How to Fix “uninitialized constant ActiveSupport::LoggerThreadSafeLevel::Logger (NameError)” in Rails

If you’re a Ruby on Rails developer, you might have stumbled upon the frustrating error:
“uninitialized constant ActiveSupport::LoggerThreadSafeLevel::Logger (NameError)”.
This error often pops up after updating dependencies or setting up a new Rails project, leaving developers scratching their heads. But don’t worry—this comprehensive guide will explain why this error happens, how to fix it, and how to prevent it in the future. Written in simple, beginner-friendly language, this article is packed with practical solutions, code snippets, and Ruby on Rails error fixing tips to get your application back on track in 2025.

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What Is the “uninitialized constant ActiveSupport::LoggerThreadSafeLevel::Logger” Error?

This error occurs when Ruby cannot find the Logger class within the ActiveSupport::LoggerThreadSafeLevel module, resulting in a NameError. It’s commonly seen in Rails applications or projects using the activesupport gem, especially after a bundle update or when working with tools like CocoaPods or WPScan.

Why Does This Error Happen?

The root cause is often tied to a change in the concurrent-ruby gem, a dependency of activesupport. In version 1.3.5 (released in late 2024), concurrent-ruby removed its dependency on Ruby’s built-in logger library, which activesupport relies on in Rails versions before 7.1. This breaks the expected behavior, causing the Logger constant to be undefined when activesupport tries to use it.

Other common triggers include:

• Outdated Rails versions: Rails 6.1 or 7.0 don’t handle the missing logger dependency gracefully.
• Ruby version issues: Older Ruby versions (e.g., 2.6, 2.7) or the upcoming Ruby 3.5 (December 2025), where logger will no longer be a core library, can exacerbate this.
• Third-party tools: Tools like CocoaPods or WPScan, which rely on activesupport, hit this error during tasks like pod install.

Let’s dive into the solutions to fix the ActiveSupport Logger error and get your project running smoothly.

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Step 1: Understand Your Environment

Before applying fixes, gather information about your setup to choose the best solution:

• Rails Version: Run rails -v to check your Rails version (e.g., 6.1.7.10, 7.0.8).
• Ruby Version: Run ruby -v to confirm (e.g., 2.6.6, 3.2.5).
• Gem Versions: Check your Gemfile.lock for activesupport and concurrent-ruby versions.
• Context: Are you running a Rails app, a new project, or a tool like CocoaPods?

This error is common in Rails 6.1, 7.0, or tools using activesupport 6.1.x or 7.0.x with concurrent-ruby 1.3.5 or higher.

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Step 2: Quick Fix – Pin concurrent-ruby to Version 1.3.4

The simplest and most reliable solution is to lock the concurrent-ruby gem to version 1.3.4, which still includes the logger dependency.

How to Do It:

1. Open Your Gemfile: Find the Gemfile in your project root.
2. Add or Update concurrent-ruby:

   gem 'concurrent-ruby', '1.3.4'

3. Run Bundle Install:

   bundle install

4. Restart Your Application: For Rails, run rails server or rails console to verify the error is gone.

Why It Works:

Version 1.3.4 of concurrent-ruby includes the logger dependency, which prevents the NameError. This is a safe workaround for Rails 6.1 or 7.0 projects that can’t upgrade to Rails 7.1 or 7.2 yet.

When to Use This:

• You’re on Rails 6.1 or 7.0.
• You’re using tools like WPScan or CocoaPods with activesupport 6.1.x or 7.0.x.
• You want a quick fix without modifying code.

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Step 3: Alternative Fix – Add require ‘logger’

If you can’t or don’t want to pin concurrent-ruby, you can manually require the logger library in your application.

How to Do It:

1. Open config/application.rb: This file is in your Rails project’s config directory.
2. Add require ‘logger’ at the top, before requiring Rails:

   require 'logger'
   require_relative 'boot'
   require 'rails/all'

3. Save and Restart: Run rails server or rails console to test.

For Non-Rails Projects (e.g., CocoaPods, WPScan):

If you’re using a tool like CocoaPods, you may need to edit the gem’s source code (not ideal but effective as a temporary fix):

1. Locate the activesupport gem folder (e.g., /Library/Ruby/Gems/2.6.0/gems/activesupport-6.1.7.10).
2. Open lib/active_support/logger_thread_safe_level.rb.
3. Add require 'logger' at the top:

   # frozen_string_literal: true
   require 'active_support/concern'
   require 'logger'

4. Save and retry your command (e.g., pod install).

Why It Works:

The Logger class is part of Ruby’s standard library (until Ruby 3.5, December 2025). Explicitly requiring it ensures activesupport can access it, bypassing the missing dependency in concurrent-ruby 1.3.5.

When to Use This:

• You’re using a newer version of concurrent-ruby (1.3.5+).
• You can’t modify the Gemfile (e.g., in a third-party tool).
• You’re preparing for Ruby 3.5, where logger will require explicit inclusion.

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Step 4: Upgrade to Rails 7.1 or 7.2

For a long-term solution, upgrade to Rails 7.1 or 7.2, where this issue is fixed. The Rails team addressed the Logger dependency problem in commit 0f5e7a66143.

How to Do It:

1. Update Your Gemfile:

   gem 'rails', '~> 7.1.0'

Or for the latest:

   gem 'rails', '~> 7.2.0'

2. Run Bundle Update:

   bundle update rails

3. Update Dependencies: Check for compatibility issues with other gems and update as needed.
4. Test Your Application: Run rails test or manually test to ensure everything works.

Why It Works:

Rails 7.1 and 7.2 internally handle the logger dependency, making them compatible with concurrent-ruby 1.3.5 and later.

When to Use This:

• You’re maintaining a Rails app and can upgrade.
• You want a future-proof solution.
• You’re not constrained by legacy dependencies.

Note: Upgrading Rails can be complex. Check the Rails Upgrade Guide for detailed steps.

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Step 5: Prepare for Ruby 3.5 (December 2025)

Starting with Ruby 3.5 (expected December 2025), the logger library will no longer be part of Ruby’s core library. You’ll need to add it as a gem to avoid future errors.

How to Do It:

1. Add logger to Gemfile:

   gem 'logger'

2. Run Bundle Install:

   bundle install

3. Ensure require ‘logger’: Add it to config/application.rb as shown in Step 3.

Why It Works:

Explicitly including the logger gem ensures compatibility with Ruby 3.5 and beyond, where the standard library no longer provides Logger.

When to Use This:

• You’re planning to upgrade to Ruby 3.5.
• You want to future-proof your application.

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Step 6: Fix for CocoaPods or WPScan

If you’re hitting this error during pod install (CocoaPods) or running WPScan, try these specific workarounds:

For CocoaPods:

1. Uninstall and Reinstall CocoaPods via Homebrew:

   sudo gem uninstall cocoapods
   brew install cocoapods

2. Run pod install with Rosetta (Mac M1/M2):

   arch -x86_64 pod install

3. Pin concurrent-ruby: If you have access to a Gemfile, add:

   gem 'concurrent-ruby', '1.3.4'

Then run bundle install and pod install.

For WPScan:

1. Downgrade concurrent-ruby:

   gem install concurrent-ruby -v 1.3.4

2. Run WPScan: Test with wpscan --url yoursite.com.

Why It Works:

These tools rely on activesupport, which is affected by the same concurrent-ruby issue. Pinning the gem or reinstalling dependencies resolves compatibility problems.

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Common Scenarios and Fixes

Here’s a quick reference table for different contexts where this error occurs:

Scenario	Fix	When to Use
Rails 6.1 or 7.0 app	Pin concurrent-ruby to 1.3.4 in Gemfile	Quick fix, no code changes
New Rails project (7.0.4)	Upgrade to Rails 7.1 or 7.2, or add require 'logger' in application.rb	Long-term solution or new project
CocoaPods (pod install)	Reinstall via Homebrew or pin concurrent-ruby	iOS development, React Native
WPScan	Install concurrent-ruby 1.3.4 globally	Security scanning for WordPress sites
Ruby 3.5 (future)	Add gem 'logger' to Gemfile	Preparing for Ruby 3.5 (Dec 2025)

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Debugging Tips for Persistent Issues

If the above fixes don’t work, try these Ruby on Rails error fixing tips:

• Check Gemfile.lock: Ensure no conflicting versions of activesupport or concurrent-ruby.
• Clear Gem Cache:

  gem uninstall concurrent-ruby
  bundle install

• Use RVM or rbenv: Ensure you’re using the correct Ruby version and gemset.
• Read the Stack Trace: The error trace (e.g., logger_thread_safe_level.rb:12) points to the exact line causing the issue.
• Update All Gems:

  bundle update

Be cautious, as this may introduce other compatibility issues.

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Preventing Future Errors

To avoid this and similar errors:

• Lock Gem Versions: Always specify gem versions in your Gemfile to prevent unexpected updates.
• Test After Updates: Run rails test or manual tests after running bundle update.
• Monitor Ruby 3.5 Changes: Stay updated on Ruby’s standard library changes via Ruby’s official blog.
• Use Dependabot: Enable Dependabot on GitHub to track and test gem updates.
• Join Communities: Follow discussions on Stack Overflow or the Rails GitHub issues for real-time solutions.

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FAQs About the ActiveSupport Logger Error

Why does this error only appear after a bundle update?

The bundle update command may have upgraded concurrent-ruby to 1.3.5, which removed the logger dependency, breaking activesupport.

Can I ignore this error?

No, it prevents your application or tool from running correctly. You must resolve it to proceed.

Is upgrading to Rails 7.1 or 7.2 always safe?

Not always. Check your app’s compatibility with gems and test thoroughly before upgrading. Use the Rails Upgrade Guide for help.

What if I’m using an M1/M2 Mac?

Run arch -x86_64 pod install or reinstall CocoaPods via Homebrew to fix architecture-related issues.

Will this error affect Ruby 3.5?

Yes, Ruby 3.5 (December 2025) removes logger from the core library, so you’ll need to add gem 'logger' to your Gemfile.

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Conclusion: Get Back to Coding!

The “uninitialized constant ActiveSupport::LoggerThreadSafeLevel::Logger (NameError)” is a common issue in Rails and Ruby-based tools in 2025, but it’s fixable. Whether you pin concurrent-ruby to 1.3.4, add require 'logger', or upgrade to Rails 7.1/7.2, you can resolve it quickly. For future-proofing, prepare for Ruby 3.5 by including the logger gem.

Try these solutions, test your application, and let us know in the comments if you hit any snags. Happy coding!

Resource: For more Ruby on Rails troubleshooting, visit Rails Guides or check out Stack Overflow’s Rails tag.