GET /v1/conversations and read one with GET /v1/conversations/{conversationId}. Use the returned identifier: it represents the phone number ID and WhatsApp user ID separated by a colon.
GET /v1/conversations/{conversationId}/messages reads the thread. POST on that path accepts {"text":"YOUR_MESSAGE"} and uses the same send flow as POST /v1/messages.
Assign a conversation
PATCH /v1/conversations/{conversationId} accepts assigneeEmail. The current contract allows the account owner’s email or null to unassign; it is not an arbitrary agent directory.
A conversation reply is a real outbound message. Read the history and verify the intended conversation before sending.