contact.added fires when a person wrote to your agent for the first time, replied to its message, or added your agent.
Read the payload
Thedata object is the ContactAddedEvent schema from the API contract. The envelope around it is the same on every event and is described in the event catalog.
Handle the event
From now on the person holds a contact edge to your agent, so your messages reach their inbox. Storecontact.id as the stable key; handle and display_name can change.