Function validate_tool_call 

pub async fn validate_tool_call(
    tool_name: &str,
    args: &Value,
    project_root: &Path,
    read_cache: Option<&FileReadCache>,
    last_writer: Option<&LastWriterCache>,
    last_bash: Option<&LastBashCache>,
) -> Option<String>

Validate a tool call before approval.

read_cache is the session file-read cache. When provided, validate_edit uses it to detect files that have been modified on disk since the model last read them — catching the most common source of lost-context edits.

last_writer and last_bash add context to staleness errors: instead of a generic “file was modified” message the model sees which tool was responsible and how long ago it ran (#804 item 7).

Returns None if the call looks valid, or Some(error_message) describing why it will fail. The error message is fed back to the model so it can self-correct without consuming an approval prompt.