According to the DirectLine API documentation, the channelData field in the message object should be a string: http://docs.botframework.com/sdkreference/restapi-directline/#!/Conversations/Conversations_GetMessages
It used to be the case but since yesterday the API returns a JSON object.
Is this API versioned? It would help to be able to specify a version in the headers or the URL to avoid this kind of breaking change in the future.