diff --git a/include/proxy/v4/detail/compatibility_check.h b/include/proxy/v4/detail/compatibility_check.h
new file mode 100644
index 0000000..0c06083
--- /dev/null
+++ b/include/proxy/v4/detail/compatibility_check.h
@@ -0,0 +1,28 @@
+// Copyright (c) 2022-2026 Microsoft Corporation.
+// Copyright (c) 2026-Present Next Gen C++ Foundation.
+// Licensed under the MIT License.
+
+#ifndef MSFT_PROXY_V4_DETAIL_COMPATIBILITY_CHECK_H_
+#define MSFT_PROXY_V4_DETAIL_COMPATIBILITY_CHECK_H_
+
+#if (defined(_MSVC_LANG) ? _MSVC_LANG : __cplusplus) < 202002L
+#error "Proxy requires C++20 or later."
+#endif
+
+// clang-cl miscalculates the layout of an empty [[msvc::no_unique_address]]
+// member in a base class (llvm/llvm-project#143245), corrupting pro::proxy.
+#if __has_cpp_attribute(msvc::no_unique_address)
+namespace pro::inline v4::detail::compatibility_check {
+struct empty {};
+struct base {
+  [[msvc::no_unique_address]] empty value;
+};
+struct derived : base {
+  char dummy;
+};
+static_assert(sizeof(derived) == sizeof(char),
+              "[[msvc::no_unique_address]] is broken");
+} // namespace pro::inline v4::detail::compatibility_check
+#endif
+
+#endif // MSFT_PROXY_V4_DETAIL_COMPATIBILITY_CHECK_H_
diff --git a/include/proxy/v4/proxy.h b/include/proxy/v4/proxy.h
index 84735c4..848c6ef 100644
--- a/include/proxy/v4/proxy.h
+++ b/include/proxy/v4/proxy.h
@@ -5,10 +5,11 @@
 #ifndef MSFT_PROXY_V4_PROXY_H_
 #define MSFT_PROXY_V4_PROXY_H_
 
-#include "detail/core.h"            // IWYU pragma: export
-#include "detail/dispatch.h"        // IWYU pragma: export
-#include "detail/facade_creation.h" // IWYU pragma: export
-#include "detail/proxy_creation.h"  // IWYU pragma: export
-#include "detail/skills.h"          // IWYU pragma: export
+#include "detail/compatibility_check.h" // IWYU pragma: keep
+#include "detail/core.h"                // IWYU pragma: export
+#include "detail/dispatch.h"            // IWYU pragma: export
+#include "detail/facade_creation.h"     // IWYU pragma: export
+#include "detail/proxy_creation.h"      // IWYU pragma: export
+#include "detail/skills.h"              // IWYU pragma: export
 
 #endif // MSFT_PROXY_V4_PROXY_H_