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FiddlerCore

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What FiddlerCore is

FiddlerCore is a .NET class library for embedding HTTP and HTTPS traffic capture and modification in .NET applications without the Fiddler interface. It supports filtering, storing and reloading traffic, and importing or exporting traffic in various formats. Developers can replace frontend or backend services, replicate APIs, alter responses, and simulate network conditions or user loads for testing. The product also describes monitoring network systems, devices, and applications to find delays, missing services, or potential dangers. Documentation says it supports virtually any client application and most devices through mobile proxy settings. FiddlerCore integrates with Telerik Fiddler and Telerik TestStudio. The latest version targets .NET Standard 2.1; .NET Framework 4.8 requires FiddlerCore 5.0.2 or older because 6.x is incompatible. Internal Applications Use costs $2,999 a year per application. A 30-day trial excludes production and redistribution use.

Who it suits

FiddlerCore suits .NET developers building traffic inspection, testing, monitoring, or network simulation into applications. It also fits teams that need request editing, replay, response mocking, WebSocket inspection, or proxy chaining across supported platforms.

What it does well

  • Captures and modifies HTTP and HTTPS traffic
  • Can replay traffic and mock responses
  • Supports API replication and service replacement
  • Imports and exports multiple traffic formats
  • Works across Windows, macOS, and Linux

What to know first

  • No free plan
  • Internal license costs $2,999 a year
  • Trial forbids production and redistribution use
  • .NET Framework requires version 5.0.2 or older

Limits

The 30-day trial cannot be used in end products or for commercial, productive, training, or redistribution purposes. .NET Framework 4.8 support requires FiddlerCore 5.0.2 or older; version 6.x is incompatible.

Plans

PlanBilled and limitsPrice
Internal Applications Useper application, per year · Object code form only · one internal business application · valid for one year · 1 Year Priority Support$2,999 a year
Commercial Applications Useper application, per year · For commercial applications · includes redistribution rights · Priority SupportOn request

As telerik.com lists them, read 8 Oct 2026. The maker's own page has the final word.

Getting started

  1. Open the FiddlerCore website
  2. Start the 30-day trial or obtain a developer license
  3. Download installation files, assemblies, or NuGet packages
  4. Integrate the .NET class library into your application
  5. Use the supported platform: API, Linux, macOS, or Windows

Verdict

FiddlerCore provides embedded traffic control for .NET software, covering capture, editing, replay, mocking, and monitoring scenarios. Version and trial restrictions matter, especially for .NET Framework applications and commercial deployment.

Questions

Is there a free plan or trial?

There is no free plan. A 30-day trial is available with restrictions on production, commercial, training, redistribution, and end-product integration.

Which platforms are supported?

FiddlerCore is listed for API, Linux, macOS, and Windows environments.

What is FiddlerCore?

It is a .NET class library that captures and modifies HTTP and HTTPS traffic inside .NET applications without the Fiddler user interface.

Which .NET frameworks are supported?

The latest version targets .NET Standard 2.1. .NET Framework 4.8 requires FiddlerCore 5.0.2 or older.

What integrations are listed?

The product page lists integration with Telerik Fiddler and Telerik TestStudio.

Facts from FiddlerCore's own pages

Client support
The documentation says FiddlerCore supports virtually any client application and most devices through mobile proxy settings. · telerik.com, 8 Oct 2026
Current framework
The latest version targets .NET Standard 2.1; support for .NET Framework 4.8 and earlier is available only in older versions. · telerik.com, 8 Oct 2026
Downloads
With an active trial or developer license, users can download standalone installation files, assemblies, NuGet packages, and old versions. · telerik.com, 8 Oct 2026
Import and export
The product page lists importing and exporting traffic to various formats as a key feature. · telerik.com, 8 Oct 2026
Integrations
The product page lists integration with Telerik Fiddler and Telerik TestStudio. · telerik.com, 8 Oct 2026
Monitoring use
The product page describes using FiddlerCore to monitor network systems, devices, and applications and identify delays, missing services, or potential dangers. · telerik.com, 8 Oct 2026
Older framework limit
FiddlerCore 6.x is incompatible with .NET Framework; the system requirements page specifies FiddlerCore 5.0.2 or older for .NET Framework 4.8. · telerik.com, 8 Oct 2026
Platforms
The latest version can be used on Windows, Linux, Mac, and other platforms that implement .NET Standard. · telerik.com, 8 Oct 2026
Testing use
FiddlerCore can replace frontend and backend services, replicate APIs, modify responses, and simulate network conditions and user loads for testing. · telerik.com, 8 Oct 2026
Traffic tools
Its features include HTTP/HTTPS capture and modification, content filtering, and storing and reloading web traffic. · telerik.com, 8 Oct 2026
Trial limits
The 30-day trial does not allow integration into end products or commercial, productive, training, or redistribution use. · telerik.com, 8 Oct 2026
Trial support
The license agreement says trial users can submit support requests with a 72-hour response time, excluding Saturdays, Sundays, and holidays, for 30 days after their initial trial download. · telerik.com, 8 Oct 2026
What it is
FiddlerCore is a .NET class library that can be integrated into .NET applications to capture and modify HTTP and HTTPS traffic without the Fiddler user interface. · telerik.com, 8 Oct 2026

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