ACE-Step 1.5 Edit Add Layer generates a single new instrument part and mixes it into your existing track, locked to the song's key, tempo, and groove. Choose the stem to add from 12 options, describe the part you want, and optionally set a start and end time so the layer lands only where you need it. An optional planning pass tightens coherence, and you can render up to 4 variations. Ideal for building arrangements incrementally, adding solos or pads, reinforcing a section, and quick production sketches.