event_id
event_id is a unique identifier attached to a conversion event and sent through two channels at once — the browser pixel and a server-side CAPI call. Facebook uses that matching id to recognize both signals as the same real-world action and avoid counting it twice.
Why deduplication matters
Without a matching event_id, the same action — a purchase, a registration — gets counted once from the pixel and again from CAPI, inflating the conversion count in the ad account and distorting cost per result. The optimization algorithm then trains on bad data and gets worse at finding the right audience.
How it plays out in a PWA chain
event_id is typically generated client-side, at the moment of the action on the lander or inside the PWA, then passed along with the event to the tracker and on to the postback that reaches CAPI with the same value that went to the pixel. APEX's postback setup (Keitaro to FB CAPI) is built to carry this id through the chain intact, so pixel and CAPI events deduplicate correctly.
FAQ
- What happens if the event_id doesn't match?
- Facebook can't recognize it as the same event, conversion counts get doubled, and campaign optimization ends up training on distorted data.
- Does event_id need a specific format?
- There's no strict format — what matters is that it's unique per event and matches exactly between the pixel firing and the CAPI call for that action.
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