A server-side connection that sends conversion events to ad platforms (most commonly Meta) directly from your server instead of relying only on the browser pixel. Since iOS privacy changes and browser tracking restrictions, the pixel alone misses a large share of conversions. CAPI restores that signal, which is what keeps the algorithm optimising against real outcomes rather than a half-blind picture.
Benchmark. Treat CAPI as non-optional infrastructure, not an optimisation. Without it your reported ROAS and the platform's targeting both degrade as browser signal keeps shrinking.