Function env_reduce_mode 

pub fn env_reduce_mode() -> ReduceMode

Global reduction mode from the environment (read once), so a whole cluster opts into deterministic + precise reductions with no code change: RLX_DETERMINISTIC_REDUCE=1 → ReduceMode::Deterministic (unset) → ReduceMode::Ring Every rank must agree (mismatched modes mix incompatible message sizes and would desync), so set it on all nodes — the same discipline as any other cluster-wide RLX_* flag.