I was interested in exploring how good Opus 4.5 has become at music composition, so I built a local-first browser-based DAW where you describe music and Claude composes it using an agentic loop with lots of tools (create tracks, write notes, read back what it wrote, apply swing, humanize timing, set panning/volume, etc.). It also supports recording via MIDI keyboard, looping, multi-track, panning, a bunch of synth instruments, all inspired by the Logic Pro UI, so you can collaborate with the AI.

Opus 4.5 is pretty good at one-shotting MIDI compositions already, even without a custom harness and tool, but it really shines when you ask it to self-review (a Ralph Wiggum loop would help here!) and correct multi-track mistakes, which it will capably do after being asked to. This results in nuanced multi-voice compositions with fewer clashes.

There's a good onboarding flow in there as well so it's easy to get set up and prompt your first few compositions.

You have to provide an Anthropic API key but it's all local to your browser and never goes to my servers. But you can import and export projects, export .WAV, etc. to get around the local storage limitation.

- https://youtu.be/vgeN_obkg50

- https://usecadenza.studio