x402 over REST · Roundlot docs

Paid REST routes answer 402 with payment requirements; retry with a signed payment header.

REST routes are gated by OKX's @okxweb3/app-x402-express middleware (x402 version 2, exact scheme).

The round trip

Call the route without payment. It answers 402 with a base64 PAYMENT-REQUIRED header listing what to pay: amount, asset, network and payTo.
Sign an EIP-3009 TransferWithAuthorization for that amount of USD₮0. The EIP-712 domain is name USD₮0 (with the ₮ character), version 1, chain id 1952.
Retry the same request with the base64 payload in PAYMENT-SIGNATURE.
The response carries the tool result, and PAYMENT-RESPONSE holds the settlement receipt with its transaction hash.

curl -i "https://api.roundlot.0xo.in/v1/quote?symbol=NVDA&side=buy&size=100"
# HTTP/1.1 402 Payment Required
# PAYMENT-REQUIRED: eyJ4NDAyVmVyc2lvbiI6Mi...

Clients

Any x402 v2 client that signs EIP-3009 works, for example x402Client with ExactEvmScheme from @okxweb3/app-x402-core and @okxweb3/app-x402-evm, backed by a plain viem account.

Refunds you never need

Input is validated before payment is requested (a bad call gets 400, not 402), and settlement happens only after the tool returns 2xx. A refused call is never charged.