Glossary
What is an event_id?
An event_id is a unique identifier attached to a conversion event so ad platforms can deduplicate multiple reports of the same conversion, most commonly a browser pixel event and a server-side event. When both copies carry the same event_id and event name, the platform keeps exactly one.
The safest event IDs are deterministic: derived from something stable like the order number (for example ord_84213) rather than generated randomly. Retries, replays and the browser copy then all resolve to the same ID, so deduplication never depends on timing or luck.
Meta’s Conversions API pairs events by event_name plus event_id; the TikTok Events API uses event_id the same way; Google Ads relies on order/transaction IDs to avoid double-counting purchases. The mechanics differ slightly per platform, but the principle is identical: one conversion, one stable identifier.
In practice the hard part is coordination: the on-page pixel and the server must agree on the ID before either fires. Setups that generate the ID once, from the order, and hand it to both sides avoid the classic failure mode: two random IDs and a double-counted purchase.