Video start failure (VSF) is the rate at which playback attempts fail to ever produce a first frame - the viewer presses play and gets an error or an endless spinner instead of video. It is the most severe QoE problem because, unlike rebuffering or low quality, the viewer gets nothing at all.
Causes span the whole chain: license or entitlement errors, manifest or CDN failures, codec/DRM incompatibility on the device, or app bugs. Because each failure is a directly lost view (and, for ads, lost revenue), VSF is a top-line reliability metric watched in real-time operations dashboards, often split by device, app version, CDN, and region to localize the cause fast.
