If you're building AI agents that trade, they need market signals. But most data APIs require human signup, API keys, and monthly subscriptions — none of which an autonomous agent can handle.
I built a trading signal API where AI agents pay per call using the x402 protocol (USDC micropayments on Base L2). No signup, no API keys. Agent shows up, pays half a cent, gets data.
How x402 Works
x402 revives the HTTP 402 ("Payment Required") status code:
- Agent calls
GET /signal/NVDA - Server responds with HTTP 402 + payment instructions
- Agent wallet signs a USDC transfer on Base L2
- Agent retries with a payment proof header
- Server verifies via Coinbase facilitator, returns data
The entire flow takes ~200ms.
The Signal Engine
Each call returns a composite momentum score built from:
- RSI (14) — oversold/overbought detection
- ADX (14) — trend strength (>25 = strong trend)
- MACD (12/26/9) — crossover + direction
- Volume ratio — current vs 20-day average
Score maps to BUY (>=30), SELL (<=-30), or HOLD, with a confidence value from 0 to 1.
Endpoints
| Endpoint | Price | Returns |
|---|---|---|
/signal/TICKER |
$0.005 | Single stock signal |
/scan/momentum |
$0.01 | Top 10 momentum setups |
/risk?tickers=X,Y |
$0.01 | Portfolio risk analysis |
Stack
- FastAPI (sync endpoints for yfinance blocking I/O)
- x402 Python SDK with ASGI middleware
- LRU cache (200 entries, 5-min TTL)
- Deployed on Fly.io
Open source: https://github.com/pmestre-Forge/signal-api
If you're thinking about building services for the emerging AI agent economy, this is a template you can fork and adapt.