Conversions API (CAPI) is a server-side measurement pathway that sends conversion events directly from the advertiser's server to the ad platform (Meta, Google, TikTok, LinkedIn), bypassing the browser-side pixel. In a post-iOS-14.5 + post-cookie environment, CAPI is the architectural answer to client-side signal decay.
Practical shape: a server-side Google Tag Manager (sGTM) container or a CDP (RudderStack, Segment) receives client-side events, enriches them server-side with first-party identifiers (hashed email, phone, address), and dispatches to each ad-platform endpoint. event_id deduplication keeps the platform from counting the same conversion twice (once via pixel, once via CAPI).
The discipline that makes CAPI work: hashing PII to spec, monitoring Event Match Quality (EMQ) weekly, validating Test Events before deploys, and verifying the dedupe rate. CAPI without monitoring is invisible signal loss; with monitoring it is the only reliable measurement layer in 2026.