Two small instruments that work together.
Chordline (CL-1). Pick chords from a key and the bass and melody write themselves around them. Every note is coloured by how it sits over the chord: home, step, clash or outside. Song mode arranges parts into sections with intros, drops and fades, and a voice lane records you over the song, tunes you to its key and can vocode you through its chords.
Boomline (BL-1). An analog-style drum machine with twelve synthesized voices, four 16-step patterns, accents and rolls, and grooves that explain why they groove.
When an idea is worth keeping, export stems (every part as a WAV and as MIDI, all starting at bar 1) and drag them into GarageBand. MIDI exported from either app carries the whole project inside it, so opening it back in Chordline restores everything.
Everything runs in the browser and nothing is uploaded.