Malsah Labs has released a macOS app that takes a monophonic MIDI melody and writes harmony parts around it. The voice leading is worked out for you, and every note it writes can be edited.
HarmonyKeen ships as a standalone app plus VST3 and Audio Unit plugins, bundled together under one license. Harmony styles run from a single added line, through close vocal harmony and four-part writing, to fuller multi-part arrangements. Output goes to a DAW as MIDI, or to the page as sheet music with PDF and MusicXML export.
The engine runs on music theory rather than a neural network. It reads the whole melody first, finds the phrases and cadences, chooses a chord sequence that fits, and then writes the voices with voice-leading rules. The same input and settings produce the same result every time. There is no account, and the app works offline once the license is on the machine.
“I wanted something that would make my music better without taking the creative control away,” said Thom Haslam, founder of Malsah Labs. “Nothing I tried could write harmony from my own melodies that was true to music theory, so I built it.”
The plugin runs the same engine inside Logic Pro, Reaper, GarageBand and most VST3 hosts. It can capture a melody from a track in the session, play the harmony back through the host’s instruments in time with the song, and drag each voice out onto its own track.
HarmonyKeen is available now for macOS at a launch price of $59 USD through September 30th, 2026 (regular $79 USD). A Windows version is coming soon, and one license covers both. A 7-day free trial is available to try the plugin.
More information: HarmonyKeen