diff --git a/libc/CMakeLists.txt b/libc/CMakeLists.txt
--- a/libc/CMakeLists.txt
+++ b/libc/CMakeLists.txt
@@ -76,7 +76,7 @@
 set(LIBC_TARGET_TRIPLE "" CACHE STRING "The target triple for the libc build.")
 
 set(LIBC_ENABLE_UNITTESTS ON)
-set(LIBC_ENABLE_HERMETIC_TESTS ON)
+set(LIBC_ENABLE_HERMETIC_TESTS ${LLVM_LIBC_FULL_BUILD})
 
 # Defines LIBC_TARGET_ARCHITECTURE and associated macros.
 include(LLVMLibCArchitectures)