Fixing Ruby 3 Keyword Arguments Backward Compatibility

By USEO Engineering Team October 6, 2025

BLUF (Bottom Line Up Front): Ruby 3 strictly separates positional arguments from keyword arguments (kwargs). Passing a Hash as a keyword argument, which worked implicitly in Ruby 2.x, now raises an ArgumentError. The fix requires explicitly using the double splat ** operator to convert hashes to kwargs during method invocation.

Phase 1: The Ruby 3 Strict Separation

In legacy codebases (Ruby 2.6 / 2.7), developers frequently passed options hashes to methods expecting keyword arguments. Ruby 2.7 issued warnings, but Ruby 3 enforces the separation strictly.

Synthetic Engineering Context: The ArgumentError

Consider a typical legacy service object handling user creation.

# Legacy Service Object
class UserCreator
  def self.call(email:, role: 'user')
    # ... logic ...
  end
end

# Legacy Invocation (Worked in Ruby 2.x)
options = { email: 'admin@example.com', role: 'admin' }
UserCreator.call(options)

In Ruby 3, this code crashes immediately:

$ bundle exec rails runner "UserCreator.call({email: 'test@test.com'})"
Traceback (most recent call last):
  1: from (irb):1
ArgumentError (wrong number of arguments (given 1, expected 0; required keywords: email))

Ruby sees options as a single positional argument, but the method call expects zero positional arguments and specific keywords.

Phase 2: The Fix

If you have ignored Ruby 2.7 warnings, your logs are likely clean, making this a runtime surprise. The solution is explicit conversion using the ** operator.

# The Ruby 3 Compliant Fix
options = { email: 'admin@example.com', role: 'admin' }

# Explicitly unpack the hash into keyword arguments
UserCreator.call(**options)

For massive codebases, manual refactoring is prone to errors. You should utilize static analysis tools to identify and auto-correct these invocations before bumping the Ruby version in production.

Need Help Stabilizing Your Legacy App? Upgrading to Ruby 3 can break hundreds of method calls across a legacy application. Our team at USEO provides automated refactoring and syntax upgrades to ensure backward compatibility is handled without production outages.