diff --git a/.rubocop.yml b/.rubocop.yml
index f38ff4393..099a61823 100644
--- a/.rubocop.yml
+++ b/.rubocop.yml
@@ -13,3 +13,8 @@ Lint/DuplicateMethods:
   Enabled: true
   Include:
     - 'test/**/*_test.rb'
+
+Style/FrozenStringLiteralComment:
+  Enabled: true
+  Include:
+    - 'lib/**/*.rb'
diff --git a/lib/rbs.rb b/lib/rbs.rb
index 9b4d65723..0559fa422 100644
--- a/lib/rbs.rb
+++ b/lib/rbs.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require "rbs/version"
 
 require "set"
diff --git a/lib/rbs/ancestor_graph.rb b/lib/rbs/ancestor_graph.rb
index ed7fb0da1..7c197746f 100644
--- a/lib/rbs/ancestor_graph.rb
+++ b/lib/rbs/ancestor_graph.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module RBS
   class AncestorGraph
     InstanceNode = _ = Struct.new(:type_name, keyword_init: true)
diff --git a/lib/rbs/annotate.rb b/lib/rbs/annotate.rb
index 1256a7120..c55568470 100644
--- a/lib/rbs/annotate.rb
+++ b/lib/rbs/annotate.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require "rdoc"
 
 require "rbs/annotate/rdoc_annotator"
diff --git a/lib/rbs/annotate/annotations.rb b/lib/rbs/annotate/annotations.rb
index 797a8e889..0030b3046 100644
--- a/lib/rbs/annotate/annotations.rb
+++ b/lib/rbs/annotate/annotations.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module RBS
   module Annotate
     class Annotations
diff --git a/lib/rbs/annotate/formatter.rb b/lib/rbs/annotate/formatter.rb
index 7e2251410..c53124e27 100644
--- a/lib/rbs/annotate/formatter.rb
+++ b/lib/rbs/annotate/formatter.rb
@@ -1,10 +1,12 @@
+# frozen_string_literal: true
+
 module RBS
   module Annotate
     class Formatter
       attr_reader :buffer
 
       def initialize()
-        @buffer = ""
+        @buffer = +""
         @pending_separator = nil
       end
 
@@ -14,7 +16,7 @@ def <<(s)
             s = self.class.translate(s) or raise
           end
 
-          s.rstrip!
+          s = s.rstrip
 
           unless s.empty?
             if ss = @pending_separator
diff --git a/lib/rbs/annotate/rdoc_annotator.rb b/lib/rbs/annotate/rdoc_annotator.rb
index eff74a57f..a1dffd518 100644
--- a/lib/rbs/annotate/rdoc_annotator.rb
+++ b/lib/rbs/annotate/rdoc_annotator.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module RBS
   module Annotate
     class RDocAnnotator
diff --git a/lib/rbs/annotate/rdoc_source.rb b/lib/rbs/annotate/rdoc_source.rb
index f0dd6622a..88a326585 100644
--- a/lib/rbs/annotate/rdoc_source.rb
+++ b/lib/rbs/annotate/rdoc_source.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module RBS
   module Annotate
     class RDocSource
diff --git a/lib/rbs/ast/annotation.rb b/lib/rbs/ast/annotation.rb
index 4894bb944..8cf5c1c0e 100644
--- a/lib/rbs/ast/annotation.rb
+++ b/lib/rbs/ast/annotation.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module RBS
   module AST
     class Annotation
diff --git a/lib/rbs/ast/comment.rb b/lib/rbs/ast/comment.rb
index 21041b3da..62db2c806 100644
--- a/lib/rbs/ast/comment.rb
+++ b/lib/rbs/ast/comment.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module RBS
   module AST
     class Comment
diff --git a/lib/rbs/ast/declarations.rb b/lib/rbs/ast/declarations.rb
index e4c312712..b52f9e387 100644
--- a/lib/rbs/ast/declarations.rb
+++ b/lib/rbs/ast/declarations.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module RBS
   module AST
     module Declarations
diff --git a/lib/rbs/ast/members.rb b/lib/rbs/ast/members.rb
index bfc466d56..9942eac52 100644
--- a/lib/rbs/ast/members.rb
+++ b/lib/rbs/ast/members.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module RBS
   module AST
     module Members
diff --git a/lib/rbs/ast/type_param.rb b/lib/rbs/ast/type_param.rb
index 612030405..3775ba2e6 100644
--- a/lib/rbs/ast/type_param.rb
+++ b/lib/rbs/ast/type_param.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module RBS
   module AST
     class TypeParam
@@ -106,7 +108,7 @@ def self.rename(params, new_names:)
       end
 
       def to_s
-        s = ""
+        s = +""
 
         if unchecked?
           s << "unchecked "
diff --git a/lib/rbs/buffer.rb b/lib/rbs/buffer.rb
index bc9a7c2ee..10f0273df 100644
--- a/lib/rbs/buffer.rb
+++ b/lib/rbs/buffer.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module RBS
   class Buffer
     attr_reader :name
diff --git a/lib/rbs/builtin_names.rb b/lib/rbs/builtin_names.rb
index cc6761af8..b36d898b0 100644
--- a/lib/rbs/builtin_names.rb
+++ b/lib/rbs/builtin_names.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module RBS
   module BuiltinNames
     class Name
diff --git a/lib/rbs/cli.rb b/lib/rbs/cli.rb
index 28e49055e..592e380f1 100644
--- a/lib/rbs/cli.rb
+++ b/lib/rbs/cli.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require "open3"
 require "optparse"
 require "shellwords"
diff --git a/lib/rbs/collection.rb b/lib/rbs/collection.rb
index aca483d1f..78d096d16 100644
--- a/lib/rbs/collection.rb
+++ b/lib/rbs/collection.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'yaml'
 require 'bundler'
 
diff --git a/lib/rbs/collection/cleaner.rb b/lib/rbs/collection/cleaner.rb
index 834f94c51..1d126d659 100644
--- a/lib/rbs/collection/cleaner.rb
+++ b/lib/rbs/collection/cleaner.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module RBS
   module Collection
     class Cleaner
diff --git a/lib/rbs/collection/config.rb b/lib/rbs/collection/config.rb
index 230aae7af..566f72a74 100644
--- a/lib/rbs/collection/config.rb
+++ b/lib/rbs/collection/config.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module RBS
   module Collection
 
diff --git a/lib/rbs/collection/config/lockfile_generator.rb b/lib/rbs/collection/config/lockfile_generator.rb
index 608bdd93a..c2db175f3 100644
--- a/lib/rbs/collection/config/lockfile_generator.rb
+++ b/lib/rbs/collection/config/lockfile_generator.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module RBS
   module Collection
 
diff --git a/lib/rbs/collection/installer.rb b/lib/rbs/collection/installer.rb
index fa375c65b..0e3e98779 100644
--- a/lib/rbs/collection/installer.rb
+++ b/lib/rbs/collection/installer.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module RBS
   module Collection
     class Installer
diff --git a/lib/rbs/collection/sources.rb b/lib/rbs/collection/sources.rb
index bd0ad252c..1af76f642 100644
--- a/lib/rbs/collection/sources.rb
+++ b/lib/rbs/collection/sources.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require_relative './sources/base'
 require_relative './sources/git'
 require_relative './sources/stdlib'
diff --git a/lib/rbs/collection/sources/base.rb b/lib/rbs/collection/sources/base.rb
index 9e77f25ba..7c7836f1f 100644
--- a/lib/rbs/collection/sources/base.rb
+++ b/lib/rbs/collection/sources/base.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module RBS
   module Collection
     module Sources
diff --git a/lib/rbs/collection/sources/git.rb b/lib/rbs/collection/sources/git.rb
index 714090e65..99f86ce9d 100644
--- a/lib/rbs/collection/sources/git.rb
+++ b/lib/rbs/collection/sources/git.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'digest/sha2'
 require 'open3'
 require 'find'
diff --git a/lib/rbs/collection/sources/rubygems.rb b/lib/rbs/collection/sources/rubygems.rb
index 6b6d80279..5a47bf749 100644
--- a/lib/rbs/collection/sources/rubygems.rb
+++ b/lib/rbs/collection/sources/rubygems.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'singleton'
 
 module RBS
diff --git a/lib/rbs/collection/sources/stdlib.rb b/lib/rbs/collection/sources/stdlib.rb
index 2276dd5ee..f2bb11b5d 100644
--- a/lib/rbs/collection/sources/stdlib.rb
+++ b/lib/rbs/collection/sources/stdlib.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require 'singleton'
 
 module RBS
diff --git a/lib/rbs/constant.rb b/lib/rbs/constant.rb
index ddebfff93..ec60af075 100644
--- a/lib/rbs/constant.rb
+++ b/lib/rbs/constant.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module RBS
   class Constant
     attr_reader :name
diff --git a/lib/rbs/constant_table.rb b/lib/rbs/constant_table.rb
index 58eaf86dc..d51dc9660 100644
--- a/lib/rbs/constant_table.rb
+++ b/lib/rbs/constant_table.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module RBS
   class ConstantTable
     attr_reader :definition_builder
diff --git a/lib/rbs/definition.rb b/lib/rbs/definition.rb
index 270979e3c..da3af8331 100644
--- a/lib/rbs/definition.rb
+++ b/lib/rbs/definition.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module RBS
   class Definition
     class Variable
diff --git a/lib/rbs/definition_builder.rb b/lib/rbs/definition_builder.rb
index f5766b3ae..f44f4c4cd 100644
--- a/lib/rbs/definition_builder.rb
+++ b/lib/rbs/definition_builder.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module RBS
   class DefinitionBuilder
     attr_reader :env
diff --git a/lib/rbs/definition_builder/ancestor_builder.rb b/lib/rbs/definition_builder/ancestor_builder.rb
index 204ed8bb6..62448acc2 100644
--- a/lib/rbs/definition_builder/ancestor_builder.rb
+++ b/lib/rbs/definition_builder/ancestor_builder.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module RBS
   class DefinitionBuilder
     class AncestorBuilder
diff --git a/lib/rbs/definition_builder/method_builder.rb b/lib/rbs/definition_builder/method_builder.rb
index 2c3283c95..27adb2422 100644
--- a/lib/rbs/definition_builder/method_builder.rb
+++ b/lib/rbs/definition_builder/method_builder.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module RBS
   class DefinitionBuilder
     class MethodBuilder
diff --git a/lib/rbs/environment.rb b/lib/rbs/environment.rb
index 56403164c..4aa35d5f3 100644
--- a/lib/rbs/environment.rb
+++ b/lib/rbs/environment.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module RBS
   class Environment
     attr_reader :declarations
diff --git a/lib/rbs/environment_loader.rb b/lib/rbs/environment_loader.rb
index 858cc4604..c399ca1fa 100644
--- a/lib/rbs/environment_loader.rb
+++ b/lib/rbs/environment_loader.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module RBS
   class EnvironmentLoader
     class UnknownLibraryError < StandardError
diff --git a/lib/rbs/environment_walker.rb b/lib/rbs/environment_walker.rb
index 384da066c..7b9674591 100644
--- a/lib/rbs/environment_walker.rb
+++ b/lib/rbs/environment_walker.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module RBS
   class EnvironmentWalker
     InstanceNode = _ = Struct.new(:type_name, keyword_init: true)
diff --git a/lib/rbs/errors.rb b/lib/rbs/errors.rb
index 246dbe9d9..2e1450518 100644
--- a/lib/rbs/errors.rb
+++ b/lib/rbs/errors.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module RBS
   module MethodNameHelper
     def method_name_string()
@@ -216,7 +218,7 @@ def initialize(type:, method_name:, members:)
       @method_name = method_name
       @members = members
 
-      message = "#{Location.to_string location}: #{qualified_method_name} has duplicated definitions"
+      message = +"#{Location.to_string location}: #{qualified_method_name} has duplicated definitions"
       if members.size > 1
         message << " in #{other_locations.map { |loc| Location.to_string loc }.join(', ')}"
       end
diff --git a/lib/rbs/factory.rb b/lib/rbs/factory.rb
index 6822e8f46..a224ebd76 100644
--- a/lib/rbs/factory.rb
+++ b/lib/rbs/factory.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module RBS
   class Factory
     def type_name(string)
diff --git a/lib/rbs/location_aux.rb b/lib/rbs/location_aux.rb
index 73e2d013e..769f91c5c 100644
--- a/lib/rbs/location_aux.rb
+++ b/lib/rbs/location_aux.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module RBS
   class Location
     def inspect
diff --git a/lib/rbs/locator.rb b/lib/rbs/locator.rb
index 539f01c0a..42d6b17e2 100644
--- a/lib/rbs/locator.rb
+++ b/lib/rbs/locator.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module RBS
   class Locator
     attr_reader :decls
diff --git a/lib/rbs/method_type.rb b/lib/rbs/method_type.rb
index 0ddb92875..e07d75e5e 100644
--- a/lib/rbs/method_type.rb
+++ b/lib/rbs/method_type.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module RBS
   class MethodType
     attr_reader :type_params
diff --git a/lib/rbs/namespace.rb b/lib/rbs/namespace.rb
index a2899689a..d41496436 100644
--- a/lib/rbs/namespace.rb
+++ b/lib/rbs/namespace.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module RBS
   class Namespace
     attr_reader :path
diff --git a/lib/rbs/parser_aux.rb b/lib/rbs/parser_aux.rb
index d18eaa7da..1b9e41324 100644
--- a/lib/rbs/parser_aux.rb
+++ b/lib/rbs/parser_aux.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module RBS
   class Parser
     def self.parse_type(source, line: 1, column: 0, variables: [])
diff --git a/lib/rbs/parser_compat/lexer_error.rb b/lib/rbs/parser_compat/lexer_error.rb
index fd150dd2f..0ed6cb3cc 100644
--- a/lib/rbs/parser_compat/lexer_error.rb
+++ b/lib/rbs/parser_compat/lexer_error.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 RBS.print_warning {
   "RBS::Parser::LexerError is deprecated and will be deleted in RBS 2.0."
 }
diff --git a/lib/rbs/parser_compat/located_value.rb b/lib/rbs/parser_compat/located_value.rb
index 6ccabaf06..99ad57a1b 100644
--- a/lib/rbs/parser_compat/located_value.rb
+++ b/lib/rbs/parser_compat/located_value.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 RBS.print_warning {
   "RBS::Parser::LocatedValue is deprecated and will be deleted in RBS 2.0."
 }
diff --git a/lib/rbs/parser_compat/semantics_error.rb b/lib/rbs/parser_compat/semantics_error.rb
index 14c9c2017..1a40743d1 100644
--- a/lib/rbs/parser_compat/semantics_error.rb
+++ b/lib/rbs/parser_compat/semantics_error.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 RBS.print_warning {
   "RBS::Parser::SemanticsError is deprecated and will be deleted in RBS 2.0."
 }
diff --git a/lib/rbs/parser_compat/syntax_error.rb b/lib/rbs/parser_compat/syntax_error.rb
index be7f68f1c..5ae5df62c 100644
--- a/lib/rbs/parser_compat/syntax_error.rb
+++ b/lib/rbs/parser_compat/syntax_error.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 RBS.print_warning {
   "RBS::Parser::SyntaxError is deprecated and will be deleted in RBS 2.0."
 }
diff --git a/lib/rbs/prototype/helpers.rb b/lib/rbs/prototype/helpers.rb
index 044243847..10baa0679 100644
--- a/lib/rbs/prototype/helpers.rb
+++ b/lib/rbs/prototype/helpers.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module RBS
   module Prototype
     module Helpers
diff --git a/lib/rbs/prototype/rb.rb b/lib/rbs/prototype/rb.rb
index cbdc3893e..d6b37ca7c 100644
--- a/lib/rbs/prototype/rb.rb
+++ b/lib/rbs/prototype/rb.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module RBS
   module Prototype
     class RB
diff --git a/lib/rbs/prototype/rbi.rb b/lib/rbs/prototype/rbi.rb
index cb50956fa..4d92154c4 100644
--- a/lib/rbs/prototype/rbi.rb
+++ b/lib/rbs/prototype/rbi.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module RBS
   module Prototype
     class RBI
diff --git a/lib/rbs/prototype/runtime.rb b/lib/rbs/prototype/runtime.rb
index 87b27dd79..460e2b447 100644
--- a/lib/rbs/prototype/runtime.rb
+++ b/lib/rbs/prototype/runtime.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module RBS
   module Prototype
     class Runtime
diff --git a/lib/rbs/repository.rb b/lib/rbs/repository.rb
index 86ce45c0a..49285975d 100644
--- a/lib/rbs/repository.rb
+++ b/lib/rbs/repository.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module RBS
   class Repository
     DEFAULT_STDLIB_ROOT = Pathname(_ = __dir__) + "../../stdlib"
diff --git a/lib/rbs/resolver/constant_resolver.rb b/lib/rbs/resolver/constant_resolver.rb
index e6fe19dff..7c75de218 100644
--- a/lib/rbs/resolver/constant_resolver.rb
+++ b/lib/rbs/resolver/constant_resolver.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module RBS
   module Resolver
     class ConstantResolver
diff --git a/lib/rbs/resolver/type_name_resolver.rb b/lib/rbs/resolver/type_name_resolver.rb
index 26896b4a5..25893a9bb 100644
--- a/lib/rbs/resolver/type_name_resolver.rb
+++ b/lib/rbs/resolver/type_name_resolver.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module RBS
   module Resolver
     class TypeNameResolver
diff --git a/lib/rbs/sorter.rb b/lib/rbs/sorter.rb
index 0a2f46208..f03e6613a 100644
--- a/lib/rbs/sorter.rb
+++ b/lib/rbs/sorter.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module RBS
   class Sorter
     include RBS::AST
diff --git a/lib/rbs/substitution.rb b/lib/rbs/substitution.rb
index faa35feaf..86ca12ca8 100644
--- a/lib/rbs/substitution.rb
+++ b/lib/rbs/substitution.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module RBS
   class Substitution
     attr_reader :mapping
diff --git a/lib/rbs/test.rb b/lib/rbs/test.rb
index cb999b9de..e24e1e412 100644
--- a/lib/rbs/test.rb
+++ b/lib/rbs/test.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require "securerandom"
 require "rbs/test/observer"
 require "rbs/test/spy"
diff --git a/lib/rbs/test/errors.rb b/lib/rbs/test/errors.rb
index 8fe8f7fbb..b3c186d54 100644
--- a/lib/rbs/test/errors.rb
+++ b/lib/rbs/test/errors.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module RBS
   module Test
     module Errors
diff --git a/lib/rbs/test/hook.rb b/lib/rbs/test/hook.rb
index b0b02f339..ed6026a13 100644
--- a/lib/rbs/test/hook.rb
+++ b/lib/rbs/test/hook.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require "rbs"
 require "pp"
 
diff --git a/lib/rbs/test/observer.rb b/lib/rbs/test/observer.rb
index 710d4c171..ff023871a 100644
--- a/lib/rbs/test/observer.rb
+++ b/lib/rbs/test/observer.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module RBS
   module Test
     module Observer
diff --git a/lib/rbs/test/setup.rb b/lib/rbs/test/setup.rb
index 71e2c51bf..df6581626 100644
--- a/lib/rbs/test/setup.rb
+++ b/lib/rbs/test/setup.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 require "rbs"
 require "rbs/test"
 require "optparse"
diff --git a/lib/rbs/test/setup_helper.rb b/lib/rbs/test/setup_helper.rb
index 58edb60f5..03ca94773 100644
--- a/lib/rbs/test/setup_helper.rb
+++ b/lib/rbs/test/setup_helper.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module RBS
   module Test
     module SetupHelper
diff --git a/lib/rbs/test/spy.rb b/lib/rbs/test/spy.rb
index 2a086064f..766b02bc9 100644
--- a/lib/rbs/test/spy.rb
+++ b/lib/rbs/test/spy.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module RBS
   module Test
   end
diff --git a/lib/rbs/test/tester.rb b/lib/rbs/test/tester.rb
index 32c588687..4fd3b517c 100644
--- a/lib/rbs/test/tester.rb
+++ b/lib/rbs/test/tester.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module RBS
   module Test
     class Tester
diff --git a/lib/rbs/test/type_check.rb b/lib/rbs/test/type_check.rb
index 8dfcbd63b..8c4c9b42c 100644
--- a/lib/rbs/test/type_check.rb
+++ b/lib/rbs/test/type_check.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module RBS
   module Test
     class TypeCheck
diff --git a/lib/rbs/type_alias_dependency.rb b/lib/rbs/type_alias_dependency.rb
index e581866bb..95cf96430 100644
--- a/lib/rbs/type_alias_dependency.rb
+++ b/lib/rbs/type_alias_dependency.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module RBS
   class TypeAliasDependency
     attr_reader :env
diff --git a/lib/rbs/type_alias_regularity.rb b/lib/rbs/type_alias_regularity.rb
index 66f85d4f9..fbe4b98c7 100644
--- a/lib/rbs/type_alias_regularity.rb
+++ b/lib/rbs/type_alias_regularity.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module RBS
   class TypeAliasRegularity
     class Diagnostic
diff --git a/lib/rbs/type_name.rb b/lib/rbs/type_name.rb
index ceead6a1c..c9245c3f9 100644
--- a/lib/rbs/type_name.rb
+++ b/lib/rbs/type_name.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module RBS
   class TypeName
     attr_reader :namespace
diff --git a/lib/rbs/type_name_resolver.rb b/lib/rbs/type_name_resolver.rb
index 0e43aec5a..87d1059ed 100644
--- a/lib/rbs/type_name_resolver.rb
+++ b/lib/rbs/type_name_resolver.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module RBS
   class TypeNameResolver
     Query = _ = Struct.new(:type_name, :context, keyword_init: true)
diff --git a/lib/rbs/types.rb b/lib/rbs/types.rb
index 7cc4617ba..1f5699344 100644
--- a/lib/rbs/types.rb
+++ b/lib/rbs/types.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module RBS
   module Types
     module NoFreeVariables
diff --git a/lib/rbs/validator.rb b/lib/rbs/validator.rb
index fae5c78da..77adfd1ca 100644
--- a/lib/rbs/validator.rb
+++ b/lib/rbs/validator.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module RBS
   class Validator
     attr_reader :env
diff --git a/lib/rbs/variance_calculator.rb b/lib/rbs/variance_calculator.rb
index 257189fd8..2c02e6f96 100644
--- a/lib/rbs/variance_calculator.rb
+++ b/lib/rbs/variance_calculator.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module RBS
   class VarianceCalculator
     class Result
diff --git a/lib/rbs/vendorer.rb b/lib/rbs/vendorer.rb
index c374ca664..9e3a35961 100644
--- a/lib/rbs/vendorer.rb
+++ b/lib/rbs/vendorer.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module RBS
   class Vendorer
     attr_reader :vendor_dir
diff --git a/lib/rbs/version.rb b/lib/rbs/version.rb
index 21cf0b4a1..6bc8c93a5 100644
--- a/lib/rbs/version.rb
+++ b/lib/rbs/version.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module RBS
   VERSION = "2.6.0"
 end
diff --git a/lib/rbs/writer.rb b/lib/rbs/writer.rb
index 22b33314d..8c1c0af3e 100644
--- a/lib/rbs/writer.rb
+++ b/lib/rbs/writer.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module RBS
   class Writer
     attr_reader :out
@@ -280,7 +282,7 @@ def write_def(member)
                "self.#{method_name(member.name)}"
              end
 
-      string = ""
+      string = +""
 
       prefix = "#{visibility}def #{name}:"
       padding = " " * (prefix.size-1)
diff --git a/lib/rdoc/discover.rb b/lib/rdoc/discover.rb
index 06b13cd37..1cb9d82cf 100644
--- a/lib/rdoc/discover.rb
+++ b/lib/rdoc/discover.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 begin
   gem 'rdoc', '~> 6.4.0'
   require 'rdoc_plugin/parser'
diff --git a/test/assets/test-gem/amber.gemspec b/test/assets/test-gem/amber.gemspec
index 6669094d4..59374af34 100644
--- a/test/assets/test-gem/amber.gemspec
+++ b/test/assets/test-gem/amber.gemspec
@@ -1,4 +1,5 @@
 # coding: utf-8
+
 lib = File.expand_path('../lib', __FILE__)
 $LOAD_PATH.unshift(lib) unless $LOAD_PATH.include?(lib)