Not sure if this is what you're asking about, but I think all messages we send via Olm should be a structured JSON blob

While decrypting, we need to manually add message type like here which I believe it's always 1, only when decrypting for the very first time is 0, and was wondering if we need to do it explicitly because it work the way it is right now

Ah, I understand the question now. I'm not sure, but I'd be worried that default initialization of the size_t messageType parameter of struct EncryptedData might not always initialize to 0. My vague memory of C / C++ default initialization is that if you don't explicitly initialize to 0, it might be a "garbage value".

Separately, my comment above was actually about the contents of the Olm-encrypted message. I think we should avoid encrypted raw text data, and instead make sure all of the text we're encrypted is "structured". This will help future-proof our system, for when we might have multiple categories of encrypted message.