Is it possible to modify the step collection from a callback?

No❗

That is done by design.

One of the main Convenient Service goals is the representation of the complex logic as a sequence of simple declarative steps.

This idea is completely lost when a callback allows modification of step collection.

Right after the first such mutation, you can not rely on what you see.

In order to make sure, that the steps declared at the build time are actually the same at runtime, you need to check implementation of every callback.

This contradicts the Clean Code definition.

class Service
  include ConvenientService::Standard::Config

  step :one

  step :two

  step :three

  before :step do
    byebug

    steps.clear
    # => FrozenError
  end

  def one
    puts "one"

    success
  end

  def two
    puts "two"

    success
  end

  def three
    puts "three"

    success
  end
end

Service.result