CSAI (client-side ad insertion)

In client-side ad insertion, the player app or web page handles ads directly. At an ad break, it calls the ad server, receives a VAST response, loads the ad video, plays it and fires tracking pixels, then returns to the content.

CSAI is the traditional approach on web and mobile video. Its advantages are rich measurement and interactivity: because the ad runs on the device, verification scripts through the Open Measurement SDK and interactive layers through SIMID can observe and interact with it, and the ad server sees the real device's IP address and user agent, which helps fraud detection.

Its drawbacks are buffering and delays when switching between content and ad, higher failure rates on less powerful devices, and vulnerability to ad blockers. That is why SSAI dominates CTV and live streaming, while CSAI remains common in web video, mobile apps and many outstream video placements. Some services use hybrid approaches, stitching server-side but running measurement client-side.

Think of it like this

CSAI is like a DJ stopping the music to play a sponsor's jingle from a separate CD, where you notice the switch but the DJ can see exactly who is in the room.

An example

A news website in Spain uses CSAI for its video player; the ad call carries the reader's real IP and browser details, and a verification tag measures viewability of the pre-roll.