I Built a Paid API That AI Agents Use Autonomously

Last week, Tempo and Stripe launched the Machine Payments Protocol. The idea: HTTP 402 (Payment Required) finally does something useful. An agent requests data, the server says “pay me first,” the agent pays in USDC, and the server delivers. I wanted to see how fast I could build a real paid service on top of this. Turns out, pretty fast. What I built FinSight is a financial portfolio analytics API. AI agents send a portfolio (a list of assets with weights, returns, and volatility numbers), pay a small amount in USDC per call, and get structured analysis back. Five paid endpoints: Risk profiling scores each asset 0-100 based on volatility and max drawdown. Stablecoins get a flat 5. Everything else runs through min(100, (vol * 80 + drawdown * 60) / 1.4). Portfolio risk is the weighted average. Rebalance computes target allocation based on a risk profile. Conservative profiles push 60% toward stablecoins. Balanced does 30%. Aggressive does 10%. The rest gets distributed by scoring non-st