I catalogued every tell that makes a UI look AI-generated. My own tool kept failing the test.

You can always tell when an AI built a UI. You can't always say why.

I maintain StyleSeed — an open-source set of design rules that Claude Code, Cursor, and Codex read automatically, so AI-built UI stops looking generated. For the past few months my actual job has been writing that "why" down: cataloguing every tell, turning each one into a rule an agent can obey, and then dogfooding the rules to see if they hold.

They kept not holding — in ways that embarrassed me three separate times. That's what this post is about, because the failures turned out to be more useful than the rules.

The catalog: generation-one tells

These are the ones everyone recognizes, even non-designers: