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A new Fiddler Beta minor update was released this evening [.NET2 or .NET4]. This build contains a few long-awaited tweaks, bug fixes, and minor features. Read on for the summary of changes, and please let us know (Help > Send Feedback) in Fiddler if you find any problems!

Web Sessions List Improvements

The Web Sessions list on the left side of Fiddler shows each Session and provides entry points into most of Fiddler’s functionality. In the new version, a number of improvements have been made to this critical interface.

Sorting Indicator

Fiddler has always allowed sorting the Web Sessions list on any column. However, until now, you had to remember how your columns were sorted if you changed away from the default sort (ascending on Session ID#). In the new build, Fiddler will show the standard sort-indicator triangle in the column header.

Sorted by Hostname ascending screenshot

Sorted by Body Size descending

Searching Columns

Fiddler offers a huge number of ways to search for sessions of interest, from the Find Sessions window (CTRL+F) to the QuickExec box. However, until now it hasn’t been simple to search based on the contents of the columns in the Web Sessions list. Now, you can simply right-click any column header and choose Search this column…


Depending on whether you’ve selected a column of text or a column of numbers, Fiddler will prompt you for a search expression:




Fiddler will then select the Sessions which match the criteria that you’ve specified. With the target entries selected, you can easily Save, Export or Delete them, or examine their size and type using the Statistics tab.

If you prefer to launch your search from the QuickExec box, you can type any of these commands:

  • select @col.Host
  • select @col.body >20k
  • select @col.Protocol Exact:HTTPS
  • select "@col.ColumnName With Spaces" value

Hint: The select command also allows you to search based on the contents of Session flags or Request or Response headers, as explained in the QuickExec documentation.

Comparing Sessions

After selecting two entries in the Web Sessions list, you can choose Compare from the context menu to compare the request and response from each Session. Recently, two Fiddler users independently pointed out that (in almost all cases) users want to compare the Sessions after decompression and unchunking. The newest Fiddler build allows you to skip using the Decode Selected Sessions menu option by decoding both Sessions automatically when a comparison is performed.

If you prefer to only decode Sessions manually, set the fiddler.differ.DecodeFirst preference to false. You can configure this and other preferences by typing about:config in the QuickExec box.

Protocol Violation Reporting

By default, Fiddler will warn you of certain HTTP protocol violations that might impact interoperability with clients, servers, or proxy servers. As a developer, these warnings may draw your attention to the root cause of otherwise difficult-to-diagnose problems.

However, these warnings could be very noisy-- they appeared individually in popup message boxes. Loading a page from a buggy server could result in dozens of alerts.

In the newest build, Fiddler aggregates these messages into a simple list UI:


You may disable notification of protocol violations by unchecking the checkbox—to reenable, see the option inside Tools > Fiddler Options > General.

Alternatively, you may enable more stringent protocol usage checks by selecting "Extended checks in the dropdown box, or by setting the fiddler.lint.HTTP preference to True and restarting Fiddler.

Assorted Improvements

The Browse button in the toolbar now permits you to launch browsers (like Chrome) which are only installed on a per-user basis.


By default, Fiddler ships with HTTPS decryption disabled:


However, that means that one of the most common support questions is “Why don’t I see my HTTPS traffic?” In the newest Fiddler, a prominent warning alerts you if you try to inspect a CONNECT tunnel and decryption hasn’t yet been enabled:


Fiddler’s Viewer mode (launched via the -viewer command line option) has been enhanced to show the last loaded SAZ filename in the title bar, enabling you to more easily switch between captures if you have multiple instances loaded:


Improved Enable Loopback tool

Windows 8 reached the general availability phase late last month, bringing with it the need for developers to configure Win8 Apps for Loopback Exemptions. Configuring such exemptions is easy for Win8 users of the .NET4 version of Fiddler. Simply click the button at the left-edge of the toolbar:

Win8 Config button screenshot

The Loopback Exemption Utility has been enhanced to simplify the display of the AppContainers on your system, enabling you to debug Win8 Apps and Internet Explorer 10 with Enhanced Protected Mode.

EnableLoopback Utility screenshot

…And more

As with most Fiddler updates, this version also includes dozens of bug fixes and performance improvements, and upgrading is thus recommended for all users.

Thanks for using Fiddler! As always, please let us know (Help > Send Feedback) in Fiddler if you find any problems.

About the author

Eric Lawrence

Eric Lawrence

(@ericlaw) has built websites and web client software since the mid-1990s. After over a decade of working on the web for Microsoft, Eric joined Telerik in October 2012 to enhance the Fiddler Web Debugger on a full-time basis. With his recent move to Austin, Texas, Eric has now lived in the American South, North, West, and East.


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