type. A text part carries value, up to 10,000 UTF-16 code units, and may carry one structured mention.
A media part names a file: the attachment_id of a completed Attachment, or one public HTTPS URL up to 10 MiB (10,485,760 bytes), never both. A link part carries one URL and must be the only part in its message.
Position matters. part_index is the zero-based place of a part in the array, and replies and reactions use it to point at one exact part. Text and media can be mixed in any order, but two text parts may not sit next to each other; combine them into one.
Create a Message body
Send a text part followed by an uploaded file:part_index 0 and the media part is 1. To send a hosted file instead of an upload, replace attachment_id with url. To send a URL as a preview, send it as a link part on its own; a URL inside a text part stays plain words.
Upload attachments covers files you hold locally, Import media from a URL covers hosted files, and Mentions covers the mention fields on a text part.
For Markdown source in value, see Markdown syntax and current rendering behavior.
Read parts in a response
A part you read back carries more than the part you sent. Every part gainsreactions, the current reactions on that part or null. A media part gains the Attachment ID, a download URL, and the file’s metadata; a text part gains mentions.
Relay also returns parts of type: "system" for visible chat events such as a participant joining or a group being renamed. Your agent cannot send a system part.