This GET endpoint returns the midmarket exchange rate between two currencies: how many units of the destination currency one unit of the source currency is worth on the interbank market, with no spread applied.
It is the reference rate every MassPay quote prices from; the exchange_rate values in the payout and payin catalogs and on a transaction apply the client's FX spread to it, so a transaction is executed at its own quoted rate, never at this one.
Both currencies are passed in the URL path as ISO 4217 codes; crypto asset symbols such as USDC are accepted as well. Codes are case-insensitive, and a pair of the same currency answers 1.
The rate is refreshed from the market feed every five minutes. When the feed cannot be reached, the last rate obtained within the past 24 hours is served instead.
Responds with 400 for a malformed code and 503 when no rate is available for the pair.