Looking for SPI handshake specification

[PoolGuy]

I have an ESP32-SOLO-1 which is interfaced over SPI to a RISC-V processor. I have the "AT Command and examples" manual but when exchanging these commands over SPI there is an additional low-level protocol involved, because SPI by itself is just a bidirectional bitstream. The format of what this protocol is varies between manufacturers, so this is something that Espressif has designed. I have looked everywhere can can not find a description of this protocol.

Somebody with a datascope was able to discover some aspects of this protocol, and that it involves the exchange of 4-byte header packets containing an operation code and a length. But I would like to see it spelled out exactly. Where can I find the full specification?