Fiddler Options

Check for updates on startup controls whether Fiddler will contact the version-checking web service on startup.
Reuse connections to servers controls whether Fiddler will reuse HTTP server connections (keep-alive).
Reuse client connections controls whether Fiddler will reuse HTTP client connections (keep-alive).
Show a message when HTTP protocol violations encountered controls whether Fiddler notifies you if it observes HTTP protocol violations.
Allow remote computers to connect controls whether or not Fiddler will accept HTTP requests from other computers.
Enable IPv6 controls whether or not Fiddler will connect to HTTP servers using IPv6, if available.
Map socket to originating process controls whether Fiddler's session list will show which process generated each HTTP request.
Encrypt using AES256 controls whether SAZ files use strong encryption when saved using password protection.
Automatically stream audio & video controls whether Fiddler will use streaming mode when downloading audio and video content-types.
The Systemwide Hotkey checkbox permits you to pick a hotkey which will restore and activate Fiddler when it is inactive or minimized.

Capture HTTPS CONNECTs controls whether Fiddler will attempt to create tunnels through which secure traffic will flow.
Decrypt HTTPS traffic controls whether Fiddler will decrypt the requests and responses within those secure tunnels.
Ignore server certificate errors controls whether Fiddler will warn you if an HTTPS server has presented a certificate that did not validate. You should not check this box when surfing the Internet at large due to the security risk.
Show data from RPASpy controls whether or not Fiddler will show HTTPS headers reported by the deprecated RPASpy addon.

Automatically reload script when changed controls whether or not Fiddler will automatically reload the FiddlerScript rules file when it detects that the file has been changed.
The Editor text box allows you to control which text editor is used to edit the FiddlerScript. The "..." button will launch a dialog to allow selection of the editor.
The References text box allows you to specify any additional assemblies that your FiddlerScript depends upon.
The Find more extensions link will open the Fiddler Add-ons page.

The Act as System Proxy on startup controls whether or not Fiddler registers as the System proxy on startup. Internet Explorer and many other applications use the system proxy by default and are notified when it changes. Learn more...
The Chain to upstream gateway proxy controls whether or not Fiddler will the system proxy configured before Fiddler registered itself as the system proxy as an upstream gateway proxy.
The Show Gateway Info shows information about that upstream gateway proxy.
The Fiddler listens on port checkbox controls which port Fiddler listens for HTTP traffic on.
The IE Should bypass Fiddler list controls which requests should not be sent through Fiddler at all when it is registered as the system proxy. Note that this list is generally only respected by WinINET clients like Internet Explorer.
The WinINET Connections list shows what network connections are configured for this machine. This may be useful if you are connected to the Internet via dialup or VPN and want Fiddler to automatically hook a connection other than your LAN connection. Learn more...

The Font Size box allows you to select the font-size Fiddler will use. A restart is required to fully complete the change.
The Hide Fiddler when minimized checkbox will show the Fiddler icon in your system tray rather than the task bar when Fiddler is minimized.
The Use SmartScroll option controls whether or not Fiddler will auto-scroll to the bottom of the session list when new sessions are added but you've manually scrolled the list to an earlier session.
Reset Session ID counter on CTRL+X will cause the session id numbering to restart from 0 when the Session list is cleared via Remove > All Sessions, or when you use the CTRL+X hotkey.