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Can a System One model steer music? Jev picks the plan (enums only); code renders sheet, audio and MIDI.
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Jev Playground — wustep
A Vite app where Jev picks only closed-enum labels for a composition plan and code expands those into notes, engraving, audio and MIDI, with an offline heuristic stub when no key is set.
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- 2
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- TypeScript
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- Author
- wustep
- Last verified
- Oct 2026
This is one of several Jev listings for the same project. The canonical entry is Jev Playground — Little-Planet-Labs.
About this project
Can a System One model steer music across styles? 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:
Quoted from the project's README. Read the full README.
Used inJev for Software & DevTools
- Languages
- TypeScript
Highlights
- Jev decides enum labels only - register, motion, accompaniment, form, key, meter, palette, tempo, dynamics - code writes the notes.
- A style is a table row plus a harmony book, and a test forces each style's most typical plan to differ on register, motion and accompaniment.
- Trolley dilemmas use typed Jev decisions, while the fictional inbox triage and match demos run on client stubs offline.
- The plan JSON is editable in the UI: change register or a bar's chord, hit Apply to renderer, hear only that label's effect.
Watch out
A style earns its place by diverging on the axes the plan can name - register, motion, accompaniment, palette, rubato - not by having a name, and the divergence test covers register, motion and accompaniment only. Reference coverage is uneven: the four classical styles are measured against committed public-domain MIDI, Glass and Elijah Fox against local files that never enter the repository, and Laufey and Hans Zimmer have no MIDI at all. The rate limit is 90 POSTs per minute per client IP, and the README is explicit that the counters are in memory per function instance, so a cold start resets them and parallel instances count separately; a hard cap would need a shared store this project does not configure. VITE_JEV_API_KEY is a local-only escape hatch that gets inlined into public JavaScript. The digest we reviewed is cut off partway through the plan schema, so later fields are not documented here. Copy reviewed against the repository README on 2026-10-04, not from running it.
Questions about this entry
Short answers drawn from the linked project or page.
Not if you use the documented path. TYPESAFE_API_KEY is read server-side only and enables the live JevPlanner through the /api/jev proxy, and the README says it never reaches the browser bundle. There is an escape hatch, VITE_JEV_API_KEY, which makes the browser call TypeSafe directly and is inlined into public JavaScript - the README flags it as local-only and says not to set it on a deployed build.
Only loosely, by the README's own account. It is keyed on headers Vercel's edge sets so a client cannot forge them, and over the limit you get 429 plus Retry-After, but the counters are in memory per function instance: a cold start resets them and parallel instances count separately. The README calls that enough to stop loops and casual abuse, and says a hard cap needs a shared store such as Vercel KV or Upstash, which this project has not configured.
A style is a table row plus a harmony book, and none of the eight styles has code of its own. The plan carries register, motion, accompaniment, palette and the rest as closed enums, and a test holds the styles to it by requiring each one's most typical plan to differ from every other's on register, motion and accompaniment. Musical evidence is uneven: classical styles are compared against committed public-domain MIDI, and Laufey and Zimmer have no MIDI at all.
Closed enums only, through an allowlist rather than a general proxy. /api/jev accepts one of seven typed ops (music: globals, phrase, score; trolley: trolley_cast, trolley_judge; inbox: inbox_triage; match: match_rank), re-validates them against the enums and builds the actual state and questions server-side; raw state or questions are rejected with 400. Responses carry only model, answers and usage, and failed upstream calls return a generic message rather than TypeSafe's body.
Use Node 20 or later, run npm install and npm run dev to open http://localhost:5173, which works offline on the heuristic stub. npm run build runs tsc --noEmit then vite build and must pass, and npm test runs vitest over renderer invariants, Jev request shapes, the proxy, MIDI round-trip and the inbox and match stubs.
/trolley/ poses absurd trolley problems where Jev makes typed decisions and code assembles every sentence; /inbox/ is fictional inbox triage and /match/ is fictional matching, both client stubs that run offline. The landing page, trolley, inbox and match demos never download VexFlow.
renderPlan is pure and deterministic, and the order is the argument: writeMelody runs first over the whole piece and owns the register, the subdivision and where the tune rests; writeAccompaniment is then handed the melody's lowest note and may not cross it; expression is applied last and changes no pitches.
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