A clef change must never fall inside a beamed group, and must be handled deliberately across a tie, slur, or system break

A clef change is never placed in the MIDDLE of a beamed group — beam groups are engraved as one visual unit, and mixing two clefs' worth of noteheads under one beam (with two different staff-position interpretations for the same physical notehead position) is not a recognized notation at all; a clef change must fall on a beam-group boundary. Where a clef change falls immediately before or after a tie or slur, the arc is drawn using each note's own (post-change) staff position, exactly as engraved without any clef change — the clef change affects only how a given staff position is READ, never the physical arc geometry between two noteheads' drawn positions. Where a clef change coincides with a system break, the courtesy-clef (clef-change-courtesy-clef-end-of-system) and repeated-clef (clef-repeated-every-system) rules both apply together — the courtesy clef announces it at the end of the old system, and the new system opens already in the new clef.

A clef change authored (once the capability exists) at a beat that falls in the middle of an eighth-note beam group rather than at the group's boundary.

Gould, Behind Bars, Clefs chapter, states the beam-boundary constraint explicitly (a clef change 'must not fall within a beamed group'). Ross, The Art of Music Engraving, agrees. Neither authority treats the tie/slur-arc-geometry point as a separate rule since it follows automatically from how ties/slurs are always drawn (relative to each note's own drawn staff position, never adjusted for clef) — this chapter states it explicitly only because the brief specifically asked for it.

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