Unprepared and unresolved dissonance

Any dissonant interval against the prevailing harmony (or CF, in species) must be accounted for by a recognized figure: passing tone (approached/left by step, same direction), neighbor tone (approached/left by step, opposite directions, returning to start), appoggiatura (approached by leap, left by step), or suspension (prepared as a consonance on the previous strong beat, tied over, resolving down by step). A dissonance matching none of these is unprepared/unresolved — forbidden in strict species writing (1st-3rd species forbid ANY strong-beat dissonance outright).

A weak-beat passing tone in 2nd species left by a LEAP rather than continuing by step — breaks the passing-tone shape with no other figure to justify it.

Fux (Gradus ad Parnassum) via Jeppesen's modern codification.

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