diff --git a/.github/workflows/c-check.yml b/.github/workflows/c-check.yml
index b1bdc66cc..eb8b0be58 100644
--- a/.github/workflows/c-check.yml
+++ b/.github/workflows/c-check.yml
@@ -12,7 +12,7 @@ jobs:
       - uses: actions/checkout@v4
       - uses: ruby/setup-ruby@v1
         with:
-          ruby-version: "3.4"
+          ruby-version: "4.0.0-preview2"
           bundler-cache: none
       - name: Set working directory as safe
         run: git config --global --add safe.directory $(pwd)
diff --git a/.github/workflows/ruby.yml b/.github/workflows/ruby.yml
index 976c781c8..6f8851fed 100644
--- a/.github/workflows/ruby.yml
+++ b/.github/workflows/ruby.yml
@@ -13,24 +13,24 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        ruby: ['3.1', '3.2', '3.3', '3.4', head]
+        ruby: ['3.2', '3.3', '3.4', '4.0.0-preview2', head]
         rubyopt: [""]
         job:
           - test
         include:
           - ruby: head
             job: stdlib_test rubocop
-          - ruby: "3.4"
+          - ruby: "4.0.0-preview2"
             job: stdlib_test
-          - ruby: "3.4"
+          - ruby: "4.0.0-preview2"
             job: test
             rubyopt: "--enable-frozen-string-literal"
-          - ruby: "3.4"
+          - ruby: "4.0.0-preview2"
             job: stdlib_test
             rubyopt: "--enable-frozen-string-literal"
-          - ruby: "3.4"
+          - ruby: "4.0.0-preview2"
             job: rubocop validate test_doc build test_generate_stdlib raap
-          - ruby: "3.4"
+          - ruby: "4.0.0-preview2"
             job: typecheck_test
     env:
       RANDOMIZE_STDLIB_TEST_ORDER: "true"
@@ -94,7 +94,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        ruby: ['3.4', head]
+        ruby: ['4.0.0-preview2', head]
     steps:
       - uses: actions/checkout@v4
       - name: Install dependencies
@@ -116,4 +116,3 @@ jobs:
         run: |
           bin/setup
       - run: bundle exec rake clean compile_c99
-
diff --git a/.github/workflows/typecheck.yml b/.github/workflows/typecheck.yml
index 5f737028a..a3f7d630f 100644
--- a/.github/workflows/typecheck.yml
+++ b/.github/workflows/typecheck.yml
@@ -16,7 +16,7 @@ jobs:
       - uses: actions/checkout@v4
       - uses: ruby/setup-ruby@v1
         with:
-          ruby-version: "3.3"
+          ruby-version: "3.4"
           bundler: none
       - name: Set working directory as safe
         run: git config --global --add safe.directory $(pwd)
diff --git a/.github/workflows/windows.yml b/.github/workflows/windows.yml
index 75aa8a716..a249e8ede 100644
--- a/.github/workflows/windows.yml
+++ b/.github/workflows/windows.yml
@@ -13,7 +13,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        ruby: ['3.2', '3.3', ucrt, mswin]
+        ruby: ['3.4', ucrt, mswin]
     steps:
       - uses: actions/checkout@v4
       - name: load ruby
diff --git a/Gemfile b/Gemfile
index 7b03f739f..a04a1b352 100644
--- a/Gemfile
+++ b/Gemfile
@@ -21,6 +21,7 @@ gem "fileutils"
 gem "raap"
 gem "activesupport", "~> 7.0"
 gem "extconf_compile_commands_json"
+gem "irb"
 
 group :libs do
   # Libraries required for stdlib test
@@ -30,7 +31,6 @@ group :libs do
   gem "dbm"
   gem "mutex_m"
   gem "nkf"
-  gem "pathname"
 end
 
 group :profilers do
diff --git a/Gemfile.lock b/Gemfile.lock
index 2fd1d6279..252c3a459 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -13,7 +13,7 @@ GEM
   remote: https://rubygems.org/
   specs:
     abbrev (0.1.2)
-    activesupport (7.2.2.1)
+    activesupport (7.2.3)
       base64
       benchmark (>= 0.3)
       bigdecimal
@@ -25,23 +25,24 @@ GEM
       minitest (>= 5.1)
       securerandom (>= 0.3)
       tzinfo (~> 2.0, >= 2.0.5)
-    addressable (2.8.7)
-      public_suffix (>= 2.0.2, < 7.0)
+    addressable (2.8.8)
+      public_suffix (>= 2.0.2, < 8.0)
     ast (2.4.3)
-    base64 (0.2.0)
-    benchmark (0.4.0)
+    base64 (0.3.0)
+    benchmark (0.5.0)
     benchmark-ips (2.14.0)
-    bigdecimal (3.1.9)
+    bigdecimal (3.3.1)
     concurrent-ruby (1.3.5)
-    connection_pool (2.5.3)
-    csv (3.3.4)
+    connection_pool (3.0.2)
+    csv (3.3.5)
     dbm (1.1.0)
-    diff-lcs (1.6.1)
-    digest (3.2.0)
-    drb (2.2.1)
+    diff-lcs (1.6.2)
+    digest (3.2.1)
+    drb (2.2.3)
+    erb (6.0.0)
     extconf_compile_commands_json (0.0.7)
     ffi (1.17.2)
-    fileutils (1.7.3)
+    fileutils (1.8.0)
     goodcheck (3.1.0)
       marcel (>= 1.0, < 2.0)
       psych (>= 3.1, < 5.0)
@@ -49,70 +50,80 @@ GEM
       strong_json (>= 1.1, < 2.2)
     i18n (1.14.7)
       concurrent-ruby (~> 1.0)
+    io-console (0.8.1)
+    irb (1.15.3)
+      pp (>= 0.6.0)
+      rdoc (>= 4.0.0)
+      reline (>= 0.4.2)
     json (2.10.2)
-    json-schema (5.1.1)
+    json-schema (6.0.0)
       addressable (~> 2.8)
       bigdecimal (~> 3.1)
-    language_server-protocol (3.17.0.4)
+    language_server-protocol (3.17.0.5)
     lint_roller (1.1.0)
     listen (3.9.0)
       rb-fsevent (~> 0.10, >= 0.10.3)
       rb-inotify (~> 0.9, >= 0.9.10)
     logger (1.7.0)
-    marcel (1.0.4)
+    marcel (1.1.0)
     memory_profiler (1.1.0)
-    mini_portile2 (2.8.8)
-    minitest (5.25.5)
+    mini_portile2 (2.8.9)
+    minitest (5.26.2)
     mutex_m (0.3.0)
     net-protocol (0.2.2)
       timeout
     net-smtp (0.5.1)
       net-protocol
     nkf (0.2.0)
-    nokogiri (1.18.8)
+    nokogiri (1.18.10)
       mini_portile2 (~> 2.8.2)
       racc (~> 1.4)
-    ostruct (0.6.1)
+    ostruct (0.6.3)
     parallel (1.27.0)
-    parser (3.3.8.0)
+    parser (3.3.10.0)
       ast (~> 2.4.1)
       racc
-    pathname (0.4.0)
-    power_assert (2.0.5)
-    prism (1.4.0)
+    power_assert (3.0.1)
+    pp (0.6.3)
+      prettyprint
+    prettyprint (0.2.0)
+    prism (1.6.0)
     pstore (0.2.0)
     psych (4.0.6)
       stringio
-    public_suffix (6.0.2)
-    raap (1.2.0)
-      rbs (~> 3.0)
+    public_suffix (7.0.0)
+    raap (1.3.0)
+      rbs (~> 3.9.0)
       timeout (~> 0.4)
     racc (1.8.1)
     rainbow (3.1.1)
-    rake (13.2.1)
+    rake (13.3.1)
     rake-compiler (1.3.0)
       rake
     rb-fsevent (0.11.2)
     rb-inotify (0.11.1)
       ffi (~> 1.0)
-    rdoc (6.16.0)
+    rdoc (6.17.0)
       erb
       psych (>= 4.0.0)
-    regexp_parser (2.10.0)
-    rspec (3.13.0)
+      tsort
+    regexp_parser (2.11.3)
+    reline (0.6.3)
+      io-console (~> 0.5)
+    rspec (3.13.2)
       rspec-core (~> 3.13.0)
       rspec-expectations (~> 3.13.0)
       rspec-mocks (~> 3.13.0)
-    rspec-core (3.13.3)
+    rspec-core (3.13.6)
       rspec-support (~> 3.13.0)
-    rspec-expectations (3.13.4)
+    rspec-expectations (3.13.5)
       diff-lcs (>= 1.2.0, < 2.0)
       rspec-support (~> 3.13.0)
-    rspec-mocks (3.13.4)
+    rspec-mocks (3.13.7)
       diff-lcs (>= 1.2.0, < 2.0)
       rspec-support (~> 3.13.0)
-    rspec-support (3.13.3)
-    rubocop (1.75.5)
+    rspec-support (3.13.6)
+    rubocop (1.81.7)
       json (~> 2.3)
       language_server-protocol (~> 3.17.0.2)
       lint_roller (~> 1.1.0)
@@ -120,13 +131,13 @@ GEM
       parser (>= 3.3.0.2)
       rainbow (>= 2.2.2, < 4.0)
       regexp_parser (>= 2.9.3, < 3.0)
-      rubocop-ast (>= 1.44.0, < 2.0)
+      rubocop-ast (>= 1.47.1, < 2.0)
       ruby-progressbar (~> 1.7)
       unicode-display_width (>= 2.4.0, < 4.0)
-    rubocop-ast (1.44.1)
+    rubocop-ast (1.48.0)
       parser (>= 3.3.7.2)
       prism (~> 1.4)
-    rubocop-on-rbs (1.6.0)
+    rubocop-on-rbs (1.8.0)
       lint_roller (~> 1.1)
       rbs (~> 3.5)
       rubocop (>= 1.72.1, < 2.0)
@@ -156,22 +167,23 @@ GEM
       strscan (>= 1.0.0)
       terminal-table (>= 2, < 5)
       uri (>= 0.12.0)
-    stringio (3.1.7)
+    stringio (3.1.9)
     strong_json (2.1.2)
     strscan (3.1.5)
     tempfile (0.3.1)
     terminal-table (4.0.0)
       unicode-display_width (>= 1.1.1, < 4)
-    test-unit (3.6.8)
+    test-unit (3.7.3)
       power_assert
-    timeout (0.4.3)
+    timeout (0.5.0)
+    tsort (0.2.0)
     tzinfo (2.0.6)
       concurrent-ruby (~> 1.0)
-    unicode-display_width (3.1.4)
-      unicode-emoji (~> 4.0, >= 4.0.4)
-    unicode-emoji (4.0.4)
-    uri (1.0.3)
-    zlib (3.2.1)
+    unicode-display_width (3.2.0)
+      unicode-emoji (~> 4.1)
+    unicode-emoji (4.1.0)
+    uri (1.1.1)
+    zlib (3.2.2)
 
 PLATFORMS
   ruby
@@ -188,6 +200,7 @@ DEPENDENCIES
   extconf_compile_commands_json
   fileutils
   goodcheck
+  irb
   json (~> 2.10.2)
   json-schema
   memory_profiler
@@ -196,7 +209,6 @@ DEPENDENCIES
   net-smtp
   nkf
   ostruct
-  pathname
   pstore
   raap
   rake
@@ -215,4 +227,4 @@ DEPENDENCIES
   test-unit
 
 BUNDLED WITH
-   2.6.3
+  4.0.0.dev
diff --git a/Rakefile b/Rakefile
index 06792575d..343704e8a 100644
--- a/Rakefile
+++ b/Rakefile
@@ -212,7 +212,14 @@ task :validate => :compile do
   end
 
   libs.each do |lib|
-    sh "#{ruby} #{rbs} -r #{lib} validate --exit-error-on-syntax-error"
+    args = ["-r", lib]
+
+    if lib == "rbs"
+      args << "-r"
+      args << "prism"
+    end
+
+    sh "#{ruby} #{rbs} #{args.join(' ')} validate --exit-error-on-syntax-error"
   end
 end
 
diff --git a/bin/benchmark-parse.rb b/bin/benchmark-parse.rb
index 98e4ce16d..64d4fd530 100644
--- a/bin/benchmark-parse.rb
+++ b/bin/benchmark-parse.rb
@@ -1,7 +1,6 @@
 require "rbs"
 require "benchmark/ips"
 require "csv"
-require "pathname"
 
 files = {}
 ARGV.each do |file|
diff --git a/lib/rbs.rb b/lib/rbs.rb
index d0b8d24d4..02bb50e4b 100644
--- a/lib/rbs.rb
+++ b/lib/rbs.rb
@@ -4,7 +4,7 @@
 
 require "set"
 require "json"
-require "pathname"
+require "pathname" unless defined?(Pathname)
 require "pp"
 require "ripper"
 require "logger"
diff --git a/steep/Gemfile.lock b/steep/Gemfile.lock
index 4ca31fc47..a60d87257 100644
--- a/steep/Gemfile.lock
+++ b/steep/Gemfile.lock
@@ -80,4 +80,4 @@ DEPENDENCIES
   steep (~> 1.10.pre)
 
 BUNDLED WITH
-   2.6.3
+   4.0.0.dev
diff --git a/test/stdlib/Marshal_test.rb b/test/stdlib/Marshal_test.rb
index 945ffa78a..f0d03fa0f 100644
--- a/test/stdlib/Marshal_test.rb
+++ b/test/stdlib/Marshal_test.rb
@@ -1,5 +1,4 @@
 require_relative "test_helper"
-require "pathname"
 require "tmpdir"
 
 class MarshalSingletonTest < Test::Unit::TestCase
diff --git a/test/stdlib/zlib/GzipReader_test.rb b/test/stdlib/zlib/GzipReader_test.rb
index d105a797c..6d2e288b6 100644
--- a/test/stdlib/zlib/GzipReader_test.rb
+++ b/test/stdlib/zlib/GzipReader_test.rb
@@ -1,7 +1,6 @@
 require_relative "../test_helper"
 require "zlib"
 require "tempfile"
-require "pathname"
 
 class ZlibGzipReaderSingletonTest < Test::Unit::TestCase
   include TestHelper