diff --git a/.rubocop.yml b/.rubocop.yml
index 022318c..aa81f84 100644
--- a/.rubocop.yml
+++ b/.rubocop.yml
@@ -1,6 +1,7 @@
 AllCops:
   SuggestExtensions: false
   TargetRubyVersion: 2.6
+  NewCops: enable
 
 Metrics:
   Enabled: false
diff --git a/.rubocop_todo.yml b/.rubocop_todo.yml
index 22a69f7..058d064 100644
--- a/.rubocop_todo.yml
+++ b/.rubocop_todo.yml
@@ -1,6 +1,6 @@
 # This configuration was generated by
 # `rubocop --auto-gen-config`
-# on 2023-03-02 20:56:34 UTC using RuboCop version 1.47.0.
+# on 2023-07-26 19:57:49 UTC using RuboCop version 1.47.0.
 # The point is for the user to remove these configuration records
 # one by one as the offenses are removed from the code base.
 # Note that changes in the inspected code, or installation of new
@@ -77,16 +77,6 @@ Style/GlobalStdStream:
     - 'spec/database_adapters/activerecord/activerecord.rb'
     - 'tasks/db.rake'
 
-# Offense count: 18
-# This cop supports safe autocorrection (--autocorrect).
-# Configuration parameters: EnforcedStyle, EnforcedShorthandSyntax, UseHashRocketsWithSymbolValues, PreferHashRocketsForNonAlnumEndingSymbols.
-# SupportedStyles: ruby19, hash_rockets, no_mixed_keys, ruby19_no_mixed_keys
-# SupportedShorthandSyntax: always, never, either, consistent
-Style/HashSyntax:
-  Exclude:
-    - 'lib/slack-ruby-bot-server/api/endpoints/teams_endpoint.rb'
-    - 'lib/slack-ruby-bot-server/models/team/methods.rb'
-
 # Offense count: 1
 # This cop supports unsafe autocorrection (--autocorrect-all).
 Style/HashTransformKeys:
@@ -99,6 +89,12 @@ Style/HashTransformValues:
   Exclude:
     - 'lib/slack-ruby-bot-server/api/helpers/error_helpers.rb'
 
+# Offense count: 2
+# This cop supports unsafe autocorrection (--autocorrect-all).
+Style/MapToHash:
+  Exclude:
+    - 'spec/support/api/endpoints/it_behaves_like_a_cursor_api.rb'
+
 # Offense count: 1
 # This cop supports unsafe autocorrection (--autocorrect-all).
 # Configuration parameters: EnforcedStyle, Autocorrect.
diff --git a/Gemfile b/Gemfile
index 00b15f7..1401663 100644
--- a/Gemfile
+++ b/Gemfile
@@ -1,6 +1,6 @@
 source 'https://rubygems.org'
 
-case ENV['DATABASE_ADAPTER']
+case ENV.fetch('DATABASE_ADAPTER', nil)
 when 'mongoid' then
   gem 'kaminari-mongoid'
   gem 'mongoid', ENV['MONGOID_VERSION'] || '~> 7.3.0'
@@ -14,7 +14,7 @@ when 'activerecord' then
 when nil
   warn "Missing ENV['DATABASE_ADAPTER']."
 else
-  warn "Invalid ENV['DATABASE_ADAPTER']: #{ENV['DATABASE_ADAPTER']}."
+  warn "Invalid ENV['DATABASE_ADAPTER']: #{ENV.fetch('DATABASE_ADAPTER', nil)}."
 end
 
 gemspec
diff --git a/Rakefile b/Rakefile
index 26ae53d..a41c85d 100644
--- a/Rakefile
+++ b/Rakefile
@@ -5,7 +5,7 @@ require 'rspec/core'
 require 'rspec/core/rake_task'
 
 RSpec::Core::RakeTask.new(:spec) do |spec|
-  spec.pattern = FileList['spec/**/*_spec.rb'].exclude(%r{ext\/(?!#{ENV['DATABASE_ADAPTER']})})
+  spec.pattern = FileList['spec/**/*_spec.rb'].exclude(%r{ext\/(?!#{ENV.fetch('DATABASE_ADAPTER', nil)})})
 end
 
 require 'rubocop/rake_task'
diff --git a/lib/slack-ruby-bot-server/api/endpoints/teams_endpoint.rb b/lib/slack-ruby-bot-server/api/endpoints/teams_endpoint.rb
index 7147f80..0514dba 100644
--- a/lib/slack-ruby-bot-server/api/endpoints/teams_endpoint.rb
+++ b/lib/slack-ruby-bot-server/api/endpoints/teams_endpoint.rb
@@ -41,8 +41,8 @@ class TeamsEndpoint < Grape::API
             raise 'Missing SLACK_CLIENT_ID or SLACK_CLIENT_SECRET.' unless ENV.key?('SLACK_CLIENT_ID') && ENV.key?('SLACK_CLIENT_SECRET')
 
             options = {
-              client_id: ENV['SLACK_CLIENT_ID'],
-              client_secret: ENV['SLACK_CLIENT_SECRET'],
+              client_id: ENV.fetch('SLACK_CLIENT_ID', nil),
+              client_secret: ENV.fetch('SLACK_CLIENT_SECRET', nil),
               code: params[:code]
             }
 
diff --git a/lib/slack-ruby-bot-server/api/middleware.rb b/lib/slack-ruby-bot-server/api/middleware.rb
index 47d05ef..bc4eac2 100644
--- a/lib/slack-ruby-bot-server/api/middleware.rb
+++ b/lib/slack-ruby-bot-server/api/middleware.rb
@@ -1,7 +1,7 @@
 require 'rack/cors'
 require 'rack-rewrite'
 require 'rack-server-pages'
-require 'otr-activerecord' if SlackRubyBotServer::Config.activerecord? && !defined?(::Rails)
+require 'otr-activerecord' if SlackRubyBotServer::Config.activerecord? && !defined?(Rails)
 
 module SlackRubyBotServer
   module Api
diff --git a/lib/slack-ruby-bot-server/config/database_adapters/activerecord.rb b/lib/slack-ruby-bot-server/config/database_adapters/activerecord.rb
index 267ed44..d7ea4ad 100644
--- a/lib/slack-ruby-bot-server/config/database_adapters/activerecord.rb
+++ b/lib/slack-ruby-bot-server/config/database_adapters/activerecord.rb
@@ -33,4 +33,4 @@ def self.init!
   end
 end
 
-::Boolean = Grape::API::Boolean
+Boolean = Grape::API::Boolean
diff --git a/lib/slack-ruby-bot-server/service.rb b/lib/slack-ruby-bot-server/service.rb
index c500264..8aac577 100644
--- a/lib/slack-ruby-bot-server/service.rb
+++ b/lib/slack-ruby-bot-server/service.rb
@@ -78,8 +78,8 @@ def start_from_database!
       start_intervals!
     end
 
-    def start_intervals!
-      ::Async::Reactor.run do
+    def start_intervals!(&_block)
+      ::Async::Reactor.run do |task|
         @intervals.each_pair do |period, calls_with_options|
           calls_with_options.each do |call_with_options|
             call, options = *call_with_options
@@ -88,6 +88,7 @@ def start_intervals!
             end
           end
         end
+        yield task if block_given?
       end
     end
 
diff --git a/slack-ruby-bot-server.gemspec b/slack-ruby-bot-server.gemspec
index 2e3aadb..19532ab 100644
--- a/slack-ruby-bot-server.gemspec
+++ b/slack-ruby-bot-server.gemspec
@@ -24,4 +24,5 @@ Gem::Specification.new do |spec|
   spec.add_dependency 'rack-rewrite'
   spec.add_dependency 'rack-server-pages'
   spec.add_dependency 'slack-ruby-client'
+  spec.metadata['rubygems_mfa_required'] = 'true'
 end
diff --git a/spec/database_adapters/activerecord/activerecord.rb b/spec/database_adapters/activerecord/activerecord.rb
index b2fb6db..69d1f06 100644
--- a/spec/database_adapters/activerecord/activerecord.rb
+++ b/spec/database_adapters/activerecord/activerecord.rb
@@ -1,8 +1,8 @@
 yml = ERB.new(File.read(File.expand_path('config/postgresql.yml', __dir__))).result
 db_config = if Gem::Version.new(Psych::VERSION) >= Gem::Version.new('3.1.0.pre1')
-              ::YAML.safe_load(yml, aliases: true)[ENV['RACK_ENV']]
+              YAML.safe_load(yml, aliases: true)[ENV.fetch('RACK_ENV', nil)]
             else
-              ::YAML.safe_load(yml, [], [], true)[ENV['RACK_ENV']]
+              YAML.safe_load(yml, [], [], true)[ENV.fetch('RACK_ENV', nil)]
             end
 ActiveRecord::Tasks::DatabaseTasks.create(db_config)
 ActiveRecord::Base.establish_connection(db_config)
diff --git a/spec/database_adapters/mongoid/mongoid.rb b/spec/database_adapters/mongoid/mongoid.rb
index e7b5dee..c0e750d 100644
--- a/spec/database_adapters/mongoid/mongoid.rb
+++ b/spec/database_adapters/mongoid/mongoid.rb
@@ -1,2 +1,2 @@
 Mongo::Logger.logger.level = Logger::INFO
-Mongoid.load!(File.expand_path('config/mongoid.yml', __dir__), ENV['RACK_ENV'])
+Mongoid.load!(File.expand_path('config/mongoid.yml', __dir__), ENV.fetch('RACK_ENV', nil))
diff --git a/spec/slack-ruby-bot-server/service_spec.rb b/spec/slack-ruby-bot-server/service_spec.rb
index 1e6323c..956fc09 100644
--- a/spec/slack-ruby-bot-server/service_spec.rb
+++ b/spec/slack-ruby-bot-server/service_spec.rb
@@ -56,10 +56,7 @@
     let(:instance) { SlackRubyBotServer::Service.instance }
     context 'without timers' do
       it 'noop' do
-        Async::Reactor.run do |task|
-          instance.start_intervals!
-          task.stop
-        end
+        instance.start_intervals!(&:stop)
         expect(instance.instance_variable_get(:@intervals).keys).to eq []
       end
     end
@@ -94,8 +91,7 @@
         end
       end
       it 'sets up timers' do
-        Async::Reactor.run do |task|
-          instance.start_intervals!
+        instance.start_intervals! do |task|
           task.sleep 3
           task.stop
         end
@@ -115,8 +111,7 @@
         end
       end
       it 'does not abort all timers on failure of the first one' do
-        Async::Reactor.run do |task|
-          instance.start_intervals!
+        instance.start_intervals! do |task|
           task.sleep 3
           task.stop
         end
@@ -132,8 +127,7 @@
           end
         end
         it 'runs the timer once within 3 seconds' do
-          Async::Reactor.run do |task|
-            instance.start_intervals!
+          instance.start_intervals! do |task|
             task.sleep 3
             task.stop
           end
@@ -148,8 +142,7 @@
           end
         end
         it 'runs the timer exactly twice within 3 seconds' do
-          Async::Reactor.run do |task|
-            instance.start_intervals!
+          instance.start_intervals! do |task|
             task.sleep 3
             task.stop
           end
diff --git a/spec/support/api/endpoints/it_behaves_like_a_cursor_api.rb b/spec/support/api/endpoints/it_behaves_like_a_cursor_api.rb
index f48139c..d998338 100644
--- a/spec/support/api/endpoints/it_behaves_like_a_cursor_api.rb
+++ b/spec/support/api/endpoints/it_behaves_like_a_cursor_api.rb
@@ -35,7 +35,7 @@
         models_ids.concat(response.map { |instance| instance._links.self._url.gsub("http://example.org/api/#{model_ps}/", '') })
         break unless response._links[:next]
 
-        next_cursor = Hash[CGI.parse(URI.parse(response._links.next._url).query).map { |a| [a[0], a[1][0]] }]
+        next_cursor = CGI.parse(URI.parse(response._links.next._url).query).map { |a| [a[0], a[1][0]] }.to_h
       end
       expect(models_ids.uniq.count).to eq model.all.count
     end
@@ -48,7 +48,7 @@
         models_ids.concat(response.map { |instance| instance._links.self._url.gsub("http://example.org/api/#{model_ps}/", '') })
         break unless response._links[:next]
 
-        next_cursor = Hash[CGI.parse(URI.parse(response._links.next._url).query).map { |a| [a[0], a[1][0]] }]
+        next_cursor = CGI.parse(URI.parse(response._links.next._url).query).map { |a| [a[0], a[1][0]] }.to_h
       end
       expect(models_ids.uniq.count).to eq model.all.count - 3
     end