CAPI - a Conversions API - is server-side conversion tracking. Instead of relying only on a browser pixel to report events like a lead or a purchase to an ad platform, your server sends the same events directly to the platform. Nothing in the browser can block it, and it can carry richer matching data (hashed email and phone), so match rates are higher.
It exists because the browser pixel has gone increasingly blind: ad blockers, browser tracking prevention, and consent rejections mean browser-only tracking now misses a substantial share of real conversions. Every lost event is a conversion the ad platform never learns from, which degrades targeting and inflates reported cost per lead. Server-side delivery recovers those events.
The recommended setup runs both the browser pixel and the server-side API, deduplicated by a shared event ID, so each conversion counts exactly once. Capturing ad-platform click IDs (fbclid, gclid, ttclid) and sending them with server events gives the strongest attribution match. The funnels guide covers pixel-versus-CAPI tracking, and the CAPI deep-dive details the failure modes.
Related terms
Lead funnel
A multi-step flow that converts a click into a qualified contact by asking one small question at a time and capturing the email at the end.
UTM parameters
Tags appended to a URL (source, medium, campaign, and more) that tell analytics tools where a visitor came from.
Webhook
An automated HTTP POST that a service sends to a URL you control whenever an event happens, so your systems react in near real time without polling.
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