{"type":"link","value":"https://..."} as the only part of a message, through either send path. The URL must be absolute and at most 2,048 characters. Relay stores the URL and the app renders the preview; a URL inside a text part stays plain words.
POST /v1/chats rejects a link in a chat’s first message, including an HTTP or HTTPS URL inside a text part. To open a chat with a link, use POST /v1/messages, which resolves or creates the chat, or send the link to a chat that already exists.
Send a link part
Send the link on its own:202 with the message. The part you read back carries value and reactions. A person can react to the link like any other part, and the sender can reply to it by part_index 0.