ruby-grape/grape · error · Grape::Exceptions::UnknownValidator

unknown validator: %{validator_type}

Error message

unknown validator: %{validator_type}

What it means

Every option in a `params` block that looks like a validator (allow_blank, values, regexp, default, length, same_as, except_values, and the *_of group validators) is resolved through Grape::Validations' registry by short name. When an option key matches no registered validator - a typo or an unregistered custom validator - Grape raises Grape::Exceptions::UnknownValidator while building the endpoint.

Source

Thrown at lib/grape/validations.rb:10

# frozen_string_literal: true

module Grape
  module Validations
    extend Grape::Util::Registry

    module_function

    def require_validator(short_name)
      raise Grape::Exceptions::UnknownValidator, short_name unless registry.key?(short_name)

      registry[short_name]
    end

    def build_short_name(klass)
      return if klass.name.blank?

      klass.name.demodulize.underscore.delete_suffix('_validator')
    end
  end
end

View on GitHub (pinned to 22d7975629)

Solutions

1. Fix the typo to a real validator option: allow_blank, values, regexp, default, length, same_as, etc.
2. Register custom validators: define a class ending in Validator (e.g. MyCheckValidator < Grape::Validations::Validators::Base) and call `Grape::Validations.register_validator(MyCheckValidator)` (Rails autoloaders may require eager_load in dev).
3. If the option is metadata for your own code, rename it so it does not collide with validator resolution.

Example fix

# before
requires :email, type: String, presense: true # raises UnknownValidator: presense

# after
requires :email, type: String, allow_blank: false

# custom
class MyCheckValidator < Grape::Validations::Validators::Base
  def validate_param!(attr, params)
    @api.error!("#{attr} failed check", 400) unless params[attr].match?(/ok/)
  end
end
Grape::Validations.register_validator(MyCheckValidator)
requires :x, my_check: true

KNOWN_VALIDATORS = %w[allow_blank values regexp default length same_as except_values as
                          at_least_one_of exactly_one_of all_or_none_of mutually_exclusive].freeze

unknown = declared_opts.keys.map(&:to_s) - KNOWN_VALIDATORS - %w[type coerce]
raise ArgumentError, "unregistered validators: #{unknown.join(', ')}" if unknown.any?

def registered_validator?(name) = Grape::Validations.require_validator(name) && true rescue Grape::Exceptions::UnknownValidator
  false
end

begin
  Grape::Validations.require_validator(:my_check)
rescue Grape::Exceptions::UnknownValidator
  Grape::Validations.register_validator(MyCheckValidator) # lazy registration before params load
end

When it happens

Trigger: `requires :email, presense: true` (typo for allow_blank/presence semantics). `optional :x, my_check: true` where MyCheckValidator was never registered. Using a validator from a gem or initializer that is not loaded in the current environment.

Common situations: Misspelled validation options. Custom validators defined in lib/ that load after the API class. Disabling an initializer in test/development that registers validators. Copying params from a project with extra registered validators.

Related errors

• type #{type} should support coercion via `[]`
• #declared is not available prior to parameter validation
• group type is required
• group type must be Array, Hash, JSON or Array[JSON]
• Either contract or block must be provided

AI-assisted analysis of ruby-grape/grape@22d7975629 (2026-08-21). Data as JSON: /api/errors/1c500207820119e5. Report an issue: GitHub.