CAPI (Conversions API)

What is CAPI?

The Conversions API (CAPI) sends conversion events server-to-server, deduplicated against the browser pixel, restoring the signal lost to iOS/ATT, ad blockers, and cookie limits, and per industry implementation data it typically recovers 20–30% of otherwise-missing conversions.

How it works

CAPI runs alongside the browser pixel, not instead of it. Both send the same event carrying a shared event_id, and Meta deduplicates so a conversion seen by both is counted once, while a conversion the pixel missed (an ATT opt-out, an ad blocker, a Safari cookie limit) still arrives through the server. The server send happens from your own infrastructure, which is what makes it both more reliable than the browser and controllable in a way the pixel is not. Match quality still matters: the more clean identifiers you pass (a hashed email, phone, or click ID), the better the platform ties a server event back to the person who saw the ad, so a well-configured CAPI feed with strong match quality recovers far more than a bare one.

A worked example

Work it with round, illustrative numbers. Say 100 patients subscribe after clicking iPhone ads but the browser pixel, degraded by iOS ATT, only registers 75 of them. A clean CAPI feed recovers most of the missing 25 server-side, so the platform optimizes on nearly all 100 conversions instead of three-quarters of them. That is often the difference between Meta reading a profitable GLP-1 campaign as a loser (and pausing it) and seeing the true conversion volume that keeps it scaling.

The configuration is the compliance surface

Because you control what the server sends, the CAPI payload is where compliance is won or lost. Send the minimum (event name, value, and a hashed identifier such as email) and never a condition, diagnosis, prescription, or intake answer, since the FTC’s GoodRx ($1.5M) and BetterHelp ($7.8M) actions in 2023 turned exactly that data flow into penalties. CAPI is the mechanism that makes server-side tracking real on Meta, and pairing the two correctly is a core piece of HIPAA-compliant marketing: recover the signal, strip the health data.

Why it matters for health and DTC brands

Health traffic skews mobile and iPhone, so it lost more signal to ATT than almost any other category, which makes the 20–30% CAPI recovers disproportionately valuable here. A clean feed is frequently the difference between the platform optimizing a campaign on a fraction of true conversions and optimizing on nearly all of them, and it does that without any protected health information leaving your stack. Implement CAPI as the server leg of server-side tracking, deduplicate carefully against the pixel with the shared event ID, and judge the payoff on blended marketing efficiency ratio (MER) and real CAC rather than the platform’s ATT-degraded reporting.

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