What they did
The poster, u/oh-keh, gives every new Claude model the same job in Claude Code. One prompt asks for an original, fast piano piece in the manner of a Chopin etude, delivered as a MIDI file. The model cannot listen to anything, so it wrote the notes, timing, dynamics and pedal marks as a MIDI file from a Python script. The poster allows one run with no rerolls and does not change any notes. For the clip they played the file through a sampled grand piano (Salamander Grand Piano) and a falling-notes visualiser they had built themselves.
How it went
Fable 5.2 at max effort ran for 47 minutes and used 262,305 output tokens, 243,553 of them thinking, about $13.30 at API prices. It produced 759 notes covering the full keyboard from A0 to C8 and named the piece "Riptide", an etude in B-flat minor. On the same prompt Opus 5.5 at max effort had taken 45 minutes, cost $7.20 and written 651 notes. Most commenters, including self-described musicians, found the result busy, repetitive and without a clear thread or resolution, while agreeing it was a neat technical feat.
Worth knowing
The music is the model's but the sound is not: the timbre comes from a sample library and the visuals from the poster's own player. Expect a long, thinking-heavy run at max effort. Commenters suggested asking for a genre listeners know better, such as a film score, since an etude is meant to be a technical exercise rather than a showpiece.