code #!/usr/bin/env python3 from urllib.request import urlopen # with fiddler open # no problem, see the slash resp = urlopen("http://www.baidu.com/") # with fiddler open # raises 400 Bad Request resp = urlopen("http://www.baidu.com") # with fiddler close # no problem resp = urlopen("http://www.baidu.com") wireshark see wireshark.png EXTRA v5.0.20181.14850 for .NET 4.6.1 64-bit AMD64, VM: 47.0mb, WS: 87.0mb .NET 4.6.2 WinNT 10.0.17134.0