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