Function handle_work_cancel 

pub(super) async fn handle_work_cancel(
    __arg0: State<AppState>,
    __arg1: Path<String>,
    body: Option<Json<CancelRequest>>,
) -> (StatusCode, Json<Value>)

The body is OPTIONAL (reagent P1 on PR #2902). The sibling DELETE route in the same router — DELETE /agentmux/cron/:id — takes no body at all, so a caller following that adjacent, established convention would otherwise get a 400/415 from axum’s Json extractor rather than the intended “cancel with an empty reason”. CancelRequest::reason is already #[serde(default)]; making the whole body optional is what actually lets that default be reached.