How to make AI email agents idempotent against duplicate webhook deliveries

AI email agents built on webhook-driven platforms like Nylas can silently send duplicate replies to customers because webhooks are delivered at least once and may fire up to three times per event. The root cause is not a model error but a systems design issue: handlers that treat every incoming POST as a new event will act on retries just as they act on originals. The article identifies two distinct IDs in each Nylas webhook payload — a notification ID that remains constant across retries and a message ID that identifies the underlying email — and explains how each serves a different deduplication purpose. Proper idempotency requires atomically persisting processed notification IDs, acknowledging the webhook before processing, making the send path itself idempotent, and using per-thread locks to prevent race conditions that deduplication alone cannot catch. These principles apply to any Nylas grant, including Agent Accounts where the agent owns the inbox rather than borrowing a human user's credentials.