Fiddler is an HTTP and HTTPS debugging proxy for Windows. The Windows application, Fiddler Classic, captures web traffic from browsers and desktop programs so users can examine requests, inspect server responses and change the exchanges being tested. It also creates requests independently of the application that originally sent them.
Fiddler Classic is free for eligible non-commercial use and is in maintenance mode. Fiddler Everywhere is a separate, actively supported product for commercial work and for Windows, macOS and Linux. The Classic application and its traffic tools are the subject of this description.
How Fiddler captures and inspects sessions
Traffic passing through Fiddler appears in the Web Sessions List. Each exchange has identifying details such as the destination hostname, URL, protocol and result code. Content type, body size, caching information and the originating process provide context for deciding which exchange needs attention. Applications must send their connections through the proxy to appear in the capture.
Selecting a session opens its contents in Inspectors. Inspectors show the request sent by the client together with the response returned by the server. Find Sessions searches the captured requests and responses when the relevant transaction is buried among other traffic.
Fiddler separates content inspection from timing analysis. Statistics shows performance information for a selected session, while Timeline draws a waterfall for one or more selected sessions. The waterfall places their transfers on a common time axis, making overlapping requests and their sequence visible.
HTTPS decryption
HTTPS payload inspection requires decryption, which Fiddler Classic leaves disabled by default. Once enabled, it makes the encrypted exchanges available for inspection. A host exclusion list lets selected destinations pass without decryption; FiddlerScript can also restrict decryption to one hostname.
Request composer
Composer builds an HTTP request without requiring the user to repeat an action in a browser or desktop application. A captured session can be dragged into Composer as a starting point, or a request can be constructed manually. Parsed mode supplies fields for composing the request; Raw mode accepts a complete, correctly formatted HTTP request.
The user can change the destination, headers or request body and issue the resulting request to the server. This supports comparisons such as sending the same request with a different header value. Composer can correct the Content-Length header to match the body and follow redirects, and it can insert local files into the outgoing body. The resulting exchange can be opened directly in Inspectors for examination.
AutoResponder
AutoResponder substitutes responses for requests that match a rule. Dragging captured sessions into its rule list creates matches that return the corresponding recorded responses instead of sending those requests to the destination server. An application can therefore receive a previously captured reply while continuing to request its usual URL.
Each rule pairs a request match with an action. Individual rules can be switched on or off, and their order controls priority.
FiddlerScript
FiddlerScript handles changes that need conditions or repeated processing. Rules run before a request or before a response, allowing different changes at each stage. A request rule can add a header or redirect a path to another resource; a response rule can remove a header or replace text in returned HTML.
A response rule runs after the server has already received the request. Editing the request at that point cannot alter what the server processed. FiddlerScript also supports returning a local file or generating an error response, extending the available ways to control the traffic an application receives.
Session archives
Fiddler saves captured traffic in SAZ session archives. Saved archives keep a capture for later investigation without reproducing the client activity.
