Browser pixels miss conversions: ad blockers stop them, visitors close the tab before the thank-you page loads, consent banners delay them, and some purchases happen off-site entirely. A conversions API is a server-to-server endpoint (Meta's Conversions API, Google Ads offline and enhanced conversions, TikTok's Events API and others) where you report conversions yourself.
Server events are matched to ad clicks using click ids and hashed customer data such as email and phone. When you run both the pixel and the API, each event carries the same event id so the platform deduplicates. The result is more complete conversion data, which helps the platform's bidding algorithms find more customers like the ones who actually bought.