diff --git a/.github/actions/Build_LLVM/action.yml b/.github/actions/Build_LLVM/action.yml
index 4a1618175..10ecfd717 100644
--- a/.github/actions/Build_LLVM/action.yml
+++ b/.github/actions/Build_LLVM/action.yml
@@ -43,7 +43,8 @@ runs:
           ninja LLVMOrcDebugging -j ${{ env.ncpus }}
           ninja clingInterpreter -j ${{ env.ncpus }}
         else
-          if [[ "${{ matrix.oop-jit }}" == "On" ]]; then
+          llvm_vers=$(echo "${{ matrix.clang-runtime }}" | tr '[:lower:]' '[:upper:]')
+          if [[ "${{ matrix.oop-jit }}" == "On" && "${llvm_vers}" == "20" ]]; then
             git apply -v ../patches/llvm/clang20-1-out-of-process.patch
             echo "Apply clang20-1-out-of-process.patch:"
           fi
diff --git a/.github/actions/Build_and_Test_CppInterOp/action.yml b/.github/actions/Build_and_Test_CppInterOp/action.yml
index 873a6a5f9..e5e6cc00d 100644
--- a/.github/actions/Build_and_Test_CppInterOp/action.yml
+++ b/.github/actions/Build_and_Test_CppInterOp/action.yml
@@ -109,9 +109,9 @@ runs:
         }
         else
         {
-          $env:CPLUS_INCLUDE_PATH="$env:LLVM_DIR\llvm\include;$env:LLVM_DIR\clang\include;$env:LLVM_BUILD_DIR\include;$env:LLVM_BUILD_DIR\tools\clang\include;$env:PWD_DIR\include;"
-          echo "CPLUS_INCLUDE_PATH=$env:CPLUS_INCLUDE_PATH"
-          echo "CPLUS_INCLUDE_PATH=$env:CPLUS_INCLUDE_PATH" >> $env:GITHUB_ENV
+          $env:IncludePath="$env:IncludePath;$env:LLVM_DIR\llvm\include;$env:LLVM_DIR\clang\include;$env:LLVM_BUILD_DIR\include;$env:LLVM_BUILD_DIR\tools\clang\include;$env:PWD_DIR\include;"
+          echo "IncludePath=$env:IncludePath"
+          echo "IncludePath=$env:IncludePath" >> $env:GITHUB_ENV
         }
 
         $env:CB_PYTHON_DIR="$env:PWD_DIR\cppyy-backend\python"
@@ -142,7 +142,7 @@ runs:
         {
           cmake -DCMAKE_BUILD_TYPE=${{ env.BUILD_TYPE }}    `
                 -DLLVM_DIR="$env:LLVM_BUILD_DIR\lib\cmake\llvm"  `
-                -DLLVM_ENABLE_WERROR=On                          `
+                -DLLVM_ENABLE_WERROR=On  -DBUILD_SHARED_LIBS=ON `
                 -DClang_DIR="$env:LLVM_BUILD_DIR\lib\cmake\clang"  -DCODE_COVERAGE=${{ env.CODE_COVERAGE }}  -DCMAKE_INSTALL_PREFIX="$env:CPPINTEROP_DIR"  ..\
         }
         cmake --build . --config ${{ env.BUILD_TYPE }} --target check-cppinterop --parallel ${{ env.ncpus }}
diff --git a/.github/actions/Build_and_Test_cppyy/action.yml b/.github/actions/Build_and_Test_cppyy/action.yml
index b4c890cfd..a4efa68b6 100644
--- a/.github/actions/Build_and_Test_cppyy/action.yml
+++ b/.github/actions/Build_and_Test_cppyy/action.yml
@@ -138,3 +138,95 @@ runs:
         tail -n1 test_xfailed.log
         echo "Return Code: ${RETCODE}"
         exit $RETCODE
+
+    - name: Build and Install cppyy-backend
+      if: ${{ matrix.cppyy == 'On' && runner.os == 'Windows' }}
+      shell: powershell
+      run: |
+        # Download cppyy-backend
+        git clone --depth=1 https://github.com/compiler-research/cppyy-backend.git
+        cd cppyy-backend
+        mkdir $env:CPPINTEROP_DIR\lib
+        mkdir build
+        cd build
+        # Install CppInterOp
+        cd $env:CPPINTEROP_BUILD_DIR
+        cmake --build . --target install --parallel ${{ env.ncpus }}
+        cd ..\cppyy-backend\build\
+        # Build and Install cppyy-backend
+        cmake -DCppInterOp_DIR=$env:CPPINTEROP_DIR -DCMAKE_BUILD_TYPE=${{ env.BUILD_TYPE }} ..
+        cmake --build . --config ${{ env.BUILD_TYPE }} --parallel ${{ env.ncpus }}
+        cp Release\libcppyy-backend.dll $env:CPPINTEROP_DIR\lib\
+        cd ..
+    - name: Install CPyCppyy
+      if: ${{ matrix.cppyy == 'On' && runner.os == 'Windows' }}
+      shell: powershell
+      run: |
+        python -m venv .venv
+        . .\.venv\Scripts\Activate.ps1
+        # Install CPyCppyy
+        git clone --depth=1 https://github.com/compiler-research/CPyCppyy.git
+        mkdir CPyCppyy\build
+        cd CPyCppyy\build
+        cmake ..
+        cmake --build . --parallel ${{ env.ncpus }}
+        #
+        env:CPYCPPYY_DIR = $PWD.Path
+        cd ../..
+        # We need CPYCPPYY_DIR later
+        echo "CPYCPPYY_DIR=$env:CPYCPPYY_DIR" >> $env:GITHUB_ENV
+    - name: Install cppyy
+      if: ${{ matrix.cppyy == 'On' && runner.os == 'Windows' }}
+      shell: powershell
+      run: |
+        # source virtual environment
+        . .\.venv\Scripts\Activate.ps1
+        # Install cppyy
+        git clone --depth=1 https://github.com/compiler-research/cppyy.git
+        cd cppyy
+        python -m pip install --upgrade . --no-deps
+        cd ..
+    - name: Run cppyy
+      if: ${{ matrix.cppyy == 'On' && runner.os == 'Windows' }}
+      shell: powershell
+      run: |
+        # Run cppyy
+        . .\.venv\Scripts\Activate.ps1
+        $env:PYTHONPATH=$env:PYTHONPATH:$env:CPYCPPYY_DIR:$env:CB_PYTHON_DIR
+        python -c "import cppyy"
+        # We need PYTHONPATH later
+        echo "PYTHONPATH=$env:PYTHONPATH" >> $GITHUB_ENV
+    - name: Run the tests
+      if: ${{ matrix.cppyy == 'On' && runner.os == 'Windows' }}
+      shell: powershell
+      run: |
+        # Run the tests
+        . .\.venv\Scripts\Activate.ps1
+        cd cppyy\test
+        echo ::group::Prepare For Testing
+        make all
+        python -m pip install --upgrade pip
+        python -m pip install pytest
+        python -m pip install pytest-xdist
+        python -m pip install numba
+        echo ::endgroup::
+        echo ::group::Run complete test suite
+        $ErrorActionPreference = "Stop"
+        python -m pytest -sv -ra 2>&1 | Tee-Object -FilePath complete_testrun.log
+        $ErrorActionPreference = "Continue"
+        echo ::group::Crashing Test Logs
+        # See if we don't have a crash that went away
+        # Comment out all xfails but the ones that have a run=False condition.
+        find.exe . -name "*.py" -exec sed.exe -i '/run=False/!s/^ *@mark.xfail\(.*\)/#&/' {} \;
+        python -m pytest -n 1 -m "xfail" --runxfail -sv -ra --max-worker-restart 512 2>&1 | Tee-Object -FilePath test_crashed.log
+        $LASTEXITCODE = 0
+        git checkout .
+        echo ::endgroup::
+        echo ::group::XFAIL Test Logs
+        # Rewrite all xfails that have a run clause to skipif. This way we will
+        # avoid conditionally crashing xfails
+        find.exe . -name "*.py" -exec sed.exe -i -E 's/(^ *)@mark.xfail\(run=(.*)/\1@mark.skipif(condition=not \2/g' {} \;
+        # See if we don't have an xfail that went away
+        python -m pytest --runxfail -sv -ra  | tee test_xfailed.log 2>&1 || true
+        git checkout .
+        echo ::endgroup::
diff --git a/.github/workflows/clang-format.yml b/.github/workflows/clang-format.yml
deleted file mode 100644
index 2be58cacb..000000000
--- a/.github/workflows/clang-format.yml
+++ /dev/null
@@ -1,50 +0,0 @@
-name: clang-format
-
-on:
-  pull_request:
-    paths:
-      - '**.h'
-      - '**.cpp'
-
-concurrency:
-  group: ${{ github.workflow }}-${{ github.event.pull_request.number }}
-  cancel-in-progress: true
-
-jobs:
-  precheckin:
-    runs-on: ubuntu-22.04
-    steps:
-      - name: Checkout PR branch
-        uses: actions/checkout@v5
-        with:
-          ref: ${{ github.event.pull_request.head.sha }}
-          fetch-depth: 0
-
-      - name: Setup Python
-        uses: actions/setup-python@v6
-        with:
-          python-version: "3.11"
-
-      - name: Install clang-format
-        run: |
-          curl https://apt.llvm.org/llvm-snapshot.gpg.key | sudo apt-key add -
-          os_codename="`cat /etc/os-release | grep UBUNTU_CODENAME | cut -d = -f 2`"
-          echo "deb https://apt.llvm.org/${os_codename}/ llvm-toolchain-${os_codename}-18 main" | sudo tee -a /etc/apt/sources.list
-          sudo apt update
-          sudo apt install -y clang-format-18
-
-      - name: Run git-clang-format
-        run: |
-          PR_BASE=$(git rev-list ${{ github.event.pull_request.head.sha }} ^${{ github.event.pull_request.base.sha }} | tail --lines 1 | xargs -I {} git rev-parse {}~1)
-          echo "running git clang-format against $PR_BASE commit"
-          git \
-          -c color.ui=always \
-          -c diff.wsErrorHighlight=all \
-          -c color.diff.whitespace='red reverse' \
-          clang-format-18 --diff --binary clang-format-18 --commit $PR_BASE -- include/ lib/ || \
-          (echo "Please run the following git-clang-format locally to fix the formatting: \n
-                git-clang-format HEAD~\n
-                for multiple commits we should place the formatting changes in the related commit with:\n
-                \t\tgit rebase -i -x \"git-clang-format-18 main && git commit -a --allow-empty --fixup=HEAD\" --strategy-option=theirs origin/main\n
-                \t\t Then inspect the results with: git log --oneline\n
-                \t\t Then squash without poluting the history with: git rebase --autosquash -i main\n" && exit 1)
diff --git a/.github/workflows/clang-tidy-review-post.yml b/.github/workflows/clang-tidy-review-post.yml
deleted file mode 100644
index f1aecb0d4..000000000
--- a/.github/workflows/clang-tidy-review-post.yml
+++ /dev/null
@@ -1,30 +0,0 @@
-name: Post clang-tidy review comments
-
-on:
-  workflow_run:
-    workflows: ["clang-tidy-review"]
-    types:
-      - completed
-
-permissions:
-  checks: write
-  pull-requests: write
-
-concurrency:
-  group: ${{ github.workflow }}-${{ github.event.pull_request.number }}
-  cancel-in-progress: true
-
-jobs:
-  build:
-    runs-on: ubuntu-22.04
-
-    steps:
-      - name: Post review comments
-        id: post-review
-        uses: ZedThree/clang-tidy-review/post@v0.21.0
-        with:
-          max_comments: 10
-
-       # If there are any comments, fail the check
-      - if: steps.post-review.outputs.total_comments > 0
-        run: exit 1
\ No newline at end of file
diff --git a/.github/workflows/clang-tidy-review.yml b/.github/workflows/clang-tidy-review.yml
deleted file mode 100644
index 70ea9b12e..000000000
--- a/.github/workflows/clang-tidy-review.yml
+++ /dev/null
@@ -1,52 +0,0 @@
-name: clang-tidy-review
-
-on:
-  pull_request:
-    paths:
-      - '**.h'
-      - '**.cpp'
-
-concurrency:
-  group: ${{ github.workflow }}-${{ github.event.pull_request.number }}
-  cancel-in-progress: true
-
-jobs:
-  review:
-    runs-on: ubuntu-22.04
-    steps:
-      - name: Checkout PR branch
-        uses: actions/checkout@v5
-
-      - name: Setup Python
-        uses: actions/setup-python@v6
-        with:
-          python-version: "3.11"
-
-      - name: Install LLVM and Clang
-        uses: KyleMayes/install-llvm-action@v2.0.7
-        with:
-          version: "20.1.4"
-
-      - name: install lit
-        run: pip install lit
-
-      - name: Run clang-tidy
-        uses: ZedThree/clang-tidy-review@v0.21.0
-        id: review
-        with:
-          build_dir: build
-          apt_packages: cmake,libxml2,libxml2-dev,libtinfo-dev,zlib1g-dev,libzstd-dev
-          split_workflow: true
-          config_file: .clang-tidy
-          cmake_command: >
-            cmake . -B build -DCMAKE_BUILD_TYPE="Release"
-            -DCMAKE_C_COMPILER="$GITHUB_WORKSPACE/llvm/bin/clang"
-            -DCMAKE_CXX_COMPILER="$GITHUB_WORKSPACE/llvm/bin/clang++"
-            -DLLVM_DIR="$GITHUB_WORKSPACE/llvm"
-            -DBUILD_SHARED_LIBS=ON
-            -DCMAKE_EXPORT_COMPILE_COMMANDS=ON &&
-            cd build &&
-            cmake --build . --target googletest --parallel $(nproc --all)
-
-      - name: Upload artifacts
-        uses: ZedThree/clang-tidy-review/upload@v0.21.0
diff --git a/.github/workflows/main.yml b/.github/workflows/main.yml
index 9db2b1d90..d5a9b0ac4 100644
--- a/.github/workflows/main.yml
+++ b/.github/workflows/main.yml
@@ -2,9 +2,9 @@ name: Native Builds
 
 on:
   pull_request:
-    branches: [main]
+    branches: [CppInterOp-main]
   push:
-    branches: [main]
+    branches: [CppInterOp-main]
   release:
     types: [published]
   schedule:
@@ -22,202 +22,20 @@ jobs:
       fail-fast: false
       matrix:
         include:
-          # Ubuntu Arm Jobs
-          - name: ubu24-arm-gcc12-clang-repl-20
-            os: ubuntu-24.04-arm
-            compiler: gcc-12
-            clang-runtime: '20'
-            cling: Off
-            cppyy: Off
-            llvm_enable_projects: "clang"
-            llvm_targets_to_build: "host;NVPTX"
-            Valgrind: On
-          - name: ubu24-arm-gcc12-clang-repl-19-cppyy
-            os: ubuntu-24.04-arm
-            compiler: gcc-12
-            clang-runtime: '19'
-            cling: Off
-            cppyy: On
-            llvm_enable_projects: "clang"
-            llvm_targets_to_build: "host;NVPTX"
-          - name: ubu24-arm-gcc12-clang-repl-18-cppyy
-            os: ubuntu-24.04-arm
-            compiler: gcc-12
-            clang-runtime: '18'
-            cling: Off
-            cppyy: On
-            llvm_enable_projects: "clang"
-            llvm_targets_to_build: "host;NVPTX"
-          - name: ubu24-arm-gcc9-clang18-cling-cppyy
-            os: ubuntu-24.04-arm
-            compiler: gcc-9
-            clang-runtime: '18'
-            cling: On
-            cppyy: Off
-            cling-version: '1.2'
-            llvm_enable_projects: "clang"
-            llvm_targets_to_build: "host;NVPTX"
-          # Ubuntu X86 Jobs
-          - name: ubu24-x86-gcc12-clang-repl-20-coverage
-            os: ubuntu-24.04
-            compiler: gcc-12
-            clang-runtime: '20'
-            cling: Off
-            cppyy: Off
-            llvm_enable_projects: "clang;compiler-rt"
-            llvm_targets_to_build: "host;NVPTX"
-            coverage: true
-            oop-jit: On
-            Valgrind: On
-          - name: ubu24-x86-gcc12-clang-repl-20
-            os: ubuntu-24.04
-            compiler: gcc-12
-            clang-runtime: '20'
-            cling: Off
-            cppyy: Off
-            llvm_enable_projects: "clang"
-            llvm_targets_to_build: "host;NVPTX"
-          - name: ubu24-x86-gcc12-clang-repl-20-out-of-process
-            os: ubuntu-24.04
-            compiler: gcc-12
-            clang-runtime: '20'
-            cling: Off
-            cppyy: Off
-            llvm_enable_projects: "clang;compiler-rt"
-            llvm_targets_to_build: "host;NVPTX"
-            oop-jit: On
-          - name: ubu24-x86-gcc12-clang-repl-19-cppyy
-            os: ubuntu-24.04
-            compiler: gcc-12
-            clang-runtime: '19'
-            cling: Off
-            cppyy: On
-            llvm_enable_projects: "clang"
-            llvm_targets_to_build: "host;NVPTX"
-          - name: ubu24-x86-gcc12-clang-repl-18-cppyy
-            os: ubuntu-24.04
-            compiler: gcc-12
-            clang-runtime: '18'
-            cling: Off
-            cppyy: On
-            llvm_enable_projects: "clang"
-            llvm_targets_to_build: "host;NVPTX"
-          - name: ubu24-x86-gcc9-clang18-cling-cppyy
-            os: ubuntu-24.04
-            compiler: gcc-9
-            clang-runtime: '18'
-            cling: On
-            cppyy: Off
-            cling-version: '1.2'
-            llvm_enable_projects: "clang"
-            llvm_targets_to_build: "host;NVPTX"
-          # MacOS Arm Jobs
-          - name: osx15-arm-clang-clang-repl-20-out-of-process
-            os: macos-15
-            compiler: clang
-            clang-runtime: '20'
-            cling: Off
-            cppyy: Off
-            llvm_enable_projects: "clang;compiler-rt"
-            llvm_targets_to_build: "host"
-            oop-jit: On
-          - name: osx15-arm-clang-clang-repl-20
-            os: macos-15
-            compiler: clang
-            clang-runtime: '20'
-            cling: Off
-            cppyy: Off
-            llvm_enable_projects: "clang"
-            llvm_targets_to_build: "host"
-          - name: osx15-arm-clang-clang-repl-19-cppyy
-            os: macos-15
-            compiler: clang
-            clang-runtime: '19'
-            cling: Off
-            cppyy: On
-            llvm_enable_projects: "clang"
-            llvm_targets_to_build: "host"
-          - name: osx15-arm-clang-clang-repl-18-cppyy
-            os: macos-15
-            compiler: clang
-            clang-runtime: '18'
-            cling: Off
-            cppyy: On
-            llvm_enable_projects: "clang"
-            llvm_targets_to_build: "host"
-          - name: osx15-arm-clang-clang18-cling-cppyy
-            os: macos-15
-            compiler: clang
-            clang-runtime: '18'
-            cling: On
-            cppyy: On
-            cling-version: '1.2'
-            llvm_enable_projects: "clang"
-            llvm_targets_to_build: "host;NVPTX"
-          # MacOS X86 Jobs
-          - name: osx15-x86-clang-clang-repl-20
-            os: macos-15-intel
-            compiler: clang
-            clang-runtime: '20'
-            cling: Off
-            cppyy: Off
-            llvm_enable_projects: "clang"
-            llvm_targets_to_build: "host"
-          - name: osx15-x86-clang-clang-repl-19-cppyy
-            os: macos-15-intel
-            compiler: clang
-            clang-runtime: '19'
-            cling: Off
-            cppyy: On
-            llvm_enable_projects: "clang"
-            llvm_targets_to_build: "host"
-          - name: osx15-x86-clang-clang-repl-18-cppyy
-            os: macos-15-intel
-            compiler: clang
-            clang-runtime: '18'
-            cling: Off
-            cppyy: On
-            llvm_enable_projects: "clang"
-            llvm_targets_to_build: "host"
-          - name: osx15-x86-clang-clang18-cling-cppyy
-            os: macos-15-intel
-            compiler: clang
-            clang-runtime: '18'
-            cling: On
-            cppyy: On
-            cling-version: '1.2'
-            llvm_enable_projects: "clang"
-            llvm_targets_to_build: "host;NVPTX"
           # Windows Arm Jobs
-          - name: win11-msvc-clang-repl-20
-            os: windows-11-arm
-            compiler: msvc
-            clang-runtime: '20'
-            cling: Off
-            llvm_enable_projects: "clang"
-            llvm_targets_to_build: "host;NVPTX"
-          - name: win11-msvc-clang18-cling
+          - name: win11-msvc-clang-repl-21
             os: windows-11-arm
             compiler: msvc
-            clang-runtime: '18'
-            cling: On
-            cling-version: '1.2'
+            clang-runtime: '21'
+            cppyy: On
             llvm_enable_projects: "clang"
             llvm_targets_to_build: "host;NVPTX"
           # Windows X86 Jobs
-          - name: win2025-msvc-clang-repl-20
+          - name: win2025-msvc-clang-repl-21
             os: windows-2025
             compiler: msvc
-            clang-runtime: '20'
-            cling: Off
-            llvm_enable_projects: "clang"
-            llvm_targets_to_build: "host;NVPTX"
-          - name: win2025-msvc-clang18-cling
-            os: windows-2025
-            compiler: msvc
-            clang-runtime: '18'
-            cling: On
-            cling-version: '1.2'
+            clang-runtime: '21'
+            cppyy: On
             llvm_enable_projects: "clang"
             llvm_targets_to_build: "host;NVPTX"
 
@@ -314,3 +132,4 @@ jobs:
       # When debugging increase to a suitable value!
       timeout-minutes: 30
 
+
diff --git a/.github/workflows/markdown-linter.yml b/.github/workflows/markdown-linter.yml
deleted file mode 100644
index 7569f9a3c..000000000
--- a/.github/workflows/markdown-linter.yml
+++ /dev/null
@@ -1,40 +0,0 @@
----
-name: Markdown-Linter
-
-on:
-  pull_request:
-    branches: [main]
-    paths:
-      - '**.md'
-
-concurrency:
-  group: ${{ github.workflow }}-${{ github.event.pull_request.number }}
-  cancel-in-progress: true
-
-jobs:
-  build:
-    name: Linter
-    runs-on: ubuntu-22.04
-
-    permissions:
-      contents: read
-      packages: read
-      # To report GitHub Actions status checks
-      statuses: write
-
-    steps:
-      - name: Checkout code
-        uses: actions/checkout@v5
-        with:
-          # super-linter needs the full git history to get the
-          # list of files that changed across commits
-          fetch-depth: 0
-
-      - name: Super-linter
-        uses: super-linter/super-linter@v7.2.0
-        env:
-          VALIDATE_ALL_CODEBASE: false
-          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
-          VALIDATE_MARKDOWN: true
-          DEFAULT_BRANCH: ${{ github.base_ref }}
-          MARKDOWN_LINT_CONFIG_FILE: .markdownlint.json
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 755abffeb..a28975329 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -68,14 +68,14 @@ include(GNUInstallDirs)
   ## Define supported version of clang and llvm
 
   set(CLANG_MIN_SUPPORTED 18.0)
-  set(CLANG_MAX_SUPPORTED "20.1.x")
-  set(CLANG_VERSION_UPPER_BOUND 21.0.0)
+  set(CLANG_MAX_SUPPORTED "21.1.x")
+  set(CLANG_VERSION_UPPER_BOUND 22.0.0)
   set(LLD_MIN_SUPPORTED 18.0)
-  set(LLD_MAX_SUPPORTED "20.1.x")
-  set(LLD_VERSION_UPPER_BOUND 21.0.0)
+  set(LLD_MAX_SUPPORTED "21.1.x")
+  set(LLD_VERSION_UPPER_BOUND 22.0.0)
   set(LLVM_MIN_SUPPORTED 18.0)
-  set(LLVM_MAX_SUPPORTED "20.1.x")
-  set(LLVM_VERSION_UPPER_BOUND 21.0.0)
+  set(LLVM_MAX_SUPPORTED "21.1.x")
+  set(LLVM_VERSION_UPPER_BOUND 22.0.0)
 
   ## Set Cmake packages search order
 
diff --git a/Emscripten-build-instructions.md b/Emscripten-build-instructions.md
index 7e9f516e1..740425483 100644
--- a/Emscripten-build-instructions.md
+++ b/Emscripten-build-instructions.md
@@ -42,11 +42,11 @@ $env:PWD_DIR= $PWD.Path
 $env:SYSROOT_PATH="$env:EMSDK/upstream/emscripten/cache/sysroot"
 ```
 
-Now clone the 20.x release of the LLVM project repository and CppInterOp (the building of the emscripten version of llvm can be
+Now clone the 21.x release of the LLVM project repository and CppInterOp (the building of the emscripten version of llvm can be
 avoided by executing micromamba install llvm -c <https://repo.mamba.pm/emscripten-forge> and setting the LLVM_BUILD_DIR/$env:LLVM_BUILD_DIR appropriately)
 
 ```bash
-git clone --depth=1 --branch release/20.x https://github.com/llvm/llvm-project.git
+git clone --depth=1 --branch release/21.x https://github.com/llvm/llvm-project.git
 git clone --depth=1 https://github.com/compiler-research/CppInterOp.git
 ```
 
@@ -55,16 +55,16 @@ executing
 
 ```bash
 cd ./llvm-project/
-git apply -v ../CppInterOp/patches/llvm/emscripten-clang20-*.patch
+git apply -v ../CppInterOp/patches/llvm/emscripten-clang21-*.patch
 ```
 
 On Windows execute the following
 
 ```powershell
 cd .\llvm-project\
-cp -r ..\patches\llvm\emscripten-clang20*
-git apply -v emscripten-clang20-2-shift-temporary-files-to-tmp-dir.patch
-git apply -v emscripten-clang20-3-enable_exception_handling.patch
+cp -r ..\patches\llvm\emscripten-clang21*
+git apply -v emscripten-clang21-1-shift-temporary-files-to-tmp-dir.patch
+git apply -v emscripten-clang21-2-enable_exception_handling.patch
 ```
 
 We are now in a position to build an emscripten build of llvm by executing the following on Linux
@@ -343,7 +343,7 @@ of llvm you are building against)
 ```bash
 cd ../..
 git clone --depth=1 https://github.com/compiler-research/xeus-cpp.git
-export LLVM_VERSION=20
+export LLVM_VERSION=21
 cd ./xeus-cpp
 mkdir build
 cd build
diff --git a/README.md b/README.md
index fd2f6a4bc..19133a965 100644
--- a/README.md
+++ b/README.md
@@ -104,10 +104,10 @@ git clone --depth=1 https://github.com/compiler-research/cppyy-backend.git
 
 #### Setup Clang-REPL
 
-Clone the 20.x release of the LLVM project repository.
+Clone the 21.x release of the LLVM project repository.
 
 ```bash
-git clone --depth=1 --branch release/20.x https://github.com/llvm/llvm-project.git
+git clone --depth=1 --branch release/21.x https://github.com/llvm/llvm-project.git
 cd llvm-project
 ```
 
@@ -456,10 +456,10 @@ git clone --depth=1 https://github.com/compiler-research/cppyy-backend.git
 
 #### Setup Clang-REPL
 
-Clone the 20.x release of the LLVM project repository.
+Clone the 21.x release of the LLVM project repository.
 
 ```bash
-git clone --depth=1 --branch release/20.x https://github.com/llvm/llvm-project.git
+git clone --depth=1 --branch release/21.x https://github.com/llvm/llvm-project.git
 cd llvm-project
 ```
 
diff --git a/docs/DevelopersDocumentation.rst b/docs/DevelopersDocumentation.rst
index 347e07fda..77e89238a 100644
--- a/docs/DevelopersDocumentation.rst
+++ b/docs/DevelopersDocumentation.rst
@@ -34,11 +34,11 @@ library
  Setup Clang-REPL
 ******************
 
-Clone the 20.x release of the LLVM project repository.
+Clone the 21.x release of the LLVM project repository.
 
 .. code:: bash
 
-   git clone --depth=1 --branch release/20.x https://github.com/llvm/llvm-project.git
+   git clone --depth=1 --branch release/21.x https://github.com/llvm/llvm-project.git
    cd llvm-project
 
 If you want to have out-of-process JIT execution enabled in CppInterOp, then apply this patch on Linux-x86_64 and MacOS arm64 environment.
diff --git a/docs/Emscripten-build-instructions.rst b/docs/Emscripten-build-instructions.rst
index c36f5e0bb..a46db3ef8 100644
--- a/docs/Emscripten-build-instructions.rst
+++ b/docs/Emscripten-build-instructions.rst
@@ -57,7 +57,7 @@ and on Windows execute in Powershell
    $env:PWD_DIR= $PWD.Path
    $env:SYSROOT_PATH="$env:EMSDK/upstream/emscripten/cache/sysroot"
 
-Now clone the 20.x release of the LLVM project repository and CppInterOp
+Now clone the 21.x release of the LLVM project repository and CppInterOp
 (the building of the emscripten version of llvm can be avoided by
 executing micromamba install llvm -c
 <https://repo.mamba.pm/emscripten-forge> and setting the LLVM_BUILD_DIR/$env:LLVM_BUILD_DIR
@@ -65,7 +65,7 @@ appropriately)
 
 .. code:: bash
 
-   git clone --depth=1 --branch release/20.x https://github.com/llvm/llvm-project.git
+   git clone --depth=1 --branch release/21.x https://github.com/llvm/llvm-project.git
    git clone --depth=1 https://github.com/compiler-research/CppInterOp.git
 
 Now move into the cloned llvm-project folder and apply the required patches. On Linux and osx this
@@ -74,16 +74,16 @@ executing
 .. code:: bash
 
    cd ./llvm-project/
-   git apply -v ../CppInterOp/patches/llvm/emscripten-clang20-*.patch
+   git apply -v ../CppInterOp/patches/llvm/emscripten-clang21-*.patch
 
 On Windows execute the following
 
 .. code:: powershell
 
    cd .\llvm-project\
-   cp -r ..\patches\llvm\emscripten-clang20*
-   git apply -v emscripten-clang20-2-shift-temporary-files-to-tmp-dir.patch
-   git apply -v emscripten-clang20-3-enable_exception_handling.patch
+   cp -r ..\patches\llvm\emscripten-clang21*
+   git apply -v emscripten-clang21-1-shift-temporary-files-to-tmp-dir.patch
+   git apply -v emscripten-clang21-2-enable_exception_handling.patch
 
 We are now in a position to build an emscripten build of llvm by executing the following on Linux
 and osx
@@ -365,7 +365,7 @@ by executing (replace LLVM_VERSION with the version of llvm you are building aga
 
    cd ../..
    git clone --depth=1 https://github.com/compiler-research/xeus-cpp.git
-   export LLVM_VERSION=20
+   export LLVM_VERSION=21
    cd ./xeus-cpp
    mkdir build
    cd build
diff --git a/docs/InstallationAndUsage.rst b/docs/InstallationAndUsage.rst
index a5b1b1892..911281c25 100644
--- a/docs/InstallationAndUsage.rst
+++ b/docs/InstallationAndUsage.rst
@@ -34,11 +34,11 @@ library
  Setup Clang-REPL
 ******************
 
-Clone the 20.x release of the LLVM project repository.
+Clone the 21.x release of the LLVM project repository.
 
 .. code:: bash
 
-   git clone --depth=1 --branch release/20.x https://github.com/llvm/llvm-project.git
+   git clone --depth=1 --branch release/21.x https://github.com/llvm/llvm-project.git
    cd llvm-project
 
 If you want to have out-of-process JIT execution enabled in CppInterOp, then apply this patch on Linux-x86_64 and MacOS arm64 environment.
diff --git a/docs/ReleaseNotes.md b/docs/ReleaseNotes.md
index 811519dea..a39ef6220 100644
--- a/docs/ReleaseNotes.md
+++ b/docs/ReleaseNotes.md
@@ -25,7 +25,7 @@ infrastructure are described first.
 ## External Dependencies
 
 - CppInterOp now works with:
-  - llvm20
+  - llvm21
 
 ## Introspection
 
diff --git a/lib/CppInterOp/Compatibility.h b/lib/CppInterOp/Compatibility.h
index 0c029910e..c5d9f7574 100644
--- a/lib/CppInterOp/Compatibility.h
+++ b/lib/CppInterOp/Compatibility.h
@@ -31,6 +31,23 @@ static inline char* GetEnv(const char* Var_Name) {
 #endif
 }
 
+#if CLANG_VERSION_MAJOR < 21
+#define Print_Canonical_Types PrintCanonicalTypes
+#else
+#define Print_Canonical_Types PrintAsCanonical
+#endif
+
+#if CLANG_VERSION_MAJOR < 21
+#define clang_LookupResult_Found clang::LookupResult::Found
+#define clang_LookupResult_Not_Found clang::LookupResult::NotFound
+#define clang_LookupResult_Found_Overloaded clang::LookupResult::FoundOverloaded
+#else
+#define clang_LookupResult_Found clang::LookupResultKind::Found
+#define clang_LookupResult_Not_Found clang::LookupResultKind::NotFound
+#define clang_LookupResult_Found_Overloaded                                    \
+  clang::LookupResultKind::FoundOverloaded
+#endif
+
 #if CLANG_VERSION_MAJOR < 19
 #define Template_Deduction_Result Sema::TemplateDeductionResult
 #define Template_Deduction_Result_Success                                      \
@@ -479,11 +496,14 @@ inline void InstantiateClassTemplateSpecialization(
 #if CLANG_VERSION_MAJOR < 20
   interp.getSema().InstantiateClassTemplateSpecialization(
       clang::SourceLocation::getFromRawEncoding(1), CTSD,
-      clang::TemplateSpecializationKind::TSK_Undeclared, /*Complain=*/true);
+
+      clang::TemplateSpecializationKind::TSK_ExplicitInstantiationDefinition,
+      /*Complain=*/true);
 #else
   interp.getSema().InstantiateClassTemplateSpecialization(
       clang::SourceLocation::getFromRawEncoding(1), CTSD,
-      clang::TemplateSpecializationKind::TSK_Undeclared, /*Complain=*/true,
+      clang::TemplateSpecializationKind::TSK_ExplicitInstantiationDefinition,
+      /*Complain=*/true,
       /*PrimaryHasMatchedPackOnParmToNonPackOnArg=*/false);
 #endif
 }
diff --git a/lib/CppInterOp/CppInterOp.cpp b/lib/CppInterOp/CppInterOp.cpp
index 46f2fe226..9b96a7abc 100755
--- a/lib/CppInterOp/CppInterOp.cpp
+++ b/lib/CppInterOp/CppInterOp.cpp
@@ -1182,16 +1182,16 @@ bool GetClassTemplatedMethods(const std::string& name, TCppScope_t parent,
   auto* DC = clang::Decl::castToDeclContext(D);
   Cpp_utils::Lookup::Named(&S, R, DC);
 
-  if (R.getResultKind() == clang::LookupResult::NotFound && funcs.empty())
+  if (R.getResultKind() == clang_LookupResult_Not_Found && funcs.empty())
     return false;
 
   // Distinct match, single Decl
-  else if (R.getResultKind() == clang::LookupResult::Found) {
+  else if (R.getResultKind() == clang_LookupResult_Found) {
     if (IsTemplatedFunction(R.getFoundDecl()))
       funcs.push_back(R.getFoundDecl());
   }
   // Loop over overload set
-  else if (R.getResultKind() == clang::LookupResult::FoundOverloaded) {
+  else if (R.getResultKind() == clang_LookupResult_Found_Overloaded) {
     for (auto* Found : R)
       if (IsTemplatedFunction(Found))
         funcs.push_back(Found);
@@ -1960,7 +1960,7 @@ static void GetDeclName(const clang::Decl* D, ASTContext& Context,
   PrintingPolicy Policy(Context.getPrintingPolicy());
   Policy.SuppressTagKeyword = true;
   Policy.SuppressUnwrittenScope = true;
-  Policy.PrintCanonicalTypes = true;
+  Policy.Print_Canonical_Types = true;
   if (const TypeDecl* TD = dyn_cast<TypeDecl>(D)) {
     // This is a class, struct, or union member.
     QualType QT;
diff --git a/lib/CppInterOp/exports.ld b/lib/CppInterOp/exports.ld
index e22490f2a..9d99a30c8 100644
--- a/lib/CppInterOp/exports.ld
+++ b/lib/CppInterOp/exports.ld
@@ -51,4 +51,6 @@
 -Wl,--export=_ZNK5clang12FunctionDecl12getNumParamsEv
 -Wl,--export=__clang_Interpreter_SetValueNoAlloc
 -Wl,--export=__clang_Interpreter_SetValueWithAlloc
+-Wl,--export=_ZN5clang11Interpreter6createENSt3__210unique_ptrINS_16CompilerInstanceENS1_14default_deleteIS3_EEEENS2_IN4llvm3orc12LLJITBuilderENS4_IS9_EEEE
+-Wl,--export=_ZNK5clang13CXXRecordDecl19isInjectedClassNameEv
 -Wl,--export=_ZN4llvm15SmallVectorBaseIjE8set_sizeEm
\ No newline at end of file
diff --git a/patches/llvm/emscripten-clang21-1-shift-temporary-files-to-tmp-dir.patch b/patches/llvm/emscripten-clang21-1-shift-temporary-files-to-tmp-dir.patch
new file mode 100644
index 000000000..32ac45c13
--- /dev/null
+++ b/patches/llvm/emscripten-clang21-1-shift-temporary-files-to-tmp-dir.patch
@@ -0,0 +1,15 @@
+diff --git a/clang/lib/Interpreter/Wasm.cpp b/clang/lib/Interpreter/Wasm.cpp
+index aa10b160ccf8..184867e2b55f 100644
+--- a/clang/lib/Interpreter/Wasm.cpp
++++ b/clang/lib/Interpreter/Wasm.cpp
+@@ -76,8 +76,8 @@ llvm::Error WasmIncrementalExecutor::addModule(PartialTranslationUnit &PTU) {
+   llvm::TargetMachine *TargetMachine = Target->createTargetMachine(
+       PTU.TheModule->getTargetTriple(), "", "", TO, llvm::Reloc::Model::PIC_);
+   PTU.TheModule->setDataLayout(TargetMachine->createDataLayout());
+-  std::string ObjectFileName = PTU.TheModule->getName().str() + ".o";
+-  std::string BinaryFileName = PTU.TheModule->getName().str() + ".wasm";
++  std::string ObjectFileName = "/tmp/" + PTU.TheModule->getName().str() + ".o";
++  std::string BinaryFileName = "/tmp/" + PTU.TheModule->getName().str() + ".wasm";
+ 
+   std::error_code Error;
+   llvm::raw_fd_ostream ObjectFileOutput(llvm::StringRef(ObjectFileName), Error);
diff --git a/patches/llvm/emscripten-clang21-2-enable_exception_handling.patch b/patches/llvm/emscripten-clang21-2-enable_exception_handling.patch
new file mode 100644
index 000000000..269fa5d24
--- /dev/null
+++ b/patches/llvm/emscripten-clang21-2-enable_exception_handling.patch
@@ -0,0 +1,67 @@
+diff --git a/clang/lib/Interpreter/Interpreter.cpp b/clang/lib/Interpreter/Interpreter.cpp
+index ed3bae59a..f1fedd532 100644
+--- a/clang/lib/Interpreter/Interpreter.cpp
++++ b/clang/lib/Interpreter/Interpreter.cpp
+@@ -144,6 +144,48 @@ CreateCI(const llvm::opt::ArgStringList &Argv) {
+   return std::move(Clang);
+ }
+ 
++static llvm::Error HandleFrontendOptions(const CompilerInstance &CI) {
++  const auto &FrontendOpts = CI.getFrontendOpts();
++
++  if (FrontendOpts.ShowHelp) {
++    driver::getDriverOptTable().printHelp(
++        llvm::outs(), "clang -cc1 [options] file...",
++        "LLVM 'Clang' Compiler: http://clang.llvm.org",
++        /*ShowHidden=*/false, /*ShowAllAliases=*/false,
++        llvm::opt::Visibility(driver::options::CC1Option));
++    return llvm::createStringError(llvm::errc::not_supported, "Help displayed");
++  }
++
++  if (FrontendOpts.ShowVersion) {
++    llvm::cl::PrintVersionMessage();
++    return llvm::createStringError(llvm::errc::not_supported,
++                                   "Version displayed");
++  }
++
++  if (!FrontendOpts.LLVMArgs.empty()) {
++    unsigned NumArgs = FrontendOpts.LLVMArgs.size();
++    auto Args = std::make_unique<const char *[]>(NumArgs + 2);
++    Args[0] = "clang-repl (LLVM option parsing)";
++    for (unsigned i = 0; i != NumArgs; ++i) {
++      Args[i + 1] = FrontendOpts.LLVMArgs[i].c_str();
++      // remove the leading '-' from the option name
++      if (Args[i + 1][0] == '-') {
++        auto *option = static_cast<llvm::cl::opt<bool> *>(
++            llvm::cl::getRegisteredOptions()[Args[i + 1] + 1]);
++        if (option) {
++          option->setInitialValue(true);
++        } else {
++          llvm::errs() << "Unknown LLVM option: " << Args[i + 1] << "\n";
++        }
++      }
++    }
++    Args[NumArgs + 1] = nullptr;
++    llvm::cl::ParseCommandLineOptions(NumArgs + 1, Args.get());
++  }
++
++  return llvm::Error::success();
++}
++
+ } // anonymous namespace
+ 
+ namespace clang {
+@@ -462,7 +504,12 @@ const char *const Runtimes = R"(
+ llvm::Expected<std::unique_ptr<Interpreter>>
+ Interpreter::create(std::unique_ptr<CompilerInstance> CI,
+                     std::unique_ptr<llvm::orc::LLJITBuilder> JB) {
+-  llvm::Error Err = llvm::Error::success();
++
++  llvm::Error Err = HandleFrontendOptions(*CI);
++  if (Err) {
++    return std::move(Err);
++  }
++
+   auto Interp = std::unique_ptr<Interpreter>(
+       new Interpreter(std::move(CI), Err, JB ? std::move(JB) : nullptr));
+   if (Err)