Inspiration
I do Olympic weightlifting, and most of the coaching happens over WhatsApp videos. You send a clip, the coach answers whenever, and payment is a mess: some pay up front and wait forever, others get feedback and "forget" to pay. I'd already built Sideline Lift Judge (AI that scores bar path from a video), so I wanted to turn it into something a coach could actually charge for.
What it does
- A lifter uploads a clip, taps the bar, and gets an instant bar path score.
- They can order a full review from a coach. PayPal authorizes the payment but doesn't charge yet.
- The AI drafts the review (score, drift, 1–2 cues), and the coach edits and approves it.
- Only when the coach delivers does the app capture the payment. If the coach misses the deadline, a sweeper voids the authorization automatically. If there's a dispute, the money is refunded.
- Coaches get an ops agent: they type things like "Invoice my 3 athletes $60 each" or "refund Noa's review" and it prepares the PayPal action, but nothing runs until they hit confirm.
How I built it
- Python + FastAPI on Heroku
- The bar path pipeline from Sideline Lift Judge (NVIDIA models on Nebius Token Factory)
- PayPal Orders v2 for authorize, capture, void and refund
- PayPal Agent Toolkit for invoicing from the ops agent
- NVIDIA Nemotron as the ops agent's brain, with a human confirm step on every money action, and an audit log
Challenges
- Getting the payment state right: authorize, deliver, capture, or void on timeout, without ever charging twice.
- Invoices kept returning 403 in sandbox until the account settings were sorted out.
- Tracking the bar is hard when there's other gym equipment in the frame. The path sometimes jumps to a hanging ring during the catch, and I'm still fixing that.
What I learned
Authorize-then-capture is perfect for "pay when delivered" services, and an AI agent touching money must always ask a human before it acts.
What's next
Better bar tracking, coach payouts, and WhatsApp sharing for reviews.
Built With
- fastapi
- heroku
- javascript
- nebius
- nvidia-nemotron
- paypal
- paypal-agent-toolkit
- paypal-orders-api
- python
