2023: Operation requires a group chat

The request uses a feature that is only supported in group chats. This is an HTTP 409 error returned by the message send endpoints (POST /v3/messages, POST /v3/chats, POST /v3/chats/{chatId}/messages) when a text part includes a mention and the target conversation is a direct (1:1) chat — including a send to a single recipient, which creates a direct chat.

@mentions address a specific participant of a group conversation, so they are rejected rather than silently dropped when the chat has only one other participant.

• Remove the mention field from text parts when messaging a direct chat
• To @mention someone, send the message to a group chat that includes them — the mention target must be one of the chat’s participants
• Check the target chat with GET /v3/chats/{chatId}: mentions require group: true