Read-only Base mainnet data: GET /base/block, /base/tx?hash=, /base/balance?address=, /base/erc20?token=&holder=, /base/gas. Small JSON payloads over a Base node; pay ~$0.002 USDC on Base per call.

Live x402 conformance pre-flight check: pass ?url=<x402 endpoint>, get a structured PASS/WARN/FAIL verdict on its 402 challenge before you trust it with a real payment.