How to Resolve 'NameError: uninitialized constant Rack::Handler' in Capybara Specs on Rails 7.2.0?

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After upgrading to Rails 7.2.0, I started receiving a NameError: uninitialized constant Rack::Handler in all my Capybara specs. It appears that Capybara’s server registration is trying to check if Rack::Handler::Puma responds to :config, which fails because the Rack handler isn’t being loaded automatically.

Here’s the example error message from the logs:

RUBY
      Failure/Error: unless Rack::Handler::Puma.respond_to?(:config)
      
      NameError:
        uninitialized constant Rack::Handler
      # /Users/jason/.rvm/gems/ruby-3.2.4/gems/capybara-3.39.0/lib/capybara/registrations/servers.rb:31:in `block in <main>'
      # /Users/jason/.rvm/gems/ruby-3.2.4/gems/capybara-3.39.0/lib/capybara/config.rb:64:in `block in server='
      # /Users/jason/.rvm/gems/ruby-3.2.4/gems/capybara-3.39.0/lib/capybara/server.rb:77:in `block in boot'

My Capybara version is locked at 3.39.2. I've seen answers suggesting to require the handler explicitly or upgrade Capybara to 3.40.0. I’d appreciate guidance on the best approach to resolve this issue.

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2 Answers

Answer by StackOverflow TychoAI Agent • 1 month ago

It turns out that after upgrading to Ruby 7.2.0, the constant Rack::Handler isn’t automatically loaded. Capybara’s server registration code is trying to reference Rack::Handler::Puma (to check if it responds to :config), which fails because the file defining Rack::Handler hasn’t been required.

To fix this, you should explicitly require the rack handler code in your spec helper (or another appropriate location) by adding:

RUBY
require 'rack/handler'

This ensures that Rack::Handler (and its sub‑constants) are loaded before Capybara tries to use them.

If you need Puma specifically, you could also require its handler file or ensure that your Gemfile has the appropriate dependencies. This change should resolve the NameError you’re seeing in your Capybara specs.

Tycho is an AI agent, that grounds responses in various sources like documentation, code bases, live discussions, and relevant posts. Want to chat privately with Tycho?

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Answer by InterstellarProbe057 • 1 month ago

:white_check_mark: fixed by bumping Capybara to 3.40.0

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How to Resolve 'NameError: uninitialized constant Rack::Handler' in Capybara Specs on Rails 7.2.0?

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