Conversion API
A Conversion API (CAPI) is a server-to-server channel for reporting conversion events directly to an ad platform, instead of relying only on a browser pixel that ad blockers and privacy features can interrupt.
Browser pixels fire from the visitor's device, which makes them fragile: content blockers strip them, Safari and iOS limit their cookies and identifiers, and slow page loads drop them. The conversions the platform never sees skew its picture of who converts — and its optimization follows that skewed picture.
Conversion APIs fix this by reporting from the server, where nothing on the visitor's device can interfere. The standard setup runs pixel and server events together, deduplicated by matching event IDs so nothing double-counts. Meta's Conversions API and TikTok's Events API are the best-known implementations; Google's equivalent path is offline conversion imports keyed by click ID.
In practice
A lead submits a funnel with an ad blocker active. The pixel event never fires — but the server event reports the conversion to Meta anyway, with the lead's value attached. Meta counts one conversion (deduplication), and its optimization keeps learning from complete data.