When the --anticache option is passed to mitmproxy, it removes headers (if-none-match and if-modified-since) that might elicit a 304 not modified response from the server. This is useful when you want to make sure you capture an HTTP exchange in its totality. It’s also often used during Client-side replay, when you want to make sure the server responds with complete data.

command-line --anticache
mitmproxy shortcut O then a