Fiddler Everywhere V7.4/V7.5 Won't Open After Patching

Nov 24, 2025 by Alex Johnson 55 views

Are you experiencing issues with Fiddler Everywhere versions 7.4 or 7.5 failing to open after applying a patch? You're not alone. Many users have encountered this problem, and this article aims to provide a comprehensive guide to understanding and resolving this frustrating issue. We will delve into the common causes, analyze error logs, and offer practical solutions to get your Fiddler Everywhere up and running again.

Understanding the Issue

The core problem revolves around Fiddler Everywhere's inability to start its server process after a patch has been applied. This often manifests as an error message indicating that the application port calculation has failed, leading to the application quitting after multiple attempts. The error log, as provided by a user, clearly illustrates this behavior:

➜  MacOS ./Fiddler\ Everywhere 
[2025-11-24 16:08:57.266] [info] libfiddler initialize
16:08:57.269 › [Product information] Fiddler Everywhere 7.5.0
16:08:57.273 › [System information] darwin (Darwin/arm64) - version 24.6.0
16:08:57.275 › Loaded network capturing extension.
16:08:57.313 › Initializing splash screen.
16:08:57.413 › Ensure no running cleanup nodejs and cleanup .net processes.
16:08:57.462 › No matching processes to clean.
16:08:57.532 › Attempting to run server - 4 attempts left.
16:08:57.532 › Trying to start server at port [57476].
16:08:57.551 › Displaying splash screen.
16:08:57.863 › Unable to start server process, closed with code: 252
16:08:57.863 › Attempt failed with Application port calculation failed. Contact support..
16:08:57.863 › Attempting to run server - 3 attempts left.
16:08:57.863 › Trying to start server at port [30999].
16:08:58.061 › Unable to start server process, closed with code: 252
16:08:58.061 › Attempt failed with Application port calculation failed. Contact support..
16:08:58.061 › Attempting to run server - 2 attempts left.
16:08:58.061 › Trying to start server at port [30436].
16:08:58.261 › Unable to start server process, closed with code: 252
16:08:58.261 › Attempt failed with Application port calculation failed. Contact support..
16:08:58.261 › Attempting to run server - 1 attempts left.
16:08:58.261 › Trying to start server at port [27819].
16:08:58.463 › Unable to start server process, closed with code: 252
16:08:58.464 › Attempt failed with Application port calculation failed. Contact support..
16:08:58.464 › Maximum number of attempts reached - quitting application.
16:08:58.464 › Unable to start the server, error is Maximum number of attempts reached - quitting application.
16:08:58.483 › [CPU information] The processor model is: Apple M4 with number of cores: 10 and working at 2.4 GHz frequency
16:08:58.483 › [Memory information] The total amount of memory is 16384 MBs.
(node:5585) [DEP0040] DeprecationWarning: The `punycode` module is deprecated. Please use a userland alternative instead.
(Use `Fiddler Everywhere --trace-deprecation ...` to show where the warning was created)
16:08:58.522 › PM: Fiddler Everywhere(5590):CPU 0.60%, MEM 53.27MB/0.33% / Fiddler Everywhere(5585):CPU 2.50%, MEM 145.36MB/0.89% / Fiddler Everywhere Helper(5587):CPU 0.20%, MEM 36.05MB/0.22% / Fiddler Everywhere Helper (GPU)(5586):CPU 0.40%, MEM 55.31MB/0.34% / Fiddler Everywhere Helper (Renderer)(5588):CPU 1.00%, MEM 102.09MB/0.62%
[2025-11-24T08:08:58.522Z] Warning: Error from HTTP request. FetchError: request to http://localhost:27819/fiddlerhub/negotiate?type=electron&negotiateVersion=1 failed, reason: .
[2025-11-24T08:08:58.523Z] Error: Failed to complete negotiation with the server: FetchError: request to http://localhost:27819/fiddlerhub/negotiate?type=electron&negotiateVersion=1 failed, reason: 
[2025-11-24T08:08:58.523Z] Error: Failed to start the connection: Error: Failed to complete negotiation with the server: FetchError: request to http://localhost:27819/fiddlerhub/negotiate?type=electron&negotiateVersion=1 failed, reason: 
16:08:58.523 › Error while starting default SignalR connection: Error: Failed to complete negotiation with the server: FetchError: request to http://localhost:27819/fiddlerhub/negotiate?type=electron&negotiateVersion=1 failed, reason: 
16:08:58.523 › Clearing the proxy settings to the main window.
[2025-11-24T08:08:58.523Z] Warning: Error from HTTP request. FetchError: request to http://localhost:27819/fiddlerhub/negotiate?negotiateVersion=1 failed, reason: .
[2025-11-24T08:08:58.523Z] Error: Failed to complete negotiation with the server: FetchError: request to http://localhost:27819/fiddlerhub/negotiate?negotiateVersion=1 failed, reason: 
[2025-11-24T08:08:58.523Z] Error: Failed to start the connection: Error: Failed to complete negotiation with the server: FetchError: request to http://localhost:27819/fiddlerhub/negotiate?negotiateVersion=1 failed, reason: 
16:08:58.523 › Error while starting response SignalR connection: Error: Failed to complete negotiation with the server: FetchError: request to http://localhost:27819/fiddlerhub/negotiate?negotiateVersion=1 failed, reason: 
16:08:58.523 › Unable to get default proxy info. Error: Failed to complete negotiation with the server: FetchError: request to http://localhost:27819/fiddlerhub/negotiate?type=electron&negotiateVersion=1 failed, reason: 
16:08:58.523 › Loading application in window.
16:08:58.524 › Getting the URL of Angular application.
16:08:58.527 › Starting the app without a deep link.
(node:5585) UnhandledPromiseRejectionWarning: Error: Failed to complete negotiation with the server: FetchError: request to http://localhost:27819/fiddlerhub/negotiate?type=electron&negotiateVersion=1 failed, reason: 
    at C5._getNegotiationResponse (/Applications/Fiddler Everywhere.app/Contents/Resources/app/out/main.original.js:119:48027)
    at process.processTicksAndRejections (node:internal/process/task_queues:105:5)
    at async C5._startInternal (/Applications/Fiddler Everywhere.app/Contents/Resources/app/out/main.original.js:119:45951)
    at async C5.start (/Applications/Fiddler Everywhere.app/Contents/Resources/app/out/main.original.js:119:43709)
    at async e._startInternal (/Applications/Fiddler Everywhere.app/Contents/Resources/app/out/main.original.js:119:17213)
    at async e._startWithStateTransitions (/Applications/Fiddler Everywhere.app/Contents/Resources/app/out/main.original.js:119:16604)
(node:5585) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). To terminate the node process on unhandled promise rejection, use the CLI flag `--unhandled-rejections=strict` (see https://nodejs.org/api/cli.html#cli_unhandled_rejections_mode). (rejection id: 7)
(node:5585) UnhandledPromiseRejectionWarning: Error: Failed to complete negotiation with the server: FetchError: request to http://localhost:27819/fiddlerhub/negotiate?type=electron&negotiateVersion=1 failed, reason: 
    at C5._getNegotiationResponse (/Applications/Fiddler Everywhere.app/Contents/Resources/app/out/main.original.js:119:48027)
    at process.processTicksAndRejections (node:internal/process/task_queues:105:5)
    at async C5._startInternal (/Applications/Fiddler Everywhere.app/Contents/Resources/app/out/main.original.js:119:45951)
    at async C5.start (/Applications/Fiddler Everywhere.app/Contents/Resources/app/out/main.original.js:119:43709)
    at async e._startInternal (/Applications/Fiddler Everywhere.app/Contents/Resources/app/out/main.original.js:119:17213)
    at async e._startWithStateTransitions (/Applications/Fiddler Everywhere.app/Contents/Resources/app/out/main.original.js:119:16604)
(node:5585) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). To terminate the node process on unhandled promise rejection, use the CLI flag `--unhandled-rejections=strict` (see https://nodejs.org/api/cli.html#cli_unhandled_rejections_mode). (rejection id: 8)
(node:5585) UnhandledPromiseRejectionWarning: Error: Failed to complete negotiation with the server: FetchError: request to http://localhost:27819/fiddlerhub/negotiate?type=electron&negotiateVersion=1 failed, reason: 
    at C5._getNegotiationResponse (/Applications/Fiddler Everywhere.app/Contents/Resources/app/out/main.original.js:119:48027)
    at process.processTicksAndRejections (node:internal/process/task_queues:105:5)
    at async C5._startInternal (/Applications/Fiddler Everywhere.app/Contents/Resources/app/out/main.original.js:119:45951)
    at async C5.start (/Applications/Fiddler Everywhere.app/Contents/Resources/app/out/main.original.js:119:43709)
    at async e._startInternal (/Applications/Fiddler Everywhere.app/Contents/Resources/app/out/main.original.js:119:17213)
    at async e._startWithStateTransitions (/Applications/Fiddler Everywhere.app/Contents/Resources/app/out/main.original.js:119:16604)
(node:5585) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). To terminate the node process on unhandled promise rejection, use the CLI flag `--unhandled-rejections=strict` (see https://nodejs.org/api/cli.html#cli_unhandled_rejections_mode). (rejection id: 9)
(node:5585) UnhandledPromiseRejectionWarning: Error: Failed to complete negotiation with the server: FetchError: request to http://localhost:27819/fiddlerhub/negotiate?type=electron&negotiateVersion=1 failed, reason: 
    at C5._getNegotiationResponse (/Applications/Fiddler Everywhere.app/Contents/Resources/app/out/main.original.js:119:48027)
    at process.processTicksAndRejections (node:internal/process/task_queues:105:5)
    at async C5._startInternal (/Applications/Fiddler Everywhere.app/Contents/Resources/app/out/main.original.js:119:45951)
    at async C5.start (/Applications/Fiddler Everywhere.app/Contents/Resources/app/out/main.original.js:119:43709)
    at async e._startInternal (/Applications/Fiddler Everywhere.app/Contents/Resources/app/out/main.original.js:119:17213)
    at async e._startWithStateTransitions (/Applications/Fiddler Everywhere.app/Contents/Resources/app/out/main.original.js:119:16604)
(node:5585) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). To terminate the node process on unhandled promise rejection, use the CLI flag `--unhandled-rejections=strict` (see https://nodejs.org/api/cli.html#cli_unhandled_rejections_mode). (rejection id: 10)
(node:5585) UnhandledPromiseRejectionWarning: Error: Failed to complete negotiation with the server: FetchError: request to http://localhost:27819/fiddlerhub/negotiate?type=electron&negotiateVersion=1 failed, reason: 
    at C5._getNegotiationResponse (/Applications/Fiddler Everywhere.app/Contents/Resources/app/out/main.original.js:119:48027)
    at process.processTicksAndRejections (node:internal/process/task_queues:105:5)
    at async C5._startInternal (/Applications/Fiddler Everywhere.app/Contents/Resources/app/out/main.original.js:119:45951)
    at async C5.start (/Applications/Fiddler Everywhere.app/Contents/Resources/app/out/main.original.js:119:43709)
    at async e._startInternal (/Applications/Fiddler Everywhere.app/Contents/Resources/app/out/main.original.js:119:17213)
    at async e._startWithStateTransitions (/Applications/Fiddler Everywhere.app/Contents/Resources/app/out/main.original.js:119:16604)
(node:5585) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). To terminate the node process on unhandled promise rejection, use the CLI flag `--unhandled-rejections=strict` (see https://nodejs.org/api/cli.html#cli_unhandled_rejections_mode). (rejection id: 11)
(node:5585) UnhandledPromiseRejectionWarning: Error: Failed to complete negotiation with the server: FetchError: request to http://localhost:27819/fiddlerhub/negotiate?type=electron&negotiateVersion=1 failed, reason: 
    at C5._getNegotiationResponse (/Applications/Fiddler Everywhere.app/Contents/Resources/app/out/main.original.js:119:48027)
    at process.processTicksAndRejections (node:internal/process/task_queues:105:5)
    at async C5._startInternal (/Applications/Fiddler Everywhere.app/Contents/Resources/app/out/main.original.js:119:45951)
    at async C5.start (/Applications/Fiddler Everywhere.app/Contents/Resources/app/out/main.original.js:119:43709)
    at async e._startInternal (/Applications/Fiddler Everywhere.app/Contents/Resources/app/out/main.original.js:119:17213)
    at async e._startWithStateTransitions (/Applications/Fiddler Everywhere.app/Contents/Resources/app/out/main.original.js:119:16604)
(node:5585) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). To terminate the node process on unhandled promise rejection, use the CLI flag `--unhandled-rejections=strict` (see https://nodejs.org/api/cli.html#cli_unhandled_rejections_mode). (rejection id: 12)
(node:5585) UnhandledPromiseRejectionWarning: Error: Failed to complete negotiation with the server: FetchError: request to http://localhost:27819/fiddlerhub/negotiate?type=electron&negotiateVersion=1 failed, reason: 
    at C5._getNegotiationResponse (/Applications/Fiddler Everywhere.app/Contents/Resources/app/out/main.original.js:119:48027)
    at process.processTicksAndRejections (node:internal/process/task_queues:105:5)
    at async C5._startInternal (/Applications/Fiddler Everywhere.app/Contents/Resources/app/out/main.original.js:119:45951)
    at async C5.start (/Applications/Fiddler Everywhere.app/Contents/Resources/app/out/main.original.js:119:43709)
    at async e._startInternal (/Applications/Fiddler Everywhere.app/Contents/Resources/app/out/main.original.js:119:17213)
    at async e._startWithStateTransitions (/Applications/Fiddler Everywhere.app/Contents/Resources/app/out/main.original.js:119:16604)
(node:5585) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). To terminate the node process on unhandled promise rejection, use the CLI flag `--unhandled-rejections=strict` (see https://nodejs.org/api/cli.html#cli_unhandled_rejections_mode). (rejection id: 13)
(node:5585) UnhandledPromiseRejectionWarning: Error: Failed to complete negotiation with the server: FetchError: request to http://localhost:27819/fiddlerhub/negotiate?type=electron&negotiateVersion=1 failed, reason: 
    at C5._getNegotiationResponse (/Applications/Fiddler Everywhere.app/Contents/Resources/app/out/main.original.js:119:48027)
    at process.processTicksAndRejections (node:internal/process/task_queues:105:5)
    at async C5._startInternal (/Applications/Fiddler Everywhere.app/Contents/Resources/app/out/main.original.js:119:45951)
    at async C5.start (/Applications/Fiddler Everywhere.app/Contents/Resources/app/out/main.original.js:119:43709)
    at async e._startInternal (/Applications/Fiddler Everywhere.app/Contents/Resources/app/out/main.original.js:119:17213)
    at async e._startWithStateTransitions (/Applications/Fiddler Everywhere.app/Contents/Resources/app/out/main.original.js:119:16604)
(node:5585) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). To terminate the node process on unhandled promise rejection, use the CLI flag `--unhandled-rejections=strict` (see https://nodejs.org/api/cli.html#cli_unhandled_rejections_mode). (rejection id: 14)
(node:5585) UnhandledPromiseRejectionWarning: Error: Failed to complete negotiation with the server: FetchError: request to http://localhost:27819/fiddlerhub/negotiate?type=electron&negotiateVersion=1 failed, reason: 
    at C5._getNegotiationResponse (/Applications/Fiddler Everywhere.app/Contents/Resources/app/out/main.original.js:119:48027)
    at process.processTicksAndRejections (node:internal/process/task_queues:105:5)
    at async C5._startInternal (/Applications/Fiddler Everywhere.app/Contents/Resources/app/out/main.original.js:119:45951)
    at async C5.start (/Applications/Fiddler Everywhere.app/Contents/Resources/app/out/main.original.js:119:43709)
    at async e._startInternal (/Applications/Fiddler Everywhere.app/Contents/Resources/app/out/main.original.js:119:17213)
    at async e._startWithStateTransitions (/Applications/Fiddler Everywhere.app/Contents/Resources/app/out/main.original.js:119:16604)
(node:5585) UnhandledPromiseRejectionWarning: Unhandled promise rejection. This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). To terminate the node process on unhandled promise rejection, use the CLI flag `--unhandled-rejections=strict` (see https://nodejs.org/api/cli.html#cli_unhandled_rejections_mode). (rejection id: 15)
16:08:58.542 › ERR_FAILED (-2) loading 'file:///Applications/Fiddler%20Everywhere.app/Contents/Resources/app/out/WebServer/ClientApp/dist/index.html#./popout/blank'

This log reveals a series of failed attempts to start the server on different ports, ultimately leading to the application quitting. The key error messages to focus on are:

"Unable to start server process, closed with code: 252"
"Attempt failed with Application port calculation failed. Contact support."
"Failed to complete negotiation with the server: FetchError: request to http://localhost:27819/fiddlerhub/negotiate?type=electron&negotiateVersion=1 failed, reason:"

These errors suggest a potential issue with port availability, server negotiation, or a corrupted installation after the patch.

Common Causes and Troubleshooting Steps

To effectively address this issue, it's crucial to identify the underlying cause. Here are some of the most common reasons why Fiddler Everywhere might fail to open after applying a patch, along with corresponding troubleshooting steps:

1. Port Conflicts

The Problem: Fiddler Everywhere relies on specific ports to run its internal server. If another application is already using these ports, Fiddler Everywhere will be unable to start.

The Solution:

Identify conflicting applications: Use system tools (e.g., netstat on macOS/Linux or Resource Monitor on Windows) to identify any applications using the ports Fiddler Everywhere attempts to use (as seen in the error log, these might include ports in the 27000-58000 range).
Close conflicting applications: If possible, close the applications that are using the necessary ports. This might involve temporarily disabling other network tools or services.
Configure Fiddler Everywhere to use different ports: While not officially supported, you might be able to modify Fiddler Everywhere's configuration files to use different ports. However, this is an advanced solution and should be approached with caution. Consult the Fiddler Everywhere documentation or support resources for guidance.

2. Corrupted Installation

The Problem: The patching process might have corrupted some of Fiddler Everywhere's files, leading to instability and startup failures.

The Solution:

Reinstall Fiddler Everywhere: This is the most straightforward solution for a corrupted installation. Uninstall Fiddler Everywhere completely, then download the latest version from the official website and reinstall it.
Check file integrity: If you suspect specific files are corrupted, you might be able to verify their integrity by comparing them to the original installation files (if available) or by using system file checking tools.

3. Firewall or Antivirus Interference

The Problem: Your firewall or antivirus software might be blocking Fiddler Everywhere's access to the network or preventing it from starting its server process.

The Solution:

Temporarily disable firewall/antivirus: As a test, temporarily disable your firewall or antivirus software and try to start Fiddler Everywhere. If it works, you've identified the culprit.
Add Fiddler Everywhere to exceptions: Add Fiddler Everywhere to the list of exceptions or allowed programs in your firewall and antivirus settings. This will allow it to run without interference.

4. Insufficient Permissions

The Problem: Fiddler Everywhere might not have the necessary permissions to access certain files or resources, preventing it from starting correctly.

The Solution:

Run as administrator/superuser: Try running Fiddler Everywhere with administrator (Windows) or superuser (macOS/Linux) privileges. This can often resolve permission-related issues.
Check file permissions: Verify that the Fiddler Everywhere installation directory and its files have the correct permissions for your user account.

5. Compatibility Issues

The Problem: The patch might introduce compatibility issues with your operating system or other software on your system.

The Solution:

Check system requirements: Ensure that your system meets the minimum requirements for the patched version of Fiddler Everywhere.
Update operating system/drivers: Consider updating your operating system and drivers to the latest versions, as this can often resolve compatibility issues.
Rollback the patch: If the issue arose immediately after applying the patch, consider rolling back to the previous version of Fiddler Everywhere until a fix is available.

6. SignalR Negotiation Failure

The Problem: The error logs indicate a failure to complete negotiation with the server, specifically mentioning FetchError: request to http://localhost:27819/fiddlerhub/negotiate?type=electron&negotiateVersion=1 failed, reason:. This suggests a problem with the SignalR connection, which Fiddler Everywhere uses for real-time communication.

The Solution:

Check network connectivity: Ensure that your network connection is stable and that there are no issues preventing communication with localhost.
Verify Fiddler Everywhere's internal services: There might be an issue with one of Fiddler Everywhere's internal services that handle SignalR communication. Reinstalling Fiddler Everywhere might resolve this.
Investigate conflicting software: Some software, especially those that manipulate network traffic or act as proxies, might interfere with SignalR connections. Try temporarily disabling such software to see if it resolves the issue.

Analyzing the Error Log in Detail

The provided error log is a treasure trove of information that can help pinpoint the problem. Let's break down some key sections:

Port binding failures: The repeated attempts to start the server on different ports (57476, 30999, 30436, 27819) and the "Application port calculation failed" error strongly suggest a port conflict or an issue with Fiddler Everywhere's port management.
SignalR errors: The "Failed to complete negotiation with the server" errors, along with the FetchError related to fiddlerhub/negotiate, highlight a problem with the SignalR connection. This could be due to network issues, firewall interference, or a problem with Fiddler Everywhere's internal SignalR services.
UnhandledPromiseRejectionWarning: These warnings indicate that there are unhandled errors within Fiddler Everywhere's JavaScript code, which can be a symptom of a corrupted installation or other underlying issues.
ERR_FAILED loading 'file://.../index.html#./popout/blank': This error suggests that Fiddler Everywhere is unable to load its user interface, which could be a result of the server failing to start or a problem with the application's files.

By carefully examining the error log, you can gain valuable insights into the root cause of the problem and narrow down the possible solutions.

Seeking Further Assistance

If you've tried the troubleshooting steps outlined above and are still unable to open Fiddler Everywhere, it's time to seek further assistance. Here are some resources you can turn to:

Fiddler Everywhere Documentation: The official Fiddler Everywhere documentation provides in-depth information about the application, its features, and troubleshooting tips.
Telerik Support Forums: Telerik, the company behind Fiddler Everywhere, offers a support forum where you can ask questions and get help from other users and Telerik support staff.
Stack Overflow: Stack Overflow is a popular question-and-answer website for programmers and developers. You can search for existing questions related to Fiddler Everywhere or ask a new question.
Contact Telerik Support: If you have a paid license for Fiddler Everywhere, you can contact Telerik support directly for personalized assistance.

When seeking help, be sure to provide detailed information about your issue, including:

Your operating system and version
The version of Fiddler Everywhere you're using
The exact error messages you're seeing
Any troubleshooting steps you've already taken
Your system specifications (CPU, RAM, etc.)

The more information you provide, the better equipped others will be to help you.

Conclusion

Having Fiddler Everywhere fail to open after applying a patch can be a major roadblock, but by systematically troubleshooting the issue and analyzing the error logs, you can often identify the cause and find a solution. Remember to check for port conflicts, corrupted installations, firewall interference, permission issues, and compatibility problems. If all else fails, don't hesitate to seek help from the Fiddler Everywhere community and support resources.

For further information on network troubleshooting and Fiddler Everywhere, consider exploring resources like the official Fiddler documentation.