diff --git a/.github/actionlint.yaml b/.github/actionlint.yaml
index eacfff24889021b..61ee08db9919bb9 100644
--- a/.github/actionlint.yaml
+++ b/.github/actionlint.yaml
@@ -1,5 +1,12 @@
 config-variables: null
 
+# Pending release of actionlint > 1.7.12 for ubuntu-26.04* support:
+# https://github.com/rhysd/actionlint/pull/683
+self-hosted-runner:
+  labels:
+    - ubuntu-26.04
+    - ubuntu-26.04-arm
+
 paths:
   .github/workflows/**/*.yml:
      ignore:
diff --git a/.github/workflows/add-issue-header.yml b/.github/workflows/add-issue-header.yml
index 09da61a470ff959..55095f17bb4cce9 100644
--- a/.github/workflows/add-issue-header.yml
+++ b/.github/workflows/add-issue-header.yml
@@ -17,7 +17,7 @@ permissions:
 
 jobs:
   add-header:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-slim
     permissions:
       issues: write
     timeout-minutes: 5
diff --git a/.github/workflows/build.yml b/.github/workflows/build.yml
index 9ef98f556cb205b..59b0361b192c9d9 100644
--- a/.github/workflows/build.yml
+++ b/.github/workflows/build.yml
@@ -53,7 +53,7 @@ jobs:
     name: 'Check if Autoconf files are up to date'
     # Don't use ubuntu-latest but a specific version to make the job
     # reproducible: to get the same tools versions (autoconf, aclocal, ...)
-    runs-on: ubuntu-24.04
+    runs-on: ubuntu-26.04
     container:
       image: ghcr.io/python/autoconf:2025.01.02.12581854023
     timeout-minutes: 60
@@ -96,7 +96,7 @@ jobs:
     name: 'Check if generated files are up to date'
     # Don't use ubuntu-latest but a specific version to make the job
     # reproducible: to get the same tools versions (autoconf, aclocal, ...)
-    runs-on: ubuntu-24.04
+    runs-on: ubuntu-26.04
     timeout-minutes: 60
     needs: build-context
     if: needs.build-context.outputs.run-tests == 'true'
@@ -239,18 +239,18 @@ jobs:
         - false
         - true
         os:
-        - ubuntu-24.04
-        - ubuntu-24.04-arm
+        - ubuntu-26.04
+        - ubuntu-26.04-arm
         exclude:
         # Do not test BOLT with free-threading, to conserve resources
         - bolt: true
           free-threading: true
         # BOLT currently crashes during instrumentation on aarch64
-        - os: ubuntu-24.04-arm
+        - os: ubuntu-26.04-arm
           bolt: true
         include:
         # Enable CPU-intensive tests on ARM (default build only)
-        - os: ubuntu-24.04-arm
+        - os: ubuntu-26.04-arm
           bolt: false
           free-threading: false
           test-opts: '-u cpu'
@@ -270,7 +270,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [ubuntu-24.04]
+        os: [ubuntu-26.04]
         ssllib:
           # See Tools/ssl/make_ssl_data.py for notes on adding a new version
           ## OpenSSL
@@ -346,7 +346,7 @@ jobs:
           - arch: aarch64
             runs-on: macos-26
           - arch: x86_64
-            runs-on: ubuntu-24.04
+            runs-on: ubuntu-26.04
 
     runs-on: ${{ matrix.runs-on }}
     steps:
@@ -393,7 +393,7 @@ jobs:
 
   test-hypothesis:
     name: "Hypothesis tests on Ubuntu"
-    runs-on: ubuntu-24.04
+    runs-on: ubuntu-26.04
     timeout-minutes: 60
     needs: build-context
     if: needs.build-context.outputs.run-ubuntu == 'true'
@@ -504,7 +504,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        os: [ubuntu-24.04]
+        os: [ubuntu-26.04]
     env:
       OPENSSL_VER: 3.5.7
       PYTHONSTRICTEXTENSIONBUILD: 1
diff --git a/.github/workflows/jit.yml b/.github/workflows/jit.yml
index 846c90d2525d253..955721e0b94ea53 100644
--- a/.github/workflows/jit.yml
+++ b/.github/workflows/jit.yml
@@ -29,7 +29,7 @@ env:
 jobs:
   interpreter:
     name: Interpreter (Debug)
-    runs-on: ubuntu-24.04
+    runs-on: ubuntu-26.04
     timeout-minutes: 60
     steps:
       - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
@@ -145,9 +145,9 @@ jobs:
           - false
         include:
           - target: x86_64-unknown-linux-gnu/gcc
-            runner: ubuntu-24.04
+            runner: ubuntu-26.04
           - target: aarch64-unknown-linux-gnu/gcc
-            runner: ubuntu-24.04-arm
+            runner: ubuntu-26.04-arm
     steps:
       - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
         with:
@@ -160,8 +160,7 @@ jobs:
           sudo ./.github/workflows/posix-deps-apt.sh
       - name: Build
         run: |
-          sudo bash -c "$(wget -O - https://apt.llvm.org/llvm.sh)" ./llvm.sh ${{ env.LLVM_VERSION }}
-          export PATH="$(llvm-config-${{ env.LLVM_VERSION }} --bindir):$PATH"
+          # On ubuntu-26.04 image, clang is clang-21 by default
           ./configure --enable-experimental-jit ${{ matrix.debug && '--with-pydebug' || '' }}
           make all --jobs 4
       - name: Test
@@ -171,7 +170,7 @@ jobs:
   linux-extras:
     name: ${{ matrix.name }}
 
-    runs-on: ubuntu-24.04
+    runs-on: ubuntu-26.04
     timeout-minutes: 60
     strategy:
       fail-fast: false
@@ -202,8 +201,7 @@ jobs:
           sudo ./.github/workflows/posix-deps-apt.sh
       - name: Build
         run: |
-          sudo bash -c "$(wget -O - https://apt.llvm.org/llvm.sh)" ./llvm.sh ${{ env.LLVM_VERSION }}
-          export PATH="$(llvm-config-${{ env.LLVM_VERSION }} --bindir):$PATH"
+          # On ubuntu-26.04 image, clang is clang-21 by default
           if [ "${{ matrix.use_clang }}" = "true" ]; then
             export CC=clang-${{ env.LLVM_VERSION }}
           fi
diff --git a/.github/workflows/new-bugs-announce-notifier.yml b/.github/workflows/new-bugs-announce-notifier.yml
index 9c19adb7ffda707..864d0f48904bcc3 100644
--- a/.github/workflows/new-bugs-announce-notifier.yml
+++ b/.github/workflows/new-bugs-announce-notifier.yml
@@ -10,7 +10,7 @@ permissions:
 
 jobs:
   notify-new-bugs-announce:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-slim
     permissions:
       issues: read
     timeout-minutes: 10
diff --git a/.github/workflows/posix-deps-apt.sh b/.github/workflows/posix-deps-apt.sh
index f241fccdeb2a32a..803204635abfe11 100755
--- a/.github/workflows/posix-deps-apt.sh
+++ b/.github/workflows/posix-deps-apt.sh
@@ -5,37 +5,23 @@ apt-get -yq --no-install-recommends install \
     build-essential \
     pkg-config \
     cmake \
+    curl \
     gdb \
     lcov \
     libb2-dev \
     libbz2-dev \
     libffi-dev \
-    libgdbm-dev \
     libgdbm-compat-dev \
+    libgdbm-dev \
     liblzma-dev \
+    libmpdec-dev \
     libncurses5-dev \
     libreadline6-dev \
     libsqlite3-dev \
     libssl-dev \
     libzstd-dev \
-    lzma \
-    lzma-dev \
     strace \
     tk-dev \
     uuid-dev \
     xvfb \
     zlib1g-dev
-
-# Workaround missing libmpdec-dev on ubuntu 24.04 by building mpdecimal
-# from source. ppa:ondrej/php (launchpad.net) are unreliable
-# (https://status.canonical.com) so fetch the tarball directly
-# from the upstream host.
-# https://www.bytereef.org/mpdecimal/
-MPDECIMAL_VERSION=4.0.1
-curl -fsSL "https://www.bytereef.org/software/mpdecimal/releases/mpdecimal-${MPDECIMAL_VERSION}.tar.gz" \
-    | tar -xz -C /tmp
-(cd "/tmp/mpdecimal-${MPDECIMAL_VERSION}" \
-    && ./configure --prefix=/usr/local \
-    && make -j"$(nproc)" \
-    && make install)
-ldconfig
diff --git a/.github/workflows/require-pr-label.yml b/.github/workflows/require-pr-label.yml
index f3e2666879530f9..8af254c10786e1b 100644
--- a/.github/workflows/require-pr-label.yml
+++ b/.github/workflows/require-pr-label.yml
@@ -11,7 +11,7 @@ jobs:
   label-dnm:
     name: DO-NOT-MERGE
     if: github.repository_owner == 'python'
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-slim
     permissions:
       pull-requests: read
     timeout-minutes: 10
@@ -28,7 +28,7 @@ jobs:
   label-reviews:
     name: Unresolved review
     if: github.repository_owner == 'python'
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-slim
     permissions:
       pull-requests: read
     timeout-minutes: 10
diff --git a/.github/workflows/reusable-docs.yml b/.github/workflows/reusable-docs.yml
index cac481d26a8d0cc..a2373df0dda7df0 100644
--- a/.github/workflows/reusable-docs.yml
+++ b/.github/workflows/reusable-docs.yml
@@ -95,7 +95,7 @@ jobs:
   # Run "doctest" on HEAD as new syntax doesn't exist in the latest stable release
   doctest:
     name: 'Doctest'
-    runs-on: ubuntu-24.04
+    runs-on: ubuntu-latest
     timeout-minutes: 60
     steps:
     - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
diff --git a/.github/workflows/reusable-emscripten.yml b/.github/workflows/reusable-emscripten.yml
index 46664522aa6d793..0205203ef77a9f5 100644
--- a/.github/workflows/reusable-emscripten.yml
+++ b/.github/workflows/reusable-emscripten.yml
@@ -12,7 +12,7 @@ env:
 jobs:
   build-emscripten-reusable:
     name: 'build and test'
-    runs-on: ubuntu-24.04
+    runs-on: ubuntu-26.04
     timeout-minutes: 40
     steps:
     - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
diff --git a/.github/workflows/reusable-san.yml b/.github/workflows/reusable-san.yml
index d29f4e83b48dac8..405fa36454a62bf 100644
--- a/.github/workflows/reusable-san.yml
+++ b/.github/workflows/reusable-san.yml
@@ -26,7 +26,7 @@ jobs:
         && ' (free-threading)'
         || ''
       }}
-    runs-on: ubuntu-24.04
+    runs-on: ubuntu-26.04
     timeout-minutes: 60
     steps:
     - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
@@ -37,14 +37,7 @@ jobs:
     - name: Install dependencies
       run: |
         sudo ./.github/workflows/posix-deps-apt.sh
-        # Install clang
-        wget https://apt.llvm.org/llvm.sh
-        chmod +x llvm.sh
-        sudo ./llvm.sh 21
-        sudo update-alternatives --install /usr/bin/clang clang /usr/bin/clang-21 100
-        sudo update-alternatives --set clang /usr/bin/clang-21
-        sudo update-alternatives --install /usr/bin/clang++ clang++ /usr/bin/clang++-21 100
-        sudo update-alternatives --set clang++ /usr/bin/clang++-21
+        # On ubuntu-26.04 image, clang is clang-21 by default
 
         if [ "${SANITIZER}" = "TSan" ]; then
           # Reduce ASLR to avoid TSan crashing
diff --git a/.github/workflows/reusable-ubuntu.yml b/.github/workflows/reusable-ubuntu.yml
index cd810babb7a9d6e..4ecc541e6aa7d02 100644
--- a/.github/workflows/reusable-ubuntu.yml
+++ b/.github/workflows/reusable-ubuntu.yml
@@ -49,7 +49,7 @@ jobs:
     - name: Install Clang and BOLT
       if: ${{ fromJSON(inputs.bolt-optimizations) }}
       run: |
-        sudo bash -c "$(wget -O - https://apt.llvm.org/llvm.sh)" ./llvm.sh 21
+        # On ubuntu-26.04 image, LLVM is LLVM-21 by default
         sudo apt-get install --no-install-recommends bolt-21
         echo PATH="$(llvm-config-21 --bindir):$PATH" >> $GITHUB_ENV
     - name: Configure OpenSSL env vars
diff --git a/.github/workflows/reusable-wasi.yml b/.github/workflows/reusable-wasi.yml
index 9675c39b86a24c4..b0dda62d0b291c2 100644
--- a/.github/workflows/reusable-wasi.yml
+++ b/.github/workflows/reusable-wasi.yml
@@ -12,7 +12,7 @@ env:
 jobs:
   build-wasi-reusable:
     name: 'build and test'
-    runs-on: ubuntu-24.04-arm
+    runs-on: ubuntu-26.04-arm
     timeout-minutes: 60
     env:
       WASMTIME_VERSION: 38.0.3
diff --git a/.github/workflows/stale.yml b/.github/workflows/stale.yml
index 37f78519dedc329..0283e9f621dcf05 100644
--- a/.github/workflows/stale.yml
+++ b/.github/workflows/stale.yml
@@ -10,7 +10,7 @@ permissions:
 jobs:
   stale:
     if: github.repository_owner == 'python'
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-slim
     permissions:
       actions: write
       pull-requests: write
diff --git a/.github/workflows/tail-call.yml b/.github/workflows/tail-call.yml
index f5b174d87d84486..f25552679c8d45c 100644
--- a/.github/workflows/tail-call.yml
+++ b/.github/workflows/tail-call.yml
@@ -66,13 +66,13 @@ jobs:
       matrix:
         include:
           - target: x86_64-unknown-linux-gnu/gcc
-            runner: ubuntu-24.04
+            runner: ubuntu-26.04
             configure_flags: --with-pydebug
           - target: x86_64-unknown-linux-gnu/gcc-free-threading
-            runner: ubuntu-24.04
+            runner: ubuntu-26.04
             configure_flags: --disable-gil
           - target: aarch64-unknown-linux-gnu/gcc
-            runner: ubuntu-24.04-arm
+            runner: ubuntu-26.04-arm
             configure_flags: --with-pydebug
     steps:
       - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
@@ -83,8 +83,7 @@ jobs:
           python-version: '3.11'
       - name: Build
         run: |
-          sudo bash -c "$(wget -O - https://apt.llvm.org/llvm.sh)" ./llvm.sh ${{ env.LLVM_VERSION }}
-          export PATH="$(llvm-config-${{ env.LLVM_VERSION }} --bindir):$PATH"
+          # On ubuntu-26.04 image, clang is clang-21 by default
           CC=clang-${{ env.LLVM_VERSION }} ./configure --with-tail-call-interp ${{ matrix.configure_flags }}
           make all --jobs 4
       - name: Test
diff --git a/.github/workflows/verify-expat.yml b/.github/workflows/verify-expat.yml
index bae93a4d9ac0a2b..b96e71b487dbe57 100644
--- a/.github/workflows/verify-expat.yml
+++ b/.github/workflows/verify-expat.yml
@@ -20,7 +20,7 @@ concurrency:
 
 jobs:
   verify:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-slim
     timeout-minutes: 5
     steps:
       - uses: actions/checkout@9c091bb21b7c1c1d1991bb908d89e4e9dddfe3e0 # v7.0.0
diff --git a/Python/bytecodes.c b/Python/bytecodes.c
index 6d5ae1b074db5b7..5175556d1ee6c74 100644
--- a/Python/bytecodes.c
+++ b/Python/bytecodes.c
@@ -6,6 +6,7 @@
 // The actual instruction definitions start at // BEGIN BYTECODES //.
 // See Tools/cases_generator/README.md for more information.
 
+
 #include "Python.h"
 #include "pycore_abstract.h"      // _PyIndex_Check()
 #include "pycore_audit.h"         // _PySys_Audit()
diff --git a/Python/getcompiler.c b/Python/getcompiler.c
index cc56ad8c8955513..1381a278f801494 100644
--- a/Python/getcompiler.c
+++ b/Python/getcompiler.c
@@ -24,6 +24,7 @@
 
 #endif /* !COMPILER */
 
+
 const char *
 Py_GetCompiler(void)
 {