The Standard Relayer API is an alternative to the Radar Relay API Feed & Trade APIs. The Standard Relayer API focuses on the blockchain being the source of truth for all trade information. This allows for less trust in a central server but is at the cost of some convenience.
Rate limit is 60 requests/minute per IP address.
https://api.radarrelay.com/0x/v3
wss://ws.radarrelay.com/0x/v3
SRA