A single item of content
OptionalmessageA unique identifier for the message this chunk belongs to.
All chunks belonging to the same message share the same messageId.
A change in messageId indicates a new message has started.
Optional_The _meta property is reserved by ACP to allow clients and agents to attach additional metadata to their interactions. Implementations MUST NOT make assumptions about values at these keys.
See protocol docs: Extensibility
A streamed item of content