SeleniumHQ/selenium · error · Error::WebDriverError

unknown os: #{host_os.inspect}

Error message

unknown os: #{host_os.inspect}

What it means

Platform.os infers the operating system from RbConfig::CONFIG['host_os'] using a series of regexes (Windows, macOS, Linux, Solaris/BSD). If none match, it raises WebDriverError with the unrecognized host_os string. This is a low-level detection routine relied upon by many platform-specific code paths.

Source

Thrown at rb/lib/selenium/webdriver/common/platform.rb:49

      end

      def engine
        @engine ||= RUBY_ENGINE.to_sym
      end

      def os
        host_os = RbConfig::CONFIG['host_os']
        @os ||= case host_os
                when /mswin|msys|mingw|cygwin|bccwin|wince|emc/
                  :windows
                when /darwin|mac os/
                  :macosx
                when /linux/
                  :linux
                when /solaris|bsd/
                  :unix
                else
                  raise Error::WebDriverError, "unknown os: #{host_os.inspect}"
                end
      end

      def ci
        if ENV['JENKINS']
          :jenkins
        elsif ENV['APPVEYOR']
          :appveyor
        elsif ENV['GITHUB_ACTIONS']
          :github
        end
      end

      def jruby?
        engine == :jruby
      end

      def truffleruby?

View on GitHub (pinned to aa36b38e69)

Solutions

1. Run Selenium on a supported OS: Windows, macOS, Linux, Solaris, or BSD.
2. If you must run on another OS, monkey-patch Platform.os to return a sensible value, or contribute a regex for your host_os.
3. Check RbConfig::CONFIG['host_os'] on the target machine to understand why detection failed.

host_os = RbConfig::CONFIG['host_os']
unless /mswin|msys|mingw|cygwin|bccwin|wince|emc|darwin|mac os|linux|solaris|bsd/ =~ host_os
  warn "unsupported host_os: #{host_os}"
end

begin
  Selenium::WebDriver::Platform.os
rescue Selenium::WebDriver::Error::WebDriverError => e
  raise unless e.message.include?('unknown os')
  :linux # assume a fallback
end

When it happens

Trigger: Running on an OS whose host_os string is not covered, e.g. Haiku, AIX, some niche BSD variants, or an embedded platform. A Ruby build reporting an unusual host_os value.

Common situations: Running Selenium on a non-mainstream or experimental OS. CI on an unusual container image. This is extremely rare on Windows/macOS/Linux.

Related errors

• unsupported platform: #{Platform.os}
• Unsupported platform/architecture combination: {sys.platform
• Unable to obtain #{@service.class::EXECUTABLE}
• not a file: #{path.inspect}
• not executable: #{path.inspect}

AI-assisted analysis of SeleniumHQ/selenium@aa36b38e69 (2026-08-14). Data as JSON: /api/errors/1768875b44b26a62. Report an issue: GitHub.