You are mistaking recommended patterns for required behavior. "...In HTTP, it SHOULD be sent whenever the message's length can be determined prior to being tra

You are mistaking recommended patterns for required behavior. "...In HTTP, it SHOULD be sent whenever the message's length can be determined prior to being transferred..." https://tools.ietf.org/html/rfc2616#section-14.13 https://tools.ietf.org/html/rfc2616#section-14.13 If this were required, the keyword would have been "MUST be sent" - there are completely legitimate reasons for not returning Content-Length; this is noted directly in the RFC, as quoted above.