diff --git a/libcxx/CMakeLists.txt b/libcxx/CMakeLists.txt
--- a/libcxx/CMakeLists.txt
+++ b/libcxx/CMakeLists.txt
@@ -133,7 +133,7 @@
     "Whether to enable support for incomplete library features. Incomplete features
     are new library features under development. These features don't guarantee
     ABI stability nor the quality of completed library features. Vendors
-    shipping the library may want to disable this option." ON)
+    shipping the library may want to disable this option." OFF)
 set(LIBCXX_TEST_CONFIG "legacy.cfg.in" CACHE STRING
     "The path to the Lit testing configuration to use when running the tests.
      If a relative path is provided, it is assumed to be relative to '<monorepo>/libcxx/test/configs'.")