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

When I try to analyse the requests on Yahoo Finance, I see different results when requesting from a browser with Fiddler turned on compared to the same request when Fiddler is turned off. I also tried different capture methods (System Proxy, Network Capture and Browser), but the same occurs. If I use this url:

 

https://query1.finance.yahoo.com/v8/finance/chart/IVV?events=capitalGain%7Cdiv%7Csplit&formatted=true&includeAdjustedClose=true&interval=1d&period1=0&period2=1747180800&symbol=IVV&userYfid=true&lang=en-US&region=US

 

Opening this url in a Chrome tab with Fiddler turned off, everything works fine (HTTP 200 response). Wih Fiddler turned on, however, the site responds with a HTTP 429 code (Too Many Requests).

 

Why this is happening?

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   
---------------------------

Following guide online to swap out the old max speed with the new max speed but i'm using a mac whereas most people used fiddler classic on windows and i'm not sure whether I put the values in the correct places within the rule.  Anyone spot my noob mistake ?  Seems like many people had success so I know it's possible and my phone with the app making the API calls is connected properly to Fiddler that's not the problem. Please any help is so appreciated ! Thanks so much

websocket message garbled

Hello, for some tests, I'd like to use Fiddler to route through the corporate proxy network and, in a specific case, process requests/responses directly with mitmproxy scripts (because Fiddler Everywhere doesn't yet offer the same functionality as FC).

So I connect Fiddler to mitmproxy by setting the gateway proxy to mitmproxy listening at localhost:9090.

But it seems that Fiddler doesn't respect this setting. It always routes through the internet normally.
Even if I intentionally configure the wrong proxy port, the connection persists.

What should I do? Maybe I got wrong something??

 

I'm using Fiddler in Linux and already added certificates of Fiddler and mitmproxy to the system (actually i'm using the same one).

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