Reduce Delays from Expect: 100-continue headers

Updated on Nov 6, 2025

To have Fiddler Classic return the 100-Continue header for a request:

Click Rules > Customize Rules....
Add the following function inside the Handlers class:

static function OnPeekAtRequestHeaders(oSession: Session) {
	if (oSession.HTTPMethodIs("POST") && oSession.oRequest.headers.ExistsAndContains("Expect", "continue"))
	{
		if (null != oSession.oRequest.pipeClient)
		{
		oSession["ui-backcolor"] = "lightyellow";
		oSession.oRequest.headers.Remove("Expect");
		oSession.oRequest.pipeClient.Send(System.Text.Encoding.ASCII.GetBytes("HTTP/1.1 100 Continue\r\nServer: Fiddler\r\n\r\n"));
		}
	}
}