Bitcoin API standard
You can check this github page about btcd, one of the concurrent of bitcoin core where they talk about their API which is fully compatible with bitcoin


  btcd provides a JSON-RPC API that is fully compatible with the original bitcoind/bitcoin-qt. There are a few key differences between btcd and bitcoind as far as how RPCs are serviced:


https://github.com/btcsuite/btcd/blob/master/docs/json_rpc_api.md