Senior Full-Stack Developer — Next.js / Openai / Voice — 3-Week Mvp

We are seeking a highly experienced Senior Full-Stack Developer to take ownership of and stabilize an existing mobile-first ai sports coaching mvp. This is a critical 3-week sprint focused on enhancing and refining current functionalities, not a greenfield build. The application is built with Next.js, React, TypeScript, and integrates with OpenAI APIs, deployed on Vercel.

The primary objective for this Phase 1 pilot is to ensure robust functionality allowing a coach to:
- Create or select a player profile.
- Record a training session.
- Record natural voice feedback directly on a phone.
- Reliably capture audio on iPhone/Safari browsers.
- Transcribe the recorded feedback using speech-to-text technology.
- Generate structured AI development recommendations based on the feedback.
- Enable the coach to approve or correct the AI-generated recommendation.
- Save the athlete's session history.
- Facilitate recording subsequent sessions to measure progress.

Critical experience required includes: production-level Next.js, React, and TypeScript development; expertise with iPhone Safari browser audio recording (MediaRecorder API); handling audio uploads; experience with speech-to-text services (e.g., Whisper); working with OpenAI for structured outputs and JSON schema validation; implementing robust validation, retries, and fallbacks; and proficiency in Vercel deployment and production debugging.

The selected developer will first audit the existing codebase, identify areas for improvement and necessary fixes, and then personally execute the required work. We are looking for a dedicated individual engineer, not an agency or subcontractors. The ability to start within 48 hours and significant availability for the next 3 weeks are essential.