diff --git a/.github/workflows/checkin.yml b/.github/workflows/checkin.yml
index 2febfde2..c897d9a1 100644
--- a/.github/workflows/checkin.yml
+++ b/.github/workflows/checkin.yml
@@ -5,7 +5,7 @@ jobs:
   check_pr:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v4
 
     - name: "npm ci"
       run: npm ci
diff --git a/.github/workflows/selftest.yml b/.github/workflows/selftest.yml
index c8f8c5e2..08be3bc6 100644
--- a/.github/workflows/selftest.yml
+++ b/.github/workflows/selftest.yml
@@ -19,6 +19,16 @@ jobs:
             - nightly
             - source
             - stable
+        msys2:  [true, false]
+        exclude:
+          - os: macos-12
+            msys2: true
+          - os: macos-14
+            msys2: true
+          - os: macos-stable
+            msys2: true
+          - os: ubuntu-latest
+            msys2: true
 
     runs-on: ${{ matrix.os }}
 
@@ -31,6 +41,7 @@ jobs:
         if: matrix.target == 'stable'
         uses: ./ # This uses the action code in the current PR
         with:
+          msys2: ${{ matrix.msys2 }}
           cache: false
           # We test without cache, as caching is tested in a separate workflow.
           # This way we make sure the cache isn't hiding any issue.
@@ -39,6 +50,7 @@ jobs:
         if: matrix.target == 'nightly'
         uses: ./
         with:
+          msys2: ${{ matrix.msys2 }}
           version: nightly
           cache: false
 
@@ -46,6 +58,7 @@ jobs:
         if: matrix.target == 'source'
         uses: ./
         with:
+          msys2: ${{ matrix.msys2 }}
           branch: master
           cache: false
 
@@ -80,3 +93,13 @@ jobs:
       - run: alr -n version | grep "arch:" | grep AARCH64
         if: runner.arch == 'ARM64'
         shell: bash
+
+      # Verify proper msys2 behavior
+
+      - run: alr -n version | grep "distribution:" | grep MSYS2
+        if: matrix.os == 'windows-latest' && matrix.msys2 == true
+        shell: bash
+
+      - run: alr -n version | grep "distribution:" | grep UNKNOWN
+        if: matrix.os == 'windows-latest' && matrix.msys2 == false
+        shell: bash
diff --git a/.github/workflows/test-cache.yml b/.github/workflows/test-cache.yml
index 038e5529..3c6f6f41 100644
--- a/.github/workflows/test-cache.yml
+++ b/.github/workflows/test-cache.yml
@@ -24,12 +24,29 @@ jobs:
             branch:  ''
           - version: ''
             branch:  'master'
+        msys2:
+          - true
+          - false
         # Exclude combos that do not make sense (we dont have aarm64 for 1.2.2)
         exclude:
           - os: macos-14
             config: {version: '1.2.2', branch: ''}
           - os: macos-latest
             config: {version: '1.2.2', branch: ''}
+          - os: windows-latest
+            config: {version: '1.2.2'}
+            msys2: false # We don't have the option to disable install on 1.2.2
+
+          # We don't exclude the following because this way we verify that the
+          # same cache is used for both msys2 true and false on non-Windows.
+          # - os: macos-12
+          #   msys2: true
+          # - os: macos-14
+          #   msys2: true
+          # - os: macos-stable
+          #   msys2: true
+          # - os: ubuntu-latest
+          #   msys2: true
 
     runs-on: ${{ matrix.os }}
 
@@ -44,7 +61,8 @@ jobs:
         uses: ./
         with:
           version: ${{matrix.config.version}}
-          branch: ${{matrix.config.branch}}
+          branch:  ${{matrix.config.branch}}
+          msys2:   ${{ matrix.msys2 }}
 
       # Next attemp should hit cache given the previous run
 
@@ -54,7 +72,8 @@ jobs:
         uses: ./
         with:
           version: ${{matrix.config.version}}
-          branch: ${{matrix.config.branch}}
+          branch:  ${{matrix.config.branch}}
+          msys2:   ${{ matrix.msys2 }}
 
       - name: Diagnose cache use
         shell: bash
@@ -77,7 +96,7 @@ jobs:
           alr exec -- gnat --version
           alr exec -- gprbuild --version
 
-      # Verify proper builds
+      # Verify proper OS
       - run: alr -n version | grep "os:" | grep LINUX
         if: matrix.os == 'ubuntu-latest'
         shell: bash
@@ -89,3 +108,23 @@ jobs:
       - run: alr -n version | grep "os:" | grep WINDOWS
         if: matrix.os == 'windows-latest'
         shell: bash
+
+      # Verify proper architecture
+
+      - run: alr -n version | grep "arch:" | grep X86_64
+        if: runner.arch == 'X64'
+        shell: bash
+
+      - run: alr -n version | grep "arch:" | grep AARCH64
+        if: runner.arch == 'ARM64'
+        shell: bash
+
+      # Verify proper msys2 behavior
+
+      - run: alr -n version | grep "distribution:" | grep MSYS2
+        if: matrix.os == 'windows-latest' && matrix.msys2 == true
+        shell: bash
+
+      - run: alr -n version | grep "distribution:" | grep UNKNOWN
+        if: matrix.os == 'windows-latest' && matrix.msys2 == false
+        shell: bash
diff --git a/README.md b/README.md
index 69928686..fcdf83db 100644
--- a/README.md
+++ b/README.md
@@ -48,10 +48,10 @@ inputs:
     description: Arguments to pass to `alr toolchain` after setup.
     required: false
     default: 'gnat_native gprbuild'
-  toolchain_dir:
-    description: Location to install the toolchain under.
+  msys2:
+    description: Whether to install MSYS2 on Windows. When false, `msys2.do_not_install` will be set to true in alr's settings.
     required: false
-    default: ''
+    default: true
   cache:
     description: Whether to reuse a cached previous install.
     required: false
diff --git a/action.yml b/action.yml
index 58f3b777..01e885e4 100644
--- a/action.yml
+++ b/action.yml
@@ -16,6 +16,10 @@ inputs:
     description: Arguments to pass to `alr toolchain` after setup.
     required: false
     default: 'gnat_native gprbuild'
+  msys2:
+    description: Whether to install MSYS2 on Windows. When false, `msys2.do_not_install` will be set to true in alr's settings.
+    required: false
+    default: true
   cache:
     description: Whether to reuse a cached previous install.
     required: false
@@ -46,11 +50,22 @@ runs:
           echo "version=${{ inputs.version }}" >> $GITHUB_OUTPUT
         fi
 
+    # This saves us duplicating caches for msys2 and non-msys2 on non-Windows
+    - name: Override msys2 for non-Windows platforms
+      id: override
+      shell: bash
+      run: |
+        if [[ "${{ runner.os }}" == "Windows" ]]; then
+          echo "msys2=${{ inputs.msys2 }}" >> $GITHUB_OUTPUT
+        else
+          echo "msys2=false" >> $GITHUB_OUTPUT
+        fi
+
     - name: Generate cache key
       id: cache-key
       shell: bash
       run: |
-        echo "key=alr[1][${{ steps.find-hash.outputs.version }}][${{ inputs.toolchain }}][${{ runner.os }}][${{ runner.arch }}][${{ steps.find-hash.outputs.hash }}]" >> $GITHUB_OUTPUT
+        echo "key=alr[1][${{steps.find-hash.outputs.version}}][${{inputs.toolchain}}][msys2=${{steps.override.outputs.msys2}}][${{runner.os}}][${{runner.arch}}][${{steps.find-hash.outputs.hash}}]" >> $GITHUB_OUTPUT
       # The first value in square brackets is to make the key unique for debugging
 
     - name: Reuse cached installation
@@ -107,7 +122,8 @@ runs:
       shell: bash
       run: echo "need=true" >> $GITHUB_OUTPUT
 
-    # Download a stable alr capable of installing a toolchain
+    # Download a stable alr capable of installing a toolchain. Avoid to install
+    # a msys2 at this point which is not needed and confuses later tests.
     - name: Install GNAT (I)
       if: steps.need-GNAT.outputs.need == 'true'
       shell: bash
@@ -124,6 +140,8 @@ runs:
         unzip -o ${alr_filename} "bin/alr*" -d tmp_alr
         rm ${alr_filename}
         echo "$(pwd -W 2>/dev/null || pwd)/tmp_alr/bin" >> $GITHUB_PATH
+        tmp_alr/bin/alr settings --global --set msys2.do_not_install true || \
+          tmp_alr/bin/alr config --global --set msys2.do_not_install true
 
     # Perform the actual `alr install` and remove the `alr` just used to avoid
     # conflicts with the `alr` being built.
diff --git a/lib/main.js b/lib/main.js
index 576984ff..41c88069 100644
--- a/lib/main.js
+++ b/lib/main.js
@@ -136,19 +136,31 @@ function run() {
         try {
             var version;
             var branch;
+            var msys2;
             var tool_args;
             if (process.argv[2]) { // e.g., node lib/script.js <json object>
                 const inputs = JSON.parse(process.argv[2]);
+                // Log the inputs for the record
+                console.log("Inputs: " + process.argv[2]);
                 version = inputs.version;
                 branch = inputs.branch;
+                msys2 = inputs.msys2 == "true";
                 tool_args = inputs.toolchain;
             }
             else {
                 // Old way in case this is fixed by GH
                 version = core.getInput('version');
                 branch = core.getInput('branch');
+                msys2 = core.getInput('msys2') == "true";
                 tool_args = core.getInput('toolchain');
             }
+            //  Identify the major version number to choose between config/settings
+            const major = parseInt(version.split(".")[0], 10);
+            const settings_cmd = (major >= 2 ? "settings" : "config");
+            // Fail if version <2 and msys2=false (we cannot honor that)
+            if (major < 2 && !msys2 && process.platform == "win32") {
+                throw new Error("MSYS2 installation is mandatory for alr<2.0");
+            }
             // Install the requested version/branch unless cached
             const cached = detect_cached();
             if (!cached) {
@@ -168,6 +180,18 @@ function run() {
                 yield exec.exec(`alr -n settings --global --set index.auto_update_asked true`);
                 console.log("Enabled index auto-refresh without further asking.");
             }
+            //  Disable msys2 installation if requested
+            if (process.platform == "win32") {
+                if (msys2) {
+                    //  Re-enable in case it was disabled during previous actions steps
+                    yield exec.exec(`alr -n ${settings_cmd} --global --set msys2.do_not_install false`);
+                    console.log(`MSYS2 installation NOT disabled (msys2=${msys2})`);
+                }
+                else {
+                    yield exec.exec(`alr -n ${settings_cmd} --global --set msys2.do_not_install true`);
+                    console.log(`MSYS2 installation DISABLED (msys2=${msys2})`);
+                }
+            }
             // And configure the toolchain
             if (tool_args.length > 0 && !cached) {
                 yield exec.exec(`alr -n toolchain ${tool_args != "--disable-assistant" ? "--select " : ""} ${tool_args}`);
diff --git a/src/main.ts b/src/main.ts
index e6ea1336..3742a958 100644
--- a/src/main.ts
+++ b/src/main.ts
@@ -113,20 +113,36 @@ async function run() {
     try {
         var version : string
         var branch  : string
+        var msys2   : boolean
         var tool_args : string
 
         if (process.argv[2]) { // e.g., node lib/script.js <json object>
             const inputs = JSON.parse(process.argv[2])
+
+            // Log the inputs for the record
+            console.log("Inputs: " + process.argv[2])
+
             version   = inputs.version
             branch    = inputs.branch
+            msys2     = inputs.msys2 == "true";
             tool_args = inputs.toolchain
         } else {
             // Old way in case this is fixed by GH
             version   = core.getInput('version');
             branch    = core.getInput('branch');
+            msys2     = core.getInput('msys2') == "true";
             tool_args = core.getInput('toolchain');
         }
 
+        //  Identify the major version number to choose between config/settings
+        const major : number = parseInt(version.split(".")[0], 10);
+        const settings_cmd : string = (major >= 2 ? "settings" : "config");
+
+        // Fail if version <2 and msys2=false (we cannot honor that)
+        if (major < 2 && !msys2 && process.platform == "win32") {
+            throw new Error("MSYS2 installation is mandatory for alr<2.0");
+        }
+
         // Install the requested version/branch unless cached
         const cached : boolean = detect_cached()
 
@@ -150,6 +166,18 @@ async function run() {
             console.log("Enabled index auto-refresh without further asking.");
         }
 
+        //  Disable msys2 installation if requested
+        if (process.platform == "win32") {
+            if (msys2) {
+                //  Re-enable in case it was disabled during previous actions steps
+                await exec.exec(`alr -n ${settings_cmd} --global --set msys2.do_not_install false`);
+                console.log(`MSYS2 installation NOT disabled (msys2=${msys2})`);
+            } else {
+                await exec.exec(`alr -n ${settings_cmd} --global --set msys2.do_not_install true`);
+                console.log(`MSYS2 installation DISABLED (msys2=${msys2})`);
+            }
+        }
+
         // And configure the toolchain
         if (tool_args.length > 0 && !cached) {
             await exec.exec(`alr -n toolchain ${tool_args != "--disable-assistant" ? "--select " : ""} ${tool_args}`);