Finalist
Mixtape
Mixtape — The Booth is a DJ instrument you perform in, built in the browser with Web Audio and Three.js.
Core trick
Read-only companion receives measured mixer state and returns strict structured JSON, with domain rules rejecting any command that would alter the mix.
Description
Mixtape — The Booth is a DJ instrument you perform in, built in the browser with Web Audio and Three.js. Two CDJ-style players or two turntables share one live mix, and switching rigs mid-set keeps every track, blend and stem exactly where it was. A practice exercise scores one real skill, bringing the second track in on bar five and owning it by bar nine, then restores the exact starting point so you can try again.
GPT-6 Astra is the companion behind the decks. It never hears audio. It reads the instrument's measured state, positions, tempo with provenance, gains, filter, EQ, stem routing and your last eight moves, and answers through the Responses API with strict Structured Outputs: one grounded observation, one next action. Change the brief and the plan changes. The domain layer rejects every model command, so your hands stay authoritative.
Built with Codex on GPT-6 Astra, including a Blender-authored mixer wired to live audio and a test suite that caught timing bugs the tolerance refused to forgive.
README excerpt
# Mixtape — The Booth A browser DJ instrument you perform in: a four-channel Web Audio mixer, crossfader, filter, EQ, tempo, stems and a Three.js booth, with **GPT-6 Astra as a read-only practice companion**. Four CDJs and their mixer strips now read **A, B, C, D from left to right** ([verified controls and audio](docs/evidence/deck-order/)). - **Live:** [Mixtape — The Booth](https://mixtape-the-booth.vercel.app/#play) — free play needs no Booth account; Astra hints need Booth sign-in. Existing Vercel deployment protection is separate. - **Video (60 s):** pending a shareable recording URL. - **Repo:** [our-mixtape/the-booth](https://github.com/our-mixtape/the-booth) · developed September 10, 2026, and prepared for the OpenAI GPT-6 Astra Hackathon NYC. [Provenance](docs/PROVENANCE.md) distinguishes work completed before the 10:30 AM hacking start; the whole repository is not claimed as event-built. ## What runs where Playback, timing, every control and the practice result run locally in the browser. Astra never receives the mix audio. It reads measured deck state: positions, BPM with provenance, gains, filter, EQ, tempo, available stem routing and the last eight commands. Hosted HTTP hints use the Responses API with strict Structured Outputs; the local live session streams plans and reviews over a Responses WebSocket. The domain layer rejects every agent-origin command: Astra can point, you play. Optional microphone conversation uses a separate speech model. ## Native Ast