All packsTransactional Email Deliverability CoreSolve
resend webhook dedupe svix id vs email id
email_id looks like the dedupe key. It isn't.
Resend's data.email_id is shared across a message's bounced, delivered, and complained events -- deduping on it collapses distinct events into one.
This is one of the things Transactional Email Deliverability Core already handles. Send transactional email that actually lands, and stop emailing people who bounce or complain.
Is this you?
Resend webhook payloads carry no top-level event id, and the obvious candidate -- data.email_id -- is shared across every event type that happens to one message: sent, delivered, bounced, and complained can all carry the same email_id. Deduplicating on it looks correct in a quick test (the same event delivered twice does dedupe) and silently collapses genuinely distinct events into one, dropping real signal like a complaint that arrived after a delivery notification for the same message.
Why this one is easy to get wrong
email_id is right there in the payload, named like an identifier, and does correctly dedupe the retry-of-the-same-event case, which is the case most webhook-handling code is written and tested against. It takes reading Resend's event-shape documentation specifically for "can two different event types share this field" to notice the collision -- not something a dedupe implementation naturally surfaces on its own.
What you get instead
The pack's webhook decisions dedupe on the svix-id header instead -- unique per delivery attempt group, supplied by the Svix transport layer Resend signs with, not by Resend's own payload. It's stored as email_events.provider_event_id with a UNIQUE constraint, so the insert itself is the idempotency check: a duplicate delivery throws a conflict, is reported as {duplicate: true}, and the route still returns 200 so Svix doesn't retry forever.
Source: ARCHITECTURE.md 'Webhooks (Resend -> Svix transport)', decision 3 — checkable in the pack you receive
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