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A server built with compression support compressed every handler response the client accepted, with no way to turn it off. Over HTTPS the length of a compressed response can give away a secret in it when the same response also carries text an attacker chooses (BREACH). set_compress(false) sends handler responses, buffered or streamed, as they are; compression stays on by default, and static file compression keeps its own setting. The README and the cookbook describe the switch and when to use it.