Hello,

I have two questions regarding Fiddler Classic, and I would appreciate your help.

1) Cloudflare blocking error

When I try to load either of the two websites below through Fiddler Classic, I receive the following Cloudflare error message: "Sorry, you have been blocked." Please see the screen shot attached.

https://www.gurufocus.com
https://et3arraf.com

Both websites load fine when Fiddler is not active. Over time, I’ve encountered the same issue with several other sites as well — the two above are just recent examples. It seems this error has become more frequent lately.

Do you know why this happens? Is there any way to bypass or prevent this issue?

2) TLS 1.3 support in Fiddler Classic

Could you please provide an update regarding the implementation of TLS 1.3 in Fiddler Classic? Is it still under development, or has the upgrade been officially discontinued?

No recent news here: https://feedback.telerik.com/fiddler/1584209-support-tls-1-3-now-that-net-framework-does

Please note that I’d consider switching to Fiddler Everywhere, but I’m currently relying on a large script that only works with the Classic version, since scripting isn’t yet supported in Everywhere.

Thank you in advance!

Best regards,
Alex

HI All,

I get the following error when trying to open .saz files.  Do you know what is happening and if I get other .saz files recorded, is there a particular setting I should look for? 


---------------------------
Awww, Fiddlesticks!
---------------------------
Fiddler has encountered an unexpected problem. If you believe this is a bug in Fiddler, please copy this message by hitting CTRL+C, and submit a bug report at http://www.telerik.com/forums/fiddler.

Input string was not in a correct format.

Type: System.FormatException
Source: mscorlib
   at System.Number.StringToNumber(String str, NumberStyles options, NumberBuffer& number, NumberFormatInfo info, Boolean parseDecimal)

   at System.Number.ParseInt32(String s, NumberStyles style, NumberFormatInfo info)

   at Fiddler.Session.TestComparator(Object )

   at Fiddler.Session.LoadMetadata(Stream strmMetadata)


Fiddler v5.0.20253.3311 (x64 AMD64) [.NET 4.0.30319.42000 on Microsoft Windows NT 10.0.22631.0] 
---------------------------
OK   
---------------------------

I keep getting this warning and as soon I click, I get it back. I think one of the security apps is monitoring and locking down the proxy server setting in Windows. I am running Fiddler as an admin and manually setting the proxy server but my setting is instantly removed.
It seems I can't use network sniffers that depend on certain proxy server settings.

Does Fiddler Anywhere suffer from the same issue?

I am running 

Progress Telerik Fiddler Classic
v5.0.20253.3311 for .NET 4.6.2
Built: Monday, March 31, 2025
64-bit AMD64, VM: 74.0mb, WS: 145.0mb
.NET 4.8 WinNT 10.0.26100.0

 

When trying to import a net-export JSON file it fails with

---------------------------

Failed to import NetLog
---------------------------
Fiddler has encountered an unexpected problem. If you believe this is a bug in Fiddler, please copy this message by hitting CTRL+C, and submit a bug report at http://www.telerik.com/forums/fiddler.

Could not load type 'JSONParseErrors' from assembly 'Fiddler, Version=5.0.20253.3311, Culture=neutral, PublicKeyToken=null'.

Type: System.TypeLoadException
Source: FiddlerImportNetlog
   at FiddlerImportNetlog.NetlogImporter..ctor(StreamReader oSR, List`1 listSessions, EventHandler`1 evtProgressNotifications)

   at FiddlerImportNetlog.NetLogFormatImport.ImportSessions(String sFormat, Dictionary`2 dictOptions, EventHandler`1 evtProgressNotifications)


Fiddler v5.0.20253.3311 (x64 AMD64) [.NET 4.0.30319.42000 on Microsoft Windows NT 10.0.26100.0] 
---------------------------
OK   
---------------------------

Today after upgrade Fiddler Classic to new version we got compilation error for our custom rules.

Fiddler version:

v5.0.20253.3311 for .NET 4.6.2
Built: Monday, March 31, 2025

64-bit AMD64, VM: 63.0mb, WS: 114.0mb
.NET 4.8 WinNT 10.0.22631.0

I don't see in release notes anything about FiddlerScript changes. So what need to be added now to resolve this issue?

I am running Windows 11 Home addition and a friend of mine is running Windows 10 Pro.  When I installed Fiddler Classic on my computer I was able to start monitoring API traffic with a couple setting changes.  Once I did that, everything worked fine including being able to go to various websites.

My friend installed the same version of Fiddler on his computer and immediately had issues going to websites.  I gave him all of my settings and he never could see the API traffic.  He did get a message indicating that "Do_Not_Trust Certificate was the cause of this.

Why would Fiddler work on my computer and not his?  I did not think that installing a certificate would cause this issue.  I would really appreciate any information that would help get Fiddler working for him

Thanks

Charles

I am using Fiddler Classic strictly for its FiddlerScript capabilities, and I have just learned that the Progress team no longer have any plans to add this awesome functionality to Fiddler Everywhere. This is very sad news for me, as I feel that probably Fiddler's most appreciated feature is dropped all together. I have just installed the Everywhere version, checked out its Rules tab, but I am most positive that all those hard-coded rules will not even come close to the functionality and the simplicity provided by a scripting language. The Rules tab in Everywhere may be a more easier and intuitive option for most users, but this comes at the price of loosing the flexibility and the powerful tools that only a script can provide. The web is becoming more and more complex with each day, and in order for Fiddler to stay on top of it, I believe it should give its users access to the most powerful tools available. Employing scripting capabilities is certainly one of it, if not the best one! Why restrict that?!

I would like to take this opportunity to kindly ask the design team to re-evaluate this decision of dropping this future. It sets Fiddler apart from all the other software out there, and dropping it all together would be a major downgrade for Fiddler, in my opinion. Having a Rules tab for those in need of a quick set-up, and preserving the current scripting capabilities for those looking for powerful functionality seems like the best way to go for me. With these two foundations, I have no doubt Fiddler will continue to stay relevant and grow in the years ahead.

Anyhow, in case no re-evaluation will be made and approved, I would like to know if you have any plans of dropping support/updates for Fiddler Classic in the future. This would be a second blow to all the 10 year+ Fiddler users.

I hope both versions will be offered to us for many years to come, no matter how much the web will be changing. So, please confirm if we can at least rely on Fiddler Classic, as some believe it is only a matter of time until it will be deprecated (including me). For the time being, hopefully the plans to add support for HTTP/2 and TLS 1.3 will indeed materialize (for Fiddler Classic).

If others feel like me on this whole issue, please do comment. Thank you!

Alex

I'm trying to learn about the API that my Amazon Echo device uses when I ask Alexa for information.

So I'd like to be able to run my Echo traffic through Fiddler (classic or everywhere).

But everything I've read requires using a browser or configuring a proxy on the remote device or some other step that I can't do on my Amazon Echo.

So is there some way to configure Fiddler to work in the scenario where I can only do stuff (e.g., VLANs, port mirroring) on an AP/PC-as-hotspot/gateway/router?

There are some sites like this https://vk.com/ that just because fiddler is open makes the site loop the capcha challenge and not stop doing it. How to resolve this? Thanks.

Video Explaining My Issue


I was trying, as a test run, to capture the traffic from Facebook. I did what someone suggested in my previous post about removing the "https" and just using "facebook." I did that, clicked "Actions," and selected "Run Filter Set Now." I then refreshed the tab I was currently on, which was Facebook, and as you can see in the video link, no traffic was captured.

So, what am I supposed to do if I want to capture only the tab with Facebook traffic ? Do I really have to close my Firefox window, reopen Firefox, and have just one tab open with Facebook? Then, I would use Fiddler Classic. How is this realistic at all?

If anyone can please inform me how I can capture all of Facebook's traffic in the session list without any other traffic being captured-thus sparing me the hassle of sorting through the list to identify which traffic belongs to Facebook- that would be great. Otherwise, I see no reason to continue using Fiddler Classic or to subscribe.