--- buildtools/third_party/libc++/BUILD.gn.orig	2026-09-13 16:45:18 UTC
+++ buildtools/third_party/libc++/BUILD.gn
@@ -595,6 +595,19 @@ target(libcxx_target_type, "libc++") {
     }
   }
 
+  if (!use_custom_libcxx) {
+    # When use_custom_libcxx=false the runtime_library config does not add
+    # -nostdinc++ or the bundled include paths.  The libc++ sources still need
+    # their own headers (e.g. __config with _LIBCPP_BEGIN_NAMESPACE_LFTS),
+    # so add them explicitly here to avoid picking up mismatched system headers.
+    cflags_cc = [
+      "-nostdinc++",
+      "-isystem" + rebase_path("//buildtools/third_party/libc++", root_build_dir),
+      "-isystem" + rebase_path("//third_party/libc++/src/include", root_build_dir),
+      "-isystem" + rebase_path("//third_party/libc++abi/src/include", root_build_dir),
+    ]
+  }
+
   # Disabling -Wexit-time-destructors, as libc++ uses `static string` objects
   # for locale code.
   configs += [ "//build/config/compiler:no_exit_time_destructors" ]