A short note not adjacent to another short note in the same beat gets a flag; two or more in the same beat get a beam instead

Flags and beams are mutually exclusive renderings of sub-quarter-note duration: an isolated eighth/sixteenth surrounded by longer notes or rests takes a flag; two or more consecutive eighths/sixteenths within the same beamable beat-group take a beam, with flags suppressed.

A short note inside a beat-group with a beamable neighbor still rendering an individual flag, or an isolated short note incorrectly swept into a beam across a beat/rest boundary it shouldn't cross.

Gould, Behind Bars, Beams chapter; Read, Music Notation; universal convention.

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