Conversions API is Meta's server-to-server event API. Instead of relying on the browser-side pixel to fire when a user converts, your backend POSTs the conversion event directly to Meta with hashed user identifiers (email, phone). Meta matches it to a user account and credits the conversion to whichever ad they saw, even if the pixel didn't fire because the user blocked tracking, used iOS, or has aggressive ad-blockers.
Why it matters
After iOS 14.5, pixel-only accounts typically miss 30-60% of conversions. CAPI recovers most of that. The accounts that didn't add CAPI in 2022-2023 are the ones still wondering why their reported ROAS keeps dropping — it's not that the ads got worse, it's that the measurement got worse.
Setup paths
- Direct API: your backend POSTs events. Most flexible, most engineering effort.
- Shopify integration (for Shopify merchants): Meta's official Shopify app routes order events through CAPI
- WooCommerce + Pixel Manager plugin: server-side event forwarding without custom code
- GTM Server-Side Tagging: route events through a Google Tag Manager server container that forwards to Meta
Deduplication
When you run pixel + CAPI together, the same purchase fires twice — once browser-side, once server-side. Meta deduplicates these by event_id: send the same event_id from both sources, and Meta counts it once. Skip event_id and you'll double-count every conversion, which makes ROAS look 2× better than it is.