Walk down any tequila aisle and you'll see '100% agave' on nearly every premium bottle. It sounds definitive: agave and nothing else. It isn't quite that.
What '100% agave' actually means
The term is a real legal distinction, and an important one: it separates 100% agave tequila from 'mixto,' which can be up to 49% other sugars. Always choose 100% agave. But the category's own rules permit up to 1% of the liquid, by volume, to be additives, and those additives do not have to be disclosed anywhere on the label.
One percent sounds trivial. It isn't. The four permitted additives, glycerin for body, sugar-based syrups for sweetness, oak extract for false aging, and caramel color, are potent at tiny doses. A few drops can round off a harsh spirit, add a vanilla sweetness that reads as 'smooth,' or darken a young tequila to look older. They are, in effect, invisible makeup.
What the stricter promise means
The stricter promise some producers make is a spirit with nothing added at all: cooked agave, water and yeast, from field to bottle. No sweetener, no glycerin, no oak extract, no color. What you taste is the agave, the terroir, the fermentation, and the still, nothing added to flatter it.
The complication is that no government body certifies any claim of this kind, and Mexico's regulator has actively challenged organizations that promote certification language around it. That leaves the burden on producer transparency, independent scrutiny, and, honestly, your own palate.
How to tell the difference in the glass
Additives tend to announce themselves once you know the tells: an artificially round, syrupy texture; a vanilla-and-butterscotch sweetness that arrives before any oak could justify it; a finish that stays sweet instead of drying out. A tequila made without them is often less immediately 'smooth' and more alive, peppery, mineral, vegetal, with edges. Those edges are the point.
The bottom line
Buy 100% agave always; but know that it's the floor, not the ceiling. If this matters to you, look for producers who say plainly what goes into the bottle and who publish their process, then calibrate your own taste for the difference.