Designing the Persona System, a Local-First AI App
Project Summary
The Persona System is a local-first, on-device AI app for iOS, built under Nupixl. Your data and your AI companion live on your phone, not on someone else's server. I designed the architecture, the rules the system can never break, and the interface, then packaged it all as a full spec and test suite so development starts on solid ground.
Spec before code
The build started as a five-document spec package: the architecture, the invariants, a milestone implementation plan, and a UI handoff. The invariants define what must always be true, so every future feature gets checked against them instead of against someone's memory.
Proven before there's a screen
A Swift Package skeleton with separate PersonaKit and FileStore targets, a full invariant test suite, and fixture bundles, including a golden-path demo persona. The system's behavior is proven before the interface exists.
Voice that stays on the device
For speech I chose Marvis-TTS, a 250-million-parameter model under the Apache 2.0 license, running through mlx-audio-swift, over a larger model with tighter licensing. Voice runs locally and the licensing stays clean.
An interface with momentum
A floating three-part bottom bar, a persona briefing card, a category-tick timeline, a narrative momentum widget, and a glass aesthetic over live camera passthrough. The interface was prototyped in Claude Design, which replaced the earlier Figma wireframes.
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