Constant MAX_HEAD_BYTES 

pub const MAX_HEAD_BYTES: usize = _; // 16_384usize

Largest whole head an answer may carry, every line counted together, in bytes.

This is the one that bounds what a head costs: a line count and a size per line multiply, and MAX_RESPONSE_HEADERS lines of MAX_HEADER_BYTES each is four mebibytes of head that both of them admit. Sixteen kibibytes is what the strictest runtime any Hook0 SDK runs on enforces by default, and matching it is the point: a lower ceiling would refuse heads another SDK accepts, and a higher one would not bind there at all.