diff --git a/.github/workflows/c-check.yml b/.github/workflows/c-check.yml
index 11ef122f5..dd7076d2c 100644
--- a/.github/workflows/c-check.yml
+++ b/.github/workflows/c-check.yml
@@ -12,7 +12,7 @@ jobs:
       - uses: actions/checkout@v6
       - 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 b2245a1d7..a04583832 100644
--- a/.github/workflows/ruby.yml
+++ b/.github/workflows/ruby.yml
@@ -13,24 +13,24 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        ruby: ['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"
@@ -74,7 +74,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        ruby: ['3.4', head]
+        ruby: ['4.0.0-preview2', head]
     steps:
       - uses: actions/checkout@v6
       - name: Install dependencies
diff --git a/.github/workflows/typecheck.yml b/.github/workflows/typecheck.yml
index 4068cd23b..109becfd1 100644
--- a/.github/workflows/typecheck.yml
+++ b/.github/workflows/typecheck.yml
@@ -16,7 +16,7 @@ jobs:
       - uses: actions/checkout@v6
       - 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 e5110cdc8..e313060d0 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@v6
       - name: load ruby
diff --git a/Gemfile b/Gemfile
index 9cef14ea8..7c9b389ae 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 403496147..890e0489a 100644
--- a/Gemfile.lock
+++ b/Gemfile.lock
@@ -43,9 +43,7 @@ GEM
     drb (2.2.3)
     erb (6.0.0)
     extconf_compile_commands_json (0.0.7)
-    ffi (1.17.2-aarch64-linux-gnu)
-    ffi (1.17.2-arm64-darwin)
-    ffi (1.17.2-x86_64-linux-gnu)
+    ffi (1.17.2)
     fileutils (1.8.0)
     goodcheck (3.1.0)
       marcel (>= 1.0, < 2.0)
@@ -54,6 +52,11 @@ 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 (2.16.0)
     json-schema (6.0.0)
       addressable (~> 2.8)
@@ -66,6 +69,7 @@ GEM
     logger (1.7.0)
     marcel (1.1.0)
     memory_profiler (1.1.0)
+    mini_portile2 (2.8.9)
     minitest (5.26.2)
     mutex_m (0.3.0)
     net-protocol (0.2.2)
@@ -73,19 +77,18 @@ GEM
     net-smtp (0.5.1)
       net-protocol
     nkf (0.2.0)
-    nokogiri (1.18.10-aarch64-linux-gnu)
-      racc (~> 1.4)
-    nokogiri (1.18.10-arm64-darwin)
-      racc (~> 1.4)
-    nokogiri (1.18.10-x86_64-linux-gnu)
+    nokogiri (1.18.10)
+      mini_portile2 (~> 2.8.2)
       racc (~> 1.4)
     ostruct (0.6.3)
     parallel (1.27.0)
     parser (3.3.10.0)
       ast (~> 2.4.1)
       racc
-    pathname (0.4.0)
     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)
@@ -107,6 +110,8 @@ GEM
       psych (>= 4.0.0)
       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)
@@ -183,9 +188,7 @@ GEM
     zlib (3.2.2)
 
 PLATFORMS
-  aarch64-linux
-  arm64-darwin
-  x86_64-linux
+  ruby
 
 DEPENDENCIES
   abbrev
@@ -199,6 +202,7 @@ DEPENDENCIES
   extconf_compile_commands_json
   fileutils
   goodcheck
+  irb
   json
   json-schema
   memory_profiler
@@ -207,7 +211,6 @@ DEPENDENCIES
   net-smtp
   nkf
   ostruct
-  pathname
   pstore
   raap
   rake
@@ -226,4 +229,4 @@ DEPENDENCIES
   test-unit
 
 BUNDLED WITH
-   2.6.3
+  4.0.0.dev
diff --git a/Rakefile b/Rakefile
index 330530a24..5e43b2cec 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 b4a8b6aa5..197d5f703 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 abfe87181..b9873e283 100644
--- a/steep/Gemfile.lock
+++ b/steep/Gemfile.lock
@@ -89,4 +89,4 @@ DEPENDENCIES
   test-unit
 
 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