ACE-Step 1.5 Edit Complete Track builds a full backing arrangement around a partial recording, such as an a cappella vocal or a solo instrument riff. Choose which stems to generate around your source from 12 options, describe the overall style, and the model arranges parts that follow the source in key, tempo, and feel. An optional planning pass improves musical coherence, and you can render up to 4 variations. Works best with a source that carries clear melody and rhythm. Great for songwriting, demos, and Vocal2BGM.