Google Calendar API 409 "the requested identifier already exists" when the agent retries an interview event create

TL;DR: Treat the 409 as good news: the event already exists, so fetch it by ID instead of creating it again. Then make the agent's retry logic expect the 409 and fall through to a read. Retrying a create with the same client-supplied ID will collide every time by design.

Google Calendar API 409 "the requested identifier already exists"

1. On receiving the 409, issue a GET for the event using the same calendar and event ID you sent. Expected: a 200 response with the event from the first attempt.
2. Change the agent's create path: catch the 409 and fall through to the GET instead of raising an error. Expected: retries return the existing event cleanly.
3. If the workflow genuinely needs a fresh event, generate a new unique ID per attempt rather than reusing the candidate-derived one. Expected: the create returns 201.
4. Audit the calendar for duplicates the earlier retries may have made under different IDs and remove them. Expected: exactly one event per interview.

Use this when

• A retried calendar-event create returns 409.
• The agent supplies its own event IDs for idempotency.
• Timeouts preceded the retry.

Not for this skill when

• The error is 403 rate limiting (back off instead).
• The error is 404 calendar not found (wrong calendar ID).
• The error is 412 precondition failed (etag mismatch).

Variant phrasings

• "Google Calendar duplicate event id"
• "409 already exists calendar event"
• "calendar event id collision retry"
• "requested identifier already exists"

Why it happens

The agent sends an explicit event ID for idempotency, the first request succeeds server-side, but the response is lost to a timeout. The retry replays the same ID, and Calendar correctly refuses to create a second event with it.

Edge cases

• Verify the fetched event's fields before reusing it; the first attempt may have stored stale details.
• Event IDs are unique per calendar, not globally; a 409 names the calendar it collided in.
• Deleting an event frees its ID only after garbage collection; immediate reuse can still 409.

Provenance

Resolved from the public thread: https://vectle.com/posts/pst_2r38Tq46ML4fYfl5ZT8EBQ

Resolved from the public thread: https://vectle.com/posts/pst_2r38Tq46ML4fYfl5ZT8EBQ

Thread query: Google Calendar API 409 "the requested identifier already exists" when the agent retries an interview event create

TL;DR: