Can Jev steer music?

# Jev Playground

Can a **System One** model steer music across styles?

[Jev](https://typesafe.ai/blog/introducing-system-one-models-and-jev) (TypeSafe's first System One model) doesn't generate text or notes — it makes fast, typed **classify / score / pick** decisions with calibrated probabilities. This playground tests whether that is enough to steer a composition:

- **Jev** (or an offline stub behind the same interface) decides *only enum labels*: the piece's character and form, key, meter, texture, palette, tempo, dynamics, instrument, and — per 4-bar slot — a harmony-book phrase plus per-bar contours. Code expands those into per-bar chords and roles.
- **App code** expands that plan into notes, engraves it (VexFlow), plays it (smplr) and exports it (`.mid`).

Styles on the dial: **Johann Sebastian Bach · Ludwig van Beethoven · Frédéric Chopin · Claude Debussy · Philip Glass · Hans Zimmer · Laufey · Elijah Fox**.

## Routes

| Route | What |
| --- | --- |
| `/` | Landing page: links to the demos. |
| `/music/` | The composer described below: Jev picks labels, code writes the notes. |
| `/trolley/` | Absurd trolley problems. Put anyone or anything on either track (counts, traits, a twist), or **Randomize**; then ask what Jev would do. Jev makes typed decisions only — `trolley_cast` (Choices over closed tables, sampled by code) and `trolley_judge` (a decision Choice, difficulty and absurdity Scores, a “most people would pull” Noul). Every sentence is assembled by code (`src/trolley/describe.ts`). Clearly hypothetical; no harm is ever described. |

One Vite entry; `src/main.tsx` switches on the path and lazy-loads each demo (the landing page and the trolley never download VexFlow). `vercel.json` rewrites `/music/*` and `/trolley/*` to `index.html` so direct visits an