Build the Windows probe with a Visual Studio the runner actually has

.github/DESIGN.md

@@ -45,4 +45,10 @@ detail.
 - `tools/perf-context-probe` links each archived library in turn and asserts
   `user_key_comparison_count` is non-zero for the enabled one and zero for the disabled one. The
   matrix's `run_probe` is false exactly where the runner cannot execute the target's binaries
-  (`darwin-x64`, both `windows-arm64` targets).
+  (`darwin-x64`, both `windows-arm64` targets). On Windows the generator and architecture are named
+  rather than left to CMake, whose default search falls back to NMake Makefiles and then fails with
+  no compiler. The generator name is read from vswhere and CMake rather than written down, so a
+  runner-image change surfaces as a clear error instead of an unbuildable default. The probe also
+  gets its own newer CMake there: the build is pinned to 3.31.10 to match the pinned vcpkg baseline,
+  and that CMake has no generator for the VS the image now ships. The probe only consumes the
+  installed package, so its CMake is independent of the one that built the libraries.

.github/scripts/verify-prebuild-archive.sh

@@ -191,12 +191,42 @@ if [[ "$RUN_PROBE" != "true" ]]; then
   exit 0
 fi
 
+# This step has no Visual Studio developer prompt, so CMake's default generator search can land on
+# NMake Makefiles and fail with no compiler.
+GENERATOR_ARGS=()
+if [[ "$windows_paths" == true ]]; then
+  vswhere="/c/Program Files (x86)/Microsoft Visual Studio/Installer/vswhere.exe"
+  if [[ ! -x "$vswhere" ]]; then
+    echo "::error::no vswhere.exe at ${vswhere}; cannot tell which Visual Studio to build the probe with" >&2
+    exit 1
+  fi
+  vs_major="$("$vswhere" -latest -property installationVersion 2>/dev/null | cut -d. -f1 | tr -cd '0-9' || true)"
+  if [[ -z "$vs_major" ]]; then
+    echo "::error::vswhere reported no Visual Studio installation" >&2
+    exit 1
+  fi
+  if ! cmake_help="$(cmake --help)"; then
+    echo "::error::cmake --help failed; cannot choose a generator" >&2
+    exit 1
+  fi
+  # head closes the pipe early, which pipefail would otherwise read as a failure.
+  generator="$(printf '%s\n' "$cmake_help" |
+    sed -n "s/^[ *]*\(Visual Studio ${vs_major} [0-9][0-9][0-9][0-9]\).*/\1/p" | head -1 || true)"
+  if [[ -z "$generator" ]]; then
+    echo "::error::$(cmake --version | head -1) has no generator for the installed Visual Studio ${vs_major}.x" >&2
+    exit 1
+  fi
+  echo "Building the probe with ${generator}"
+  GENERATOR_ARGS=(-G "$generator")
+fi
+
 echo "=== Behavioural probe ==="
 run_probe() {
   local name="$1" expectation="$2" build_dir="${SCRATCH}/probe-$1"
   shift 2
 
   cmake -S "$PROBE_DIR" -B "$build_dir" \
+    ${GENERATOR_ARGS[@]+"${GENERATOR_ARGS[@]}"} \
     -DCMAKE_BUILD_TYPE=Release \
     -DCMAKE_PREFIX_PATH="$PREFIX" \
     ${EXTRA_CMAKE_ARGS[@]+"${EXTRA_CMAKE_ARGS[@]}"} \

.github/workflows/build.yml

@@ -467,12 +467,12 @@ jobs:
             target: windows-x64
             triplet: x64-windows-static
             run_probe: true
-            probe_cmake_args: -DCMAKE_MSVC_RUNTIME_LIBRARY=MultiThreaded
+            probe_cmake_args: -A x64 -DCMAKE_MSVC_RUNTIME_LIBRARY=MultiThreaded
           - runner: windows-latest
             target: windows-x64-static-md
             triplet: x64-windows-static-md
             run_probe: true
-            probe_cmake_args: -DCMAKE_MSVC_RUNTIME_LIBRARY=MultiThreadedDLL
+            probe_cmake_args: -A x64 -DCMAKE_MSVC_RUNTIME_LIBRARY=MultiThreadedDLL
           - runner: windows-latest
             target: windows-arm64
             triplet: arm64-windows-static
@@ -490,7 +490,7 @@ jobs:
 
       - name: Install cmake
         if: ${{ matrix.platform.target != 'linux-x64-musl' && matrix.platform.target != 'linux-arm64-musl' }}
-        uses: lukka/get-cmake@latest
+        uses: lukka/get-cmake@a89e43e22a96abafcd7a62db652a65ecd33052c9 # latest as of 2026-10-04
         with:
           cmakeVersion: 3.31.10
 
@@ -628,6 +628,13 @@ jobs:
           tar -cf - . | xz -9 > $ARCHIVE
           echo "archive=$ARCHIVE" >> $GITHUB_OUTPUT
 
+      # Separate from the build's 3.31.10, which has no generator for the VS this image ships.
+      - name: Install a CMake that knows the installed Visual Studio (probe only)
+        if: ${{ startsWith(matrix.platform.target, 'windows-') && matrix.platform.run_probe }}
+        uses: lukka/get-cmake@a89e43e22a96abafcd7a62db652a65ecd33052c9 # latest as of 2026-10-04
+        with:
+          cmakeVersion: 4.4.3
+
       - name: Verify release archive
         if: ${{ matrix.platform.target != 'linux-x64-musl' && matrix.platform.target != 'linux-arm64-musl' }}
         shell: bash