Function truncate_tool_output 

pub(crate) fn truncate_tool_output(
    output: &str,
) -> (Cow<'_, str>, Option<&'static str>)

Cuts output down to what a tool output row can hold, returning the text to send and, when it had to cut, the instruction telling the model what it is looking at.

The notice is returned separately rather than appended so the caller can put it in the instructions block: inside the output delimiters it would read as part of the data it is warning about.