It is a cool problem space! What I'm doing is using a single buffer for body handling (since you dispatch that away and then reuse it for chunked encoding) so it never takes unbounded stack space. This might be a bit different in HTTP/3 where you can have multiple body transmissions multiplexing; I have to look into how this works (but it's UDP as well)

What we never need to do in OxCaml is to keep a giant body buffer list in the stack; with effects, we can fork the stack any time, so the request object is shared naturally. The only way to free the stack is to return from a function, but you can have a tree of these that share values earlier in the callchain.