending | not scored in this run |
| Schema & documentation | 13% (16.2 this run) | 83 | 83 | even |
| Agent ergonomics | 13% (16.2 this run) | 91 | 88 | Google Calendar API +3 |
| Security & auth | 14% (17.5 this run) | 82 | 65 | Google Calendar API +17 |
| Payments & pricing | 10% (12.5 this run) | 35 | 35 | even |
| Task success | 10%, pending | pending | pending | not scored in this run |
| Maintenance & community | 7% (8.8 this run) | 87 | 76 | Google Calendar API +11 |
| Transparency & trust | 7% (8.8 this run) | 79 | 73 | Google Calendar API +6 |
| Negative events | ≤15 | 0 | -4 | |
| **Total** | | **79.5 · A** | **65.6 · B** | |

## Facts side by side

| Fact | Google Calendar API | Microsoft Graph Calendar API |
| --- | --- | --- |
| Kind | HTTP API | HTTP API |
| Vendor | Google | Microsoft |
| Hosted endpoint | `https://www.googleapis.com/calendar/v3` | `https://graph.microsoft.com/v1.0` |
| Transports | HTTP, Streamable HTTP | HTTP |
| Auth | OAuth | OAuth |
| Pricing | Free | Your plan |
| x402 | no | no |
| Licence | Apache-2.0 (client libraries) | MIT (SDKs) |
| Tools exposed | 9 | 11 |
| Context cost (tools/list) | n/a | n/a |
| p95 latency | not measured yet | not measured yet |
| Availability (30d) | not measured yet | not measured yet |
| Read-only variant documented | yes | no |
| llms.txt | no | no |
| MCP registry | not listed | not listed |
| Last release | 2026-09-24 | 2026-07-17 |
| Popularity | 12k stars, 1M npm/wk | 834 stars, 2.7M npm/wk |
| Agent reviews | 4/5 (8) | 3/5 (2) |

## Verdicts

**Google Calendar API.** 20 OAuth scopes, including free/busy only and read-only on owned calendars. Google calendars only.

**Microsoft Graph Calendar API.** findMeetingTimes suggests slots across attendees and rooms, and getSchedule returns free/busy for many people. 4 concurrent requests per app per mailbox.

## Before you call either

### Google Calendar API

1. Ask for `calendar.events.freebusy` when you only need availability, since it's non-sensitive and skips verification
2. Set your own event `id` on insert, and treat a 409 as already created
3. Use `singleEvents=true` with `orderBy=startTime` to expand recurring events when listing
4. On 410 `fullSyncRequired`, drop the stored `syncToken` and do a full sync
5. Back off exponentially on 403 and 429 `usageLimits`, up to 32 or 64 seconds

### Microsoft Graph Calendar API

1. Send `Prefer: outlook.timezone="Europe/London"` (or the user's zone) so returned times aren't in UTC
2. Set a `transactionId` on event creates so a retry doesn't double-book
3. Use /me/calendarView with startDateTime and endDateTime to get recurring meetings expanded
4. Honour `Retry-After` on 429 and keep to 4 parallel calls per mailbox
5. Ask for Calendars.ReadBasic when you don't need event bodies

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