422 ERROR WHEN TRYING TO SEND POST REQUEST USING SSE

[Andrew-Kiswaga-John]

I've been trying to connect my postgresql with the MCP server using sse instead of stdio and no matter what I try, I still a 422 error saying
"Failed to deserialize the JSON body into the target type: data did not match any variant of untagged enum JsonRpcMessage". At first, i thought it was a format issue so I went to the rmcp repo to check for the JSON format it's expecting and I tried to match but im still receiving the same error. i would appreciate a little help. The JSON format I'm sending is the one below:
{
"jsonrpc": "2.0",
"id": 3,
"method": "register",
"params": {
"conn_str": "postgresql://postgres:admin123@localhost:5432/whatsapp-supplier-agent"
}
}