diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index e845bd98f..0a13f3640 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -22,13 +22,23 @@ jobs:
       matrix:
         include:
           - name: ubu22-x86-gcc12-clang-repl-19
+            os: ubuntu-22.04
+            compiler: gcc-12
+            clang-runtime: '19'
+            cling: Off
+            cppyy: On
+            llvm_enable_projects: "clang"
+            llvm_targets_to_build: "host;NVPTX"
+          - name: ubu22-x86-gcc12-clang-repl-19-emscripten
             os: ubuntu-22.04
             compiler: gcc-12
             clang-runtime: '19'
             cling: Off
             cppyy: On
             llvm_enable_projects: "clang;lld"
-            llvm_targets_to_build: "WebAssembly;host;NVPTX"
+            llvm_targets_to_build: "WebAssembly"
+            emscripten: On
+            emsdk_ver: "3.1.45"
           - name: ubu22-x86-gcc12-clang-repl-18
             os: ubuntu-22.04
             compiler: gcc-12
@@ -87,13 +97,23 @@ jobs:
             llvm_enable_projects: "clang"
             llvm_targets_to_build: "host;NVPTX"
           - name: osx15-arm-clang-clang-repl-19
+            os: macos-15
+            compiler: clang
+            clang-runtime: '19'
+            cling: Off
+            cppyy: On
+            llvm_enable_projects: "clang"
+            llvm_targets_to_build: "host;NVPTX"
+          - name: osx15-arm-clang-clang-repl-19-emscripten
             os: macos-15
             compiler: clang
             clang-runtime: '19'
             cling: Off
             cppyy: On
             llvm_enable_projects: "clang;lld"
-            llvm_targets_to_build: "WebAssembly;host;NVPTX"
+            llvm_targets_to_build: "WebAssembly"
+            emscripten: On
+            emsdk_ver: "3.1.45"
           - name: osx15-arm-clang-clang-repl-18
             os: macos-15
             compiler: clang
@@ -128,13 +148,23 @@ jobs:
             llvm_enable_projects: "clang"
             llvm_targets_to_build: "host;NVPTX"
           - name: osx13-x86-clang-clang-repl-19
+            os: macos-13
+            compiler: clang
+            clang-runtime: '19'
+            cling: Off
+            cppyy: On
+            llvm_enable_projects: "clang"
+            llvm_targets_to_build: "host;NVPTX"
+          - name: osx13-x86-clang-clang-repl-19-emscripten
             os: macos-13
             compiler: clang
             clang-runtime: '19'
             cling: Off
             cppyy: On
             llvm_enable_projects: "clang;lld"
-            llvm_targets_to_build: "WebAssembly;host;NVPTX"
+            llvm_targets_to_build: "WebAssembly"
+            emscripten: On
+            emsdk_ver: "3.1.45"
           - name: osx13-x86-clang-clang-repl-18
             os: macos-13
             compiler: clang
@@ -198,7 +228,7 @@ jobs:
         fi
 
         echo "CLING_HASH=$CLING_HASH" >> $GITHUB_ENV
-        echo "LLVM_HASH=$LLVM_HASH" >> $GITHUB_ENV
+        echo "LLVM_HASH=$LLVM_HASH" >> $GITHUB_ENV        
 
     - name: Save PR Info on Windows systems
       if: ${{ runner.os == 'windows' }}
@@ -227,10 +257,24 @@ jobs:
         
         echo "CLING_HASH=$env:CLING_HASH" >> $GITHUB_ENV
         echo "LLVM_HASH=$env:LLVM_HASH" >> $GITHUB_ENV
+        echo "KEY=${{ env.CLING_HASH }}-${{ runner.os }}-${{ matrix.os }}-${{ matrix.compiler }}-clang-${{ matrix.clang-runtime }}.x-patch-${{ hashFiles(format('patches/llvm/clang{0}-*.patch', matrix.clang-runtime)) || 'none' }}" >> $env:GITHUB_ENV
 
-    - name: Restore Cache LLVM/Clang runtime build directory
+    #FIXME: Shouldn't need 3 distinct restore steps for different situations.
+    - name: Restore Cache LLVM/Clang runtime build directory (Windows)
+      if: ${{ runner.os == 'windows' }}
       uses: actions/cache/restore@v4
-      id: cache
+      id: cachewindows
+      with:
+        path: |
+          llvm-project
+          ${{ matrix.cling=='On' && 'cling' || '' }}
+        key: ${{ env.key }}
+        lookup-only: true
+
+    - name: Restore Cache LLVM/Clang runtime build directory (Unix like systems)
+      if: ${{ runner.os != 'windows' && matrix.emscripten != 'On' }}
+      uses: actions/cache/restore@v4
+      id: cachenotemscripten
       with:
         path: |
           llvm-project
@@ -238,8 +282,38 @@ jobs:
         key: ${{ env.CLING_HASH }}-${{ runner.os }}-${{ matrix.os }}-${{ matrix.compiler }}-clang-${{ matrix.clang-runtime }}.x-patch-${{ hashFiles(format('patches/llvm/clang{0}-*.patch', matrix.clang-runtime)) || 'none' }}
         lookup-only: true
 
+    - name: Restore Cache LLVM/Clang runtime build directory (Unix like systems emscripten)
+      if: ${{ runner.os != 'windows' && matrix.emscripten == 'On' }}
+      uses: actions/cache/restore@v4
+      id: cacheemscripten
+      with:
+        path: |
+          llvm-project
+          ${{ matrix.cling=='On' && 'cling' || '' }}
+        key: ${{ env.CLING_HASH }}-${{ runner.os }}-${{ matrix.os }}-${{ matrix.compiler }}-clang-${{ matrix.clang-runtime }}.x-emscripten
+        lookup-only: true
+
+    - name: Setup emsdk
+      if: ${{ runner.os != 'windows' && steps.cacheemscripten.outputs.cache-hit != 'true' && matrix.emscripten == 'On' }}
+      run: |
+          git clone https://github.com/emscripten-core/emsdk.git
+          cd emsdk
+          ./emsdk install  ${{ matrix.emsdk_ver }}
+
     - name: Setup default Build Type on *nux
-      if: ${{ runner.os != 'windows' && steps.cache.outputs.cache-hit != 'true' }}
+      if: ${{ runner.os != 'windows' && steps.cachenotemscripten.outputs.cache-hit != 'true' && matrix.emscripten != 'On' }}
+      run: |
+        echo "BUILD_TYPE=Release" >> $GITHUB_ENV
+        echo "CODE_COVERAGE=0" >> $GITHUB_ENV
+        os="${{ matrix.os }}"
+        if [[ "${os}" == "macos"* ]]; then
+          echo "ncpus=$(sysctl -n hw.ncpu)" >> $GITHUB_ENV
+        else
+          echo "ncpus=$(nproc --all)" >> $GITHUB_ENV
+        fi
+
+    - name: Setup default Build Type on *nux
+      if: ${{ runner.os != 'windows' && steps.cacheemscripten.outputs.cache-hit != 'true' && matrix.emscripten == 'On' }}
       run: |
         echo "BUILD_TYPE=Release" >> $GITHUB_ENV
         echo "CODE_COVERAGE=0" >> $GITHUB_ENV
@@ -251,7 +325,7 @@ jobs:
         fi
 
     - name: Setup default Build Type on Windows
-      if: ${{ runner.os == 'windows' && steps.cache.outputs.cache-hit != 'true' }}
+      if: ${{ runner.os == 'windows' && steps.cachewindows.outputs.cache-hit != 'true' }}
       run: |
         echo "BUILD_TYPE=Release" >> $env:GITHUB_ENV
         echo "CODE_COVERAGE=0" >>  $env:GITHUB_ENV
@@ -259,7 +333,7 @@ jobs:
         echo "ncpus=$env:ncpus" >> $env:GITHUB_ENV
 
     - name: Setup compiler on Linux
-      if: ${{ runner.os == 'Linux' && steps.cache.outputs.cache-hit != 'true' }}
+      if: ${{ runner.os == 'Linux' && steps.cachenotemscripten.outputs.cache-hit != 'true' && matrix.emscripten != 'On' }}
       run: |
         # https://www.gnu.org/software/bash/manual/html_node/Shell-Parameter-Expansion.html
         vers="${compiler#*-}"
@@ -282,8 +356,69 @@ jobs:
       env:
         compiler: ${{ matrix.compiler }}
 
+    - name: Setup compiler on Linux
+      if: ${{ runner.os == 'Linux' && steps.cacheemscripten.outputs.cache-hit != 'true' && matrix.emscripten == 'On' }}
+      run: |
+        # https://www.gnu.org/software/bash/manual/html_node/Shell-Parameter-Expansion.html
+        vers="${compiler#*-}"
+        os_codename="`cat /etc/os-release | grep UBUNTU_CODENAME | cut -d = -f 2`"
+        ##sudo apt update
+        if [[ "${{ matrix.compiler }}" == *"gcc"* ]]; then
+          sudo apt install -y gcc-${vers} g++-${vers} lld
+          echo "CC=gcc-${vers}" >> $GITHUB_ENV
+          echo "CXX=g++-${vers}" >> $GITHUB_ENV
+        else
+          if ! sudo apt install -y clang-${vers}; then
+            curl https://apt.llvm.org/llvm-snapshot.gpg.key | sudo apt-key add -
+            echo "deb https://apt.llvm.org/${os_codename}/ llvm-toolchain-${os_codename}-${vers} main" | sudo tee -a /etc/apt/sources.list
+            sudo apt-get update
+            sudo apt-get install -y clang-${vers}
+          fi
+          echo "CC=clang-${vers}" >> $GITHUB_ENV
+          echo "CXX=clang++-${vers}" >> $GITHUB_ENV
+        fi
+      env:
+        compiler: ${{ matrix.compiler }}
+
+    - name: Setup compiler on macOS
+      if: ${{ runner.os == 'macOS' && steps.cachenotemscripten.outputs.cache-hit != 'true' && matrix.emscripten != 'On' }}
+      run: |
+        vers="${compiler#*-}"
+        if [[ "${{ matrix.compiler }}" == *"gcc"* ]]; then
+          brew install "gcc@$vers"
+          echo "CC=gcc-${vers}" >> $GITHUB_ENV
+          echo "CXX=g++-${vers}" >> $GITHUB_ENV
+        else
+          export ARCHITECHURE=$(uname -m)
+          if [[ "$ARCHITECHURE" == "arm64" ]]; then
+            cling_on=$(echo "${{ matrix.cling }}" | tr '[:lower:]' '[:upper:]')
+            if [[ "${cling_on}" == "ON" ]]; then  
+                brew install llvm@15
+                brew remove llvm@18
+                brew cleanup
+                #FIXME: Do not believe setting all these environment variables are necessary
+                #       They were set to avoid using Xcodes libc++ and to stop CppInterOp using llvm@18 in tests
+                echo 'LDFLAGS="-L/opt/homebrew/opt/llvm@15/lib/ -L/opt/homebrew/opt/llvm@15/c++/"' >> $GITHUB_ENV
+                echo 'CPPFLAGS="-I/opt/homebrew/opt/llvm@15/include"' >> $GITHUB_ENV
+                echo 'CPATH="/opt/homebrew/include/"' >> $GITHUB_ENV
+                echo 'LIBRARY_PATH="/opt/homebrew/lib/"' >> $GITHUB_ENV
+                echo "CC=$(brew --prefix llvm@15)/bin/clang" >> $GITHUB_ENV
+                echo "CXX=$(brew --prefix llvm@15)/bin/clang++" >> $GITHUB_ENV
+            else
+              echo "CC=$(brew --prefix llvm@18)/bin/clang" >> $GITHUB_ENV
+              echo "CXX=$(brew --prefix llvm@18)/bin/clang++" >> $GITHUB_ENV
+            fi
+          else
+            echo "CC=$(brew --prefix llvm@15)/bin/clang" >> $GITHUB_ENV
+            echo "CXX=$(brew --prefix llvm@15)/bin/clang++" >> $GITHUB_ENV
+          fi
+        fi
+        echo "SDKROOT=$(xcrun --sdk macosx --show-sdk-path)" >> $GITHUB_ENV
+      env:
+        compiler: ${{ matrix.compiler }}
+
     - name: Setup compiler on macOS
-      if: ${{ runner.os == 'macOS' && steps.cache.outputs.cache-hit != 'true' }}
+      if: ${{ runner.os == 'macOS' && steps.cacheemscripten.outputs.cache-hit != 'true' && matrix.emscripten == 'On' }}
       run: |
         vers="${compiler#*-}"
         if [[ "${{ matrix.compiler }}" == *"gcc"* ]]; then
@@ -322,7 +457,7 @@ jobs:
     #Section slightly modified version of
     #https://github.com/vgvassilev/clad/blob/40d8bec11bde47b14a281078183a4f6147abeac5/.github/workflows/ci.yml#L510C1-L534C10
     - name: Setup compiler on Windows
-      if: ${{ runner.os == 'windows' && steps.cache.outputs.cache-hit != 'true' }}
+      if: ${{ runner.os == 'windows' && steps.cachewindows.outputs.cache-hit != 'true' }}
       run: |
         if ( "${{ matrix.compiler }}" -imatch "clang" )
         {
@@ -345,13 +480,40 @@ jobs:
         }
 
     - name: Install deps on Windows
-      if: ${{ runner.os == 'windows' && steps.cache.outputs.cache-hit != 'true' }}
+      if: ${{ runner.os == 'windows' && steps.cachewindows.outputs.cache-hit != 'true' }}
       run: |
         choco install findutils
         $env:PATH="C:\Program Files (x86)\GnuWin32\bin;$env:PATH"
 
     - name: Install deps on Linux
-      if: ${{ runner.os == 'Linux' && steps.cache.outputs.cache-hit != 'true' }}
+      if: ${{ runner.os == 'Linux' && steps.cachenotemscripten.outputs.cache-hit != 'true' && matrix.emscripten != 'On' }}
+      run: |
+        # Install deps
+        sudo apt-get update
+        sudo apt-get install valgrind
+        sudo apt-get autoremove
+        sudo apt-get clean
+
+    - name: Install deps on MacOS
+      if: ${{ runner.os == 'macOS' && steps.cachenotemscripten.outputs.cache-hit != 'true' && matrix.emscripten != 'On' }}
+      run: |
+        brew update
+        export ARCHITECHURE=$(uname -m)
+        if [[ "$ARCHITECHURE" != "x86_64" ]]; then
+          brew remove unxip
+        else
+          brew remove ruby@3.0
+        fi
+        # workaround for https://github.com/actions/setup-python/issues/577
+        for pkg in $(brew list | grep '^python@'); do
+          brew unlink "$pkg"
+          brew link --overwrite "$pkg"
+        done
+        brew upgrade openssl >/dev/null 2>&1 
+        brew upgrade
+
+    - name: Install deps on Linux
+      if: ${{ runner.os == 'Linux' && steps.cacheemscripten.outputs.cache-hit != 'true' && matrix.emscripten == 'On' }}
       run: |
         # Install deps
         sudo apt-get update
@@ -360,7 +522,7 @@ jobs:
         sudo apt-get clean
 
     - name: Install deps on MacOS
-      if: ${{ runner.os == 'macOS' && steps.cache.outputs.cache-hit != 'true' }}
+      if: ${{ runner.os == 'macOS' && steps.cacheemscripten.outputs.cache-hit != 'true' && matrix.emscripten == 'On' }}
       run: |
         brew update
         export ARCHITECHURE=$(uname -m)
@@ -378,7 +540,7 @@ jobs:
         brew upgrade
 
     - name: Build LLVM/Cling on Unix systems if the cache is invalid
-      if: ${{ runner.os != 'windows' && steps.cache.outputs.cache-hit != 'true' }}
+      if: ${{ runner.os != 'windows' && steps.cachenotemscripten.outputs.cache-hit != 'true' && matrix.emscripten != 'On' }}
       run: |
         cling_on=$(echo "${{ matrix.cling }}" | tr '[:lower:]' '[:upper:]')
         if [[ "${cling_on}" == "ON" ]]; then
@@ -433,11 +595,8 @@ jobs:
                 -DLLVM_ENABLE_TERMINFO=OFF                          \
                 -DLLVM_ENABLE_LIBXML2=OFF                           \
                 ../llvm
-          if [[ "${llvm_vers}" == "16" ]]||[[ "${llvm_vers}" == "17" ]]||[[ "${llvm_vers}" == "18" ]]; then
-            cmake --build . --target clang clang-repl --parallel ${{ env.ncpus }}
-          else
-            cmake --build . --target clang clang-repl lld --parallel ${{ env.ncpus }}
-          fi
+          cmake --build . --target clang clang-repl --parallel ${{ env.ncpus }}
+          
         fi
         cd ../
         rm -rf $(find . -maxdepth 1 ! -name "build" ! -name "llvm" ! -name "clang" ! -name ".")
@@ -455,8 +614,74 @@ jobs:
           cd ../..
         fi
 
+    - name: Build LLVM/Cling on Unix systems if the cache is invalid (emscripten)
+      if: ${{ runner.os != 'windows' && steps.cacheemscripten.outputs.cache-hit != 'true' && matrix.emscripten == 'On' }}
+      run: |
+        ./emsdk/emsdk activate ${{matrix.emsdk_ver}}
+        source ./emsdk/emsdk_env.sh
+        cling_on=$(echo "${{ matrix.cling }}" | tr '[:lower:]' '[:upper:]')
+        if [[ "${cling_on}" == "ON" ]]; then
+          git clone https://github.com/root-project/cling.git
+          cd ./cling 
+          git checkout tags/v${{ matrix.cling-version }}
+          cd ..
+          git clone --depth=1 -b cling-llvm${{ matrix.clang-runtime }} https://github.com/root-project/llvm-project.git
+        else # repl
+          git clone --depth=1 -b release/${{ matrix.clang-runtime }}.x https://github.com/llvm/llvm-project.git
+        fi
+        cd llvm-project
+        # Build
+        mkdir build
+        if [[ "${cling_on}" == "ON" ]]; then
+          cd build
+          emcmake cmake -DLLVM_EXTERNAL_PROJECTS=cling \
+                        -DLLVM_EXTERNAL_CLING_SOURCE_DIR=../../cling  \
+                        -DCMAKE_BUILD_TYPE=Release \
+                        -DLLVM_HOST_TRIPLE=wasm32-unknown-emscripten \
+                        -DLLVM_ENABLE_ASSERTIONS=ON                        \
+                        -DLLVM_TARGETS_TO_BUILD="${{ matrix.llvm_targets_to_build }}" \
+                        -DLLVM_ENABLE_LIBEDIT=OFF \
+                        -DLLVM_ENABLE_PROJECTS="${{ matrix.llvm_enable_projects }}" \
+                        -DLLVM_ENABLE_ZSTD=OFF \
+                        -DLLVM_ENABLE_LIBXML2=OFF \
+                        -DCLANG_ENABLE_STATIC_ANALYZER=OFF \
+                        -DCLANG_ENABLE_ARCMT=OFF \
+                        -DCLANG_ENABLE_BOOTSTRAP=OFF \
+                        -DCMAKE_CXX_FLAGS="-Dwait4=__syscall_wait4" \
+                         ../llvm
+          emmake make clang -j ${{ env.ncpus }}
+          emmake make  cling -j ${{ env.ncpus }}
+          # Now build gtest.a and gtest_main for CppInterOp to run its tests.
+          emmake make gtest_main -j ${{ env.ncpus }}
+        else
+          # Apply patches
+          llvm_vers=$(echo "${{ matrix.clang-runtime }}" | tr '[:lower:]' '[:upper:]')
+          if [[ "${llvm_vers}" == "19" ]]; then
+            git apply -v ../patches/llvm/emscripten-clang${{ matrix.clang-runtime }}-*.patch
+            echo "Apply emscripten-clang${{ matrix.clang-runtime }}-*.patch patches:"
+          fi
+          cd build
+          emcmake cmake -DCMAKE_BUILD_TYPE=Release \
+                        -DLLVM_HOST_TRIPLE=wasm32-unknown-emscripten \
+                        -DLLVM_ENABLE_ASSERTIONS=ON                        \
+                        -DLLVM_TARGETS_TO_BUILD="${{ matrix.llvm_targets_to_build }}" \
+                        -DLLVM_ENABLE_LIBEDIT=OFF \
+                        -DLLVM_ENABLE_PROJECTS="${{ matrix.llvm_enable_projects }}" \
+                        -DLLVM_ENABLE_ZSTD=OFF \
+                        -DLLVM_ENABLE_LIBXML2=OFF \
+                        -DCLANG_ENABLE_STATIC_ANALYZER=OFF \
+                        -DCLANG_ENABLE_ARCMT=OFF \
+                        -DCLANG_ENABLE_BOOTSTRAP=OFF \
+                        -DCMAKE_CXX_FLAGS="-Dwait4=__syscall_wait4" \
+                        ../llvm
+          emmake make clang -j ${{ env.ncpus }}
+          emmake make clang-repl -j ${{ env.ncpus }}
+          emmake make lld -j ${{ env.ncpus }}
+        fi
+        cd ../..
+
     - name: Build LLVM/Cling on Windows systems if the cache is invalid
-      if: ${{ runner.os == 'windows' && steps.cache.outputs.cache-hit != 'true' }}
+      if: ${{ runner.os == 'windows' && steps.cachewindows.outputs.cache-hit != 'true' }}
       run: |
       
         if ( "${{ matrix.cling }}" -imatch "On" )
@@ -549,14 +774,32 @@ jobs:
           cd ..\..    
         } 
 
-    - name: Save Cache LLVM/Clang runtime build directory
+    - name: Save Cache LLVM/Clang runtime build directory (Windows)
+      uses: actions/cache/save@v4
+      if: ${{ runner.os == 'windows' && steps.cachewindows.outputs.cache-hit != 'true' }}
+      with:
+        path: |
+          llvm-project
+          ${{ matrix.cling=='On' && 'cling' || '' }}
+        key: ${{ steps.cachewindows.outputs.cache-primary-key }}
+
+    - name: Save Cache LLVM/Clang runtime build directory (Unix Systems)
       uses: actions/cache/save@v4
-      if: ${{ steps.cache.outputs.cache-hit != 'true' }}
+      if: ${{ runner.os != 'windows' && steps.cachenotemscripten.outputs.cache-hit != 'true' && matrix.emscripten != 'On' }}
       with:
         path: |
           llvm-project
           ${{ matrix.cling=='On' && 'cling' || '' }}
-        key: ${{ steps.cache.outputs.cache-primary-key }}
+        key: ${{ steps.cachenotemscripten.outputs.cache-primary-key }}
+
+    - name: Save Cache LLVM/Clang runtime build directory (Unix Systems Emscripten)
+      uses: actions/cache/save@v4
+      if: ${{ runner.os != 'windows' && steps.cacheemscripten.outputs.cache-hit != 'true' && matrix.emscripten == 'On' }}
+      with:
+        path: |
+          llvm-project
+          ${{ matrix.cling=='On' && 'cling' || '' }}
+        key: ${{ steps.cacheemscripten.outputs.cache-primary-key }}
 
   cppinterop_and_cppyy_build:
     needs: [build_cache]
@@ -1293,7 +1536,7 @@ jobs:
         path: |
           llvm-project
           ${{ matrix.cling=='On' && 'cling' || '' }}
-        key: ${{ env.CLING_HASH }}-${{ runner.os }}-${{ matrix.os }}-${{ matrix.compiler }}-clang-${{ matrix.clang-runtime }}.x-patch-${{ hashFiles(format('patches/llvm/clang{0}-*.patch', matrix.clang-runtime)) || 'none' }}
+        key: ${{ env.CLING_HASH }}-${{ runner.os }}-${{ matrix.os }}-${{ matrix.compiler }}-clang-${{ matrix.clang-runtime }}.x-emscripten
         
     - name: Emscripten build of CppInterOp on Unix systems
       if: ${{ runner.os != 'windows' }}
@@ -1317,12 +1560,11 @@ jobs:
         else
           CPLUS_INCLUDE_PATH="${LLVM_DIR}/llvm/include:${LLVM_DIR}/clang/include:${LLVM_BUILD_DIR}/include:${LLVM_BUILD_DIR}/tools/clang/include:$PWD/include"
         fi
-
+        
         # Build CppInterOp next to cling and llvm-project.
         mkdir build
         cd build
         export CPPINTEROP_BUILD_DIR=$PWD
-
         if [[ "${cling_on}" == "ON" ]]; then
           emcmake  cmake -DCMAKE_BUILD_TYPE=${{ env.BUILD_TYPE }}        \
                 -DUSE_CLING=ON                                  \
@@ -1340,8 +1582,8 @@ jobs:
           emcmake cmake -DCMAKE_BUILD_TYPE=${{ env.BUILD_TYPE }}    \
                 -DUSE_CLING=OFF                             \
                 -DUSE_REPL=ON                               \
-                -DCMAKE_PREFIX_PATH=$PREFIX                      \
-                -DLLVM_DIR=$LLVM_BUILD_DIR/lib/cmake/llvm       \
+                -DCMAKE_PREFIX_PATH=$PREFIX                     \
+                -DLLVM_DIR=$LLVM_BUILD_DIR/lib/cmake/llvm      \
                 -DClang_DIR=$LLVM_BUILD_DIR/lib/cmake/clang     \
                 -DBUILD_SHARED_LIBS=OFF                      \
                 -DCODE_COVERAGE=${{ env.CODE_COVERAGE }}    \
diff --git a/.github/workflows/deploy-pages.yml b/.github/workflows/deploy-pages.yml
index ebccaf279..027d8846d 100644
--- a/.github/workflows/deploy-pages.yml
+++ b/.github/workflows/deploy-pages.yml
@@ -87,7 +87,7 @@ jobs:
         path: |
           llvm-project
           ${{ matrix.cling=='On' && 'cling' || '' }}
-        key: ${{ env.CLING_HASH }}-${{ runner.os }}-${{ matrix.os }}-${{ matrix.compiler }}-clang-${{ matrix.clang-runtime }}.x-patch-${{ hashFiles(format('patches/llvm/clang{0}-*.patch', matrix.clang-runtime)) || 'none' }}
+        key: ${{ env.CLING_HASH }}-${{ runner.os }}-${{ matrix.os }}-${{ matrix.compiler }}-clang-${{ matrix.clang-runtime }}.x-emscripten
         
     - name: Emscripten build of CppInterOp on Unix systems
       if: ${{ runner.os != 'windows' }}
diff --git a/patches/llvm/emscripten-clang19-1-CrossCompile.patch b/patches/llvm/emscripten-clang19-1-CrossCompile.patch
new file mode 100644
index 000000000..4aa6b1539
--- /dev/null
+++ b/patches/llvm/emscripten-clang19-1-CrossCompile.patch
@@ -0,0 +1,16 @@
+diff --git a/llvm/cmake/modules/CrossCompile.cmake b/llvm/cmake/modules/CrossCompile.cmake
+index 6af47b51d4c6..c635e7f5be9e 100644
+--- a/llvm/cmake/modules/CrossCompile.cmake
++++ b/llvm/cmake/modules/CrossCompile.cmake
+@@ -70,8 +70,8 @@ function(llvm_create_cross_target project_name target_name toolchain buildtype)
+   add_custom_command(OUTPUT ${${project_name}_${target_name}_BUILD}/CMakeCache.txt
+     COMMAND ${CMAKE_COMMAND} -G "${CMAKE_GENERATOR}"
+         -DCMAKE_MAKE_PROGRAM="${CMAKE_MAKE_PROGRAM}"
+-        -DCMAKE_C_COMPILER_LAUNCHER="${CMAKE_C_COMPILER_LAUNCHER}"
+-        -DCMAKE_CXX_COMPILER_LAUNCHER="${CMAKE_CXX_COMPILER_LAUNCHER}"
++        -DCMAKE_C_COMPILER="clang"
++        -DCMAKE_CXX_COMPILER="clang++"
+         ${CROSS_TOOLCHAIN_FLAGS_${target_name}} ${CMAKE_CURRENT_SOURCE_DIR}
+         ${CROSS_TOOLCHAIN_FLAGS_${project_name}_${target_name}}
+         -DLLVM_TARGET_IS_CROSSCOMPILE_HOST=TRUE
+