I can send/reply to an existing chat thread, but can not send a message to someone whom does not have any established chats yet
I am using this POST API - https://graph.microsoft.com/beta/users/{ID}/chats/messages and getting this error { "error": { "code": "UnknownError", "message": "", "innerError": { "request-id": "72bb5b10-7268-4014-9e87-0f5f685d4184", "date": "2019-10-03T07:50:54" } } }
Currently there are no API's to start a conversation in 1:1 scope. If you want to create a new chat thread in a team please take a look at Create chatMessage in a channel. Please let us know if you need any further assistance.
Update
Based on documentation you can now create new chats through the Graph API https://docs.microsoft.com/en-us/graph/api/chat-post?view=graph-rest-1.0&tabs=http
You can also test it out in the Graph Explorer https://developer.microsoft.com/en-us/graph/graph-explorer
