diff --git a/stl/inc/format b/stl/inc/format
index 1ed63d4ec7d..fdcf65e7908 100644
--- a/stl/inc/format
+++ b/stl/inc/format
@@ -181,7 +181,7 @@ concept _Parse_spec_callbacks = _Parse_align_callbacks<_Ty, _CharT>
                                 && _Parse_precision_callbacks<_Ty, _CharT>
                                 && _Width_adapter_callbacks<_Ty, _CharT>
                                 && _Precision_adapter_callbacks<_Ty, _CharT>
-                                && requires(_Ty _At, basic_string_view<_CharT> _Sv, _Fmt_align _Aln, _Fmt_sign _Sgn) {
+                                && requires(_Ty _At, _Fmt_sign _Sgn) {
                                     { _At._On_sign(_Sgn) } -> same_as<void>;
                                     { _At._On_hash() } -> same_as<void>;
                                     { _At._On_zero() } -> same_as<void>;
@@ -2089,19 +2089,22 @@ public:
     }
 
     _NODISCARD size_t _Estimate_required_capacity() const noexcept {
-        using _CharT        = typename _Context::char_type;
-        size_t _Result      = 0;
-        const auto _Visitor = [&_Result]<class _ArgTy>(const _ArgTy _Arg) noexcept {
-            if constexpr (is_same_v<_ArgTy, basic_string_view<_CharT>>) {
-                _Result += _Arg.size();
-            } else if constexpr (is_same_v<_ArgTy, const _CharT*>) {
+        using _CharType = typename _Context::char_type;
+        size_t _Result  = 0;
+
+        for (size_t _Idx = 0; _Idx < _Num_args; ++_Idx) {
+            const auto _Packed_index = _Index_array[_Idx];
+            const auto _Arg_type     = _Packed_index._Type();
+            if (_Arg_type == _Basic_format_arg_type::_String_type) {
+                const auto _Arg_storage =
+                    reinterpret_cast<const unsigned char*>(_Index_array + _Num_args) + _Packed_index._Index;
+                const auto _View = _Get_value_from_memory<basic_string_view<_CharType>>(_Arg_storage);
+                _Result += _View.size();
+            } else if (_Arg_type == _Basic_format_arg_type::_CString_type) {
                 _Result += 32; // estimate for length of null-terminated strings
             } else {
                 _Result += 8; // estimate for length of all other arguments
             }
-        };
-        for (size_t _Idx = 0; _Idx < _Num_args; ++_Idx) {
-            _STD visit_format_arg(_Visitor, get(_Idx));
         }
         return _Result;
     }
@@ -2589,9 +2592,11 @@ _NODISCARD _OutputIt _Write_sign(_OutputIt _Out, const _Fmt_sign _Sgn, const boo
     return _Out;
 }
 
-inline void _Buffer_to_uppercase(char* _First, const char* _Last) {
+inline void _Buffer_to_uppercase(char* _First, const char* _Last) noexcept {
     for (; _First != _Last; ++_First) {
-        *_First = static_cast<char>(_CSTD toupper(*_First));
+        if (*_First >= 'a' && *_First <= 'z') {
+            *_First -= 'a' - 'A';
+        }
     }
 }
 
@@ -2722,19 +2727,14 @@ _NODISCARD _OutputIt _Write_integral(
         _Specs._Sgn = _Fmt_sign::_Minus;
     }
 
-    int _Base      = 10;
-    bool _To_upper = false;
+    int _Base = 10;
 
     switch (_Specs._Type) {
     case 'B':
-        _To_upper = true;
-        [[fallthrough]];
     case 'b':
         _Base = 2;
         break;
     case 'X':
-        _To_upper = true;
-        [[fallthrough]];
     case 'x':
         _Base = 16;
         break;
@@ -2760,7 +2760,7 @@ _NODISCARD _OutputIt _Write_integral(
         _Buffer_start += 1;
     }
 
-    if (_To_upper) {
+    if (_Specs._Type == 'X') {
         _Buffer_to_uppercase(_Buffer_start, _End);
     }
 
@@ -2965,9 +2965,10 @@ _NODISCARD _OutputIt _Fmt_write(
         }
     }
 
+    _STL_INTERNAL_CHECK(_Exponent == 'e' || _Exponent == 'p');
     if (_To_upper) {
         _Buffer_to_uppercase(_Buffer_start, _Result.ptr);
-        _Exponent = static_cast<char>(_CSTD toupper(_Exponent));
+        _Exponent -= 'a' - 'A';
     }
 
     const auto _Is_finite = (_STD isfinite)(_Value);