Snap CAPI sent no event_id — 180% purchase inflation and no way to deduplicate

The situation

A home fitness brand running Snap Ads had implemented both the Snap pixel and Snap Conversions API. Their reported purchase count in Snap was 2.8x Shopify order volume. The media buyer dismissed it as "Snap always overcounts" and didn't investigate further. Budget was increasing.

The problem

The Snap pixel was firing purchase events on the order confirmation page. The Snap Conversions API was also sending purchase events via a Shopify webhook. Neither the pixel event nor the CAPI event included an event_id. Snap's deduplication requires matching event_id fields on both the pixel event and the CAPI event — without them, every purchase generates two separate conversion records with no way to identify them as duplicates.

What we did

1. Confirmed pixel events had no event_id field in Snap Pixel Helper audit
2. Confirmed CAPI server events had no event_id field in Snap Events Manager logs
3. Added event_id = "purchase_" + Shopify order ID to pixel event via GTM dataLayer push
4. Added same event_id format to Shopify webhook → CAPI server payload
5. Verified dedup: after 7 days, Snap purchase count dropped to match Shopify within 5%

Why it worked

"Snap always overcounts" is a common excuse that masks a fixable deduplication problem. Snap's Conversions API deduplication works exactly like Meta's — it requires a shared event_id on both the pixel event and the CAPI event. Brands that implement both without the dedup key will always see 2x inflation. It's not Snap overcounting — it's the advertiser not configuring dedup.