diff --git a/libcxx/include/string b/libcxx/include/string
--- a/libcxx/include/string
+++ b/libcxx/include/string
@@ -1725,7 +1725,7 @@
         }
         else
         {
-            __grow_by(__cap, __sz + __n - __cap, __sz, __ip, 0, __n);
+            __grow_by_without_replace(__cap, __sz + __n - __cap, __sz, __ip, 0, __n);
             __p = std::__to_address(__get_long_pointer());
         }
         __sz += __n;
@@ -1839,9 +1839,12 @@
     template <class _ForwardIterator, __enable_if_t<__has_forward_iterator_category<_ForwardIterator>::value, int> = 0>
     inline _LIBCPP_CONSTEXPR_SINCE_CXX20 void __init(_ForwardIterator __first, _ForwardIterator __last);
 
-    _LIBCPP_CONSTEXPR_SINCE_CXX20
+    _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI_AFTER_V1 _LIBCPP_DEPRECATED_("use __grow_by_without_replace")
     void __grow_by(size_type __old_cap, size_type __delta_cap, size_type __old_sz,
                    size_type __n_copy,  size_type __n_del,     size_type __n_add = 0);
+    _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI
+    void __grow_by_without_replace(size_type __old_cap, size_type __delta_cap, size_type __old_sz,
+                   size_type __n_copy,  size_type __n_del,     size_type __n_add = 0);
     _LIBCPP_CONSTEXPR_SINCE_CXX20
     void __grow_by_and_replace(size_type __old_cap, size_type __delta_cap, size_type __old_sz,
                                size_type __n_copy,  size_type __n_del,
@@ -2277,9 +2280,12 @@
     traits_type::assign(__p[__old_sz], value_type());
 }
 
+// __grow_by is deprecated because it does not set size. It not only may not update when size is changed,
+// but also not set size at all when string was short initially, leading to unpredictable size value.
+// It is not removed or changed to avoid breaking the ABI.
 template <class _CharT, class _Traits, class _Allocator>
 void
-_LIBCPP_CONSTEXPR_SINCE_CXX20
+_LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI_AFTER_V1 _LIBCPP_DEPRECATED_("use __grow_by_without_replace")
 basic_string<_CharT, _Traits, _Allocator>::__grow_by(size_type __old_cap, size_type __delta_cap, size_type __old_sz,
                                                      size_type __n_copy,  size_type __n_del,     size_type __n_add)
 {
@@ -2308,6 +2314,38 @@
     __set_long_cap(__allocation.count);
 }
 
+template <class _CharT, class _Traits, class _Allocator>
+void _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI
+basic_string<_CharT, _Traits, _Allocator>::__grow_by_without_replace(
+    size_type __old_cap,
+    size_type __delta_cap,
+    size_type __old_sz,
+    size_type __n_copy,
+    size_type __n_del,
+    size_type __n_add) {
+    size_type __ms = max_size();
+    if (__delta_cap > __ms - __old_cap)
+        __throw_length_error();
+    pointer __old_p = __get_pointer();
+    size_type __cap =
+        __old_cap < __ms / 2 - __alignment ? __recommend(std::max(__old_cap + __delta_cap, 2 * __old_cap)) : __ms - 1;
+    auto __allocation = std::__allocate_at_least(__alloc(), __cap + 1);
+    pointer __p       = __allocation.ptr;
+    __begin_lifetime(__p, __allocation.count);
+    std::__debug_db_invalidate_all(this);
+    if (__n_copy != 0)
+        traits_type::copy(std::__to_address(__p), std::__to_address(__old_p), __n_copy);
+    size_type __sec_cp_sz = __old_sz - __n_del - __n_copy;
+    if (__sec_cp_sz != 0)
+        traits_type::copy(
+            std::__to_address(__p) + __n_copy + __n_add, std::__to_address(__old_p) + __n_copy + __n_del, __sec_cp_sz);
+    if (__libcpp_is_constant_evaluated() || __old_cap + 1 != __min_cap)
+        __alloc_traits::deallocate(__alloc(), __old_p, __old_cap + 1);
+    __set_long_pointer(__p);
+    __set_long_cap(__allocation.count);
+    __set_long_size(__old_sz - __n_del + __n_add);
+}
+
 // assign
 
 template <class _CharT, class _Traits, class _Allocator>
@@ -2367,7 +2405,7 @@
     if (__cap < __n)
     {
         size_type __sz = size();
-        __grow_by(__cap, __n - __cap, __sz, 0, __sz);
+        __grow_by_without_replace(__cap, __n - __cap, __sz, 0, __sz);
     }
     value_type* __p = std::__to_address(__get_pointer());
     traits_type::assign(__p, __n, __c);
@@ -2487,7 +2525,7 @@
         if (__cap < __n)
         {
             size_type __sz = size();
-            __grow_by(__cap, __n - __cap, __sz, 0, __sz);
+            __grow_by_without_replace(__cap, __n - __cap, __sz, 0, __sz);
         }
         pointer __p = __get_pointer();
         for (; __first != __last; ++__p, (void) ++__first)
@@ -2584,7 +2622,7 @@
         size_type __cap = capacity();
         size_type __sz = size();
         if (__cap - __sz < __n)
-            __grow_by(__cap, __sz + __n - __cap, __sz, __sz, 0);
+            __grow_by_without_replace(__cap, __sz + __n - __cap, __sz, __sz, 0);
         pointer __p = __get_pointer();
         traits_type::assign(std::__to_address(__p) + __sz, __n, __c);
         __sz += __n;
@@ -2603,7 +2641,7 @@
         size_type __cap = capacity();
         size_type __sz = size();
         if (__cap - __sz < __n)
-            __grow_by(__cap, __sz + __n - __cap, __sz, __sz, 0);
+            __grow_by_without_replace(__cap, __sz + __n - __cap, __sz, __sz, 0);
         pointer __p = __get_pointer();
         __sz += __n;
         __set_size(__sz);
@@ -2631,7 +2669,7 @@
     }
     if (__sz == __cap)
     {
-        __grow_by(__cap, 1, __sz, __sz, 0);
+        __grow_by_without_replace(__cap, 1, __sz, __sz, 0);
         __is_short = false; // the string is always long after __grow_by
     }
     pointer __p = __get_pointer();
@@ -2664,7 +2702,7 @@
             !__addr_in_range(*__first))
         {
             if (__cap - __sz < __n)
-                __grow_by(__cap, __sz + __n - __cap, __sz, __sz, 0);
+              __grow_by_without_replace(__cap, __sz + __n - __cap, __sz, __sz, 0);
             pointer __p = __get_pointer() + __sz;
             for (; __first != __last; ++__p, (void) ++__first)
                 traits_type::assign(*__p, *__first);
@@ -2777,7 +2815,7 @@
         }
         else
         {
-            __grow_by(__cap, __sz + __n - __cap, __sz, __pos, 0, __n);
+            __grow_by_without_replace(__cap, __sz + __n - __cap, __sz, __pos, 0, __n);
             __p = std::__to_address(__get_long_pointer());
         }
         traits_type::assign(__p + __pos, __n, __c);
@@ -2874,7 +2912,7 @@
     value_type* __p;
     if (__cap == __sz)
     {
-        __grow_by(__cap, 1, __sz, __ip, 0, 1);
+        __grow_by_without_replace(__cap, 1, __sz, __ip, 0, 1);
         __p = std::__to_address(__get_long_pointer());
     }
     else
@@ -2965,7 +3003,7 @@
     }
     else
     {
-        __grow_by(__cap, __sz - __n1 + __n2 - __cap, __sz, __pos, __n1, __n2);
+        __grow_by_without_replace(__cap, __sz - __n1 + __n2 - __cap, __sz, __pos, __n1, __n2);
         __p = std::__to_address(__get_long_pointer());
     }
     traits_type::assign(__p + __pos, __n2, __c);