A free, open source Fiddler Classic alternative

Fiddler Classic is now for non-commercial use only. VioLens is an open source debugging proxy that works the way Fiddler Classic does, and it runs on Mac and Linux as well as Windows: it opens your SAZ files, runs your CustomRules.js, and takes the same AutoResponder rules and QuickExec commands. It's free and MIT licensed, so you can use it at work.

Download VioLens How to switch

What changed with Fiddler Classic

On August 3, 2026, Progress Software announced that Fiddler Classic’s license would allow non-commercial use only. Commercial users had until September 17, 2026 to move to Fiddler Everywhere, Progress’s paid, subscription-based tool, or to stop using Fiddler Classic for work. Progress doesn’t sell a commercial license for Fiddler Classic itself. The details are on Progress’s commercial use page.

If you use Fiddler at work, you have a few options:

• Move to Fiddler Everywhere. It’s Progress’s supported product, with a yearly subscription per user. It works differently from Fiddler Classic: it has its own C# scripting (in beta) rather than FiddlerScript.
• Use another debugging proxy, such as Charles, mitmproxy, HTTP Toolkit or Proxyman.
• Use VioLens, if you want to keep working the way you did in Fiddler Classic, with your files and scripts, for free.

Why VioLens feels like Fiddler Classic

I used Fiddler for more than 15 years, and I built VioLens to replace it in my own work. Most of what you used in Fiddler Classic has a direct equivalent:

In Fiddler Classic	In VioLens
Session list and Inspectors	The session list, with Headers, Raw, JSON, WebForms, ImageView, Hex and Cookies views
.saz archives	File > Open SAZ and Save SAZ, plus HAR import and export
FiddlerScript (CustomRules.js)	Tools > FiddlerScript: OnBeforeRequest, OnBeforeResponse, the Rules menu, context actions and columns
AutoResponder	The same match syntax (EXACT:, regex:, NOT:, METHOD:) and actions (*redir:, *delay:, *drop, *reset, *bpu and more)
Breakpoints	Break on Requests, Break on Responses, and narrower conditions by URL, method or status
QuickExec	The same box (Alt+Q) and commands: ?text, @host, =404, bpu, bpafter, go, cls, select, tail
Composer and Reissue	Composer, including binary bodies as hex or from a file, and Replay (press R)
TextWizard	TextWizard (Ctrl+E), now with JWT, SAML and Unix time conversions
Find Sessions	Find (Ctrl+F), searching decompressed bodies
Compare (with WinDiff)	A built-in, side-by-side Compare (Ctrl+W)
Timeline and Statistics	A DevTools-style waterfall, a Timing tab and Statistics
Allow remote computers to connect	The same setting, with the certificate at http://violens/
Gateway and upstream proxies	Upstream proxies, PAC scripts, and Negotiate, NTLM or Basic sign-in
Extensions	.NET extensions, ported from their source
FiddlerCap	VioLens Capture, a one-window app for someone else to record a capture
ExecAction.exe	violens-exec, to drive a capture from scripts
Import packet captures	File > Import Packet Capture (.pcap, .pcapng)
FiddlerCore	Violens.ProxyCore, on NuGet

It also does a few things Fiddler Classic never did. It runs on Mac and Linux, captures HTTP/2, shows live WebSocket messages, decompresses Brotli and zstd, and warns about cookies today’s browsers will refuse.

Moving from Fiddler Classic to VioLens

1. Close Fiddler Classic. Only one tool can be the system proxy at a time.
2. Download VioLens and install it.
3. Click Start, and trust the root certificate. VioLens creates its own certificate, DO_NOT_TRUST_ViolensRoot, on your computer. That’s what lets it decrypt HTTPS. Your browser traffic then shows up in the session list.
4. Open your saved sessions. Use File > Open SAZ for any .saz files you kept.
5. Load your FiddlerScript. Fiddler Classic keeps it at Documents\Fiddler2\Scripts\CustomRules.js. In VioLens, check Tools > FiddlerScript, click Browse, pick the file and click Load. The status line shows which handlers it found, or the error if something doesn’t load.
6. Add your AutoResponder rules. Check Tools > AutoResponder and add your rules. The match and action syntax is the same as Fiddler’s.
7. Set up phones and other devices again. Turn on Allow remote computers to connect in Tools > Connections. Then, on the device, open http://violens/ to install VioLens’s certificate. How to debug iPhone and Android traffic walks through it.
8. Port the extensions you rely on. The User Guide explains how. Three ported extensions come with the installers: Eric Lawrence’s NetLog importer, the CSP Rule Collector and the Privacy Scanner.

When you’re done, you can remove Fiddler’s root certificate from your computer if you no longer use Fiddler Classic.

Questions

Can I still use Fiddler Classic at work?

Progress has changed Fiddler Classic’s license to allow non-commercial use only. It set September 17, 2026 as the deadline for commercial users to move to Fiddler Everywhere or stop using Fiddler Classic for work. Progress’s FAQ explains what it counts as commercial use. I’m not a lawyer, so check with whoever handles software licenses where you work.

Is VioLens free for commercial use?

Yes. VioLens is released under the MIT license, so you can use it for anything, at work or at home. There’s no account, no subscription and no paid tier.

Will my .saz files open in VioLens?

Yes. File > Open SAZ opens the sessions in a .saz file saved by Fiddler Classic, and File > Save SAZ writes them. VioLens also imports and exports HAR files. WebSocket messages are saved in HAR files but not in SAZ files yet.

Does my CustomRules.js work in VioLens?

VioLens loads FiddlerScript files like CustomRules.js, with OnBeforeRequest, OnBeforeResponse, Rules menu options, context-menu actions, extra columns and QuickExec commands. Load yours from Tools > FiddlerScript. If something in your script doesn’t work, please open an issue.

Is there a Fiddler Classic for Mac or Linux?

No. Fiddler Classic only runs on Windows. VioLens runs on Windows, Apple silicon Macs (macOS 11 or later) and 64-bit Linux, with the same features on each.

Can I keep using my Fiddler extensions?

Usually, if you have the extension’s source. It can be rebuilt for VioLens with small changes, often just removing its using Fiddler; lines, and then it runs on Windows, Mac and Linux. A compiled Fiddler .dll can’t be dropped in as it is. Extensions that rely on old Windows Forms controls can run in VioLens’s separate Legacy Extension Host on Windows.

What doesn't VioLens do yet?

It can’t use SOCKS upstream proxies or Digest proxy sign-in, and it sends only one client certificate (mutual TLS) at a time, without support for Fiddler’s ClientCertificate.cer file. The Mac and Linux versions have had less use than the Windows one. The Download page lists the gaps.

Is VioLens made by Progress or Telerik?

No. VioLens is an independent open source project, written from scratch. It isn’t affiliated with or endorsed by Progress Software or Telerik.

Missing something you relied on in Fiddler Classic? Tell me in Discussions or on Discord. That's how I decide what to build next.