Pending Review
Last Updated: 23 Aug 2019 13:15 by Gary


I have a connectivity issue when I run Fiddler (Progress Telerik Fiddler Web Debugger). I work behind a corporate proxy server and the server IP is changed every few hours. The change in IP is normally okay, but when I am running Fiddler with the 'Automatically Authenticate' option enabled, then Fiddler shows an error every time the proxy is changed; it shows a yellow error message suggesting "The system proxy was changed. Click to reenable capturing".

Clicking on the error in Fiddler works fine and it reconnects, but I find this frustrating because if i don't realise there was an error in fiddler and don't click it immediately then it affects my connectivity and some activities on my pc start to fail without me realising.

My suggestion is that you should let Fiddler auto-reconnect when this type of proxy change happens and when the 'Automatically Authenticate' option is enabled. I know it can't try to reconnect forever because it will cause an endless cycle for other people who have other types of proxy disconnects, but in my case it would be very helpful to let fiddler at least try once to auto reconnect (for me 1 retry is usually fine and Fiddler is able to connect to the new proxy address, but perhaps you can let the number of retries be a menu option so that users can set the number of retries that they want Fiddler to do after there was a proxy change or proxy error).


Thanks so much for your work, I really like Fiddler!

I hope you can bug fix this or add it as a feature, it will be very helpful!



Last Updated: 16 Jul 2019 07:40 by ADMIN

Repro steps:

1. Inspect a websocket connections traffic

2. Expand the preview column so that the message body should show more than 65 characters.

3. Observe that even though the column is wide enough to support more than 65 characters, only 65 characters are displayed.

Pending Review
Last Updated: 08 Jul 2019 19:05 by James


Fiddler does not set up it's proxy server for the websocket protocol.  Applications looking at the global proxy settings are told by WebRequest.DefaultWebProxy.IsBypassed(Uri) that no proxy server should be used for the specified wss Uri.


Replication steps:

1. Have a .NET application that uses websockets and checks WebRequest.DefaultWebProxy.IsBypassed(Uri) to see if a proxy should be used for a wss Uri.

2. Try to use Fiddler to observe websocket traffic



After turning on fiddler, enable "Use the same proxy server for all protocols" in the Internet Options / Advanced Proxy settings screen.

Last Updated: 04 Jul 2019 11:41 by Daniel


  • Have a list of several AutoResponder rules.

Steps to reproduce:

  • Select a rule in the list.
  • Press the M hotkey to set a comment for the rule.
  • Edit the comment.
  • Save the comment by confirming with OK.

Expected result:

  • The comment edit dialogue displays the current comment of the rule selected in the list.
  • Confirming the dialogue updates the comment of the rule selected in the list with the text box content.
  • The list selection does not change.

Actual result:

  • The comment edit dialogue displays the current comment of the rule selected in the list (expected).
  • As the edit dialogue opens, the selection in the AutoResponder list changes and a different rule is highlighted.
  • Confirming the dialogue updates the comment of the newly selected rule, not the one the user manually selected and whose initial comment was shown in the edit dialogue.

Version: 5.0.20192.25091 (2019-06-04)
Platform: Windows 10 build 17134, .NET 4.7.1

Pending Review
Last Updated: 24 Jun 2019 20:56 by Eric

Hello Sir , 

            I've discovered the security bug which is Bypassing UAC in Fiddler Software . any reward or acknowledgement in bug report program ? 


Sai Wynn Myat. (@404death)

Pending Review
Last Updated: 31 May 2019 11:43 by ADMIN
Created by: James
Comments: 3
Type: Bug Report

It's incredibly an annoying prompt that occurs every. single. time. i open up fiddler.


Check for updates behind the scenes if an update exists and only let me know when an update exists

Pending Review
Last Updated: 22 May 2019 20:35 by Jonathan
Created by: Jonathan
Comments: 3
Type: Bug Report


My windows DPI scaling is set to 200%, as my resolutions to 2000x3000.

If I load fiddler normally, I see the following blurry text.



So i saw this bug report, which suggested to load up fiddler with -dpiaware, and now some text is better, but most of the tabs and buttons are incorrect sizes in the composer. In my opinion its more usable with the blurry text. I also tried to change the Fiddler.exe.config - EnableWindowsFormsHighDpiAutoResizing from true to false, but the only affect this had was to change the size of the mouse cursor.


Any ideas on something else to try, or is this a limitation of Fiddler / .Net WinForms.

Pending Review
Last Updated: 22 May 2019 16:23 by ADMIN
Created by: Eric
Comments: 4
Type: Bug Report

Fiddler's "Web Browsers" filter does not include msedge.exe as a web browser. This is the executable name of the new Chromium-based version of Edge that Microsoft is now working on.

This should be a one-line fix and very much appreciated by the many thousands of Fiddler+Edge users.

Pending Review
Last Updated: 15 May 2019 15:55 by Eric

I want to capture traffic from a Chrome Incognito window only.

When I click the 'pick target' button on the toolbar and point it at the Chrome window it picks up some particular process from chrome, but that does not seem to be the one from where network requests are sent from.

Looking at Chrome's Task Manager and comparing with what Fiddler shows, Fiddler is picking up the "Browser" process, while requests are coming from the "Utility: Network Service" process, thus the difference.

An acceptable solution for this would be if we could enter the process ID manually in the UI, instead of using the target picker.




Pending Review
Last Updated: 23 Apr 2019 15:22 by Eric

Reformatter.cs, around line 880, needs to support the new JavaScript feature of allowing underscores as (ignorable) digit separators. Otherwise, the parser treats _ as the end of the numeric value which breaks reformatting.



Pending Review
Last Updated: 11 Apr 2019 13:02 by ADMIN

This is a copy of Fiddler's new Feedback Portal not working properly submitted to the Fiddler groups forum:

I have submitted probably a dozen or two bugs to the Fiddler Feedback portal . Fiddler recently changed the feedback portal software and attempted to import all existing bugs. The import was not done properly. Every bug I reported the old links do not redirect to the imported reports. If I search for a report by title only arbitrary comments of the reports are included. There are no screenshots or attachments. Also I cannot login to the portal with my yahoo account, it says "Oh, no! Something's not right, but we're sorting it out." I was able to salvage some of what I reported from screenshots of the old site but I don't remember all of what I reported. Please retry the import so what users reported remains open for discussion and fixing. One particular bug is really a nuisance and hasn't been fixed when I have one session open raw in inspector and I click on another session I have to wait a long time. I don't remember what bug I filed over it but I know I filed something.

(Since the original post I have been able to login with my yahoo account. It does not show any bugs for my e-mail and they're still broken missing content and comments as "Imported User")


Pending Review
Last Updated: 27 Mar 2019 13:20 by ADMIN

Since I installed Fiddler on my machine, I am not able to connect to Team Foundation Server in Visual Studio (Version: 2013 Professional) if Fiddler is not running. I need to start the Fiddler first in order to connect to Team Foundation Server. I have tried various options that I found online. I even uninstalled Fiddler but still no luck.

Thanks in advance.


PS: I was not finding any other medium through which I should have communicated this, So, I've chosen to report it as a bug.

Pending Review
Last Updated: 26 Mar 2019 12:56 by ADMIN
Created by: Ravindhranad
Comments: 1
Type: Bug Report
I have Windows UWP app called HP Smart downloaded from Microsoft Store. When I enabled HP Smart application using Win Config menu and later I have enabled HTTPS traffic. After that I have started HP Smart application. But I see traffic from app as tunnel.. but app is not receiving any responses from the servers. I have exported the capture, please help me to fix this issue. Please notice packet no.2, I am not able to get any response. But if I stop fiddler then HP Smart App is able to communicate and get the data response for this request.
Pending Review
Last Updated: 22 Mar 2019 21:47 by Eric
Pending Review
Last Updated: 18 Mar 2019 15:45 by Alex
Created by: Alex
Comments: 0
Type: Bug Report

make windows have 2 languages for keyboard, e.g. en-us, and ru-ru; 

make key shortcut to switch langs: alt+shift as usual;

select current lang: eng

run fiddler until it accumulates some traces;

select one of the trace records, click on request panel, click on response panel, notice: lang has switched to RU now.

Pending Review
Last Updated: 11 Mar 2019 20:25 by Eric

"Target Any Process" feature no longer working with Chrome. It used to work but no longer works specifically with Chrome. Still works with other processes. Capturing still works with Chrome as long as I am not using the "Target Any Process" feature.

Pending Review
Last Updated: 25 Feb 2019 23:34 by Eric

The HTTP2 protocol has specific requirements about the characters that are valid in header names. If a server attempts to use one of these over a HTTP2 connection, the client will complain of a ERR_SPDY_PROTOCOL_ERROR.

This is particularly problematic because the problem will magically "disappear" when Fiddler is in use because Fiddler doesn't support HTTP2, and thus the problem characters are silently allowed by the browser using HTTP/1.1.

Therefore, Fiddler's HTTP Protocol Violation feature should warn if a HTTPS response contains an illegal character in a HTTP header name or value.

Legal characters for names: abcdefghijklmnopqrstuvwxyz0123456789!#$%&'*+-.^_`|~

Legal characters for values '\t', '  ', 0x21 to 0x7E, and 0x80 to 0xFF.

Pending Review
Last Updated: 20 Feb 2019 10:42 by ADMIN

This only occurs when double-clicking a SAZ file from Explorer when Fiddler isn't open.

v5.0.20182.28034 for .NET 4.6.1

Script Failure
Failed to load script.
 Microsoft.JScript.JScriptException: Type mismatch

   at Microsoft.JScript.Convert.CoerceT(Object value, Type t, Boolean explicitOK)

   at Fiddler.ScriptNamespace._Startup.Startup()

   at Microsoft.JScript.Vsa.BaseVsaEngine.Run()
Microsoft.JScript.JScriptException: Type mismatch

   at Microsoft.JScript.Convert.CoerceT(Object value, Type t, Boolean explicitOK)

   at Fiddler.ScriptNamespace._Startup.Startup()

   at Microsoft.JScript.Vsa.BaseVsaEngine.Run()   at Microsoft.JScript.Vsa.BaseVsaEngine.Run()

   at Fiddler.FiddlerScript._LoadScript(String sScriptFilename, Boolean bPlaySounds) in C:\JenkinsHome\jobs\FiddlerReleaseBuild\workspace\Fiddler2\Common\Application\Scripting\Scripting.cs:line 315

   at Fiddler.ScriptBase.LoadRulesScript(Boolean bPlaySounds) in C:\JenkinsHome\jobs\FiddlerReleaseBuild\workspace\Fiddler2\Common\Application\Scripting\ScriptBase.cs:line 198


This reproduces even when the FiddlerScript is basically empty, like so:

import Fiddler;

class Handlers




Sadly, because Fiddler's code has been obfuscated, it's difficult to see what caused this regression.

Pending Review
Last Updated: 13 Feb 2019 23:29 by Fergal
Response headers added from Fiddler-script (in reverse-proxy setup) are not sent to client. Even though the session shows them as sent. However, if I set a bpafter for that particular url it works! I Don't need to do anything else, just set the bpafter ang type 'g' to let it go. The client then see the response header.
Pending Review
Last Updated: 01 Feb 2019 16:12 by Eric
Created by: Ahsen
Comments: 2
Type: Bug Report


I am getting the following error in fiddler and without Capture HTTPS CONNECTs it works, also in the Protocols I have enabled: <client>;ssl2;ssl3;tls1.0;tls1.1;tls1.2> HTTPS handshake to <domain> (for #6) failed. System.Security.Authentication.AuthenticationException A call to SSPI failed, see inner exception. < The message received was unexpected or badly formatted

Win32 (SChannel) Native Error Code: 0x80090326


[Edited by Telerik Staff to remove personal information and convert to bug report]

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