diff --git a/.github/workflows/checks.yml b/.github/workflows/checks.yml
index 9013ab11f7..690d22eb00 100644
--- a/.github/workflows/checks.yml
+++ b/.github/workflows/checks.yml
@@ -48,8 +48,8 @@ jobs:
           PX4_SBOM_DISABLE: 1
         run: make ${{ matrix.check }}
 
-  tests:
-    name: Unit Tests
+  tests-linux:
+    name: Unit Tests (linux)
     runs-on: [runs-on,runner=8cpu-linux-x64,image=ubuntu24-full-x64,"run-id=${{ github.run_id }}",spot=false,extras=s3-cache]
     container:
       image: ghcr.io/px4/px4-dev:v1.17.0-rc2
@@ -97,3 +97,105 @@ jobs:
       - name: Check for EKF functional changes
         run: git diff --exit-code
         working-directory: src/modules/ekf2/test/change_indication
+
+  tests-windows:
+    name: Unit Tests (windows-${{ matrix.compiler }})
+    # Native Windows host is required: the gtest binaries under
+    # platforms/posix/src/px4/windows/tests/ exercise the _MSC_VER paths
+    # in the shim headers and the MSVC-only sources (socket_msvc.cpp,
+    # sys/time.cpp, etc.). The shim CMakeLists is gated on WIN32 in
+    # platforms/posix/CMakeLists.txt, so a Linux runner cannot stand in.
+    # The compiler axis covers both native MSVC (cl.exe) and the LLVM
+    # clang-cl.exe driver against the same MSVC headers/CRT so we catch
+    # toolchain-specific shim regressions before they hit the SITL build.
+    runs-on: windows-2022
+    strategy:
+      fail-fast: false
+      matrix:
+        compiler: [msvc, clang-cl]
+    steps:
+      - uses: actions/checkout@v6
+        with:
+          submodules: recursive
+          fetch-depth: 1
+
+      - name: Setup Python
+        uses: actions/setup-python@v6
+        with:
+          python-version: '3.11'
+
+      - name: Install Python deps
+        shell: pwsh
+        run: |
+          python -m pip install --upgrade pip
+          # empy pinned <4: PX4 generators use the empy 3.x Interpreter API
+          # removed in 4.x. Matches Tools/setup/requirements.txt and the
+          # compile_windows workflow.
+          python -m pip install jinja2 pyyaml toml numpy packaging jsonschema future "empy>=3.3,<4" pyros-genmsg kconfiglib
+          # Verify kconfiglib import on the same Python that CMake will
+          # pick so we fail loud here rather than during cmake configure.
+          python -c "import kconfiglib; print(kconfiglib.__file__)"
+
+      - name: Install Ninja
+        uses: seanmiddleditch/gha-setup-ninja@v5
+
+      - name: Enable MSVC environment (x64)
+        uses: ilammy/msvc-dev-cmd@v1
+        with:
+          arch: x64
+
+      # Configure with the px4_sitl_test board, which flips
+      # CMAKE_TESTING=ON and pulls in the Windows shim test subdirectory
+      # via px4_posix_windows_add_tests() (see platforms/posix/CMakeLists.txt).
+      # Pin Python3_ROOT_DIR to the setup-python interpreter so cmake's
+      # find_package(Python3) does not pick the runner image's other Python
+      # where kconfiglib is not installed. The CC/CXX env vars decide which
+      # toolchain CMake picks: empty for native MSVC (the default cl.exe
+      # discovered via vcvars), or clang-cl.exe for the LLVM driver. We
+      # invoke cmake directly here rather than `make px4_sitl_test` so we
+      # only configure and can then build a narrow set of targets next.
+      - name: Configure (px4_sitl_test)
+        shell: pwsh
+        env:
+          CC: ${{ matrix.compiler == 'clang-cl' && 'clang-cl' || '' }}
+          CXX: ${{ matrix.compiler == 'clang-cl' && 'clang-cl' || '' }}
+        run: |
+          cmake -S . -B build/px4_sitl_test -G Ninja `
+            -DCONFIG=px4_sitl_test `
+            -DPython3_ROOT_DIR="$env:pythonLocation" `
+            -DPython3_FIND_STRATEGY=LOCATION
+
+      # Build only the Windows shim gtest binaries. The full test_results
+      # ctest target also pulls in fuzztest and the px4 binary, neither of
+      # which we need to gate the shim layer here. Names are emitted by
+      # px4_add_unit_gtest() with a `unit-` prefix (see cmake/px4_add_gtest.cmake).
+      - name: Build Windows shim unit tests
+        shell: pwsh
+        run: |
+          cmake --build build/px4_sitl_test --target unit-test_windows_shim_headers unit-test_windows_shim_runtime unit-test_windows_shim_poll unit-test_windows_shim_io
+
+      - name: Run Windows shim unit tests
+        shell: pwsh
+        run: |
+          $tests = @(
+            'unit-test_windows_shim_headers',
+            'unit-test_windows_shim_runtime',
+            'unit-test_windows_shim_poll',
+            'unit-test_windows_shim_io'
+          )
+          $failed = $false
+          foreach ($name in $tests) {
+            $exe = Get-ChildItem -Path build\px4_sitl_test -Filter "$name.exe" -Recurse -ErrorAction SilentlyContinue | Select-Object -First 1
+            if ($null -eq $exe) {
+              Write-Host "::error::Test binary not found: $name"
+              $failed = $true
+              continue
+            }
+            Write-Host "=== Running $($exe.Name) ==="
+            & $exe.FullName --gtest_color=yes
+            if ($LASTEXITCODE -ne 0) {
+              Write-Host "::error::$name failed with exit $LASTEXITCODE"
+              $failed = $true
+            }
+          }
+          if ($failed) { exit 1 }
diff --git a/.github/workflows/compile_macos.yml b/.github/workflows/compile_macos.yml
index 899d21e3a0..81526eeeea 100644
--- a/.github/workflows/compile_macos.yml
+++ b/.github/workflows/compile_macos.yml
@@ -46,7 +46,18 @@ jobs:
     - name: setup
       run: |
         ./Tools/setup/macos.sh
+        # Make the venv created by macos.sh the active Python for all
+        # subsequent steps. Prepending .venv/bin to PATH alone is not
+        # enough: actions/setup-python exports Python3_ROOT_DIR /
+        # pythonLocation pointing at the hostedtoolcache interpreter,
+        # and find_package(Python3) honours those hints over PATH. The
+        # PX4 Python deps (kconfiglib, empy, etc.) only live in the
+        # venv, so configure fails at the kconfiglib import otherwise.
         echo "${{ github.workspace }}/.venv/bin" >> $GITHUB_PATH
+        echo "VIRTUAL_ENV=${{ github.workspace }}/.venv" >> $GITHUB_ENV
+        echo "Python3_ROOT_DIR=${{ github.workspace }}/.venv" >> $GITHUB_ENV
+        echo "Python_ROOT_DIR=${{ github.workspace }}/.venv" >> $GITHUB_ENV
+        echo "pythonLocation=${{ github.workspace }}/.venv" >> $GITHUB_ENV
 
     - uses: ./.github/actions/setup-ccache
       id: ccache
diff --git a/.github/workflows/compile_windows.yml b/.github/workflows/compile_windows.yml
new file mode 100644
index 0000000000..8fc65892a2
--- /dev/null
+++ b/.github/workflows/compile_windows.yml
@@ -0,0 +1,157 @@
+name: Windows SITL build
+
+# Build px4_sitl_default for Windows on both supported toolchains:
+#   - MinGW-w64 cross-compile from Ubuntu (gate for the platforms/posix
+#     Windows shim layer and ld --wrap socket plumbing)
+#   - Native MSVC on a Windows runner (gate for the _MSC_VER paths in
+#     the shim headers and the MSVC-only sources under
+#     src/px4/windows/posix, e.g. socket_msvc.cpp / sys/time.cpp)
+
+on:
+  push:
+    branches:
+      - 'main'
+      - 'stable'
+      - 'beta'
+      - 'release/**'
+    paths-ignore:
+      - 'docs/**'
+  pull_request:
+    branches:
+      - '**'
+    paths-ignore:
+      - 'docs/**'
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+permissions:
+  contents: read
+
+jobs:
+  build:
+    name: Windows SITL (${{ matrix.toolchain }})
+    runs-on: ${{ matrix.runner }}
+    strategy:
+      fail-fast: false
+      matrix:
+        include:
+          - toolchain: MinGW
+            runner: ubuntu-24.04
+            artifact: px4-sitl-windows-x86_64-mingw
+          - toolchain: MSVC
+            runner: windows-2022
+            artifact: px4-sitl-windows-x86_64-msvc
+
+    steps:
+      - uses: actions/checkout@v4
+        with:
+          submodules: recursive
+          fetch-depth: 0
+
+      # ---- MinGW cross-build path (Ubuntu host) ----
+
+      - name: Install MinGW-w64 + PX4 base deps
+        if: matrix.toolchain == 'MinGW'
+        run: |
+          sudo apt-get update
+          # The *-posix variants of gcc/g++ ship winpthreads which PX4
+          # needs for std::thread and pthread_*.
+          sudo apt-get install -y --no-install-recommends \
+              mingw-w64 \
+              g++-mingw-w64-x86-64-posix \
+              gcc-mingw-w64-x86-64-posix \
+              cmake ninja-build ccache \
+              python3 python3-pip python3-jinja2 python3-yaml \
+              python3-toml python3-numpy python3-packaging \
+              python3-jsonschema python3-future \
+              python3-kconfiglib
+          # python3-pyros-genmsg is not packaged in Ubuntu noble; install
+          # it via pip alongside empy. PX4's msg/uxrce_dds/flight_tasks/
+          # zenoh generators rely on the empy 3.x Interpreter API, hence
+          # the <4 pin (matches Tools/setup/requirements.txt).  Use
+          # --break-system-packages on Ubuntu 24.04 where pip refuses to
+          # touch the system site-packages by default.
+          python3 -m pip install --user --break-system-packages "empy>=3.3,<4" pyros-genmsg || \
+              python3 -m pip install --user "empy>=3.3,<4" pyros-genmsg
+
+      - name: Setup ccache (MinGW)
+        if: matrix.toolchain == 'MinGW'
+        uses: hendrikmuhs/ccache-action@v1.2
+        with:
+          key: ccache-windows-sitl-mingw
+          max-size: 500M
+
+      - name: Cross-build px4.exe (MinGW)
+        if: matrix.toolchain == 'MinGW'
+        env:
+          CMAKE_ARGS: -DCMAKE_TOOLCHAIN_FILE=Toolchain-mingw-w64-x86_64
+        run: make px4_sitl_default
+
+      # ---- MSVC native path (Windows host) ----
+
+      - name: Setup Python (MSVC)
+        if: matrix.toolchain == 'MSVC'
+        uses: actions/setup-python@v6
+        with:
+          python-version: '3.11'
+
+      - name: Install Python deps (MSVC)
+        if: matrix.toolchain == 'MSVC'
+        shell: pwsh
+        run: |
+          python -m pip install --upgrade pip
+          # empy pinned <4: PX4's msg/uxrce_dds/flight_tasks/zenoh generators
+          # use the empy 3.x Interpreter(...options={em.RAW_OPT: True,
+          # em.BUFFERED_OPT: True}) API removed in empy 4.x. Matches the pin
+          # in Tools/setup/requirements.txt and Tools/setup/windows.ps1.
+          python -m pip install jinja2 pyyaml toml numpy packaging jsonschema future "empy>=3.3,<4" pyros-genmsg kconfiglib
+          # Verify kconfiglib import on the same Python that CMake will pick
+          # so we fail loud here rather than deep in cmake configure. Use
+          # `import kconfiglib` (not `menuconfig`) because the menuconfig
+          # entry point pulls in the standard `curses` module which is not
+          # shipped on Windows Pythons by default.
+          python -c "import kconfiglib; print(kconfiglib.__file__)"
+
+      - name: Install Ninja (MSVC)
+        if: matrix.toolchain == 'MSVC'
+        uses: seanmiddleditch/gha-setup-ninja@v5
+
+      - name: Install GNU make (MSVC)
+        if: matrix.toolchain == 'MSVC'
+        shell: pwsh
+        run: choco install -y make --no-progress
+
+      - name: Enable MSVC environment (x64)
+        if: matrix.toolchain == 'MSVC'
+        uses: ilammy/msvc-dev-cmd@v1
+        with:
+          arch: x64
+
+      # Run the standard top-level make target. With cl.exe + ninja on
+      # PATH (the previous step exports them via $GITHUB_ENV), the
+      # Makefile's existing Ninja branch picks the Ninja generator and
+      # cmake auto-detects MSVC as the host compiler — same entry point
+      # as Linux/macOS, no toolchain file needed.
+      #
+      # Pin Python3_ROOT_DIR to the setup-python interpreter so cmake's
+      # find_package(Python3) does not pick up the runner-image's other
+      # Python (3.12) where kconfiglib is not installed.
+      - name: Native-build px4.exe (MSVC)
+        if: matrix.toolchain == 'MSVC'
+        shell: bash
+        env:
+          CMAKE_ARGS: -DPython3_ROOT_DIR=${{ env.pythonLocation }} -DPython3_FIND_STRATEGY=LOCATION
+        run: make px4_sitl_default
+
+      # ---- Common ----
+
+      - name: Upload px4.exe artifact
+        if: success()
+        uses: actions/upload-artifact@v4
+        with:
+          name: ${{ matrix.artifact }}
+          path: build/px4_sitl_default/bin/px4.exe
+          if-no-files-found: error
+          retention-days: 14