diff --git a/config/flay.yml b/config/flay.yml
index bdea6a8b..aaff9513 100644
--- a/config/flay.yml
+++ b/config/flay.yml
@@ -1,3 +1,3 @@
 ---
 threshold: 18
-total_score: 1172
+total_score: 1168
diff --git a/config/rubocop.yml b/config/rubocop.yml
index e44a2050..64d93f50 100644
--- a/config/rubocop.yml
+++ b/config/rubocop.yml
@@ -67,7 +67,7 @@ MultilineOperationIndentation:
 
 # Prefer String#% over Kernel#sprintf
 FormatString:
-  Enabled: false
+  EnforcedStyle: percent
 
 # Use square brackets for literal Array objects
 PercentLiteralDelimiters:
diff --git a/lib/mutant/matcher/method/singleton.rb b/lib/mutant/matcher/method/singleton.rb
index 51c274c1..4d5d2d59 100644
--- a/lib/mutant/matcher/method/singleton.rb
+++ b/lib/mutant/matcher/method/singleton.rb
@@ -16,9 +16,10 @@ module Mutant
 
         # Singleton method evaluator
         class Evaluator < Evaluator
-          SUBJECT_CLASS  = Subject::Method::Singleton
-          RECEIVER_INDEX = 0
-          NAME_INDEX     = 1
+          SUBJECT_CLASS    = Subject::Method::Singleton
+          RECEIVER_INDEX   = 0
+          NAME_INDEX       = 1
+          RECEIVER_WARNING = 'Can only match :defs on :self or :const got %p unable to match'.freeze
 
         private
 
@@ -65,7 +66,7 @@ module Mutant
             when :const
               receiver_name?(receiver)
             else
-              env.warn(format('Can only match :defs on :self or :const got %s unable to match', receiver.type.inspect))
+              env.warn(RECEIVER_WARNING % receiver.type)
               nil
             end
           end
diff --git a/lib/mutant/reporter/cli/format.rb b/lib/mutant/reporter/cli/format.rb
index ba480adc..533ec8bf 100644
--- a/lib/mutant/reporter/cli/format.rb
+++ b/lib/mutant/reporter/cli/format.rb
@@ -2,6 +2,8 @@ module Mutant
   class Reporter
     class CLI
       # CLI output format
+      #
+      # rubocop:disable FormatString
       class Format
         include AbstractType, Anima.new(:tty)
 
diff --git a/lib/mutant/reporter/cli/printer.rb b/lib/mutant/reporter/cli/printer.rb
index fd5ec776..04716829 100644
--- a/lib/mutant/reporter/cli/printer.rb
+++ b/lib/mutant/reporter/cli/printer.rb
@@ -75,14 +75,14 @@ module Mutant
         #
         # @return [undefined]
         def info(string, *arguments)
-          puts(format(string, *arguments))
+          puts(string % arguments)
         end
 
         # Print a status line to output
         #
         # @return [undefined]
         def status(string, *arguments)
-          puts(colorize(status_color, format(string, *arguments)))
+          puts(colorize(status_color, string % arguments))
         end
 
         # Print a line to output
diff --git a/spec/support/corpus.rb b/spec/support/corpus.rb
index 68dd3945..3a5c0a80 100644
--- a/spec/support/corpus.rb
+++ b/spec/support/corpus.rb
@@ -24,6 +24,11 @@ module MutantSpec
     # rubocop:disable ClassLength
     class Project
       MUTEX = Mutex.new
+
+      MUTATION_GENERATION_MESSAGE = 'Total Mutations/Time/Parse-Errors: %s/%0.2fs - %0.2f/s'.freeze
+      START_MESSAGE               = 'Starting - %s'.freeze
+      FINISH_MESSAGE              = 'Mutations - %4i - %s'.freeze
+
       include Adamantium, Anima.new(
         :exclude,
         :expect_coverage,
@@ -91,12 +96,7 @@ module MutantSpec
           count
         end.inject(0, :+)
         took = Time.now - start
-        puts format(
-          'Total Mutations/Time/Parse-Errors: %s/%0.2fs - %0.2f/s',
-          total,
-          took,
-          total / took
-        )
+        puts MUTATION_GENERATION_MESSAGE % [total, took, total / took]
         self
       end
 
@@ -193,7 +193,7 @@ module MutantSpec
       #
       def start(path, _index)
         MUTEX.synchronize do
-          puts format('Starting - %s', path)
+          puts START_MESSAGE % path
         end
       end
 
@@ -207,7 +207,7 @@ module MutantSpec
       #
       def finish(path, _index, count)
         MUTEX.synchronize do
-          puts format('Mutations - %4i - %s', count, path)
+          puts FINISH_MESSAGE % [count, path]
         end
       end