What activity confirmed 3I/ATLAS's classification as a true comet?

Outgassing of volatile materials forming a coma and tail

The confirmation that 3I/ATLAS functions as a comet, rather than a simple inactive, rocky body like some asteroids, relies entirely on observing its cometary activity. This activity is defined as the outgassing or sublimation of volatile materials present within the nucleus when that nucleus is heated sufficiently by the Sun's radiation. This process creates the characteristic fuzzy atmosphere known as the coma and the visible stream of material forming a tail, which are the signature traits distinguishing an active comet from an inert object, even one with a hyperbolic orbit.