Constant MEDIA_PLACEHOLDER 

pub const MEDIA_PLACEHOLDER: &'static str;

Text a degraded media marker is replaced with before the history reaches a model that cannot consume the payload. The model may echo it verbatim into a reply, so it is plain prose rather than bracket syntax: it must never parse as an outbound delivery marker ([KIND:target], which the channel parsers key on [ and :), and it contains no JSON-special characters so a marker replaced inside a native tool-result blob leaves the surrounding object valid.