Proxy-Connection Is a Compatibility Header, Not a Tunnel Switch

Proxy-Connection: Keep-Alive is a nonstandard compatibility field encountered in HTTP proxy traffic. CONNECT is the method that requests a tunnel; this header neither opens one nor proves that a request used a proxy.

If suppressing curl's generated proxy header is the required compatibility fix, use its proxy-header interface:

curl --proxy http://proxy.example:8080   --proxy-header 'Proxy-Connection:'   https://example.com/

The empty value removes the generated field in this interface. That is different from sending an empty header. Keep the actual proxy option: header removal does not select direct routing, change proxy protocol or grant access to a denied destination.

Before making the change, establish which exchange contains the field. Client-to-proxy output does not show whether it was forwarded to the website. An origin rejecting a forwarded field requires evidence from that later hop.

Older intermediaries may behave differently when the field is removed. Record the request form, recipient, client version and observed failure, and make this a targeted compatibility choice. Neither always adding it nor removing it for every proxy error follows from the header's name.