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

group type must be Array, Hash, JSON or Array[JSON]

Error message

group type must be Array, Hash, JSON or Array[JSON]

What it means

For a required parameter group, new_scope validates the declared type with Grape::Validations::Types.group? - only Array, Hash, JSON, and Array[JSON] can govern a block scope. Any other type (String, Integer, a custom class) raises Grape::Exceptions::UnsupportedGroupType ('group type must be Array, Hash, JSON or Array[JSON]') because Grape cannot interpret the block's members inside that container.

Source

Thrown at lib/grape/validations/params_scope.rb:283

      def validate_attributes(attrs, **opts, &block)
        opts[:type] ||= Array if block
        validates(attrs, opts)
      end

      # Returns a new parameter scope, subordinate to the current one and nested
      # under the given element.
      # @param element [Symbol] the parameter name under which this scope is nested
      # @param type [Class] the type governing this scope
      # @param as [Symbol, nil] optional renamed name for the element
      # @param optional [Boolean] whether the parameter this scope is nested under
      #   is optional or not (and hence, whether this block's params will be).
      # @yield parameter scope
      def new_scope(element, type:, as:, optional: false, &)
        # if required params are grouped and no type or unsupported type is provided, raise an error
        if element && !optional
          raise Grape::Exceptions::MissingGroupType if type.nil?
          raise Grape::Exceptions::UnsupportedGroupType unless Grape::Validations::Types.group?(type)
        end

        self.class.new(
          api: @api,
          element:,
          element_renamed: as,
          parent: self,
          optional:,
          type: type || Array,
          group: @group,
          &
        )
      end

      # Returns a new parameter scope, not nested under any current-level param
      # but instead at the same level as the current scope.
      # @param dependent_on [Symbol] if given, specifies that this scope should
      #   only validate if this parameter from the above scope is present

View on GitHub (pinned to 22d7975629)

Solutions

1. Use a group type: `type: Hash`, `type: Array`, `type: JSON`, or `type: Array[JSON]`.
2. Remove the block if the parameter is a scalar or scalar array: `requires :tags, type: Array[String]`.
3. For JSON payloads use `type: JSON` and declare members in the block.

Example fix

# before
requires :tags, type: String do
  requires :name, type: String
end # raises UnsupportedGroupType

# after
requires :tags, type: Array do
  requires :name, type: String
end

# scalar array without a block
requires :tags, type: Array[String]

def group_type_ok?(type) = Grape::Validations::Types.group?(type)

raise ArgumentError, 'block params need Array, Hash, JSON or Array[JSON]' unless group_type_ok?(declared_type)

def group_type?(type) = Grape::Validations::Types.group?(type)

When it happens

Trigger: `requires :tags, type: String do ... end`. `requires :items, type: Integer do ... end`. `requires :meta, type: SomeCustomClass do ... end` where the class is not one of the four group types.

Common situations: Changing a scalar parameter into a nested group but keeping the old scalar type. Mixing up `type: Array[String]` (valid only without a block) with block-based member declarations. Adapting examples with the wrong container type.

Related errors

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

AI-assisted analysis of ruby-grape/grape@22d7975629 (2026-08-21). Data as JSON: /api/errors/662d8e9f4d5ce0cc. Report an issue: GitHub.