diff --git a/.github/actions/setup/directories/action.yml b/.github/actions/setup/directories/action.yml
index 7b955d4ff0dab2..78b57c17619a51 100644
--- a/.github/actions/setup/directories/action.yml
+++ b/.github/actions/setup/directories/action.yml
@@ -108,7 +108,7 @@ runs:
         git config --global init.defaultBranch garbage
 
     - if: inputs.checkout
-      uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
+      uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
       with:
         path: ${{ inputs.srcdir }}
         fetch-depth: ${{ inputs.fetch-depth }}
diff --git a/.github/workflows/annocheck.yml b/.github/workflows/annocheck.yml
index 1400dcc38bc546..f58363f76a23e6 100644
--- a/.github/workflows/annocheck.yml
+++ b/.github/workflows/annocheck.yml
@@ -61,7 +61,7 @@ jobs:
       - run: id
         working-directory:
 
-      - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
         with:
           sparse-checkout-cone-mode: false
           sparse-checkout: /.github
diff --git a/.github/workflows/auto_review_pr.yml b/.github/workflows/auto_review_pr.yml
index b3f73a206afeda..866029a487bacb 100644
--- a/.github/workflows/auto_review_pr.yml
+++ b/.github/workflows/auto_review_pr.yml
@@ -25,7 +25,7 @@ jobs:
 
     steps:
       - name: Checkout repository
-        uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
+        uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
         with:
           persist-credentials: false
 
diff --git a/.github/workflows/baseruby.yml b/.github/workflows/baseruby.yml
index ce517b25c586b7..301366f4787583 100644
--- a/.github/workflows/baseruby.yml
+++ b/.github/workflows/baseruby.yml
@@ -53,7 +53,7 @@ jobs:
           ruby-version: ${{ matrix.ruby }}
           bundler: none
 
-      - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
         with:
           persist-credentials: false
 
diff --git a/.github/workflows/bundled_gems.yml b/.github/workflows/bundled_gems.yml
index 28df5172ca037a..dce02c03093c7f 100644
--- a/.github/workflows/bundled_gems.yml
+++ b/.github/workflows/bundled_gems.yml
@@ -34,7 +34,7 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-      - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
         with:
           token: ${{ (github.repository == 'ruby/ruby' && !startsWith(github.event_name, 'pull')) && secrets.MATZBOT_AUTO_UPDATE_TOKEN || secrets.GITHUB_TOKEN }}
 
diff --git a/.github/workflows/check_dependencies.yml b/.github/workflows/check_dependencies.yml
index 7f142eb6a3bb1a..709af602ff908b 100644
--- a/.github/workflows/check_dependencies.yml
+++ b/.github/workflows/check_dependencies.yml
@@ -30,7 +30,7 @@ jobs:
     runs-on: ${{ matrix.os }}
 
     steps:
-      - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
         with:
           persist-credentials: false
 
diff --git a/.github/workflows/check_misc.yml b/.github/workflows/check_misc.yml
index 4cde7b8e294b6a..b3a56c8c8e24e9 100644
--- a/.github/workflows/check_misc.yml
+++ b/.github/workflows/check_misc.yml
@@ -18,7 +18,7 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-      - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
         with:
           token: ${{ (github.repository == 'ruby/ruby' && !startsWith(github.event_name, 'pull')) && secrets.MATZBOT_AUTO_UPDATE_TOKEN || secrets.GITHUB_TOKEN }}
           persist-credentials: false
@@ -100,7 +100,7 @@ jobs:
           { echo version=$2; echo ref=$4; } >> $GITHUB_OUTPUT
 
       - name: Checkout rdoc
-        uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
+        uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
         with:
           repository: ruby/rdoc
           ref: ${{ steps.rdoc.outputs.ref }}
diff --git a/.github/workflows/check_sast.yml b/.github/workflows/check_sast.yml
index 291cc37a1ff257..943bb71685f8a6 100644
--- a/.github/workflows/check_sast.yml
+++ b/.github/workflows/check_sast.yml
@@ -40,7 +40,7 @@ jobs:
       security-events: write
 
     steps:
-      - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
         with:
           persist-credentials: false
 
@@ -73,7 +73,7 @@ jobs:
 
     steps:
       - name: Checkout repository
-        uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
+        uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
         with:
           persist-credentials: false
 
diff --git a/.github/workflows/compilers.yml b/.github/workflows/compilers.yml
index da0eff2008121a..51f16b09be730a 100644
--- a/.github/workflows/compilers.yml
+++ b/.github/workflows/compilers.yml
@@ -56,7 +56,7 @@ jobs:
     timeout-minutes: 60
 
     steps:
-      - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
         with: { sparse-checkout-cone-mode: false, sparse-checkout: /.github, persist-credentials: false }
       # Set fetch-depth: 10 so that Launchable can receive commits information.
       - { uses: './.github/actions/setup/directories', with: { srcdir: 'src', builddir: 'build', makeup: true, fetch-depth: 10 } }
@@ -79,7 +79,7 @@ jobs:
     timeout-minutes: 60
 
     steps:
-      - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
         with: { sparse-checkout-cone-mode: false, sparse-checkout: /.github, persist-credentials: false }
       - { uses: './.github/actions/setup/directories', with: { srcdir: 'src', builddir: 'build', makeup: true, fetch-depth: 10 } }
       - name: 'GCC 15 LTO'
@@ -107,7 +107,7 @@ jobs:
     timeout-minutes: 60
 
     steps:
-      - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
         with: { sparse-checkout-cone-mode: false, sparse-checkout: /.github, persist-credentials: false }
       - { uses: './.github/actions/setup/directories', with: { srcdir: 'src', builddir: 'build', makeup: true, fetch-depth: 10 } }
       - { uses: './.github/actions/compilers', name: 'clang 20', with: { tag: 'clang-20' }, timeout-minutes: 8 }
@@ -126,7 +126,7 @@ jobs:
     timeout-minutes: 60
 
     steps:
-      - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
         with: { sparse-checkout-cone-mode: false, sparse-checkout: /.github, persist-credentials: false }
       - { uses: './.github/actions/setup/directories', with: { srcdir: 'src', builddir: 'build', makeup: true, fetch-depth: 10 } }
       - { uses: './.github/actions/compilers', name: 'clang 13', with: { tag: 'clang-13' }, timeout-minutes: 8 }
@@ -147,7 +147,7 @@ jobs:
     timeout-minutes: 60
 
     steps:
-      - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
         with: { sparse-checkout-cone-mode: false, sparse-checkout: /.github, persist-credentials: false }
       - { uses: './.github/actions/setup/directories', with: { srcdir: 'src', builddir: 'build', makeup: true, fetch-depth: 10 } }
       # -Wno-strict-prototypes is necessary with current clang-15 since
@@ -173,7 +173,7 @@ jobs:
     timeout-minutes: 60
 
     steps:
-      - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
         with: { sparse-checkout-cone-mode: false, sparse-checkout: /.github, persist-credentials: false }
       - { uses: './.github/actions/setup/directories', with: { srcdir: 'src', builddir: 'build', makeup: true, fetch-depth: 10 } }
       - { uses: './.github/actions/compilers', name: 'C++20', with: { CXXFLAGS: '-std=c++20 -Werror=pedantic -pedantic-errors -Wno-c++11-long-long' }, timeout-minutes: 8 }
@@ -193,7 +193,7 @@ jobs:
     timeout-minutes: 60
 
     steps:
-      - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
         with: { sparse-checkout-cone-mode: false, sparse-checkout: /.github, persist-credentials: false }
       - { uses: './.github/actions/setup/directories', with: { srcdir: 'src', builddir: 'build', makeup: true, fetch-depth: 10 } }
       - { uses: './.github/actions/compilers', name: 'disable-jit',         with: { append_configure: '--disable-yjit --disable-zjit' }, timeout-minutes: 8 }
@@ -213,7 +213,7 @@ jobs:
     timeout-minutes: 60
 
     steps:
-      - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
         with: { sparse-checkout-cone-mode: false, sparse-checkout: /.github, persist-credentials: false }
       - { uses: './.github/actions/setup/directories', with: { srcdir: 'src', builddir: 'build', makeup: true, fetch-depth: 10 } }
       - { uses: './.github/actions/compilers', name: 'NDEBUG',                         with: { cppflags: '-DNDEBUG' }, timeout-minutes: 8 }
@@ -232,7 +232,7 @@ jobs:
     timeout-minutes: 60
 
     steps:
-      - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
         with: { sparse-checkout-cone-mode: false, sparse-checkout: /.github, persist-credentials: false }
       - { uses: './.github/actions/setup/directories', with: { srcdir: 'src', builddir: 'build', makeup: true, fetch-depth: 10 } }
       - { uses: './.github/actions/compilers', name: 'HASH_DEBUG',                     with: { cppflags: '-DHASH_DEBUG' }, timeout-minutes: 8 }
@@ -252,7 +252,7 @@ jobs:
     timeout-minutes: 60
 
     steps:
-      - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
         with: { sparse-checkout-cone-mode: false, sparse-checkout: /.github, persist-credentials: false }
       - { uses: './.github/actions/setup/directories', with: { srcdir: 'src', builddir: 'build', makeup: true, fetch-depth: 10 } }
       - { uses: './.github/actions/compilers', name: 'USE_LAZY_LOAD',                  with: { cppflags: '-DUSE_LAZY_LOAD' }, timeout-minutes: 8 }
@@ -273,7 +273,7 @@ jobs:
     timeout-minutes: 60
 
     steps:
-      - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
         with: { sparse-checkout-cone-mode: false, sparse-checkout: /.github, persist-credentials: false }
       - { uses: './.github/actions/setup/directories', with: { srcdir: 'src', builddir: 'build', makeup: true, fetch-depth: 10 } }
       - { uses: './.github/actions/compilers', name: 'GC_DEBUG_STRESS_TO_CLASS',       with: { cppflags: '-DGC_DEBUG_STRESS_TO_CLASS' }, timeout-minutes: 8 }
@@ -292,7 +292,7 @@ jobs:
     timeout-minutes: 60
 
     steps:
-      - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
         with: { sparse-checkout-cone-mode: false, sparse-checkout: /.github, persist-credentials: false }
       - { uses: './.github/actions/setup/directories', with: { srcdir: 'src', builddir: 'build', makeup: true, fetch-depth: 10 } }
       - { uses: './.github/actions/compilers', name: 'VM_DEBUG_BP_CHECK',              with: { cppflags: '-DVM_DEBUG_BP_CHECK' }, timeout-minutes: 8 }
@@ -322,7 +322,7 @@ jobs:
       - 'compileB'
       - 'compileC'
     steps:
-      - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
         with: { sparse-checkout-cone-mode: false, sparse-checkout: /.github, persist-credentials: false }
       - uses: ./.github/actions/slack
         with:
diff --git a/.github/workflows/crosscompile.yml b/.github/workflows/crosscompile.yml
index 9d08ce96c250ce..79b2868161d7c5 100644
--- a/.github/workflows/crosscompile.yml
+++ b/.github/workflows/crosscompile.yml
@@ -52,7 +52,7 @@ jobs:
       )}}
 
     steps:
-      - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
         with:
           sparse-checkout-cone-mode: false
           sparse-checkout: /.github
diff --git a/.github/workflows/cygwin.yml b/.github/workflows/cygwin.yml
index 126aa8546c69c5..aeb98b95eca222 100644
--- a/.github/workflows/cygwin.yml
+++ b/.github/workflows/cygwin.yml
@@ -40,7 +40,7 @@ jobs:
     steps:
       - run: git config --global core.autocrlf input
 
-      - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
         with:
           persist-credentials: false
 
diff --git a/.github/workflows/default_gems_list.yml b/.github/workflows/default_gems_list.yml
index b21ff7b42ab9f6..457311239caf76 100644
--- a/.github/workflows/default_gems_list.yml
+++ b/.github/workflows/default_gems_list.yml
@@ -23,7 +23,7 @@ jobs:
     if: ${{ github.repository == 'ruby/ruby' }}
 
     steps:
-      - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
         with:
           token: ${{ (github.repository == 'ruby/ruby' && !startsWith(github.event_name, 'pull')) && secrets.MATZBOT_AUTO_UPDATE_TOKEN || secrets.GITHUB_TOKEN }}
 
diff --git a/.github/workflows/macos.yml b/.github/workflows/macos.yml
index c8e32674832359..c4fd3faa14b8cf 100644
--- a/.github/workflows/macos.yml
+++ b/.github/workflows/macos.yml
@@ -70,7 +70,7 @@ jobs:
       )}}
 
     steps:
-      - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
         with:
           sparse-checkout-cone-mode: false
           sparse-checkout: /.github
diff --git a/.github/workflows/mingw.yml b/.github/workflows/mingw.yml
index 91a3571ad4235d..409a835cae5483 100644
--- a/.github/workflows/mingw.yml
+++ b/.github/workflows/mingw.yml
@@ -168,7 +168,7 @@ jobs:
           [ ${#failed[@]} -eq 0 ]
         shell: sh
 
-      - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
         with:
           sparse-checkout-cone-mode: false
           sparse-checkout: /.github
diff --git a/.github/workflows/modgc.yml b/.github/workflows/modgc.yml
index 7e423bc7d139af..d0a980700821a8 100644
--- a/.github/workflows/modgc.yml
+++ b/.github/workflows/modgc.yml
@@ -53,7 +53,7 @@ jobs:
       )}}
 
     steps:
-      - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
         with:
           sparse-checkout-cone-mode: false
           sparse-checkout: /.github
diff --git a/.github/workflows/parse_y.yml b/.github/workflows/parse_y.yml
index 13610a9bad46f9..1087ac946b7b51 100644
--- a/.github/workflows/parse_y.yml
+++ b/.github/workflows/parse_y.yml
@@ -51,7 +51,7 @@ jobs:
       )}}
 
     steps:
-      - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
         with:
           sparse-checkout-cone-mode: false
           sparse-checkout: /.github
diff --git a/.github/workflows/post_push.yml b/.github/workflows/post_push.yml
index fb1c324b32aa2d..53a4644b3bfebe 100644
--- a/.github/workflows/post_push.yml
+++ b/.github/workflows/post_push.yml
@@ -35,7 +35,7 @@ jobs:
           REDMINE_SYS_API_KEY: ${{ secrets.REDMINE_SYS_API_KEY }}
         if: ${{ github.ref == 'refs/heads/master' || startsWith(github.ref, 'refs/heads/ruby_') }}
 
-      - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
         with:
           fetch-depth: 500 # for notify-slack-commits
           token: ${{ secrets.MATZBOT_AUTO_UPDATE_TOKEN }}
diff --git a/.github/workflows/publish.yml b/.github/workflows/publish.yml
index 8ec6c75e5e5b1d..daa852fc3cba64 100644
--- a/.github/workflows/publish.yml
+++ b/.github/workflows/publish.yml
@@ -18,7 +18,7 @@ jobs:
   release:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
         with:
           persist-credentials: false
 
diff --git a/.github/workflows/rust-warnings.yml b/.github/workflows/rust-warnings.yml
index ab209587240a38..6a88a4247d996a 100644
--- a/.github/workflows/rust-warnings.yml
+++ b/.github/workflows/rust-warnings.yml
@@ -41,7 +41,7 @@ jobs:
       )}}
 
     steps:
-      - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
         with:
           persist-credentials: false
 
diff --git a/.github/workflows/scorecards.yml b/.github/workflows/scorecards.yml
index dae9640a9d0f36..bdbd8090d2f719 100644
--- a/.github/workflows/scorecards.yml
+++ b/.github/workflows/scorecards.yml
@@ -34,7 +34,7 @@ jobs:
 
     steps:
       - name: "Checkout code"
-        uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
+        uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
         with:
           persist-credentials: false
 
diff --git a/.github/workflows/spec_guards.yml b/.github/workflows/spec_guards.yml
index 128b96887e030c..6823b94fd9bfb4 100644
--- a/.github/workflows/spec_guards.yml
+++ b/.github/workflows/spec_guards.yml
@@ -45,7 +45,7 @@ jobs:
       fail-fast: false
 
     steps:
-      - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
         with:
           persist-credentials: false
 
diff --git a/.github/workflows/sync_default_gems.yml b/.github/workflows/sync_default_gems.yml
index a5a5f4243840a2..5795d4abf03066 100644
--- a/.github/workflows/sync_default_gems.yml
+++ b/.github/workflows/sync_default_gems.yml
@@ -34,7 +34,7 @@ jobs:
     if: ${{ github.repository == 'ruby/ruby' }}
 
     steps:
-      - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
         name: Check out ruby/ruby
         with:
           token: ${{ github.repository == 'ruby/ruby' && secrets.MATZBOT_AUTO_UPDATE_TOKEN || secrets.GITHUB_TOKEN }}
diff --git a/.github/workflows/tarball-macos.yml b/.github/workflows/tarball-macos.yml
index 1302b653f0c401..d248338b916d6c 100644
--- a/.github/workflows/tarball-macos.yml
+++ b/.github/workflows/tarball-macos.yml
@@ -87,7 +87,7 @@ jobs:
           "$PREFIX/bin/gem" -v
           "$PREFIX/bin/bundle" -v
         if: matrix.test_task == 'check'
-      - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
         with:
           sparse-checkout: .github/actions/slack
           sparse-checkout-cone-mode: false
diff --git a/.github/workflows/tarball-test.yml b/.github/workflows/tarball-test.yml
index f27f842332ba4f..99724fd2eff485 100644
--- a/.github/workflows/tarball-test.yml
+++ b/.github/workflows/tarball-test.yml
@@ -46,7 +46,7 @@ jobs:
           || contains(github.event.pull_request.labels.*.name, 'Documentation')
           || (github.event.pull_request.user.login == 'dependabot[bot]')
           )}}
-      - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
         with:
           fetch-depth: 1 # actions/checkout fetches all heads/tags unless > 0
           persist-credentials: false
diff --git a/.github/workflows/tarball-ubuntu.yml b/.github/workflows/tarball-ubuntu.yml
index e458c9f8079517..8d3dbe1037a378 100644
--- a/.github/workflows/tarball-ubuntu.yml
+++ b/.github/workflows/tarball-ubuntu.yml
@@ -126,7 +126,7 @@ jobs:
         if: matrix.test_task == 'check'
       - name: Show .local
         run: find $HOME/.local -ls
-      - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
         with:
           sparse-checkout: .github/actions/slack
           sparse-checkout-cone-mode: false
diff --git a/.github/workflows/tarball-windows.yml b/.github/workflows/tarball-windows.yml
index 75661eef71f469..c3f12fe970cea7 100644
--- a/.github/workflows/tarball-windows.yml
+++ b/.github/workflows/tarball-windows.yml
@@ -150,7 +150,7 @@ jobs:
         timeout-minutes: 70
         continue-on-error: ${{ matrix.continue-on-error || false }}
 
-      - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
         with:
           sparse-checkout: .github/actions/slack
           sparse-checkout-cone-mode: false
diff --git a/.github/workflows/ubuntu.yml b/.github/workflows/ubuntu.yml
index b3ff647df679a4..da918d06d794c5 100644
--- a/.github/workflows/ubuntu.yml
+++ b/.github/workflows/ubuntu.yml
@@ -69,7 +69,7 @@ jobs:
       )}}
 
     steps: &make-steps
-      - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
         with:
           sparse-checkout-cone-mode: false
           sparse-checkout: /.github
@@ -243,7 +243,7 @@ jobs:
       )}}
 
     steps:
-      - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
         with:
           persist-credentials: false
 
@@ -261,7 +261,7 @@ jobs:
       - run: make install
 
       - name: Checkout ruby-bench
-        uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
+        uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
         with:
           repository: ruby/ruby-bench
           persist-credentials: false
diff --git a/.github/workflows/wasm.yml b/.github/workflows/wasm.yml
index e9841ac95e95a4..b4cc2a6c8ff8c1 100644
--- a/.github/workflows/wasm.yml
+++ b/.github/workflows/wasm.yml
@@ -59,7 +59,7 @@ jobs:
       )}}
 
     steps:
-      - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
         with:
           sparse-checkout-cone-mode: false
           sparse-checkout: /.github
diff --git a/.github/workflows/windows.yml b/.github/workflows/windows.yml
index 5e6282c36dcbed..43b025e0c8ea4a 100644
--- a/.github/workflows/windows.yml
+++ b/.github/workflows/windows.yml
@@ -70,7 +70,7 @@ jobs:
           bundler: none
           windows-toolchain: none
 
-      - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
         with:
           persist-credentials: false
           sparse-checkout-cone-mode: false
diff --git a/.github/workflows/yjit-macos.yml b/.github/workflows/yjit-macos.yml
index a17bffcf41e709..7d93dbafd92672 100644
--- a/.github/workflows/yjit-macos.yml
+++ b/.github/workflows/yjit-macos.yml
@@ -65,7 +65,7 @@ jobs:
       )}}
 
     steps:
-      - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
         with:
           sparse-checkout-cone-mode: false
           sparse-checkout: /.github
diff --git a/.github/workflows/yjit-ubuntu.yml b/.github/workflows/yjit-ubuntu.yml
index c52bc40f215bdf..ed1202c25f77ab 100644
--- a/.github/workflows/yjit-ubuntu.yml
+++ b/.github/workflows/yjit-ubuntu.yml
@@ -41,7 +41,7 @@ jobs:
       )}}
 
     steps:
-      - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
         with:
           persist-credentials: false
 
@@ -75,7 +75,7 @@ jobs:
       )}}
 
     steps:
-      - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
         with:
           persist-credentials: false
 
@@ -130,7 +130,7 @@ jobs:
       )}}
 
     steps:
-      - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
         with:
           sparse-checkout-cone-mode: false
           sparse-checkout: /.github
diff --git a/.github/workflows/zjit-macos.yml b/.github/workflows/zjit-macos.yml
index f109342fd076e4..b6afbc223af1cd 100644
--- a/.github/workflows/zjit-macos.yml
+++ b/.github/workflows/zjit-macos.yml
@@ -74,7 +74,7 @@ jobs:
       )}}
 
     steps:
-      - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
         with:
           sparse-checkout-cone-mode: false
           sparse-checkout: /.github
@@ -98,7 +98,7 @@ jobs:
           rustup install ${{ matrix.rust_version }} --profile minimal
           rustup default ${{ matrix.rust_version }}
 
-      - uses: taiki-e/install-action@07b4745e0c39a41822af610387492e3e53aa222b # v2.83.4
+      - uses: taiki-e/install-action@a6b2e2dcd845ddd7f509ce4f3ed3d922b80cc5d9 # v2.84.0
         with:
           tool: nextest@0.9
         if: ${{ matrix.test_task == 'zjit-check' }}
@@ -204,7 +204,7 @@ jobs:
       )}}
 
     steps:
-      - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
         with:
           persist-credentials: false
 
@@ -226,7 +226,7 @@ jobs:
         run: echo "MAKEFLAGS=" >> "$GITHUB_ENV"
 
       - name: Checkout ruby-bench
-        uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
+        uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
         with:
           persist-credentials: false
           repository: ruby/ruby-bench
diff --git a/.github/workflows/zjit-ubuntu.yml b/.github/workflows/zjit-ubuntu.yml
index 31c3c97c507f6f..1f4f663a744f6d 100644
--- a/.github/workflows/zjit-ubuntu.yml
+++ b/.github/workflows/zjit-ubuntu.yml
@@ -46,7 +46,7 @@ jobs:
       )}}
 
     steps:
-      - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
         with:
           persist-credentials: false
 
@@ -129,7 +129,7 @@ jobs:
       )}}
 
     steps:
-      - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
         with:
           sparse-checkout-cone-mode: false
           sparse-checkout: /.github
@@ -142,7 +142,7 @@ jobs:
           ruby-version: '3.1'
           bundler: none
 
-      - uses: taiki-e/install-action@07b4745e0c39a41822af610387492e3e53aa222b # v2.83.4
+      - uses: taiki-e/install-action@a6b2e2dcd845ddd7f509ce4f3ed3d922b80cc5d9 # v2.84.0
         with:
           tool: nextest@0.9
         if: ${{ matrix.test_task == 'zjit-check' }}
@@ -278,7 +278,7 @@ jobs:
       )}}
 
     steps:
-      - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
+      - uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
         with:
           persist-credentials: false
 
@@ -296,7 +296,7 @@ jobs:
       - run: make install
 
       - name: Checkout ruby-bench
-        uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
+        uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
         with:
           repository: ruby/ruby-bench
           persist-credentials: false
diff --git a/NEWS.md b/NEWS.md
index 08c75c1a679085..f0ff01ec810043 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -120,7 +120,7 @@ releases.
 * bundler 4.1.0.dev
   * 4.0.3 to [v4.0.4][bundler-v4.0.4], [v4.0.5][bundler-v4.0.5], [v4.0.6][bundler-v4.0.6], [v4.0.7][bundler-v4.0.7], [v4.0.8][bundler-v4.0.8], [v4.0.9][bundler-v4.0.9], [v4.0.10][bundler-v4.0.10], [v4.0.11][bundler-v4.0.11], [v4.0.12][bundler-v4.0.12], [v4.0.13][bundler-v4.0.13], [v4.0.14][bundler-v4.0.14], [v4.0.15][bundler-v4.0.15], [v4.0.16][bundler-v4.0.16]
 * erb 6.0.6
-  * 6.0.1 to [v6.0.1.1][erb-v6.0.1.1], [v6.0.2][erb-v6.0.2], [v6.0.3][erb-v6.0.3], [v6.0.4][erb-v6.0.4], [v6.0.5][erb-v6.0.5]
+  * 6.0.1 to [v6.0.1.1][erb-v6.0.1.1], [v6.0.2][erb-v6.0.2], [v6.0.3][erb-v6.0.3], [v6.0.4][erb-v6.0.4], [v6.0.5][erb-v6.0.5], [v6.0.6][erb-v6.0.6]
 * error_highlight 0.7.2
 * ipaddr 1.2.9
   * 1.2.8 to [v1.2.9][ipaddr-v1.2.9]
@@ -287,6 +287,7 @@ A lot of work has gone into making Ractors more stable, performant, and usable.
 [erb-v6.0.3]: https://github.com/ruby/erb/releases/tag/v6.0.3
 [erb-v6.0.4]: https://github.com/ruby/erb/releases/tag/v6.0.4
 [erb-v6.0.5]: https://github.com/ruby/erb/releases/tag/v6.0.5
+[erb-v6.0.6]: https://github.com/ruby/erb/releases/tag/v6.0.6
 [ipaddr-v1.2.9]: https://github.com/ruby/ipaddr/releases/tag/v1.2.9
 [json-v2.18.1]: https://github.com/ruby/json/releases/tag/v2.18.1
 [json-v2.19.0]: https://github.com/ruby/json/releases/tag/v2.19.0
diff --git a/builtin.c b/builtin.c
index 03c9d03bc3183c..094bd18c222168 100644
--- a/builtin.c
+++ b/builtin.c
@@ -1,5 +1,7 @@
 #include "internal.h"
 #include "internal/box.h"
+#include "internal/string.h"
+#include "internal/variable.h"
 #include "vm_core.h"
 #include "iseq.h"
 #include "builtin.h"
@@ -91,14 +93,17 @@ rb_define_gem_modules(VALUE flags_value, VALUE _)
 
     if (flags->gem) {
         rb_define_module("Gem");
+        // Make the error decoration gems autoload on first constant access.
+        // error.c loads them for error display on the first error
+        // (or eagerly via Process.warmup).
         if (flags->error_highlight) {
-            rb_define_module("ErrorHighlight");
+            rb_autoload_str(rb_cObject, rb_intern("ErrorHighlight"), rb_fstring_cstr("error_highlight"));
         }
         if (flags->did_you_mean) {
-            rb_define_module("DidYouMean");
+            rb_autoload_str(rb_cObject, rb_intern("DidYouMean"), rb_fstring_cstr("did_you_mean"));
         }
         if (flags->syntax_suggest) {
-            rb_define_module("SyntaxSuggest");
+            rb_autoload_str(rb_cObject, rb_intern("SyntaxSuggest"), rb_fstring_cstr("syntax_suggest"));
         }
     }
 
diff --git a/common.mk b/common.mk
index 4202dbddb2a124..e47d030724310a 100644
--- a/common.mk
+++ b/common.mk
@@ -1469,8 +1469,9 @@ run.gdb:
 	echo '# handle SIGINT nostop'         >> run.gdb
 	echo '# handle SIGPIPE nostop'        >> run.gdb
 	echo '# b rb_longjmp'                 >> run.gdb
-	echo source $(srcdir)/breakpoints.gdb >> run.gdb
-	echo source $(srcdir)/.gdbinit        >> run.gdb
+	echo directory $(srcdir)              >> run.gdb
+	echo source -s breakpoints.gdb        >> run.gdb
+	echo source -s .gdbinit               >> run.gdb
 	echo 'set $$_exitcode = -999'         >> run.gdb
 	echo run                              >> run.gdb
 	echo 'if $$_exitcode != -999'         >> run.gdb
diff --git a/depend b/depend
index e9e8bdc3fed063..70a18b4c9a35d1 100644
--- a/depend
+++ b/depend
@@ -1029,6 +1029,7 @@ builtin.$(OBJEXT): $(top_srcdir)/internal/sanitizers.h
 builtin.$(OBJEXT): $(top_srcdir)/internal/serial.h
 builtin.$(OBJEXT): $(top_srcdir)/internal/set_table.h
 builtin.$(OBJEXT): $(top_srcdir)/internal/static_assert.h
+builtin.$(OBJEXT): $(top_srcdir)/internal/string.h
 builtin.$(OBJEXT): $(top_srcdir)/internal/struct.h
 builtin.$(OBJEXT): $(top_srcdir)/internal/variable.h
 builtin.$(OBJEXT): $(top_srcdir)/internal/vm.h
@@ -1085,6 +1086,7 @@ builtin.$(OBJEXT): {$(VPATH)}builtin_binary.rbbin
 builtin.$(OBJEXT): {$(VPATH)}config.h
 builtin.$(OBJEXT): {$(VPATH)}constant.h
 builtin.$(OBJEXT): {$(VPATH)}defines.h
+builtin.$(OBJEXT): {$(VPATH)}encindex.h
 builtin.$(OBJEXT): {$(VPATH)}encoding.h
 builtin.$(OBJEXT): {$(VPATH)}id.h
 builtin.$(OBJEXT): {$(VPATH)}id_table.h
diff --git a/error.c b/error.c
index 751300565e2aa8..4ff96115f727ee 100644
--- a/error.c
+++ b/error.c
@@ -47,6 +47,7 @@
 #include "ruby/encoding.h"
 #include "ruby/st.h"
 #include "ruby/util.h"
+#include "internal/vm.h"
 #include "ruby_assert.h"
 #include "vm_core.h"
 #include "yjit.h"
@@ -1760,6 +1761,71 @@ exc_message(VALUE exc)
     return rb_funcallv(exc, idTo_s, 0, 0);
 }
 
+// Whether error_highlight, did_you_mean, and syntax_suggest have already
+// been loaded (lazily on the first error, or eagerly via Process.warmup).
+// Ractors run in parallel, so the load is claimed by an atomic exchange.
+static rb_atomic_t decoration_gems_loaded = 0;
+
+static VALUE
+load_decoration_gem(VALUE feature)
+{
+    // The C-level require bypasses Kernel#require monkeypatches;
+    // displaying an error must not invoke or depend on them.
+    return rb_require_string(feature);
+}
+
+// Require error_highlight, did_you_mean, and syntax_suggest.
+// rb_define_gem_modules registers an autoload entry for each enabled gem;
+// disabled gems have no entry and are skipped.
+static void
+require_decoration_gems(void)
+{
+    // Loading must not disturb the caller's $!: it is called while
+    // displaying an exception. rb_protect does not preserve errinfo, so
+    // save and restore it around the requires, leaving $! untouched
+    // whether or not a require raises.
+    VALUE saved_errinfo = rb_errinfo();
+    static const char *const gems[] = {"ErrorHighlight", "DidYouMean", "SyntaxSuggest"};
+    for (size_t i = 0; i < numberof(gems); i++) {
+        VALUE feature = rb_autoload_p(rb_cObject, rb_intern(gems[i]));
+        if (NIL_P(feature)) continue;
+        int state;
+        rb_protect(load_decoration_gem, feature, &state);
+        (void)state;
+    }
+    rb_set_errinfo(saved_errinfo);
+}
+
+// Load the decoration gems on the first error display instead of at boot.
+// In non-main Ractors rb_require_string delegates the require to the main
+// Ractor, so this works from any Ractor.
+// Returns whether the caller should re-dispatch to pick up the
+// detailed_message decorators the gems prepend.
+static bool
+lazy_load_decoration_gems(VALUE exc)
+{
+    if (ATOMIC_EXCHANGE(decoration_gems_loaded, 1)) return false;
+
+    // When entered through super from a decorator already sitting above
+    // this method, the caller decorates the result; re-dispatching would
+    // decorate it twice.
+    bool redispatch = rb_method_basic_definition_p(CLASS_OF(exc), id_detailed_message);
+
+    require_decoration_gems();
+    return redispatch;
+}
+
+// Load the decoration gems eagerly, e.g. from Process.warmup before a
+// pre-forking server forks, so the prepended detailed_message decorators
+// land in shared memory and do not bust method caches at runtime.
+void
+rb_eager_load_detailed_message_extension(void)
+{
+    if (ATOMIC_EXCHANGE(decoration_gems_loaded, 1)) return;
+
+    require_decoration_gems();
+}
+
 /*
  * call-seq:
  *   detailed_message(highlight: false, **kwargs) -> string
@@ -1808,6 +1874,10 @@ exc_message(VALUE exc)
 static VALUE
 exc_detailed_message(int argc, VALUE *argv, VALUE exc)
 {
+    if (lazy_load_decoration_gems(exc)) {
+        return rb_funcallv_kw(exc, id_detailed_message, argc, argv, RB_PASS_CALLED_KEYWORDS);
+    }
+
     VALUE opt;
 
     rb_scan_args(argc, argv, "0:", &opt);
diff --git a/gc.c b/gc.c
index e03d1362b84eb2..4980c1b8859709 100644
--- a/gc.c
+++ b/gc.c
@@ -3135,13 +3135,10 @@ static inline bool
 rb_obj_using_gen_fields_table_p(VALUE obj)
 {
     switch (BUILTIN_TYPE(obj)) {
+      case T_STRUCT:
       case T_DATA:
         return false;
 
-      case T_STRUCT:
-        if (!FL_TEST_RAW(obj, RSTRUCT_GEN_FIELDS)) return false;
-        break;
-
       default:
         break;
     }
@@ -3335,9 +3332,7 @@ rb_gc_mark_children(void *objspace, VALUE obj)
             gc_mark_internal(ptr[i]);
         }
 
-        if (rb_obj_shape_has_fields(obj) && !FL_TEST_RAW(obj, RSTRUCT_GEN_FIELDS)) {
-            gc_mark_internal(RSTRUCT_FIELDS_OBJ(obj));
-        }
+        gc_mark_internal(RSTRUCT_FIELDS_OBJ(obj));
 
         break;
       }
@@ -4287,14 +4282,7 @@ rb_gc_update_object_references(void *objspace, VALUE obj)
                 UPDATE_IF_MOVED(objspace, ptr[i]);
             }
 
-            if (RSTRUCT_EMBED_LEN(obj)) {
-                if (!FL_TEST_RAW(obj, RSTRUCT_GEN_FIELDS)) {
-                    UPDATE_IF_MOVED(objspace, ptr[len]);
-                }
-            }
-            else {
-                UPDATE_IF_MOVED(objspace, RSTRUCT(obj)->as.heap.fields_obj);
-            }
+            UPDATE_IF_MOVED(objspace, RSTRUCT(obj)->fields_obj);
         }
         break;
       default:
diff --git a/gem_prelude.rb b/gem_prelude.rb
index 1b78d80c726625..1a0af96aedff20 100644
--- a/gem_prelude.rb
+++ b/gem_prelude.rb
@@ -7,21 +7,3 @@
 else
   require 'bundled_gems'
 end if defined?(Gem)
-
-begin
-  require 'error_highlight'
-rescue LoadError
-  warn "`error_highlight' was not loaded."
-end if defined?(ErrorHighlight)
-
-begin
-  require 'did_you_mean'
-rescue LoadError
-  warn "`did_you_mean' was not loaded."
-end if defined?(DidYouMean)
-
-begin
-  require 'syntax_suggest/core_ext'
-rescue LoadError
-  warn "`syntax_suggest' was not loaded."
-end if defined?(SyntaxSuggest)
diff --git a/internal/error.h b/internal/error.h
index fead2aee9562d8..8cda52827360d6 100644
--- a/internal/error.h
+++ b/internal/error.h
@@ -189,6 +189,7 @@ void rb_bug_without_die(const char *fmt, ...);
 NORETURN(void rb_no_implicit_conversion(VALUE val, const char *tname));
 NORETURN(void rb_cant_convert(VALUE val, const char *tname));
 NORETURN(void rb_cant_convert_invalid_return(VALUE val, const char *tname, const char *method_name, VALUE ret));
+void rb_eager_load_detailed_message_extension(void);
 
 RUBY_SYMBOL_EXPORT_BEGIN
 /* error.c (export) */
diff --git a/internal/range.h b/internal/range.h
index 80493ce13e4008..2e6d0cdac5b0b6 100644
--- a/internal/range.h
+++ b/internal/range.h
@@ -10,6 +10,9 @@
  */
 #include "internal/struct.h"    /* for RSTRUCT */
 
+#define RANGE_FL_INIT FL_USER18
+#define RANGE_FL_EXCL FL_USER19
+
 /* range.c */
 static inline VALUE RANGE_BEG(VALUE r);
 static inline VALUE RANGE_END(VALUE r);
@@ -30,7 +33,10 @@ RANGE_END(VALUE r)
 static inline VALUE
 RANGE_EXCL(VALUE r)
 {
-    return RSTRUCT_GET_RAW(r, 2);
+    if (FL_TEST_RAW(r, RANGE_FL_INIT)) {
+        return RBOOL(FL_TEST_RAW(r, RANGE_FL_EXCL));
+    }
+    return Qnil;
 }
 
 VALUE
diff --git a/internal/struct.h b/internal/struct.h
index d3c81573931cfa..ffe1786a896bac 100644
--- a/internal/struct.h
+++ b/internal/struct.h
@@ -17,26 +17,20 @@
  *          If non-zero, the struct is embedded (its contents follow the
  *          header, rather than being on a separately allocated buffer) and
  *          these bits are the length of the Struct.
- * 8:   RSTRUCT_GEN_FIELDS
- *          The struct is embedded and has no space left to store the
- *          IMEMO/fields reference. Any ivar this struct may have will be in
- *          the generic_fields_tbl. This flag doesn't imply the struct has
- *          ivars.
  */
 enum {
     RSTRUCT_EMBED_LEN_MASK = RUBY_FL_USER7 | RUBY_FL_USER6 | RUBY_FL_USER5 | RUBY_FL_USER4 |
                                  RUBY_FL_USER3 | RUBY_FL_USER2 | RUBY_FL_USER1,
     RSTRUCT_EMBED_LEN_SHIFT = (RUBY_FL_USHIFT+1),
-    RSTRUCT_GEN_FIELDS = RUBY_FL_USER8,
 };
 
 struct RStruct {
     struct RBasic basic;
+    VALUE fields_obj;
     union {
         struct {
             long len;
             const VALUE *ptr;
-            VALUE fields_obj;
         } heap;
         /* This is a length 1 array because:
          *   1. GCC has a bug that does not optimize C flexible array members
@@ -113,28 +107,12 @@ RSTRUCT_GET_RAW(VALUE st, long k)
 static inline VALUE
 RSTRUCT_FIELDS_OBJ(VALUE st)
 {
-    const long embed_len = RSTRUCT_EMBED_LEN(st);
-    VALUE fields_obj;
-    if (embed_len) {
-        RUBY_ASSERT(!FL_TEST_RAW(st, RSTRUCT_GEN_FIELDS));
-        fields_obj = RSTRUCT_GET_RAW(st, embed_len);
-    }
-    else {
-        fields_obj = RSTRUCT(st)->as.heap.fields_obj;
-    }
-    return fields_obj;
+    return RSTRUCT(st)->fields_obj;
 }
 
 static inline void
 RSTRUCT_SET_FIELDS_OBJ(VALUE st, VALUE fields_obj)
 {
-    const long embed_len = RSTRUCT_EMBED_LEN(st);
-    if (embed_len) {
-        RUBY_ASSERT(!FL_TEST_RAW(st, RSTRUCT_GEN_FIELDS));
-        RSTRUCT_SET_RAW(st, embed_len, fields_obj);
-    }
-    else {
-        RB_OBJ_WRITE(st, &RSTRUCT(st)->as.heap.fields_obj, fields_obj);
-    }
+    RB_OBJ_WRITE(st, &RSTRUCT(st)->fields_obj, fields_obj);
 }
 #endif /* INTERNAL_STRUCT_H */
diff --git a/lib/bundler/compact_index_client/cache_file.rb b/lib/bundler/compact_index_client/cache_file.rb
index 299d683438b887..03659aeac19e0a 100644
--- a/lib/bundler/compact_index_client/cache_file.rb
+++ b/lib/bundler/compact_index_client/cache_file.rb
@@ -103,7 +103,7 @@ def open(write_mode = "wb", perm = @perm, &block)
       # Returns false without appending when no digests since appending is too error prone to do without digests.
       def append(data)
         return false unless digests?
-        open("a") {|f| f.write data }
+        open("ab") {|f| f.write data }
         verify && commit
       end
 
diff --git a/lib/rubygems/commands/setup_command.rb b/lib/rubygems/commands/setup_command.rb
index ad4aaab384a0c6..e6c3c7d0de2a81 100644
--- a/lib/rubygems/commands/setup_command.rb
+++ b/lib/rubygems/commands/setup_command.rb
@@ -355,16 +355,30 @@ def install_default_bundler_gem(bin_dir)
       loaded_from = current_default_spec.loaded_from
       File.delete(loaded_from)
 
-      # Remove previous default gem executables if they were not shadowed by a regular gem
-      FileUtils.rm_rf current_default_spec.full_gem_path if all_specs_current_version.size == 1
+      previous_specs_dir = File.dirname(loaded_from)
+
+      # Remove previous default gem executables if they were not shadowed by a
+      # regular gem. They live under the same root as the previous default
+      # gemspec, which is not necessarily default_dir.
+      if all_specs_current_version.size == 1
+        previous_root = File.dirname(File.dirname(previous_specs_dir))
+        FileUtils.rm_rf File.join(previous_root, "gems", current_default_spec.full_name)
+      end
 
-      File.dirname(loaded_from)
+      previous_specs_dir
     else
       target_specs_dir = File.join(default_dir, "specifications", "default")
       mkdir_p target_specs_dir, mode: 0o755
       target_specs_dir
     end
 
+    # Root directory that specs_dir belongs to. It may differ from default_dir
+    # when Gem.default_specifications_dir is customized to live under a
+    # different root, like Homebrew does. Executables must be extracted under
+    # the same root as the default gemspec, since that's where activation of
+    # the default gem will look for them.
+    bundler_install_dir = File.dirname(File.dirname(specs_dir))
+
     new_bundler_spec = Gem::Specification.load("bundler.gemspec")
     full_name = new_bundler_spec.full_name
     gemspec_path = "#{full_name}.gemspec"
@@ -372,19 +386,20 @@ def install_default_bundler_gem(bin_dir)
     default_spec_path = File.join(specs_dir, gemspec_path)
     Gem.write_binary(default_spec_path, new_bundler_spec.to_ruby)
 
-    bundler_spec = Gem::Specification.load(default_spec_path)
-
     # Remove gemspec that was same version of vendored bundler.
     normal_gemspec = File.join(default_dir, "specifications", gemspec_path)
     if File.file? normal_gemspec
       File.delete normal_gemspec
     end
 
-    # Remove gem files that were same version of vendored bundler.
-    if File.directory? bundler_spec.gems_dir
-      Dir.entries(bundler_spec.gems_dir).
-        select {|default_gem| File.basename(default_gem) == full_name }.
-        each {|default_gem| rm_r File.join(bundler_spec.gems_dir, default_gem) }
+    # Remove gem files that were same version of vendored bundler. A regular
+    # gem lives under default_dir, which is not necessarily the same root as
+    # the default gemspec.
+    normal_gems_dir = File.join(default_dir, "gems")
+    if File.directory? normal_gems_dir
+      Dir.entries(normal_gems_dir).
+        select {|normal_gem| File.basename(normal_gem) == full_name }.
+        each {|normal_gem| rm_r File.join(normal_gems_dir, normal_gem) }
     end
 
     require_relative "../installer"
@@ -397,14 +412,14 @@ def install_default_bundler_gem(bin_dir)
         format_executable: options[:format_executable],
         force: options[:force],
         bin_dir: bin_dir,
-        install_dir: default_dir,
+        install_dir: bundler_install_dir,
         wrappers: true
       )
-      # We need to install only executable and default spec files.
-      # lib/bundler.rb and lib/bundler/* are available under the site_ruby directory.
+      # We only need to install the executables here. The default spec was
+      # already written above, and lib/bundler.rb and lib/bundler/* are
+      # available under the site_ruby directory.
       installer.extract_bin
       installer.generate_bin
-      installer.write_default_spec
     ensure
       FileUtils.rm_f built_gem
     end
diff --git a/lib/rubygems/compact_index_client/cache_file.rb b/lib/rubygems/compact_index_client/cache_file.rb
index 3656645d822089..00e5f541ddf6f0 100644
--- a/lib/rubygems/compact_index_client/cache_file.rb
+++ b/lib/rubygems/compact_index_client/cache_file.rb
@@ -99,7 +99,7 @@ def open(write_mode = "wb", perm = @perm, &block)
     # Returns false without appending when no digests since appending is too error prone to do without digests.
     def append(data)
       return false unless digests?
-      open("a") {|f| f.write data }
+      open("ab") {|f| f.write data }
       verify && commit
     end
 
diff --git a/process.c b/process.c
index 82e3e376fe6f1e..34ef4897ec7e29 100644
--- a/process.c
+++ b/process.c
@@ -8713,11 +8713,18 @@ static VALUE rb_mProcID_Syscall;
  *  * Frees all empty heap pages and increments the allocatable pages counter
  *    by the number of pages freed.
  *  * Invoke +malloc_trim+ if available to free empty malloc pages.
+ *  * Eagerly loads the +error_highlight+, +did_you_mean+, and +syntax_suggest+
+ *    gems, which are otherwise loaded lazily on the first error display.
  */
 
 static VALUE
 proc_warmup(VALUE _)
 {
+    // Load the error decoration gems now so that their detailed_message
+    // decorators land in shared memory before a pre-forking server forks,
+    // instead of being loaded lazily on the first error at runtime.
+    rb_eager_load_detailed_message_extension();
+
     RB_VM_LOCKING() {
         rb_gc_prepare_heap();
     }
diff --git a/range.c b/range.c
index fba73f4a39d1c3..e751c282c32964 100644
--- a/range.c
+++ b/range.c
@@ -40,9 +40,8 @@ static VALUE r_cover_p(VALUE, VALUE, VALUE, VALUE);
 
 #define RANGE_SET_BEG(r, v) (RSTRUCT_SET(r, 0, v))
 #define RANGE_SET_END(r, v) (RSTRUCT_SET(r, 1, v))
-#define RANGE_SET_EXCL(r, v) (RSTRUCT_SET(r, 2, v))
 
-#define EXCL(r) RTEST(RANGE_EXCL(r))
+#define EXCL(r) RTEST(FL_TEST(r, RANGE_FL_EXCL))
 
 static void
 range_init(VALUE range, VALUE beg, VALUE end, VALUE exclude_end)
@@ -56,7 +55,12 @@ range_init(VALUE range, VALUE beg, VALUE end, VALUE exclude_end)
             rb_raise(rb_eArgError, "bad value for range");
     }
 
-    RANGE_SET_EXCL(range, exclude_end);
+    if (RTEST(exclude_end)) {
+        FL_SET_RAW(range, RANGE_FL_EXCL);
+    }
+
+    FL_SET_RAW(range, RANGE_FL_INIT);
+
     RANGE_SET_BEG(range, beg);
     RANGE_SET_END(range, end);
 
@@ -79,7 +83,7 @@ range_modify(VALUE range)
 {
     rb_check_frozen(range);
     /* Ranges are immutable, so that they should be initialized only once. */
-    if (RANGE_EXCL(range) != Qnil) {
+    if (FL_TEST(range, RANGE_FL_INIT)) {
         rb_name_err_raise("'initialize' called twice", range, ID2SYM(idInitialize));
     }
 }
@@ -115,6 +119,7 @@ static VALUE
 range_initialize_copy(VALUE range, VALUE orig)
 {
     range_modify(range);
+    FL_SET_RAW(range, FL_TEST_RAW(orig, RANGE_FL_EXCL|RANGE_FL_INIT));
     rb_struct_init_copy(range, orig);
     return range;
 }
@@ -2960,7 +2965,7 @@ Init_Range(void)
 
     rb_cRange = rb_struct_define_without_accessor(
         "Range", rb_cObject, range_alloc,
-        "begin", "end", "excl", NULL);
+        "begin", "end", NULL);
 
     rb_include_module(rb_cRange, rb_mEnumerable);
     rb_marshal_define_compat(rb_cRange, rb_cObject, range_dumper, range_loader);
diff --git a/shape.c b/shape.c
index 5c59d2aee22aca..7815591d2d8877 100644
--- a/shape.c
+++ b/shape.c
@@ -1219,19 +1219,24 @@ rb_shape_expected_layout(VALUE obj)
       case T_OBJECT: {
           return SHAPE_ID_LAYOUT_ROBJECT;
       }
+
       case T_CLASS:
       case T_MODULE:
         if (FL_TEST_RAW(obj, RCLASS_BOXABLE)) {
             return SHAPE_ID_LAYOUT_OTHER;
         }
         return SHAPE_ID_LAYOUT_RCLASS;
+
+      case T_STRUCT:
       case T_DATA:
         return SHAPE_ID_LAYOUT_EXTENDED;
+
       case T_IMEMO:
         if (IMEMO_TYPE_P(obj, imemo_fields)) {
             return SHAPE_ID_LAYOUT_ROBJECT;
         }
         return SHAPE_ID_LAYOUT_OTHER;
+
       default:
         return SHAPE_ID_LAYOUT_OTHER;
     }
diff --git a/shape.h b/shape.h
index 5a7dc76c4d12cd..14b9a00bcc8ea5 100644
--- a/shape.h
+++ b/shape.h
@@ -107,7 +107,7 @@ typedef uint32_t redblack_id_t;
 enum shape_type {
     SHAPE_ROOT,
     SHAPE_IVAR,
-    SHAPE_OBJ_ID,
+    SHAPE_OBJ_ID
 };
 
 struct rb_shape {
diff --git a/spec/bundler/bundler/compact_index_client/cache_file_spec.rb b/spec/bundler/bundler/compact_index_client/cache_file_spec.rb
new file mode 100644
index 00000000000000..c9a87c3c2d447a
--- /dev/null
+++ b/spec/bundler/bundler/compact_index_client/cache_file_spec.rb
@@ -0,0 +1,30 @@
+# frozen_string_literal: true
+
+require "bundler/compact_index_client"
+require "bundler/compact_index_client/cache_file"
+require "tmpdir"
+
+RSpec.describe Bundler::CompactIndexClient::CacheFile do
+  let(:path) { Pathname.new(Dir.mktmpdir("localpath")).join("versions") }
+
+  def sha256(data)
+    { "sha-256" => Digest::SHA256.base64digest(data) }
+  end
+
+  it "appends in binary mode so line endings are preserved" do
+    path.binwrite "created_at: 2026-06-10\n---\n"
+
+    appended = nil
+    described_class.copy(path) do |file|
+      file.digests = sha256("created_at: 2026-06-10\n---\nrake 13.0.0\n")
+      appended = file.append("rake 13.0.0\n")
+    end
+
+    expect(appended).to be_truthy
+    # On Windows a text-mode append rewrites the appended LF as CRLF while the
+    # digest is computed over the pre-write bytes, so verify passes but the file
+    # on disk is corrupted. Read raw bytes to catch any stray carriage return.
+    expect(path.binread).not_to include("\r")
+    expect(path.binread).to eq("created_at: 2026-06-10\n---\nrake 13.0.0\n")
+  end
+end
diff --git a/spec/bundler/support/shards.rb b/spec/bundler/support/shards.rb
index 396ee8049d6b66..9fd67f921b4ff7 100644
--- a/spec/bundler/support/shards.rb
+++ b/spec/bundler/support/shards.rb
@@ -147,6 +147,7 @@ module Shards
         "spec/bundler/ci_detector_spec.rb",
       ],
       shard_d: [
+        "spec/bundler/compact_index_client/cache_file_spec.rb",
         "spec/bundler/rubygems_ext_spec.rb",
         "spec/bundler/resolver/cooldown_spec.rb",
         "spec/install/cooldown_spec.rb",
diff --git a/struct.c b/struct.c
index 32da26860b993a..0c6d7330dfc590 100644
--- a/struct.c
+++ b/struct.c
@@ -818,6 +818,8 @@ struct_heap_alloc(VALUE st, size_t len)
     return ALLOC_N(VALUE, len);
 }
 
+STATIC_ASSERT(robject_rstruct_fields_offset, offsetof(struct RObject, as.extended) == offsetof(struct RStruct, fields_obj));
+
 static VALUE
 struct_alloc(VALUE klass)
 {
@@ -833,40 +835,26 @@ struct_alloc(VALUE klass)
 
     if (n > 0 && n <= embed_len_max && rb_gc_size_allocatable_p(embedded_size)) {
         flags |= n << RSTRUCT_EMBED_LEN_SHIFT;
-        if (RCLASS_MAX_IV_COUNT(klass) == 0) {
-            // We set the flag before calling `NEWOBJ_OF` in case a NEWOBJ tracepoint does
-            // attempt to write fields. We'll remove it later if no fields was written to.
-            flags |= RSTRUCT_GEN_FIELDS;
-        }
-
-        NEWOBJ_OF(st, struct RStruct, klass, flags, embedded_size);
-        if (RCLASS_MAX_IV_COUNT(klass) == 0) {
-            if (!rb_obj_shape_has_fields((VALUE)st)
-                    && embedded_size < rb_obj_shape_slot_size((VALUE)st)) {
-                FL_UNSET_RAW((VALUE)st, RSTRUCT_GEN_FIELDS);
-                RSTRUCT_SET_FIELDS_OBJ((VALUE)st, 0);
-            }
-        }
-        else {
-            RSTRUCT_SET_FIELDS_OBJ((VALUE)st, 0);
-        }
 
-        rb_mem_clear((VALUE *)st->as.ary, n);
+        VALUE st = rb_newobj(GET_EC(), klass, flags, ROOT_SHAPE_ID | SHAPE_ID_LAYOUT_EXTENDED, true, embedded_size);
+        RSTRUCT_SET_FIELDS_OBJ(st, 0);
+        rb_mem_clear((VALUE *)RSTRUCT(st)->as.ary, n);
 
-        return (VALUE)st;
+        return st;
     }
     else {
-        NEWOBJ_OF(st, struct RStruct, klass, flags, sizeof(struct RStruct));
+        VALUE obj = rb_newobj(GET_EC(), klass, flags, ROOT_SHAPE_ID | SHAPE_ID_LAYOUT_EXTENDED, true, sizeof(struct RStruct));
+        struct RStruct *st = RSTRUCT(obj);
 
+        st->fields_obj = 0;
         st->as.heap.ptr = NULL;
-        st->as.heap.fields_obj = 0;
         st->as.heap.len = 0;
 
         st->as.heap.ptr = struct_heap_alloc((VALUE)st, n);
         rb_mem_clear((VALUE *)st->as.heap.ptr, n);
         st->as.heap.len = n;
 
-        return (VALUE)st;
+        return obj;
     }
 }
 
diff --git a/test/ruby/test_box.rb b/test/ruby/test_box.rb
index 3d19bcfc4b2ebb..ab6f53b92ad72e 100644
--- a/test/ruby/test_box.rb
+++ b/test/ruby/test_box.rb
@@ -869,7 +869,7 @@ def test_prelude_gems_and_loaded_features
       assert_match EXPERIMENTAL_WARNING_LINE_PATTERNS[1], error[1]
 
       assert_includes output.grep(/^before:/).join("\n"), '/bundled_gems.rb'
-      assert_includes output.grep(/^before:/).join("\n"), '/error_highlight.rb'
+      refute_includes output.grep(/^before:/).join("\n"), '/error_highlight.rb'
       assert_includes output.grep(/^after:/).join("\n"), '/bundled_gems.rb'
       assert_includes output.grep(/^after:/).join("\n"), '/error_highlight.rb'
     end
diff --git a/test/ruby/test_process.rb b/test/ruby/test_process.rb
index d99e356e69bfd4..2dc999703cfddc 100644
--- a/test/ruby/test_process.rb
+++ b/test/ruby/test_process.rb
@@ -2786,6 +2786,18 @@ def test_warmup_frees_pages
     end;
   end
 
+  def test_warmup_eager_loads_error_decoration_gems
+    assert_separately(["--enable=gems"], "#{<<~"begin;"}\n#{<<~'end;'}")
+    begin;
+      features = ["error_highlight", "did_you_mean", "syntax_suggest"]
+      assert_empty($LOADED_FEATURES.grep(/\/(#{features.join("|")})\.rb\z/))
+      Process.warmup
+      features.each do |feature|
+        assert_includes($LOADED_FEATURES.map { File.basename(it, ".rb") }, feature)
+      end
+    end;
+  end
+
   def test_concurrent_group_and_pid_wait
     # Use a pair of pipes that will make long_pid exit when this test exits, to avoid
     # leaking temp processes.
diff --git a/test/ruby/test_ractor.rb b/test/ruby/test_ractor.rb
index 9c4bcad7f00742..2b844f4ceff8ac 100644
--- a/test/ruby/test_ractor.rb
+++ b/test/ruby/test_ractor.rb
@@ -401,6 +401,24 @@ def test_ractor_does_not_inherit_fiber_storage
     RUBY
   end
 
+  def test_detailed_message_in_ractor
+    # The error decoration gems (error_highlight, did_you_mean, and
+    # syntax_suggest) are loaded lazily on the first error display. In a
+    # non-main Ractor the require is delegated to the main Ractor, so the
+    # decorations must appear there too. [Feature #21951]
+    assert_ractor(<<~'RUBY', args: ["--enable=gems"], ignore_stderr: true)
+      message = Ractor.new do
+        begin
+          1.timess
+        rescue NoMethodError => e
+          e.detailed_message(highlight: false)
+        end
+      end.value
+      assert_include(message, "timess")
+      assert_include(message, "Did you mean?")
+    RUBY
+  end
+
   def assert_make_shareable(obj)
     refute Ractor.shareable?(obj), "object was already shareable"
     Ractor.make_shareable(obj)
diff --git a/test/ruby/test_shapes.rb b/test/ruby/test_shapes.rb
index 8dc52eee54bf04..9d7b4a34663bfa 100644
--- a/test/ruby/test_shapes.rb
+++ b/test/ruby/test_shapes.rb
@@ -1108,12 +1108,12 @@ def test_shape_layout
 
     assert_equal :extended_or_rdata, RubyVM::Shape.of(Thread.current).layout
     assert_equal :extended_or_rdata, RubyVM::Shape.of(lambda {}).layout
+    assert_equal :extended_or_rdata, RubyVM::Shape.of(Struct.new(:x).new(1)).layout
+    assert_equal :extended_or_rdata, RubyVM::Shape.of(2..3).layout
 
-    assert_equal :other, RubyVM::Shape.of(Struct.new(:x).new(1)).layout
     assert_equal :other, RubyVM::Shape.of([]).layout
     assert_equal :other, RubyVM::Shape.of("hello").layout
     assert_equal :other, RubyVM::Shape.of(/foo/).layout
-    assert_equal :other, RubyVM::Shape.of(2..3).layout
     assert_equal :other, RubyVM::Shape.of(2**67).layout
     assert_equal :other, RubyVM::Shape.of(:"aaroniscool#{123}").layout
   end
diff --git a/test/rubygems/test_gem_commands_setup_command.rb b/test/rubygems/test_gem_commands_setup_command.rb
index 7df59ecfc6dbbb..ba410d64e635cd 100644
--- a/test/rubygems/test_gem_commands_setup_command.rb
+++ b/test/rubygems/test_gem_commands_setup_command.rb
@@ -241,17 +241,44 @@ def test_install_default_bundler_gem
   def test_install_default_bundler_gem_with_default_gems_not_installed_at_default_dir
     @cmd.extend FileUtils
 
+    # Simulate Homebrew's layout, where Gem.default_dir is moved to a
+    # different root, while Gem.default_specifications_dir stays inside the
+    # ruby install tree.
     gemhome2 = File.join(@tempdir, "gemhome2")
     Gem.instance_variable_set(:@default_dir, gemhome2)
 
     FileUtils.mkdir_p gemhome2
     bin_dir = File.join(gemhome2, "bin")
 
+    previous_default_gem_dir = File.join(@gemhome, "gems", "bundler-1.15.4")
+    write_file File.join(previous_default_gem_dir, "exe", "bundle")
+
+    # A regular gem of the same version installed at the default dir, which is
+    # promoted to the default gem and must be removed. Its files live under
+    # default_dir, not under the default gemspec root.
+    normal_gem_dir = File.join(gemhome2, "gems", "bundler-#{bundler_version}")
+    write_file File.join(normal_gem_dir, "lib", "bundler.rb")
+
     @cmd.install_default_bundler_gem bin_dir
 
     # expect to remove other versions of bundler gemspecs on default specification directory.
     assert_path_not_exist previous_bundler_specification_path
     assert_path_exist new_bundler_specification_path
+
+    # expect to remove the previous default version executables, which live
+    # next to the removed gemspec, not under the new default dir.
+    assert_path_not_exist previous_default_gem_dir
+
+    # expect to remove the promoted regular gem's files under default_dir.
+    assert_path_not_exist normal_gem_dir
+
+    # expect executables to be installed under the same root as the default
+    # gemspec, since that's where activation of the default gem looks for them.
+    spec = Gem::Specification.load(new_bundler_specification_path)
+    spec.executables.each do |e|
+      assert_path_exist File.join(@gemhome, "gems", spec.full_name, spec.bindir, e)
+    end
+    assert_path_not_exist File.join(gemhome2, "gems", spec.full_name)
   end
 
   def test_install_default_bundler_gem_with_force_flag
diff --git a/test/rubygems/test_gem_compact_index_client_cache_file.rb b/test/rubygems/test_gem_compact_index_client_cache_file.rb
index 193ae327e9c24b..f2e2633646b4a0 100644
--- a/test/rubygems/test_gem_compact_index_client_cache_file.rb
+++ b/test/rubygems/test_gem_compact_index_client_cache_file.rb
@@ -82,6 +82,23 @@ def test_append_with_matching_digests
     assert_equal "abcdef", @path.read
   end
 
+  def test_append_writes_in_binary_mode
+    @path.binwrite "created_at: 2026-06-10\n---\n"
+
+    appended = nil
+    CacheFile.copy(@path) do |file|
+      file.digests = sha256("created_at: 2026-06-10\n---\nrake 13.0.0\n")
+      appended = file.append("rake 13.0.0\n")
+    end
+
+    assert appended
+    # On Windows a text-mode append rewrites the appended LF as CRLF while the
+    # digest is computed over the pre-write bytes, so verify passes but the file
+    # on disk is corrupted. Read raw bytes to catch any stray carriage return.
+    refute_includes @path.binread, "\r"
+    assert_equal "created_at: 2026-06-10\n---\nrake 13.0.0\n", @path.binread
+  end
+
   def test_append_with_mismatched_digests_keeps_original
     @path.binwrite "abc"
 
diff --git a/tool/runruby.rb b/tool/runruby.rb
index 1f7268bd4114b7..d610e2ec89ead7 100755
--- a/tool/runruby.rb
+++ b/tool/runruby.rb
@@ -163,10 +163,12 @@ def File.realpath(*args)
 if debugger
   case debugger
   when :gdb
-    debugger = %W'gdb -x #{srcdir}/.gdbinit'
+    debugger = %W'gdb'
     if File.exist?(gdb = 'run.gdb') or
       File.exist?(gdb = File.join(abs_archdir, 'run.gdb'))
       debugger.push('-x', gdb)
+    else
+      debugger.push('-x', "#{srcdir}/.gdbinit")
     end
     debugger << '--args'
   when :lldb
diff --git a/variable.c b/variable.c
index 2f4e4a387faa3a..e0b4d4a45db2a7 100644
--- a/variable.c
+++ b/variable.c
@@ -1281,10 +1281,9 @@ obj_use_generic_fields_tbl_p(VALUE obj)
       case T_OBJECT:
       case T_CLASS:
       case T_MODULE:
+      case T_STRUCT:
       case T_DATA:
         return false;
-      case T_STRUCT:
-        return !FL_TEST_RAW(obj, RSTRUCT_GEN_FIELDS);
       default:
         return true;
     }
@@ -1311,12 +1310,9 @@ rb_obj_fields(VALUE obj, ID field_name)
         return RTYPEDDATA(obj)->fields_obj;
 
       case T_STRUCT:
-        if (LIKELY(!FL_TEST_RAW(obj, RSTRUCT_GEN_FIELDS))) {
-            return RSTRUCT_FIELDS_OBJ(obj);
-        }
-        goto generic_fields;
+        return RSTRUCT_FIELDS_OBJ(obj);
+
       default:
-      generic_fields:
         {
             VALUE fields_obj = 0;
 
@@ -1348,13 +1344,10 @@ rb_free_generic_ivar(VALUE obj)
             RB_OBJ_WRITE(obj, &RTYPEDDATA(obj)->fields_obj, 0);
             break;
           case T_STRUCT:
-            if (LIKELY(!FL_TEST_RAW(obj, RSTRUCT_GEN_FIELDS))) {
-                RSTRUCT_SET_FIELDS_OBJ(obj, 0);
-                break;
-            }
-            goto generic_fields;
+            RSTRUCT_SET_FIELDS_OBJ(obj, 0);
+            break;
+
           default:
-          generic_fields:
             {
                 // Other EC may have stale caches, so fields_obj should be
                 // invalidated and the GC will replace with Qundef
@@ -1400,13 +1393,10 @@ rb_obj_set_fields(VALUE obj, VALUE fields_obj, ID field_name, VALUE original_fie
             RB_OBJ_WRITE(obj, &RTYPEDDATA(obj)->fields_obj, fields_obj);
             break;
           case T_STRUCT:
-            if (LIKELY(!FL_TEST_RAW(obj, RSTRUCT_GEN_FIELDS))) {
-                RSTRUCT_SET_FIELDS_OBJ(obj, fields_obj);
-                break;
-            }
-            goto generic_fields;
+            RSTRUCT_SET_FIELDS_OBJ(obj, fields_obj);
+            break;
+
           default:
-          generic_fields:
             {
                 RB_VM_LOCKING() {
                     st_insert(generic_fields_tbl_, (st_data_t)obj, (st_data_t)fields_obj);
diff --git a/yjit/src/cruby.rs b/yjit/src/cruby.rs
index a5514115f11ad8..7e945f5db7643f 100644
--- a/yjit/src/cruby.rs
+++ b/yjit/src/cruby.rs
@@ -761,8 +761,8 @@ mod manual_defs {
     pub const RUBY_OFFSET_RARRAY_AS_HEAP_PTR: i32 = 32; // struct RArray, subfield "as.heap.ptr"
     pub const RUBY_OFFSET_RARRAY_AS_ARY: i32 = 16; // struct RArray, subfield "as.ary"
 
-    pub const RUBY_OFFSET_RSTRUCT_AS_HEAP_PTR: i32 = 24; // struct RStruct, subfield "as.heap.ptr"
-    pub const RUBY_OFFSET_RSTRUCT_AS_ARY: i32 = 16; // struct RStruct, subfield "as.ary"
+    pub const RUBY_OFFSET_RSTRUCT_AS_HEAP_PTR: i32 = 32; // struct RStruct, subfield "as.heap.ptr"
+    pub const RUBY_OFFSET_RSTRUCT_AS_ARY: i32 = 24; // struct RStruct, subfield "as.ary"
 
     pub const RUBY_OFFSET_RSTRING_AS_HEAP_PTR: i32 = 24; // struct RString, subfield "as.heap.ptr"
     pub const RUBY_OFFSET_RSTRING_AS_ARY: i32 = 24; // struct RString, subfield "as.embed.ary"
diff --git a/zjit/src/cruby.rs b/zjit/src/cruby.rs
index ec897ac538f15f..c0c242fde8ecd5 100644
--- a/zjit/src/cruby.rs
+++ b/zjit/src/cruby.rs
@@ -1212,8 +1212,8 @@ mod manual_defs {
     pub const RUBY_OFFSET_RARRAY_AS_HEAP_PTR: i32 = 32; // struct RArray, subfield "as.heap.ptr"
     pub const RUBY_OFFSET_RARRAY_AS_ARY: i32 = 16; // struct RArray, subfield "as.ary"
 
-    pub const RUBY_OFFSET_RSTRUCT_AS_HEAP_PTR: i32 = 24; // struct RStruct, subfield "as.heap.ptr"
-    pub const RUBY_OFFSET_RSTRUCT_AS_ARY: i32 = 16; // struct RStruct, subfield "as.ary"
+    pub const RUBY_OFFSET_RSTRUCT_AS_HEAP_PTR: i32 = 32; // struct RStruct, subfield "as.heap.ptr"
+    pub const RUBY_OFFSET_RSTRUCT_AS_ARY: i32 = 24; // struct RStruct, subfield "as.ary"
 
     pub const RUBY_OFFSET_RSTRING_AS_HEAP_PTR: i32 = 24; // struct RString, subfield "as.heap.ptr"
     pub const RUBY_OFFSET_RSTRING_AS_ARY: i32 = 24; // struct RString, subfield "as.embed.ary"
diff --git a/zjit/src/hir/opt_tests.rs b/zjit/src/hir/opt_tests.rs
index 376080b560951c..f1faa88b9f1215 100644
--- a/zjit/src/hir/opt_tests.rs
+++ b/zjit/src/hir/opt_tests.rs
@@ -9352,8 +9352,8 @@ mod hir_opt_tests {
           v11:HeapBasicObject = GuardType v6, HeapBasicObject
           v12:CShape = LoadField v11, :shape_id@0x1000
           v13:CShape[0x1001] = GuardBitEquals v12, CShape(0x1001) recompile
-          v14:CAttrIndex[0] = Const CAttrIndex(0)
-          v15:BasicObject = CCall v11, :rb_ivar_get_at_no_ractor_check@0x1002, v14
+          v14:RubyValue = LoadField v11, :fields_obj@0x1002
+          v15:BasicObject = LoadField v14, :@a@0x1002
           CheckInterrupts
           Return v15
         ");