diff --git a/libcxx/utils/ci/buildkite-pipeline.yml b/libcxx/utils/ci/buildkite-pipeline.yml
--- a/libcxx/utils/ci/buildkite-pipeline.yml
+++ b/libcxx/utils/ci/buildkite-pipeline.yml
@@ -767,6 +767,19 @@
             limit: 2
       timeout_in_minutes: 120
 
+    - label: "MinGW (Incomplete Sysroot)"
+      command: "bash libcxx/utils/ci/run-buildbot mingw-incomplete-sysroot"
+      artifact_paths:
+        - "**/test-results.xml"
+        - "**/*.abilist"
+      agents:
+        queue: "windows"
+      retry:
+        automatic:
+          - exit_status: -1  # Agent was lost
+            limit: 2
+      timeout_in_minutes: 120
+
     - label: "MinGW (DLL, i686)"
       command: "bash libcxx/utils/ci/run-buildbot mingw-dll-i686"
       artifact_paths:
diff --git a/libcxx/utils/ci/run-buildbot b/libcxx/utils/ci/run-buildbot
--- a/libcxx/utils/ci/run-buildbot
+++ b/libcxx/utils/ci/run-buildbot
@@ -645,6 +645,45 @@
           -C "${MONOREPO_ROOT}/libcxx/cmake/caches/MinGW.cmake"
     check-runtimes
 ;;
+mingw-incomplete-sysroot)
+    clean
+    # When bringing up a new cross compiler from scratch, we build
+    # libunwind/libcxx in a setup where the toolchain is incomplete and
+    # unable to perform the normal linker checks; this requires a few
+    # special cases in the CMake files.
+    #
+    # The CI environment uses prebuilt complete toolchains, but make a
+    # copy of a sysroot into the build dir, and remove libunwind/libcxx
+    # to simulate the real case of bringing up a new toolchain.
+    # Pass this sysroot to CMake via CMAKE_*_FLAGS_INIT to make sure it
+    # is used throughout all the early CMake probing stages.
+    mkdir -p "${BUILD_DIR}/sysroot"
+    TOOLCHAIN="$(dirname $(command -v x86_64-w64-mingw32-clang))/.."
+    cp -a "${TOOLCHAIN}/x86_64-w64-mingw32/lib" "${BUILD_DIR}/sysroot"
+    cp -a "${TOOLCHAIN}/include" "${BUILD_DIR}/sysroot"
+    rm -rf "${BUILD_DIR}"/sysroot/lib/lib{c++,unwind}*
+    rm -rf "${BUILD_DIR}"/sysroot/include/c++
+    # Building in an incomplete setup requires setting CMAKE_*_COMPILER_WORKS,
+    # as CMake fails to probe the compiler. This case also requires
+    # setting CMAKE_CXX_COMPILER_TARGET, as LLVM's heuristics for setting
+    # the triple fails when CMake hasn't been able to probe the environment.
+    # (This is what one has to do when building the initial libunwind/libcxx
+    # for a new toolchain.)
+    generate-cmake \
+          -DCMAKE_C_COMPILER=x86_64-w64-mingw32-clang \
+          -DCMAKE_CXX_COMPILER=x86_64-w64-mingw32-clang++ \
+          -DCMAKE_C_FLAGS_INIT="--sysroot=${BUILD_DIR}/sysroot" \
+          -DCMAKE_CXX_FLAGS_INIT="--sysroot=${BUILD_DIR}/sysroot" \
+          -DCMAKE_C_COMPILER_WORKS=TRUE \
+          -DCMAKE_CXX_COMPILER_WORKS=TRUE \
+          -DCMAKE_CXX_COMPILER_TARGET=x86_64-w64-windows-gnu \
+          -C "${MONOREPO_ROOT}/libcxx/cmake/caches/MinGW.cmake"
+    # Only test that building succeeds; there's no extra value in running the
+    # tests here, as it would be equivalent to the mingw-dll config above.
+    # Additionally, the sysroot flags from CMAKE_*_FLAGS/CMAKE_*_FLAGS_INIT
+    # don't even propagate into the tests at the moment.
+    ${NINJA} -vC "${BUILD_DIR}"
+;;
 aix)
     clean
     generate-cmake -C "${MONOREPO_ROOT}/libcxx/cmake/caches/AIX.cmake" \