A tuplet (and its beam) must not straddle an authored barline

Like an ordinary beam, a tuplet bracket and any beam under it must be fully contained within one notated measure.

A triplet's three notes are authored with indices that span an author-inserted barline (e.g., the first two triplet notes end bar 3 and the third begins bar 4), producing a Tuplet bracket and beam that visually straddles the barline.

Gould, Behind Bars, ch. Tuplets — tuplet brackets, like beams, respect barlines; extends the same never-cross-a-barline principle from ordinary beaming.

This rule can be decided from pitches, durations and voices — no layout required. A validator can check it before anything is drawn.