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Every outbound Message contains 1 to 100 ordered parts.

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Part types

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Text

Text must contain 1 to 10,000 UTF-16 code units. A mention adds mention and mention_range; read Mentions before calculating that range.

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Media

Reference one completed Relay Attachment:
For public files up to 10 MB, use one HTTPS URL instead:
Use exactly one of attachment_id or url.
A link must be the only part in its Message. Put a URL inside a text part when it should remain ordinary text.

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Ordering and composition

part_index is the zero-based position in the array. Replies and reactions use it to target one exact part. Text and media may be mixed in order. Consecutive text parts are rejected; combine adjacent text into one part. A link must remain the only part in its Message.

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Response-only parts

Relay can return type: "system" for visible Chat history such as membership or group metadata changes. A developer cannot send a system part. Response parts can also contain current reactions. Media responses add the Attachment ID, opaque download URL, filename, MIME type, size, and available dimensions or duration.