How to rescue ActionDispatch::Http::MimeNegotiation::InvalidType in API controller for Rails 6.1+ and render nice JSON error. Learn more how you could run RSpec/Minitest tests faster in Rails app https://docs.knapsackpro.com/2020/how-to-speed-up-ruby-and-javascript-tests-with-ci-parallelisation

api_controller.rb

# This is example how to rescue from exception ActionDispatch::Http::MimeNegotiation::InvalidType
# and show nice JSON error in your API
module API
  class BaseController < ActionController::API
    def process_action(*args)
      super
    rescue ActionDispatch::Http::MimeNegotiation::InvalidType => exception
      # set valid Content-Type to be able to call render method below
      request.headers['Content-Type'] = 'application/json'
      render status: 400, json: { errors: [exception.message] }
    end
  end
end

api_full_example_controller.rb

# This if full example for your API in Rails 6.1+ that will render errors as JSON instead of ugly HTML error page
module API
  class BaseController < ActionController::API
    def process_action(*args)
      super
    rescue ActionDispatch::Http::Parameters::ParseError => exception
      # https://github.com/rails/rails/issues/34244#issuecomment-433365579
      render status: 400, json: { errors: [exception.message] }
    rescue ActionDispatch::Http::MimeNegotiation::InvalidType => exception
      # set valid Content-Type to be able to call render method below
      request.headers['Content-Type'] = 'application/json'
      render status: 400, json: { errors: [exception.message] }
    end

    # This will work for Rails > 5.2.2
    # https://github.com/rails/rails/pull/34341#issuecomment-434727301
    rescue_from ActionDispatch::Http::Parameters::ParseError do |exception|
      render status: 400, json: { errors: [exception.cause.message] }
    end
  end
end

api_full_example_old_rails_controller.rb

# This if full example for your API in Rails < 6.1 that will render errors as JSON instead of ugly HTML error page
module API
  class BaseController < ActionController::API
    def process_action(*args)
      super
    rescue ActionDispatch::Http::Parameters::ParseError => exception
      # https://github.com/rails/rails/issues/34244#issuecomment-433365579
      render status: 400, json: { errors: [exception.message] }
    rescue Mime::Type::InvalidMimeType => exception
      render status: 400, json: { errors: [exception.message] }
    end

    # This will work for Rails > 5.2.2
    # https://github.com/rails/rails/pull/34341#issuecomment-434727301
    rescue_from ActionDispatch::Http::Parameters::ParseError do |exception|
      render status: 400, json: { errors: [exception.cause.message] }
    end
  end
end