Constant MAX_CALL_DEPTH 

pub const MAX_CALL_DEPTH: usize = 4096;

Hard limit on call nesting. Frames are heap-allocated (see [Frame]), so unbounded recursion would grow the Flow’s memory instead of crashing the worker thread’s native stack — which is worse, not better, without a limit. 4096 comfortably covers real recursive algorithms while keeping a runaway fn f() { f() } a Fault, not an OOM.