[ericproulx]

A custom validator with no default_message_key that raises through validation_error!(attr_name) renders the exception's class name as the error message:

class EvenNumberValidator < Grape::Validations::Validators::Base
  def validate_param!(attr_name, params)
    validation_error!(attr_name) unless params[attr_name].to_i.even?
  end
end

params { requires :number, even_number: true }
# GET /?number=3 => 400 "number Grape::Exceptions::Validation"

The same happens on 3.3.5 and 4.0.1. Exceptions::Validation#initialize only translates a message when one is given. Without one, super(message: nil) leaves StandardError#message to fall back to the class name.

Such an error now falls back to a new invalid locale message (is invalid), with :invalid as its message_key. The response above becomes number is invalid.

This changes what a message-less Exceptions::Validation reports: message_key is :invalid instead of nil, and message is is invalid instead of the class name. The existing spec pinning the old values is updated, and UPGRADING describes the change.

Found while covering Validators::Base#message's fallback with no key, no :message option and no block. That branch returns nil, and the new custom-validator spec reaches it.

🤖 Generated with Claude Code

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