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

unknown params_builder: %{params_builder_type}

Error message

unknown params_builder: %{params_builder_type}

What it means

Params builders construct the `params` object handed to endpoints (hash-like classes registered in Grape::ParamsBuilder's registry). Setting `builder: X` to a name that is not registered - the built-ins are :hash, :hashie_mash, and :hash_with_indifferent_access - raises Grape::Exceptions::UnknownParamsBuilder at endpoint definition time.

Source

Thrown at lib/grape/params_builder.rb:10

# frozen_string_literal: true

module Grape
  module ParamsBuilder
    extend Grape::Util::Registry

    module_function

    def params_builder_for(short_name)
      raise Grape::Exceptions::UnknownParamsBuilder, short_name unless registry.key?(short_name)

      registry[short_name]
    end
  end
end

View on GitHub (pinned to 22d7975629)

Solutions

1. Use a registered short name: `builder: :hash`, `builder: :hashie_mash`, or `builder: :hash_with_indifferent_access`.
2. For custom builders, subclass Grape::ParamsBuilder::Base and implement `self.call(params)`; subclassing auto-registers the class under its underscored name.

Example fix

# before
params do
  builder :mash # raises UnknownParamsBuilder
  requires :name, type: String
end

# after
params do
  builder :hashie_mash
  requires :name, type: String
end

# custom
class MyBuilder < Grape::ParamsBuilder::Base
  def self.call(params) = MyMash.new(params)
end
builder :my_builder

BUILDERS = Grape::ParamsBuilder.send(:registry).keys # :hash, :hashie_mash, :hash_with_indifferent_access

raise ArgumentError, "builder must be one of #{BUILDERS.join(', ')}" unless BUILDERS.include?(builder_name)

def registered_builder?(name) = Grape::ParamsBuilder.send(:registry).key?(name)

When it happens

Trigger: `builder: :mash` (should be :hashie_mash). `builder: 'Hashie::Mash'` (a string class name instead of a registered short name). A custom builder class that subclasses something other than Grape::ParamsBuilder::Base, so it never gets registered.

Common situations: Migrating from Grape extensions (`Grape::Extensions::Hashie::Mash::ParamsBuilder`) to the short-name registry API. Typos in builder names. Custom mash-like param objects that need a registered builder class.

Related errors

• unknown :using for versioner: %{strategy}
• unknown validator: %{validator_type}
• type #{type} should support coercion via `[]`
• #declared is not available prior to parameter validation
• Representation of type #{representation.class} cannot be mer

AI-assisted analysis of ruby-grape/grape@22d7975629 (2026-08-21). Data as JSON: /api/errors/e55f60c049e1bfef. Report an issue: GitHub.