diff --git a/test/lit.cfg b/test/lit.cfg
index a46e1426..4b917081 100644
--- a/test/lit.cfg
+++ b/test/lit.cfg
@@ -238,8 +238,7 @@ else:
 
 # This test suite supports testing against either the system library or the
 # locally built one; the former mode is useful for testing ABI compatibility
-# between the current headers and a shipping dynamic library. We require the
-# user to explicitly pick one of the two modes.
+# between the current headers and a shipping dynamic library.
 use_system_lib_str = lit.params.get('use_system_lib', None)
 if use_system_lib_str is not None:
     if use_system_lib_str.lower() in ('1', 'true'):
@@ -249,7 +248,8 @@ if use_system_lib_str is not None:
     else:
         lit.fatal('user parameter use_system_lib should be 0 or 1')
 else:
-    use_system_lib = True
+    # Default to testing against the locally built libc++ library.
+    use_system_lib = False
 
 # Configure extra compiler flags.
 include_paths = ['-I' + libcxx_src_root + '/include', '-I' + libcxx_src_root + '/test/support']